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1. Explained the steps of the cell cycle and the stages involved in each step; including interphase and mitotic stage.

2. Explored the process of apoptosis.

3. Summarized and identified the major events that occur during mitosis and cytokinesis.

4. Described the basic characteristics of a cancer cell.

5. Distinguished between the structure of a prokaryotic and eukaryotic chromosome.

6. Described the phases of meiosis and the major events that occur during each phase.

7. Compared and contrasted the effects of asexual and sexual reproduction on genetic variation.

8. Examined how crossing over and independent assortment contribute to genetic variation.

9. Contrasted the differences between mitosis and meiosis.

10. Examined any changes in the chromosomal structure and number.

11. Described Gregor Mendel’s experiments and why he is considered the “father of genetics”.

12. Constructed and used a Punnett square to genotypic ratios of offspring.

13. Explained Mendel’s law of separation and law of independent assortment.

14. Explained the inheritance pattern of traits when more than two alleles for the trait exist; including incomplete dominance, codominance, sex linked traits, pleiotropy and polygenic traits.

15. Identified the chemical structure of DNA as defined by the Watson and Crick model, including the historical significance of its discovery.

16. Examined the steps involved in the replication of DNA in both prokaryotes and eukaryotes.

17. Examined the central dogma of molecular biology, and thus be able to identify the amino acids sequence from a strand of DNA.

18. Described the events that occur during transcription and translation.

19. Examined the different roles of tRNA, mRNA and rRNA in the central dogma of molecular biology.

20. Described the structure of the eukaryotic chromosome and how it is packaged inside a nucleus.

21. Identified the mechanisms used to regulate prokaryotic genes.

22. Summarized the levels of control of gene expression in eukaryotic cells.

23. Identified some of the techniques used in the field of biotechnology and genomics.

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1. Explained the steps of the cell cycle and the stages involved in each step; including interphase and mitotic stage.

Work citied: Khanaacademy.org

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Cell cycle

The series of growth and development steps a cell undergoes between its formation and reproduction

Interphase

Phase of the cell cycle where the cell grows and makes a copy of its DNA

Mitosis

Phase of the cell cycle where the cell separates its DNA into two sets and divides, forming two new cells

Cancer

A disease of uncontrolled cell growth

The cell cycle

In eukaryotic cells, the cell cycle is divided into two major phases: interphase and mitosis (or the mitotic (M) phase).

Interphase is the longest part of the cell cycle. This is when the cell grows and copies its DNA before moving into mitosis. During mitosis, chromosomes will align, separate, and move into new daughter cells.

The prefix inter- means between, so interphase takes place between one mitotic (M) phase and the next.

Interphase

Interphase consists of three steps:

G phase: first gap phase; the cell grows larger and organelles are copied

S phase: synthesis phase; the cell synthesizes a complete copy of the DNA in its nucleus

G phase: second gap phase; the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis

Mitosis (the M phase)

The process of mitosis, or cell division, is also known as the M phase. This is where the cell divides its previously-copied DNA and cytoplasm to make two new, identical daughter cells.

Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase.

Stages of mitosis

Cancer and cell cycle regulation

Checkpoints

The cell cycle is usually regulated by checkpoints. These are the factors that a cell considers when deciding whether or not to move forward through the cell cycle, and include both external cues (like molecular signals) and internal cues (like DNA damage).

Cancer is a term that describes many different diseases caused by the same problem: uncontrolled cell growth.

Most cancers occur due to a series of mutations that make them divide more quickly, bypass checkpoints during cell division, and avoid apoptosis (programmed cell death).

Generally, mutations of two types of cell cycle regulators can promote the development of cancer:

Positive regulators, which normally promote cell growth, may become hyperactivated (oncogenic).

Negative regulators (tumor suppressors), which prevent the formation of tumors, may become inactivated.

Common mistakes and misconceptions

Interphase is not part of mitosis. Although we often talk about interphase and mitosis together, interphase is technically not part of mitosis. However, both processes are part of the larger cell cycle, where interphase consists of the G, S, and G stages of the cell cycle.

DNA replication occurs during interphase, not prophase. A common misconception is that DNA copies itself during prophase, but this is not true. In prophase, the DNA has already been copied while the cell was in interphase.

The chromosome number is the same in the daughter cells as it was in the parent cell. Because DNA is duplicated during interphase before the cell undergoes mitosis, the amount of DNA in the original parent cell and the daughter cells are exactly the same.

Both genetics, as well as external factors, can play a role in the development of cancer. Many types of cancer have a genetic component, so inheriting certain genes may make someone more likely to get these types of cancer. However, having these genes does not necessarily mean that cancer will develop, as factors such as lifestyle and environment also play a part.

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The cell cycle and mitosis review (article) | Khan Academy

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Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for anyone, anywhere.

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2. Explored the process of apoptosis.
Work Citied: https://www.researchgate.net/figure/Cytology-of-apoptosis-The-different-stages-of-apoptotic-cell-death-start-by-cellular_fig3_274013152

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3. Summarized and identified the major events that occur during mitosis and cytokinesis.

Work Citied: https://bio.libretexts.org/Bookshelves/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.3%3A_Mitotic_Phase_-_Mitosis_and_Cytokinesis

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Page ID

16755

1. Divide and Split

2. The Forms of DNA

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

3. Cytokinesis

4. Review

5. Explore More

6. Attributions

Divide and Split

Can you guess what this colorful image represents? It shows a eukaryotic cell during the process of cell division. In particular, the image shows the nucleus of the cell dividing. In eukaryotic cells, the nucleus divides before the cell itself splits in two; and before the nucleus divides, the cell's DNA is replicated, or copied. There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of genetic material? To answer that question, you first need to know more about DNA and the forms it takes.

Figure \(\PageIndex{1}\): Dividing cell stained with fluorescent dyes. You can see chromosomes in blue and spindles in green.

The Forms of DNA

Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. Only when a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a chromosome, like the one shown in Figure \(\PageIndex{2}\). Because DNA has already replicated, each chromosome actually consists of two identical copies. The two copies of a chromosome are called sister chromatids. Sister chromatids are joined together at a region called a centromere.

Figure \(\PageIndex{2}\): Chromosome. After DNA replicates, it forms X-shaped chromosomes like the one shown here. 1. Chromatid, 2. Centromere, 3. short arm, 4. long arm. Centromere contains proteins called kinetochores (not shown) where spindles attach during mitosis.

The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. Mitosis occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase. They are shown in Figure \(\PageIndex{3}\) and described in detail below.

Figure \(\PageIndex{3}\): Mitosis is the phase of the eukaryotic cell cycle that occurs between DNA replication and the formation of two daughter cells. Mitosis has four substages, prophase, metaphase, anaphase, and telophase.

Prophase

Figure \(\PageIndex{4}\): Prophase in later stage is called prometaphase. The spindle starts to form during the prophase of mitosis. The spindles start to attach to the Kinetochores of centromeres of sister chromatids during Prometaphase.

The first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells that help ensure the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The blue spindle, shown in Figure \(\PageIndex{4}\), consists of fibers made of microtubules.

Metaphase

During metaphase, spindle fibers fully attach to the centromere of each pair of sister chromatids. As you can see in Figure \(\PageIndex{5}\), the sister chromatids line up at the equator, or center, of the cell. The spindle fibers ensure that sister chromatids will separate and go to different daughter cells when the cell divides. Some spindles do not attach to the kinetochore protein of the centromeres. These spindles are called non-kinetochore spindles that help in the elongation of the cell. This is visible in Figure \(\PageIndex{5}\).

Figure \(\PageIndex{5}\): Chromosomes, consisting of sister chromatids, line up at the equator or middle of the cell during metaphase. The blue lines are spindles, and the orange rectangles at the cell poles are centrioles. Some spindles from the opposing centrioles attach with each other, and some spindles attach to the kinetochores of the sister chromosomes from their respective sides. Each chromosome is attached to two spindles.

Anaphase

During anaphase, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibers. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole (see Figure \(\PageIndex{6}\)). At the end of anaphase, each pole of the cell has a complete set of chromosomes

Figure \(\PageIndex{6}\): Anaphase: Sister chromatids break apart and move to the opposite pole with the help of spindles. The newly separated sister chromatids are called chromosomes now.

Telophase

The chromosomes reach the opposite poles and begin to decondense (unravel), relaxing once again into a stretched-out chromatin configuration. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area (see Figure \(\PageIndex{7}\).

Figure \(\PageIndex{7}\): Telophase: The chromosomes decondense, spindles start to disappear, two nuclei form in a cell.

Cytokinesis

Cytokinesis is the final stage of cell division in eukaryotes as well as prokaryotes. During cytokinesis, the cytoplasm splits in two and the cell divides. The process is different in plant and animal cells, as you can see in Figure \(\PageIndex{8}\). In animal cells, the plasma membrane of the parent cell pinches inward along the cell's equator until two daughter cells form. In the plant cells, a cell plate forms along the equator of the parent cell. Then, a new plasma membrane and cell wall form along each side of the cell plate.

Figure \(\PageIndex{8}\): Cytokinesis is the final stage of eukaryotic cell division. It occurs differently in animal (left) and plant (right) cells. You can see a microfilament ring forming at the center of the elongated animal cell. This creates a depression called cleavage furrow. This invagination ultimately separates the cell cytoplasm into two cells. A cell plate forms at the center of the elongated plant cell. Then a new plasma membrane and cell wall form along each side of the cell plate.

Review

1. Describe the different forms that DNA takes before and during cell division in a eukaryotic cell.

2. Identify the four phases of mitosis in an animal cell, and summarize what happens during each phase.

3. Explain what happens during cytokinesis in an animal cell.

4. What are the main differences between mitosis and cytokinesis?

5. The familiar X-shaped chromosome represents:

1. How DNA always looks in eukaryotic cells

2. How DNA in eukaryotic cells looks once it is replicated and the cell is about to divide

3. Female sex chromosomes only

4. How DNA appears immediately after cytokinesis

6. Which of the following is not part of a chromosome in eukaryotic cells?

1. Centriole

2. Centromere

3. Chromatid

4. DNA

7. What do you think would happen if the sister chromatids of one of the chromosomes did not separate during mitosis?

8. Put the following processes in order of when they occur during cell division, from first to last:

1. separation of sister chromatids

2. DNA replication

3. cytokinesis

4. lining up of chromosomes in the center of the cell

5. condensation and coiling of DNA into a chromosome

9. Why do you think the nuclear envelope breaks down at the start of mitosis?

10. What are the fibers made of microtubules that attach to the centromeres during mitosis are called?

11. True or False. Chromosomes begin to uncoil during anaphase.

12. True or False. During cytokinesis in animal cells, sister chromatids line up along the equator of the cell.

13. True or False. After mitosis, the result is typically two daughter cells with identical DNA to each other.

Explore More

Watch the video below to visualize mitosis.

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7.3: Mitotic Phase - Mitosis and Cytokinesis

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Page ID 16755 Divide and Split The Forms of DNA Prophase Metaphase Anaphase Telophase Cytokinesis Review Explore More Attributions Divide and Split Can you guess what this colorful image represents? It shows a eukaryotic cell during the process of cell division. In particular, the image shows the nucleus of the cell dividing.

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4. Described the basic characteristics of a cancer cell.

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Introduction to Cancer Biology (Part 3): Tissue Invasion and Metastasis

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5. Distinguished between the structure of a prokaryotic and eukaryotic chromosome.

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6. Described the phases of meiosis and the major events that occur during each phase.

Work Citied: thoughtco.com

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Meiosis occurs in eukaryotic organisms that reproduce sexually. This includes plants and animals. Meiosis is a two-part cell division process that produces sex cells with one half the number of chromosomes as the parent cell.

Interphase

There are two stages or phases of meiosis: meiosis I and meiosis II. Before a dividing cell enters meiosis, it undergoes a period of growth called interphase. At the end of the meiotic process, four daughter cells are produced.

G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.

S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.

G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.

In the latter part of interphase, the cell still has nucleoli present.

The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.

In animal cells, two pairs of centrioles formed from the replication of one pair are located outside of the nucleus.

At the end of interphase, the cell enters the next phase of meiosis: Prophase I.

In prophase I of meiosis, the following events occur:

Chromosomes condense and attach to the nuclear envelope.​

Synapsis occurs (a pair of homologous chromosomes lines up closely together) and a tetrad is formed. Each tetrad is composed of four chromatids.​

Genetic recombination via crossing over may occur.​

Chromosomes thicken and detach from the nuclear envelope.​

Similar to mitosis, the centrioles migrate away from one another and both the nuclear envelope and nucleoli break down.​

Likewise, the chromosomes begin their migration to the metaphase plate.

At the end of prophase I of meiosis, the cell enters into metaphase I.

Metaphase I

In metaphase I of meiosis, the following events occur:

Tetrads align at the metaphase plate.​

Note that the centromeres of homologous chromosomes are oriented toward the opposite cell poles.

At the end of metaphase I of meiosis, the cell enters into anaphase I.

In anaphase I of meiosis, the following events occur:

Chromosomes move to the opposite cell poles. Similar to mitosis, microtubules such as the kinetochore fibers interact to pull the chromosomes to the cell poles.

Unlike in mitosis, sister chromatids remain together after the homologous chromosomes move to opposite poles.

At the end of anaphase I of meiosis, the cell enters into telophase I.

Telophase I

In telophase I of meiosis, the following events occur:

The spindle fibers continue to move the homologous chromosomes to the poles.

Once movement is complete, each pole has a haploid number of chromosomes.

In most cases, cytokinesis (the division of the cytoplasm) occurs at the same time as telophase I.

At the end of telophase I and cytokinesis, two daughter cells are produced, each with one-half the number of chromosomes of the original parent cell.

Depending on the kind of cell, various processes occur in preparation for meiosis II. There is, however, a constant: The genetic material does not replicate again.

At the end of telophase I of meiosis, the cell enters into prophase II.

Prophase II

In prophase II of meiosis, the following events occur:

The nuclear membrane and nuclei break up while the spindle network appears.​

Chromosomes do not replicate any further in this phase of meiosis.​

The chromosomes begin migrating to the metaphase II plate (at the cell's equator).

At the end of prophase II of meiosis, the cell enters into metaphase II.

Metaphase II

In metaphase II of meiosis, the following events occur:

The chromosomes line up at the metaphase II plate at the cell's center.​

The kinetochore fibers of the sister chromatids point toward opposite poles.

At the end of metaphase II of meiosis, the cell enters into anaphase II.

Anaphase II

In anaphase II of meiosis, the following events occur:

Sister chromatids separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell.​

Once the paired sister chromatids separate from one another, each is considered a full chromosome. They are referred to as daughter chromosomes.​

In preparation for the next stage of meiosis, the two cell poles also move further apart during the course of anaphase II. At the end of anaphase II, each pole contains a complete compilation of chromosomes.

Following anaphase II of meiosis, the cell enters into telophase II.

Telophase II

In telophase II of meiosis, the following events occur:

Distinct nuclei form at the opposite poles.

Cytokinesis (division of the cytoplasm and the formation of two distinct cells) occurs.

At the end of meiosis II, four daughter cells are produced. Each cell has one-half the number of chromosomes as the original parent cell.

Stages of Meiosis: Daughter Cells

The final result of meiosis is the production of four daughter cells. These cells have one half the number of chromosomes as the original cell. Only sex cells are produced by meiosis. Other cell types are produced by mitosis. When sex cells unite during fertilization, these haploid cells become a diploid cell. Diploid cells have the full complement of homologous chromosomes.

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What Occurs During the Different Stages of Meiosis?

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Meiosis occurs in eukaryotic organisms that reproduce sexually. Explore what occurs in each phase of this cell division process.

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7. Compared and contrasted the effects of asexual and sexual reproduction on genetic variation.

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8. Examined how crossing over and independent assortment contribute to genetic variation.

Work Citied: biology.stackexchange.com

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During meiosis, the independent assortment will be made first and then cross over will be made.

No, independent assortment occurs after crossing over. Crossing over occurs in prophase I while independent assortment occurs in metaphase I and anaphase I.

I am so confused, what is the difference between this two process?

During prophase I, a process called synapsis occurs. That is, homologous chromosomes (one from your mother and one from your father) come together and bind together, forming a tetrad. This binding is due to crossing over, which is when the genetic material on the maternal chromosome and the paternal chromosome exchange places. The maternal chromosome gives some of its genes to the paternal chromosome, and the paternal chromosome gives some of its genes to the maternal chromosome. Crossing over increases genetic variety by creating a new chromosome with a new combination of genes.

During metaphase I, the tetrads line up at the equator of the cell. During anaphase one, the tetrads are broken apart and each homologous chromosome that made each tetrad up is pulled toward one end of the cell. Look at the image below, where the blue represents paternal chromosomes and the red represents maternal chromosomes.

Independent assortment describes the phenomenon where the paternal and maternal chromosomes can be lined up randomly at the equator. There are many combinations. For example,

Both red chromosomes could face the top pole and both blue chromosomes could face the bottom pole.

Both blue chromosomes could face the top pole and both red could face the bottom.

One red on the left and one blue on the right could face the top, as shown in the picture.

One blue on the left and one red on the right could face the top.

This random alignment at the equator makes it so that during anaphase, random proportions of maternal and paternal chromosomes get assorted into each of the resulting daughter cells. Some cells could get more maternal chromosomes and the other would get more paternal chromosomes, and vice versa. On the other hand, they could get similar amounts. Independent assortment increases genetic variation by allowing daughter cells to each randomly receive a different proportion of paternal and maternal chromosomes.

In conclusion, crossing over and independent assortment (sometimes called random assortment) are different independent processes that both lead to an increase in genetic diversity.
References

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What is the relationship between independent assortment and cross over?

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begingroup$ During meiosis, the independent assortment will be made first and then cross over will be made. No, independent assortment occurs after crossing over. Crossing over occurs in prophase I while independent assortment occurs in metaphase I and anaphase I. I am so confused, what is the difference between this two process?

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9. Contrasted the differences between mitosis and meiosis.

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Mitosis vs. Meiosis: Side by Side Comparison

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10. Examined any changes in the chromosomal structure and number.

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Alteration of Chromosome Number and Structure

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11. Described Gregor Mendel’s experiments and why he is considered the “father of genetics”.

Work Citied: bio.libretexts.org

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8.2: Mendel’s Experiments and Laws of Inheritance

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These purplish-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology.

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12. Constructed and used a Punnett square to genotypic ratios of offspring.

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Genotypic Ratios and Phenotypic Ratios for Punnett Squares

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13. Explained Mendel’s law of separation and law of independent assortment.

Work Citied: biologydictionary.net

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The Austrian monk Gregor Mendel conducted revolutionary experiments with pea plants in the early 1800s showing the existence of traits (he called them "factors") that offspring inherit from their parents. His work culminates in the three principles of Mendelian inheritance; the law of segregation, the law of independent assortment, and the law of dominance.

The law of dominance says dominant alleles (a variation of a gene) are always expressed in the phenotype (appearance) of an organism because they mask the effects of recessive alleles. Recessive traits are seen only when the offspring inherit the recessive allele for the trait from both parents. The other two laws of Mendelian inheritance deal with creating gametes and the independent nature of their inheritance.

Law of Segregation

Meiosis is the process of creating sperm and egg gametes. Each gamete contains half ( haploid) the number of chromosomes an individual needs, so fertilization results in the offspring receiving one allele for a trait from each parent. The law of segregation states that the parental genes must separate randomly and equally into gametes during meiosis so there is an equal chance of the offspring inheriting either allele. No allele is favored or has an advantage over another.

Mendel observed segregation in his experiments when parental pea plants with two traits produced offspring that all expressed the dominant traits, but their offspring expressed dominant and recessive traits in a 3:1 ratio. As part of this work, Mendel discovered that he could predict the color and height of the offspring.

Law of Independent Assortment

This law says inheriting an allele has nothing to do with inheriting an allele for any other trait. The alleles from parents are passed on independently to the offspring. After fertilization, the resulting zygote(s) can end up with any combination of chromosomes from the parents and all the possible combinations occur with equal frequency.

Like segregation, independent assortment occurs during meiosis, specifically in prophase I when the chromosomes line up in random orientation along the metaphase plate. Crossing over, the exchange and recombination of genetic information between chromosomes also occurs in prophase I and adds to the genetic diversity of the offspring.

The image above illustrates the laws of Mendelian inheritance. The segregation of gametes and the independent assortment of traits occurs in meiosis. As a result, each offspring ends up with the full number of chromosomes containing randomly assorted alleles from each parent.

References

Mendelian inheritance. (2018, May 5). In Wikipedia. Retrieved from https://en.wikipedia.org/w/index.php?title=Mendelian_inheritance&oldid=839776145

OpenStax College. (2018). . Houston, TX. OpenStax CNX. Retrieved from http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.119

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Law of Segregation vs Law of Independent Assortment | Biology Dictionary

# data[].attributes.attachment_link.description
The Austrian monk Gregor Mendel conducted revolutionary experiments with pea plants in the early 1800s showing the existence of traits (he called them "factors") that offspring inherit from their parents. His work culminates in the three principles of Mendelian inheritance; the law of segregation, the law of independent assortment, and the law of dominance.

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14. Explained the inheritance pattern of traits when more than two alleles for the trait exist; including incomplete dominance, codominance, sex linked traits, pleiotropy and polygenic traits.

Work Citied: khanacadmy.org

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Multiple alleles, incomplete dominance, and codominance

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In the real world, genes often come in many versions (alleles). Alleles aren't always fully dominant or recessive to one another, but may instead display codominance or incomplete dominance.

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15. Identified the chemical structure of DNA as defined by the Watson and Crick model, including the historical significance of its discovery.

Work Citied: bbc.co.uk

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Find out how the structure of DNA was discovered by Francis Crick and James Watson, and why the discovery revolutionized science and medicine

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16. Examined the steps involved in the replication of DNA in both prokaryotes and eukaryotes.

Work Citied: bio.libretexts.org

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The prokaryotic chromosome is a circular molecule with a less extensive coiling structure than eukaryotic chromosomes. The eukaryotic chromosome is linear and highly coiled around proteins. While there are many similarities in the DNA replication process, these structural differences necessitate some differences in the DNA replication process in these two life forms. DNA replication in prokaryotes has been extensively studied, so we will learn the basic process of prokaryotic DNA replication, then focus on the differences between prokaryotes and eukaryotes.

How does the replication machinery know where to start? It turns out that there are specific nucleotide sequences called origins of replication where replication begins. E. coli has a single origin of replication on its one chromosome, as do most prokaryotes ( Figure \(\PageIndex{1}\)). The origin of replication is approximately 245 base pairs long and is rich in AT sequences. This sequence of base pairs is recognized by certain proteins that bind to this site. An enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs. ATP hydrolysis is required for this process because it requires energy. As the DNA opens up, Y-shaped structures called replication forks are formed ( Figure \(\PageIndex{1}\)). Two replication forks are formed at the origin of replication and these get extended bi-directionally as replication proceeds. Single-strand binding proteins (Figure \(\PageIndex{2}\)) coat the single strands of DNA near the replication fork to prevent the single-stranded DNA from winding back into a double helix.

The next important enzyme is DNA polymerase III, also known as DNA pol III, which adds nucleotides one by one to the growing DNA chain (Figure \(\PageIndex{2}\)). The addition of nucleotides requires energy; this energy is obtained from the nucleotides that have three phosphates attached to them. ATP structurally is an adenine nucleotide which has three phosphate groups attached; breaking off the third phosphate releases energy. In addition to ATP, there are also TTP, CTP, and GTP. Each of these is made up of the corresponding nucleotide with three phosphates attached. When the bond between the phosphates is broken, the energy released is used to form the phosphodiester bond between the incoming nucleotide and the existing chain.

In prokaryotes, three main types of polymerases are known: DNA pol I, DNA pol II, and DNA pol III. DNA pol III is the enzyme required for DNA synthesis; DNA pol I is used later in the process and DNA pol II is used primarily for repair (this is another irritating example of naming that was done based on the order of discovery rather than an order that makes sense).

DNA polymerase is able to add nucleotides only in the 5′ to 3′ direction (a new DNA strand can be only extended in this direction). It requires a free 3′-OH group (located on the sugar) to which it can add the next nucleotide by forming a phosphodiester bond between the 3′-OH end and the 5′ phosphate of the next nucleotide. This essentially means that it cannot add nucleotides if a free 3′-OH group is not available. Then how does it add the first nucleotide? The problem is solved with the help of a primer that provides the free 3′-OH end. Another enzyme, RNA primase, synthesizes an RNA primer that is about five to ten nucleotides long and complementary to the DNA. RNA primase does not require a free 3′-OH group. Because this sequence primes the DNA synthesis, it is appropriately called the primer. DNA polymerase can now extend this RNA primer, adding nucleotides one by one that are complementary to the template strand ( Figure \(\PageIndex{2}\)).

The replication fork moves at the rate of 1000 nucleotides per second. DNA polymerase can only extend in the 5′ to 3′ direction, which poses a slight problem at the replication fork. As we know, the DNA double helix is anti-parallel; that is, one strand is in the 5′ to 3′ direction and the other is oriented in the 3′ to 5′ direction. One strand, which is complementary to the 3′ to 5′ parental DNA strand, is synthesized continuously towards the replication fork because the polymerase can add nucleotides in this direction. This continuously synthesized strand is known as the leading strand. The other strand, complementary to the 5′ to 3′ parental DNA, is extended away from the replication fork, in small fragments known as Okazaki fragments, each requiring a primer to start the synthesis. Okazaki fragments are named after the Japanese scientist who first discovered them. The strand with the Okazaki fragments is known as the lagging strand.

The leading strand can be extended by one primer alone, whereas the lagging strand needs a new primer for each of the short Okazaki fragments. The overall direction of the lagging strand will be 3′ to 5′, and that of the leading strand 5′ to 3′. A protein called the sliding clamp holds the DNA polymerase in place as it continues to add nucleotides. The sliding clamp is a ring-shaped protein that binds to the DNA and holds the polymerase in place. Topoisomerase prevents the over-winding of the DNA double helix ahead of the replication fork as the DNA is opening up; it does so by causing temporary nicks in the DNA helix and then resealing it. As synthesis proceeds, the RNA primers are replaced by DNA pol I, which breaks down the RNA and fills the gaps with DNA nucleotides. The nicks that remain between the newly synthesized DNA (that replaced the RNA primer) and the previously synthesized DNA are sealed by the enzyme DNA ligase that catalyzes the formation of phosphodiester linkage between the 3′-OH end of one nucleotide and the 5′ phosphate end of the other fragment.

Lisa's Note

I think this process is almost impossible to visualize from reading text. I strongly recommend that you watch a couple of animations / videos like the one available here. There are additional links in Blackboard.

Once the chromosome has been completely replicated, the two DNA copies move into two different cells during cell division. The process of DNA replication can be summarized as follows:

1. DNA unwinds at the origin of replication.

2. Helicase opens up the DNA-forming replication forks; these are extended in both directions.

3. Single-strand binding proteins coat the DNA around the replication fork to prevent rewinding of the DNA.

4. Topoisomerase binds at the region ahead of the replication fork to prevent supercoiling (over-winding).

5. Primase synthesizes RNA primers complementary to the DNA strand.

6. DNA polymerase III starts adding nucleotides to the 3′-OH (sugar) end of the primer.

7. Elongation of both the lagging and the leading strand continues.

8. RNA primers are removed and gaps are filled with DNA by DNA pol I.

9. The gaps between the DNA fragments are sealed by DNA ligase.

Table \(\PageIndex{1}\): The enzymes involved in prokaryotic DNA replication and the functions of each.

Prokaryotic DNA Replication: Enzymes and Their Function

Enzyme/protein

Specific Function

DNA pol I

Exonuclease activity removes RNA primer and replaces with newly synthesized DNA

DNA pol II

Repair function

DNA pol III

Main enzyme that adds nucleotides in the 5′-3′ direction

Helicase

Opens the DNA helix by breaking hydrogen bonds between the nitrogenous bases

Ligase

Seals the gaps between the Okazaki fragments to create one continuous DNA strand

Primase

Synthesizes RNA primers needed to start replication

Sliding Clamp

Helps to hold the DNA polymerase in place when nucleotides are being added

Topoisomerase

Helps relieve the stress on DNA when unwinding by causing breaks and then resealing the DNA

Single-strand binding proteins (SSB)

Binds to single-stranded DNA to avoid DNA rewinding back.

DNA replication has been extremely well-studied in prokaryotes, primarily because of the small size of the genome and large number of variants available. Escherichia coli has 4.6 million base pairs in a single circular chromosome, and all of it gets replicated in approximately 42 minutes, starting from a single origin of replication and proceeding around the chromosome in both directions. This means that approximately 1000 nucleotides are added per second. The process is much more rapid than in eukaryotes.

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The prokaryotic chromosome is a circular molecule with a less extensive coiling structure than eukaryotic chromosomes. The eukaryotic chromosome is linear and highly coiled around proteins.

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17. Examined the central dogma of molecular biology, and thus be able to identify the amino acids sequence from a strand of DNA.

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Central dogma of molecular biology | Chemical processes | MCAT | Khan Academy

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18. Described the events that occur during transcription and translation.

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How are Proteins Made? Transcription & Translation

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19. Examined the different roles of tRNA, mRNA and rRNA in the central dogma of molecular biology.

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mRNA, tRNA, and rRNA function | Types of RNA

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20. Described the structure of the eukaryotic chromosome and how it is packaged inside a nucleus.
Work Citied: bio.libretexts.org

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In eukaryotes, chromosomes consist of a single molecule of DNA associated with many copies of 5 kinds of histones. Histones are proteins rich in lysine and arginine residues and thus positively-charged. For this reason they bind tightly to the negatively-charged phosphates in DNA. Cchromosomes have a small number of copies of many different kinds of non-histone proteins. Most of these are transcription factors that regulate which parts of the DNA will be transcribed into RNA.

Structure

For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope. However, before a cell is ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle). As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope. These duplicated chromosomes are called dyads.

Figure 7.1.1 Dyads

When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains 1-10 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays. While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes.

The kinetochore is a complex of >80 different proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase. The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm. Staining with the trypsin-giemsa method reveals a series of alternating light and dark bands called G bands. G bands are numbered and provide "addresses" for the assignment of gene loci.

Chromosome Numbers

All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number. These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined. The gametes contain the haploid number ( n) of chromosomes. In plants, the haploid stage takes up a larger part of its life cycle.

Karyotypes

The complete set of chromosomes in the cells of an organism is its karyotype. It is most often studied when the cell is at metaphase of mitosis when all the chromosomes are present as dyads. The karyotype of the human female contains 23 pairs of homologous chromosomes: 22 pairs of autosomes and an additional 1 pair of X chromosomes. In contrast, the karyotype of the human male contains the same 22 pairs of autosomes with one X chromosome and one Y chromosome. A gene on the Y chromosome designated SRY is the master switch for making a male. Both X and Y chromosomes are called the sex chromosomes.

Figure 7.1.2: Human Karyotype

Above is a human karyotype (of which sex?). It differs from a normal human karyotype in having an extra #21 dyad. As a result, this individual suffered from a developmental disorder called Down Syndrome. The inheritance of an extra chromosome, is called trisomy, in this case trisomy 21. It is an example of aneuploidy

Translocations

Karyotype analysis can also reveal translocations between chromosomes. A number of these are associated with cancers, for example

the Philadelphia chromosome (Ph) formed by a translocation between chromosomes 9 and 22 and a cause of Chronic Myelogenous Leukemia (CML)

a translocation between chromosomes 8 and 14 that causes Burkitt's lymphoma

a translocation between chromosomes 18 and 14 that causes B-cell leukemia

Fluorescence in situ Hybridization (FISH)

Figure 7.1.3 provides dramatic evidence of the truth of the story of chromosomes. A piece of single-stranded DNA was prepared that was complementary to the DNA of the human gene encoding the enzyme muscle glycogen phosphorylase. A fluorescent molecule was attached to this DNA. The dyads in a human cell were treated to denature their DNA; that is, to make the DNA single-stranded. When this preparation was treated with the fluorescent DNA, the complementary sequences found and bound each other. This produced a fluorescent spot close to the centromere of each sister chromatid of two homologous dyads (of chromosome 11, upper right). This analytical procedure, which here revealed the gene locus for the muscle glycogen phosphorylase gene, is called fluorescence in situ hybridization or FISH.

Figure 7.1.3: Location of the gene for muscle glycogen phosphorylase on human chromosome 11 courtesy of David C. Ward

DNA Content

The molecule of DNA in a single human chromosome ranges in size from 50 x 10 nucleotide pairs in the smallest chromosome (stretched full-length this molecule would extend 1.7 cm) up to 250 x 10nucleotide pairs in the largest (which would extend 8.5 cm). Stretched end-to-end, the DNA in a single human diploid cell would extend over 2 meters. In the intact chromosome, however, this molecule is packed into a much more compact structure. The packing reaches its extreme during mitosis when a typical chromosome is condensed into a structure about 5 µm long (a 10,000-fold reduction in length).

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In eukaryotes, chromosomes consist of a single molecule of DNA associated with many copies of 5 kinds of histones. Histones are proteins rich in lysine and arginine residues and thus positively-charged. For this reason they bind tightly to the negatively-charged phosphates in DNA. Cchromosomes have a small number of copies of many different kinds of non-histone proteins.

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21. Identified the mechanisms used to regulate prokaryotic genes.
Work Citied: courses.lumenlearning.com

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Discuss different components of prokaryotic gene regulation

The DNA of prokaryotes is organized into a circular chromosome supercoiled in the nucleoid region of the cell cytoplasm. Proteins that are needed for a specific function are encoded together in blocks called operons. For example, all of the genes needed to use lactose as an energy source are coded next to each other in the lactose (or lac) operon.

In prokaryotic cells, there are three types of regulatory molecules that can affect the expression of operons: repressors, activators, and inducers. Repressors are proteins that suppress transcription of a gene in response to an external stimulus, whereas activators are proteins that increase the transcription of a gene in response to an external stimulus. Finally, inducers are small molecules that either activate or repress transcription depending on the needs of the cell and the availability of substrate.

Learning Objectives

Understand the basic steps in gene regulation in prokaryotic cells

Explain the roles of repressors in negative gene regulation

Explain the role of activators and inducers in positive gene regulation

Gene Regulation in Prokaryotes

In bacteria and archaea, structural proteins with related functions-such as the genes that encode the enzymes that catalyze the many steps in a single biochemical pathway-are usually encoded together within the genome in a block called an operon and are transcribed together under the control of a single promoter. This forms a polycistronic transcript (Figure 1). The promoter then has simultaneous control over the regulation of the transcription of these structural genes because they will either all be needed at the same time, or none will be needed.

French scientists François Jacob (1920-2013) and Jacques Monod at the Pasteur Institute were the first to show the organization of bacterial genes into operons, through their studies on the of E. coli. They found that in E. coli, all of the structural genes that encode enzymes needed to use lactose as an energy source lie next to each other in the lactose (or lac) operon under the control of a single promoter, the lac promoter. For this work, they won the Nobel Prize in Physiology or Medicine in 1965.

Although eukaryotic genes are not organized into operons, prokaryotic operons are excellent models for learning about gene regulation generally. There are some gene clusters in eukaryotes that function similar to operons. Many of the principles can be applied to eukaryotic systems and contribute to our understanding of changes in gene expression in eukaryotes that can result pathological changes such as cancer.

Each operon includes DNA sequences that influence its own transcription; these are located in a region called the regulatory region. The regulatory region includes the promoter and the region surrounding the promoter, to which transcription factors, proteins encoded by regulatory genes, can bind. Transcription factors influence the binding of RNA polymerase to the promoter and allow its progression to transcribe structural genes. A repressor is a transcription factor that suppresses transcription of a gene in response to an external stimulus by binding to a DNA sequence within the regulatory region called the operator, which is located between the RNA polymerase binding site of the promoter and the transcriptional start site of the first structural gene. Repressor binding physically blocks RNA polymerase from transcribing structural genes. Conversely, an activator is a transcription factor that increases the transcription of a gene in response to an external stimulus by facilitating RNA polymerase binding to the promoter. An inducer, a third type of regulatory molecule, is a small molecule that either activates or represses transcription by interacting with a repressor or an activator.

In prokaryotes, there are examples of operons whose gene products are required rather consistently and whose expression, therefore, is unregulated. Such operons are constitutively expressed, meaning they are transcribed and translated continuously to provide the cell with constant intermediate levels of the protein products. Such genes encode enzymes involved in housekeeping functions required for cellular maintenance, including DNA replication, repair, and expression, as well as enzymes involved in core metabolism. In contrast, there are other prokaryotic operons that are expressed only when needed and are regulated by repressors, activators, and inducers.

Practice Questions

What are the parts in the DNA sequence of an operon?

What types of regulatory molecules are there?

The trp Operon: A Repressor Operon

Bacteria such as E. coli need amino acids to survive. Tryptophan is one such amino acid that E. coli can ingest from the environment. E. coli can also synthesize tryptophan using enzymes that are encoded by five genes. These five genes are next to each other in what is called the tryptophan (trp) operon (Figure 1). If tryptophan is present in the environment, then E. coli does not need to synthesize it and the switch controlling the activation of the genes in the trp operon is switched off. However, when tryptophan availability is low, the switch controlling the operon is turned on, transcription is initiated, the genes are expressed, and tryptophan is synthesized.

A DNA sequence that codes for proteins is referred to as the coding region. The five coding regions for the tryptophan biosynthesis enzymes are arranged sequentially on the chromosome in the operon. Just before the coding region is the transcriptional start site. This is the region of DNA to which RNA polymerase binds to initiate transcription. The promoter sequence is upstream of the transcriptional start site; each operon has a sequence within or near the promoter to which proteins (activators or repressors) can bind and regulate transcription.

A DNA sequence called the operator sequence is encoded between the promoter region and the first trp coding gene. This operator contains the DNA code to which the repressor protein can bind. When tryptophan is present in the cell, two tryptophan molecules bind to the trp repressor, which changes shape to bind to the trp operator. Binding of the tryptophan-repressor complex at the operator physically prevents the RNA polymerase from binding, and transcribing the downstream genes.

When tryptophan is not present in the cell, the repressor by itself does not bind to the operator; therefore, the operon is active and tryptophan is synthesized. Because the repressor protein actively binds to the operator to keep the genes turned off, the trp operon is negatively regulated and the proteins that bind to the operator to silence trp expression are negative regulators.

Watch this video to learn more about the trp operon.

Catabolite Activator Protein (CAP): An Activator Regulator

Just as the trp operon is negatively regulated by tryptophan molecules, there are proteins that bind to the operator sequences that act as a positive regulator to turn genes on and activate them. For example, when glucose is scarce, E. coli bacteria can turn to other sugar sources for fuel. To do this, new genes to process these alternate genes must be transcribed. When glucose levels drop, cyclic AMP (cAMP) begins to accumulate in the cell. The cAMP molecule is a signaling molecule that is involved in glucose and energy metabolism in E. coli. When glucose levels decline in the cell, accumulating cAMP binds to the positive regulator catabolite activator protein (CAP), a protein that binds to the promoters of operons that control the processing of alternative sugars. When cAMP binds to CAP, the complex binds to the promoter region of the genes that are needed to use the alternate sugar sources (Figure 1). In these operons, a CAP binding site is located upstream of the RNA polymerase binding site in the promoter. This increases the binding ability of RNA polymerase to the promoter region and the transcription of the genes.

The lac Operon: An Inducer Operon

The third type of gene regulation in prokaryotic cells occurs through inducible operons, which have proteins that bind to activate or repress transcription depending on the local environment and the needs of the cell. The lac operon is a typical inducible operon. As mentioned previously, E. coli is able to use other sugars as energy sources when glucose concentrations are low. To do so, the cAMP-CAP protein complex serves as a positive regulator to induce transcription. One such sugar source is lactose. The encodes the genes necessary to acquire and process the lactose from the local environment. CAP binds to the operator sequence upstream of the promoter that initiates transcription of the lac operon. However, for the lac operon to be activated, two conditions must be met. First, the level of glucose must be very low or non-existent. Second, lactose must be present. Only when glucose is absent and lactose is present will the lac operon be transcribed. This makes sense for the cell, because it would be energetically wasteful to create the proteins to process lactose if glucose was plentiful or lactose was not available.

Practice Question

Transcription of the lac operon is carefully regulated so that its expression only occurs when glucose is limited and lactose is present to serve as an alternative fuel source.

In E. coli, the trp operon is on by default, while the lac operon is off. Why do you think this is the case?

If glucose is absent, then CAP can bind to the operator sequence to activate transcription. If lactose is absent, then the repressor binds to the operator to prevent transcription. If either of these requirements is met, then transcription remains off. Only when both conditions are satisfied is the lac operon transcribed (Table 1).

Table 1. Signals that Induce or Repress Transcription of the lac Operon

Watch an animated tutorial about the workings of lac operon here.

Practice Questions

If glucose is absent, but so is lactose, the lac operon will be ________.

1. activated

2. repressed

3. activated, but only partially

4. mutated

Describe how transcription in prokaryotic cells can be altered by external stimulation such as excess lactose in the environment.

What is the difference between a repressible and an inducible operon?

Prokaryotic Gene Regulation at Work

As we've just learned, there are three types of regulatory molecules that can affect the expression of operons: repressors, activators, and inducers.

Repressors are proteins that suppress transcription of a gene in response to an external stimulus. In other words, a repressor keeps a gene "off."

Activators are proteins that increase the transcription of a gene in response to an external stimulus. In other words, an activator turns a gene "on."

Inducers are small molecules that either activate or repress transcription depending on the needs of the cell and the availability of substrate. Inducers basically help speed up or slow down "on" or "off" by binding to a repressor or activator. In other words: they don't work alone.

In the interactive below, we will focus on the differences between activators and repressors:
Click here for a text-only version of the activity.
Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

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The DNA of prokaryotes is organized into a circular chromosome supercoiled in the nucleoid region of the cell cytoplasm. Proteins that are needed for a specific function are encoded together in blocks called operons. For example, all of the genes needed to use lactose as an energy source are coded next to each other in the lactose (or lac) operon.

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22. Summarized the levels of control of gene expression in eukaryotic cells.

Work Citied: khanacademy.org

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Gene regulation is the process of controlling which genes in a cell's DNA are expressed (used to make a functional product such as a protein).

Different cells in a multicellular organism may express very different sets of genes, even though they contain the same DNA.

The set of genes expressed in a cell determines the set of proteins and functional RNAs it contains, giving it its unique properties.

In eukaryotes like humans, gene expression involves many steps, and gene regulation can occur at any of these steps. However, many genes are regulated primarily at the level of transcription.

Introduction

Your amazing body contains hundreds of different cell types, from immune cells to skin cells to neurons. Almost all of your cells contain the same set of DNA instructions - so why do they look so different, and do such different jobs? The answer: different gene regulation!

Gene regulation makes cells different

Gene regulation is how a cell controls which genes, out of the many genes in its genome, are "turned on" (expressed). Thanks to gene regulation, each cell type in your body has a different set of active genes - despite the fact that almost all the cells of your body contain the exact same DNA. These different patterns of gene expression cause your various cell types to have different sets of proteins, making each cell type uniquely specialized to do its job.

For example, one of the jobs of the liver is to remove toxic substances like alcohol from the bloodstream. To do this, liver cells express genes encoding subunits (pieces) of an enzyme called alcohol dehydrogenase. This enzyme breaks alcohol down into a non-toxic molecule. The neurons in a person's brain don't remove toxins from the body, so they keep these genes unexpressed, or "turned off." Similarly, the cells of the liver don't send signals using neurotransmitters, so they keep neurotransmitter genes turned off.

There are many other genes that are expressed differently between liver cells and neurons (or any two cell types in a multicellular organism like yourself).

How do cells "decide" which genes to turn on?

Now there's a tricky question! Many factors can affect which genes a cell expresses. Different cell types express different sets of genes, as we saw above. However, two different cells of the same type may also have different gene expression patterns depending on their environment and internal state.

Broadly speaking, we can say that a cell's gene expression pattern is determined by information from both inside and outside the cell.

Examples of information from inside the cell: the proteins it inherited from its mother cell, whether its DNA is damaged, and how much ATP it has.

Examples of information from outside the cell: chemical signals from other cells, mechanical signals from the extracellular matrix, and nutrient levels.

How do these cues help a cell "decide" what genes to express? Cells don't make decisions in the sense that you or I would. Instead, they have molecular pathways that convert information - such as the binding of a chemical signal to its receptor - into a change in gene expression.

As an example, let's consider how cells respond to growth factors. A growth factor is a chemical signal from a neighboring cell that instructs a target cell to grow and divide. We could say that the cell "notices" the growth factor and "decides" to divide, but how do these processes actually occur?

The cell detects the growth factor through physical binding of the growth factor to a receptor protein on the cell surface.

Binding of the growth factor causes the receptor to change shape, triggering a series of chemical events in the cell that activate proteins called transcription factors.

The transcription factors bind to certain sequences of DNA in the nucleus and cause transcription of cell division-related genes.

The products of these genes are various types of proteins that make the cell divide (drive cell growth and/or push the cell forward in the cell cycle).

This is just one example of how a cell can convert a source of information into a change in gene expression. There are many others, and understanding the logic of gene regulation is an area of ongoing research in biology today.

Growth factor signaling is complex and involves the activation of a variety of targets, including both transcription factors and non-transcription factor proteins. You can learn more about how growth factor signaling works in the article on intracellular signal transduction.

Eukaryotic gene expression can be regulated at many stages

In the articles that follow, we'll examine different forms of eukaryotic gene regulation. That is, we'll see how the expression of genes in eukaryotes (like us!) can be controlled at various stages, from the availability of DNA to the production of mRNAs to the translation and processing of proteins.

Eukaryotic gene expression involves many steps, and almost all of them can be regulated. Different genes are regulated at different points, and it's not uncommon for a gene (particularly an important or powerful one) to be regulated at multiple steps.

Chromatin accessibility. The structure of chromatin (DNA and its organizing proteins) can be regulated. More open or "relaxed" chromatin makes a gene more available for transcription.

Transcription. Transcription is a key regulatory point for many genes. Sets of transcription factor proteins bind to specific DNA sequences in or near a gene and promote or repress its transcription into an RNA.

RNA processing. Splicing, capping, and addition of a poly-A tail to an RNA molecule can be regulated, and so can exit from the nucleus. Different mRNAs may be made from the same pre-mRNA by alternative splicing.

RNA stability. The lifetime of an mRNA molecule in the cytosol affects how many proteins can be made from it. Small regulatory RNAs called miRNAs can bind to target mRNAs and cause them to be chopped up.

Translation. Translation of an mRNA may be increased or inhibited by regulators. For instance, miRNAs sometimes block translation of their target mRNAs (rather than causing them to be chopped up).

Protein activity. Proteins can undergo a variety of modifications, such as being chopped up or tagged with chemical groups. These modifications can be regulated and may affect the activity or behavior of the protein.

Although all stages of gene expression can be regulated, the main control point for many genes is transcription. Later stages of regulation often refine the gene expression patterns that are "roughed out" during transcription.

Gene regulation and differences between species

Differences in gene regulation makes the different cell types in a multicellular organism (such as yourself) unique in structure and function. If we zoom out a step, gene regulation can also help us explain some of the differences in form and function between different species with relatively similar gene sequences.

For instance, humans and chimpanzees have genomes that are about identical at the DNA level. The protein-coding sequences of some genes are different between humans and chimpanzees, contributing to the differences between the species. However, researchers also think that changes in gene regulation play a major role in making humans and chimps different from one another. For instance, some DNA regions that are present in the chimpanzee genome but missing in the human genome contain known gene-regulatory sequences that control when, where, or how strongly a gene is expressed.

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Overview: Eukaryotic gene regulation (article) | Khan Academy

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Gene regulation is the process of controlling which genes in a cell's DNA are expressed (used to make a functional product such as a protein). Different cells in a multicellular organism may express very different sets of genes, even though they contain the same DNA.

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23. Identified some of the techniques used in the field of biotechnology and genomics.

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Introduction to Biotechnology | Don't Memorise

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Test 3 College Biology (chpt. 9, 10) Flashcards

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Study with Quizlet and memorize flashcards containing terms like Types of cell division, Why cells divide, Reproductive signal and more.