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Deoxyribonucleic acid (DNA) is the genetic material in eukaryotic (and prokaryotic) cells.

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Deoxyribonucleic acid (DNA) is the genetic material in eukaryotic (and prokaryotic) cells.

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Part A. Location, general structure and function of the genetic material in eukaryotes

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Part A. Location, general structure and function of the genetic material in eukaryotes

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Eukaryotic genetic material appear as thread-like structures called chromosomes, which are made up of chromatin. Chromatin consists of long linear DNA that is packaged around proteins.

Eukaryotic chromosome(s) are primarily located in the nucleus.
Eukaryotic chromosomes are usually linear and are generally larger than prokaryotes. Eukaryotic cells may have one or more chromosomes, depending on the species. Most eukaryotic cells have two copies of each chromosome. Most eukaryotic cells do not have plasmids.

Extranuclear DNA are also found in mitochondria and chloroplasts.

Video link: https://youtu.be/C8OL1MTbGpU

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1. To control and regulate cellular function as it carries the information needed to produce proteins

2. To be able to pass on the information to the cell’s descendants (inheritance).

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Part B. What does a eukaryotic chromosome look like?

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Part B. What does a eukaryotic chromosome look like?

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The figure above shows the structure of a replicated and condensed chromosome and its parts.
The constricted region along the chromosome is called the centromere, while the region at its ends is called telomere.
Depending on the location of the centromere, chromosomes may have have a shorter arm called the p arm (petite) and a longer arm called the q arm.

Types of chromosomes based on the location of the centromere:
Metacentric – in the middle
Submetacentric – near the middle
Acrocentric – near the end
Telocentric – at the end

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Video link: https://youtu.be/bDYbbi5S_zQ

Genes are segments of DNA encoding for an RNA/protein. Genes are found in chromosomes, arranged in a linear order. The location of each specific gene is referred to as the gene locus/loci.

Chromosomes usually occur in pairs, one inherited maternally and the other paternally. These are called homologous chromosomes and they have the same length, centromere location and gene loci.

The total number of chromosomes in a eukaryotic organism/species may vary. For example, humans have 23 pairs, corn plants have 10 pairs, chickens have 39 pairs, etc.

Some chromosomes are involved in sex determination, which are called sex chromosomes (i.e. X and Y chromosomes). The non-sex chromosomes are called autosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (females XX, males XY).

A karyotype is an organised profile of and organism/cell/individual’s complete set of chromosomes.

It allows us to determine how many chromosomes an individual/organism/a species has, and the structure of each chromosome. It also allows determination of chromosomal abnormalities.

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Sister chromatids vs homologous chromosomes

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Sister chromatids vs homologous chromosomes

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In its replicated and condensed form, each chromosome will have two sister chromatids which have exactly the same DNA sequence. These are attached to each other by the centromere.

On the other hand, homologous chromosomes are inherited from each parent, hence will have differences in sequence--- they will have the same arrangement of genes but they may have different versions of certain genes.

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Part C. How is DNA packaged in a eukaryotic cell?

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Part C. How is DNA packaged in a eukaryotic cell?

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Video link: https://youtu.be/J0Sz_o0ksew

a) DNA wraps around histone proteins, forming nucleosomes.

b) The chain of nucleosomes fold into a stacked spiral structure called solenoid.

c) Solenoids coil around each other forming looped domains, which then make up the chromatin fiber.

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Video link: https://www.viddler.com/embed/eb18dc40/

A cell cycle is a series of events that takes place in a cell as it grows and divides.
A cell spends most of its time in interphase, which has three stages:
Gap 1 (G1) phase – normal cell functions occur as well as cell growth

Synthesis (S) – DNA is replicated

Gap 2 (G2) – cell prepares for division.

When the conditions are met and the cell is ready to divide, the cell then leaves interphase, and undergoes cell division. The resulting cells, known as daughter cells, each enter their own interphase and begin a new round of the cell cycle.

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Video link: https://www.viddler.com/embed/bac45be2/

Mitosis is a type of cell division in eukaryotic cells where one cell (mother cell) divides to produce two new cells (daughter cells) that are genetically identical to the mother cell. This type of cell division creates somatic cells.

Phases of mitosis (PMAT):

P rophase - Chromosomes (which have already replicated in the S phase of interphase) condense, nuclear membrane disintegrates, nucleolus becomes less visible, centrosomes move apart, spindle apparatus begin to form.

M etaphase - Chromosomes line up in the middle of the cell along the metaphase plate.

A naphase - Sister chromatids (now considered individual chromosomes) are pulled to either side of the cell as the spindle fibres shorten. The two poles of the cell move further apart.

T elophase - Chromosomes reach opposite poles of the cell and new nuclei form on each side. Nucleoli also reappear.

Cytokinesis (splitting of the cell) occurs right after telophase. In animals, cleavage furrow forms. In plants, cell plate forms in between daughter cells. Organelles are segregated randomly into daughter cells.

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Video links:
Meiosis I https://www.viddler.com/embed/e19eae44/ (2:31)

Meiosis II https://www.viddler.com/embed/e1ab7b6a/ (1:14)

Meiosis is a type of cell division in sexually reproducing organisms. In this type of cell division, one cell divides into four new cells which are not genetically identical to the mother cell. This happens as there are two rounds of cell division: Meiosis I and Meiosis II. In each of the four daughter cells, the total number of chromosomes is half (haploid) the number of chromosomes compared to the original mother cell. In meiosis I, there is an exchange of genetic information between homologous chromosomes (crossing over), which contributes to genetic variation.

This type of cell division creates gametic cells (sex cells).

Phases of meiosis:
Prophase I - Chromosomes (which have already replicated in the S phase of interphase) condense, nuclear membrane disintegrates, nucleolus becomes less visible, centrosomes move apart, spindle apparatus begin to form. Main difference from mitotic prophase is that homologous chromosomes pair up and align (process called synapsis) forming tetrads. Crossing over may occur, allowing the exchange of genetic material from one chromosome to another.

Metaphase I - Chromosomes line up in the middle of the cell along the metaphase plate. The main difference from mitotic metaphase is that the homologue pairs—not individual chromosomes—line up at the metaphase plate for separation.

Anaphase I - The homologous chromosomes are pulled apart from each other and move apart to opposite ends of the cell. The sister chromatids of each chromosome remain attached to one another and don't come apart. The two poles of the cell move further apart.

Telophase I - Chromosomes reach opposite poles of the cell. Nuclear membrane may reform on each side, though in many cell types this is skipped as the cells will soon divide again.

Cytokinesis I - Splitting of cell into two new cells. Organelles are segregated into daughter cells.

Prophase II - Chromosomes condense, nuclear membrane disintegrates (if it was reformed in Telophase I), centrosomes move apart, spindle apparatus begin to form.

Metaphase II - Chromosomes line up in the middle of the cell along the metaphase plate.

Anaphase II - Sister chromatids (now considered individual chromosomes) are pulled to either side of the cell as the spindle fibres shorten. The two poles of the cell move further apart.

Telophase II - Chromosomes reach opposite poles of the cell and new nuclei form on each side. Nucleoli also reappear.

Cytokinesis II - Splitting of each cell into two new cells, resulting in four cells with half the number of chromosomes. Organelles are segregated into daughter cells.

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Part F. Prokaryotic vs Eukaryotic cell division compared

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Part F. Prokaryotic vs Eukaryotic cell division compared

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Similarities:

- DNA first needs to be replicated before cell division occurs.

- Genetic material/ chromosomes are segregated between the daughter cells.

- Involves cytokinesis.
- Binary fission and mitosis results in daughter cells that are genetically identical.

Differences:
- The process of cell division in prokaryotes is binary fission, while in eukaryotes it is either mitosis or meiosis depending on the type of cell.

- Mitosis occurs in eukaryotic somatic cells (body cells). Meiosis occurs in eukaryotic gametic cells (sex cells).

- Mitosis and meiosis involve formation of spindle fibres. In binary fission, there are no spindle fibres.

- In meiosis – crossing over occurs, bringing about genetic variation.

- In meiosis – the two rounds of cell division results in four daughter cells with half the amount of genetic material.