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https://tinyurl.com/cycles-overview

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https://biologydictionary.net/phosphorus-cycle/

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https://tinyurl.com/nitrogen-recycled

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https://scied.ucar.edu/carbon-cycle

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Phosphorus Cycle Explanation- A biogeochemical cycle

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The Carbon Cycle - What Is The Carbon Cycle? - GCSE Biology

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https://tinyurl.com/j-curves-and-s-curves

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Ecological Succession: Primary and Secondary

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Ecological Succession: Primary and Secondary

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Ecological Succession: Nature's Great Grit

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Ecological Succession: Nature's Great Grit

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Ecological Succession: Nature's Great Grit

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https://tinyurl.com/disturbances-succession

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Notes: Human Impacts on the Environment 2

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Notes: Human Impacts on the Environment 2

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https://tinyurl.com/human-impacts-2

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Notes: Human Impacts on the Environment 1

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Notes: Human Impacts on the Environment 1

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https://tinyurl.com/human-impacts-1

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5 Human Impacts on the Environment: Crash Course Ecology #10

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https://tinyurl.com/what-ecosystems-provide

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https://tinyurl.com/population-capacity

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Exponential and Logistic Growth in Populations

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Exponential and Logistic Growth in Populations

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Exponential and logistic growth in populations  | Ecology | Khan Academy

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https://tinyurl.com/ecological-limits

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https://tinyurl.com/there-are-limit

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Keystone Species: Predators, Ecosystem Engineers, and Mutualists

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Keystone Species: Predators, Ecosystem Engineers, and Mutualists

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https://tinyurl.com/Key-species

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https://tinyurl.com/indirect-effects

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Notes: Tropic Cascades and Keystone Species

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Notes: Tropic Cascades and Keystone Species

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https://tinyurl.com/Ecosystem-Engineering

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https://tinyurl.com/symbiosis-basics

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Vocabulary: Community and Population Ecology

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Vocabulary: Community and Population Ecology

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https://tinyurl.com/community-population

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https://tinyurl.com/10-per-rule

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Flow of Energy and Matter Through Ecosystems

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Flow of Energy and Matter Through Ecosystems

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Flow of energy and matter through ecosystems (video) | Khan Academy

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See how energy flows and matter is recycled in ecosystems.

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Food Webs and Energy Pyramids: Bedrocks of Biodiversity

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Food Webs and Energy Pyramids: Bedrocks of Biodiversity

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Food Webs and Energy Pyramids: Bedrocks of Biodiversity

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https://tinyurl.com/habitat-niche

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https://tinyurl.com/vocab-ecology

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https://tinyurl.com/mendels-rules

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https://tinyurl.com/probability-heredity

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https://tinyurl.com/pedigree-review

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https://tinyurl.com/understanding-pedigree

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Blood Typing: Multiple Alleles and Codominance

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Blood Typing: Multiple Alleles and Codominance

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https://tinyurl.com/Blood-type-codominance

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https://www.redcrossblood.org/donate-blood/blood-types.html

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Multiple Alleles (ABO Blood Types) and Punnett Squares

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Multiple Alleles (ABO Blood Types) and Punnett Squares

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Multiple Alleles (ABO Blood Types) and Punnett Squares

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Example Punnet Square for Sex-Linked Recessive Trait

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Example Punnet Square for Sex-Linked Recessive Trait

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Example punnet square for sex-linked recessive trait | High school biology | Khan Academy

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https://tinyurl.com/x-linked-inheritance

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https://biologydictionary.net/codominance/

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https://biologydictionary.net/incomplete-dominance/

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Co-Dominance and Incomplete Dominance (Khan Academy)

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Co-Dominance and Incomplete Dominance (Khan Academy)

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Co-dominance and Incomplete Dominance | Biomolecules | MCAT | Khan Academy

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Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!

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Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!

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Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!

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Monohybrids and the Punnett Square Guinea Pigs

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Monohybrids and the Punnett Square Guinea Pigs

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Monohybrids and the Punnett Square Guinea Pigs

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https://tinyurl.com/inheritance-patterns

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How Mendel's Pea Plants Helped us Understand Genetics

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How Mendel's Pea Plants Helped us Understand Genetics

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How Mendel's pea plants helped us understand genetics - Hortensia Jiménez Díaz

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What are the ethical issues surrounding gene therapy?

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What are the ethical issues surrounding gene therapy?

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Because gene therapy involves making changes to the body's basic building blocks (DNA), it raises many unique ethical concerns. The ethical questions surrounding gene therapy and genome editing include:

How can "good" and "bad" uses of these technologies be distinguished?

Who decides which traits are normal and which constitute a disability or disorder?

Will the high costs of gene therapy make it available only to the wealthy?

Could the widespread use of gene therapy make society less accepting of people who are different?

Should people be allowed to use gene therapy to enhance basic human traits such as height, intelligence, or athletic ability?

Current research on gene therapy treatment has focused on targeting body (somatic) cells such as bone marrow or blood cells. This type of genetic alteration cannot be passed to a person's children. Gene therapy could be targeted to egg and sperm cells (germ cells), however, which would allow the genetic changes to be passed to future generations. This approach is known as germline gene therapy.

The idea of these germline alterations is controversial. While it could spare future generations in a family from having a particular genetic disorder, it might affect the development of a fetus in unexpected ways or have long-term side effects that are not yet known. Because people who would be affected by germline gene therapy are not yet born, they can't choose whether to have the treatment. Because of these ethical concerns, the U.S. Government does not allow federal funds to be used for research on germline gene therapy in people.

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What are the ethical issues surrounding gene therapy?: MedlinePlus Genetics

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Because gene therapy involves making changes to the body's basic building blocks (DNA), it raises many unique ethical concerns. The ethical questions surrounding gene therapy and genome editing include: How can "good" and "bad" uses of these technologies be distinguished? Who decides which traits are normal and which constitute a disability or disorder?

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Gene therapy works by altering the genetic code to recover the functions of critical proteins. Proteins are the workhorses of the cell and the structural basis of the body’s tissues. The instructions for making proteins are carried in a person’s genetic code, and variants (or mutations) in this code can impact the production or function of proteins that may be critical to how the body works. Fixing or compensating for disease-causing genetic changes may recover the role of these important proteins and allow the body to function as expected.

Gene therapy can compensate for genetic alterations in a couple different ways.

Gene transfer therapy introduces new genetic material into cells. If an altered gene causes a necessary protein to be faulty or missing, gene transfer therapy can introduce a normal copy of the gene to recover the function of the protein. Alternatively, the therapy can introduce a different gene that provides instructions for a protein that helps the cell function normally, despite the genetic alteration.

Genome editing is a newer technique that may potentially be used for gene therapy. Instead of adding new genetic material, genome editing introduces gene-editing tools that can change the existing DNA in the cell. Genome editing technologies allow genetic material to be added, removed, or altered at precise locations in the genome. CRISPR-Cas9 is a well-known type of genome editing.

Genetic material or gene-editing tools that are inserted directly into a cell usually do not function. Instead, a carrier called a vector is genetically engineered to carry and deliver the material. Certain viruses are used as vectors because they can deliver the material by infecting the cell. The viruses are modified so they can't cause disease when used in people. Some types of virus, such as retroviruses, integrate their genetic material (including the new gene) into a chromosome in the human cell. Other viruses, such as adenoviruses, introduce their DNA into the nucleus of the cell, but the DNA is not integrated into a chromosome. Viruses can also deliver the gene-editing tools to the nucleus of the cell.

The vector can be injected or given intravenously (by IV) directly into a specific tissue in the body, where it is taken up by individual cells. Alternately, a sample of the patient's cells can be removed and exposed to the vector in a laboratory setting. The cells containing the vector are then returned to the patient. If the treatment is successful, the new gene delivered by the vector will make a functioning protein or the editing molecules will correct a DNA error and restore protein function.

Gene therapy with viral vectors has been successful, but it does carry some risk. Sometimes the virus triggers a dangerous immune response. In addition, vectors that integrate the genetic material into a chromosome can cause errors that lead to cancer. Researchers are developing newer technologies that can deliver genetic material or gene-editing tools without using viruses. One such technique uses special structures called nanoparticles as vectors to deliver the genetic material or gene-editing components into cells. Nanoparticles are incredibly small structures that have been developed for many uses. For gene therapy, these tiny particles are designed with specific characteristics to target them to particular cell types. Nanoparticles are less likely to cause immune reactions than viral vectors, and they are easier to design and modify for specific purposes.

Researchers continue to work to overcome the many technical challenges of gene therapy. For example, scientists are finding better ways to deliver genes or gene-editing tools and target them to particular cells. They are also working to more precisely control when the treatment is functional in the body.

Scientific journal articles for further reading

Bulcha JT, Wang Y, Ma H, Tai PWL, Gao G. Viral vector platforms within the gene therapy landscape. Signal Transduct Target Ther. 2021 Feb 8;6(1):53. doi: 10.1038/s41392-021-00487-6. PMID: 33558455. Free full-text article from PubMed Central: PMC7868676.

Duan L, Ouyang K, Xu X, Xu L, Wen C, Zhou X, Qin Z, Xu Z, Sun W, Liang Y. Nanoparticle Delivery of CRISPR/Cas9 for Genome Editing. Front Genet. 2021 May 12;12:673286. doi: 10.3389/fgene.2021.673286. PubMed: 34054927. Free full-text article from PubMed Central: PMC8149999.

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How does gene therapy work?: MedlinePlus Genetics

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Gene therapy works by altering the genetic code to recover the functions of critical proteins. Discover how this process works.

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Biotechnology review (article) | Khan Academy

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Introduction to genetic engineering (video) | Khan Academy

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Gel Electrophoresis and DNA Fingerprinting

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Gel Electrophoresis and DNA Fingerprinting

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Discover natural selection as a mechanism of evolution with the Amoeba Sisters

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Discover natural selection as a mechanism of evolution with the Amoeba Sisters

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Artificial selection and domestication - Khan Academy

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Artificial selection and domestication - Khan Academy

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Artificial selection and domestication | Natural selection | AP Biology | Khan Academy

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Trusted information on everyday and complex child health conditions from The Hospital for Sick Children.

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Video: Karyotype, Gender and Non-Disjunction

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Video: Karyotype, Gender and Non-Disjunction

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4.2.7 Analyze a human karyotype to determine gender and whether non-disjunction has occurred

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The genetic code is the sequence of nucleotide bases in nucleic acids ( DNA and RNA) that code for amino acid chains in proteins. DNA consists of the four nucleotide bases: adenine (A), guanine (G), cytosine (C) and thymine (T). RNA contains the nucleotides adenine, guanine, cytosine and uracil (U). When three continuous nucleotide bases code for an amino acid or signal the beginning or end of protein synthesis, the set is known as a codon. These triplet sets provide the instructions for the production of amino acids. Amino acids are linked together to form proteins.

Dissecting the Genetic Code

RNA codons designate specific amino acids. The order of the bases in the codon sequence determines the amino acid that is to be produced. Any of the four nucleotides in RNA may occupy one of three possible codon positions. Therefore, there are 64 possible codon combinations. Sixty-one codons specify amino acids and three (UAA, UAG, UGA) serve as stop signals to designate the end of protein synthesis. The codon AUG codes for the amino acid methionine and serves as a start signal for the beginning of translation.

Multiple codons may also specify the same amino acid. For example, the codons UCU, UCC, UCA, UCG, AGU, and AGC all specify the amino acid serine. The RNA codon table above lists codon combinations and their designated amino acids. Reading the table, if uracil (U) is in the first codon position, adenine (A) in the second, and cytosine (C) in the third, the codon UAC specifies the amino acid tyrosine.

Amino Acids

The abbreviations and names of all 20 amino acids are listed below.

Ala: Alanine Arg: Arginine Asn: Asparagine Asp: Aspartic acid

Cys: Cysteine Glu: Glutamic acid Gln: Glutamine Gly: Glycine

His: Histidine Ile: Isoleucine Leu: Leucine Lys: Lysine

Met: Methionine Phe: Phenylalanine Pro: Proline Ser: Serine

Thr: Threonine Trp: Tryptophan Tyr: Tyrosine Val: Valine

Protein Production

Proteins are produced through the processes of DNA transcription and translation. The information in DNA is not directly converted into proteins, but must first be copied into RNA. DNA transcription is the process in protein synthesis that involves the transcribing of genetic information from DNA to RNA. Certain proteins called transcription factors unwind the DNA strand and allow the enzyme RNA polymerase to transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA (mRNA). When RNA polymerase transcribes the DNA, guanine pairs with cytosine and adenine pairs with uracil.

Since transcription occurs in the nucleus of a cell, the mRNA molecule must cross the nuclear membrane to reach the cytoplasm. Once in the cytoplasm, mRNA along with ribosomes and another RNA molecule called transfer RNA, work together to translate the transcribed message into chains of amino acids. During translation, each RNA codon is read and the appropriate amino acid is added to the growing polypeptide chain by transfer RNA. The mRNA molecule will continue to be translated until a termination or stop codon is reached. Once transcription has ended, the amino acid chain is modified before becoming a fully functioning protein.

How Mutations Effect Codons

A gene mutation is an alteration in the sequence of nucleotides in DNA. This change can affect a single nucleotide pair or larger segments of a chromosomes. Altering nucleotide sequences most often results in non-functioning proteins. This is because changes in the nucleotide sequences change the codons. If the codons are changed, the amino acids and thus the proteins that are synthesized will not be the ones coded for in the original gene sequence.

Gene mutations can be generally categorized into two types: point mutations and base-pair insertions or deletions. Point mutations alter a single nucleotide. Base-pair insertions or deletions result when nucleotide bases are inserted into or deleted from the original gene sequence. Gene mutations are most commonly the result of two types of occurrences. First, environmental factors such as chemicals, radiation, and ultraviolet light from the sun can cause mutations. Secondly, mutations may also be caused by errors made during the division of the cell ( mitosis and meiosis).

Key Takeaways: Genetic Code

The genetic code is a sequence of nucleotide bases in DNA and RNA that code for the production of specific amino acids. Amino acids are linked together to form proteins.

The code is read in triplet sets of nucleotide bases, called codons, that designate specific amino acids. For example, the codon UAC (uracil, adenine, and cytosine) specifies the amino acid tyrosine.

Some codons represent start (AUG) and stop (UAG) signals for RNA transcription and protein production.

Gene mutations can alter codon sequences and negatively impact protein synthesis.

Griffiths, Anthony JF, et al. "Genetic Code." An Introduction to Genetic Analysis. 7th Edition., U.S. National Library of Medicine, 1 Jan. 1970, www.ncbi.nlm.nih.gov/books/NBK21950/.

"Introduction to Genomics." NHGRI, www.genome.gov/About-Genomics/Introduction-to-Genomics.

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Decoding the Elements of Your Genetic Code

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The genetic code is the sequence of nucleotide bases in nucleic acids ( DNA and RNA) that code for amino acid chains in proteins. DNA consists of the four nucleotide bases: adenine (A), guanine (G), cytosine (C) and thymine (T). RNA contains the nucleotides adenine, guanine, cytosine and uracil (U).

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RNA and protein synthesis 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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Translation: Making Protein Synthesis Possible

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Translation: Making Protein Synthesis Possible

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Protein synthesis is accomplished through a process called translation. After DNA is transcribed into a messenger RNA (mRNA) molecule during transcription, the mRNA must be translated to produce a protein. In translation, mRNA along with transfer RNA (tRNA) and ribosomes work together to produce proteins.

Stages of Translation in Protein Synthesis

1. Initiation: Ribosomal subunits bind to mRNA.

2. Elongation: The ribosome moves along the mRNA molecule linking amino acids and forming a polypeptide chain.

3. Termination: The ribosome reaches a stop codon, which terminates protein synthesis and releases the ribosome.

Transfer RNA

Transfer RNA plays a huge role in protein synthesis and translation. Its job is to translate the message within the nucleotide sequence of mRNA to a specific amino acid sequence. These sequences are joined together to form a protein. Transfer RNA is shaped like a clover leaf with three loops. It contains an amino acid attachment site on one end and a special section in the middle loop called the anticodon site. The anticodon recognizes a specific area on a mRNA called a codon.

Messenger RNA Modifications

Translation occurs in the cytoplasm. After leaving the nucleus, mRNA must undergo several modifications before being translated. Sections of the mRNA that do not code for amino acids, called introns, are removed. A poly-A tail, consisting of several adenine bases, is added to one end of the mRNA, while a guanosine triphosphate cap is added to the other end. These modifications remove unneeded sections and protect the ends of the mRNA molecule. Once all modifications are complete, mRNA is ready for translation.

Translation

Once messenger RNA has been modified and is ready for translation, it binds to a specific site on a ribosome. Ribosomes consist of two parts, a large subunit and a small subunit. They contain a binding site for mRNA and two binding sites for transfer RNA (tRNA) located in the large ribosomal subunit.

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Translation: Making Protein Synthesis Possible

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Protein synthesis is accomplished through a process called translation. After DNA is transcribed into a messenger RNA (mRNA) molecule during transcription, the mRNA must be translated to produce a protein. In translation, mRNA along with transfer RNA (tRNA) and ribosomes work together to produce proteins. Initiation: Ribosomal subunits bind to mRNA.

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DNA transcription is a process that involves transcribing genetic information from DNA to RNA. The transcribed DNA message, or RNA transcript, is used to produce proteins. DNA is housed within the nucleus of our cells. It controls cellular activity by coding for the production of proteins. The information in DNA is not directly converted into proteins, but must first be copied into RNA. This ensures that the information contained within the DNA does not become tainted.

Key Takeaways: DNA Transcription

How DNA Transcription Works

DNA consists of four nucleotide bases that are paired together to give DNA its double helical shape. These bases are: adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G). Nucleotide base sequences are the genetic code or instructions for protein synthesis.

There are three main steps to the process of DNA transcription:

1. Initiation: RNA Polymerase Binds to DNA
DNA is transcribed by an enzyme called RNA polymerase. Specific nucleotide sequences tell RNA polymerase where to begin and where to end. RNA polymerase attaches to the DNA at a specific area called the promoter region. The DNA in the promoter region contains specific sequences that allow RNA polymerase to bind to the DNA.

2. Elongation
Certain enzymes called transcription factors unwind the DNA strand and allow RNA polymerase to transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA (mRNA). The strand that serves as the template is called the antisense strand. The strand that is not transcribed is called the sense strand.
Like DNA, RNA is composed of nucleotide bases. RNA however, contains the nucleotides adenine, guanine, cytosine, and uracil (U). When RNA polymerase transcribes the DNA, guanine pairs with cytosine (G-C) and adenine pairs with uracil (A-U).

3. Termination
RNA polymerase moves along the DNA until it reaches a terminator sequence. At that point, RNA polymerase releases the mRNA polymer and detaches from the DNA.

Transcription in Prokaryotic and Eukaryotic Cells

While transcription occurs in both prokaryotic and eukaryotic cells, the process is more complex in eukaryotes. In prokaryotes, such as bacteria, the DNA is transcribed by one RNA polymerase molecule without the assistance of transcription factors. In eukaryotic cells, transcription factors are needed for transcription to occur and there are different types of RNA polymerase molecules that transcribe the DNA depending on the type of genes. Genes that code for proteins are transcribed by RNA polymerase II, genes coding for ribosomal RNAs are transcribed by RNA polymerase I, and genes that code for transfer RNAs are transcribed by RNA polymerase III. In addition, organelles such as mitochondria and chloroplasts have their own RNA polymerases which transcribe the DNA within these cell structures.

From Transcription to Translation

In translation, the message coded in mRNA is converted into a protein. Since proteins are constructed in the cytoplasm of the cell, mRNA must cross the nuclear membrane to reach the cytoplasm in eukaryotic cells. Once in the cytoplasm, ribosomes and another RNA molecule called transfer RNA work together to translate mRNA into a protein. This process is called translation. Proteins can be manufactured in large quantities because a single DNA sequence can be transcribed by many RNA polymerase molecules at once.

Reverse Transcription

In reverse transcription, RNA is used as a template to produce DNA. The enzyme reverse transcriptase transcribes RNA to generate a single strand of complementary DNA (cDNA). The enzyme DNA polymerase converts the single-stranded cDNA into a double-stranded molecule as it does in DNA replication. Special viruses known as retroviruses use reverse transcription to replicate their viral genomes. Scientists also use reverse transcriptase processes to detect retroviruses.

Eukaryotic cells also use reverse transcription to extend the end sections of chromosomes known as telomeres. The enzyme telomerase reverse transcriptase is responsible for this process. The extension of telomeres produces cells that are resistant to apoptosis, or programmed cell death, and become cancerous. The molecular biology technique known as reverse transcription-polymerase chain reaction (RT-PCR) is used to amplify and measure RNA. Since RT-PCR detects gene expression, it can also be used to detect cancer and in aid genetic disease diagnosis.

Watch Now: Scientists Identify Mechanism for Cellular Mutation and Cancer

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DNA transcription is a process that involves transcribing genetic information from DNA to RNA. The transcribed DNA message, or RNA transcript, is used to produce proteins. DNA is housed within the nucleus of our cells. It controls cellular activity by coding for the production of proteins.

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Thought Company

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Why Replicate DNA?

DNA is the genetic material that defines every cell. Before a cell duplicates and is divided into new daughter cells through either mitosis or meiosis, biomolecules and organelles must be copied to be distributed among the cells. DNA, found within the nucleus, must be replicated in order to ensure that each new cell receives the correct number of chromosomes. The process of DNA duplication is called DNA replication. Replication follows several steps that involve multiple proteins called replication enzymes and RNA. In eukaryotic cells, such as animal cells and plant cells, DNA replication occurs in the S phase of interphase during the cell cycle. The process of DNA replication is vital for cell growth, repair, and reproduction in organisms.

Key Takeaways

DNA Structure

DNA or deoxyribonucleic acid is a type of molecule known as a nucleic acid. It consists of a 5-carbon deoxyribose sugar, a phosphate, and a nitrogenous base. Double-stranded DNA consists of two spiral nucleic acid chains that are twisted into a double helix shape. This twisting allows DNA to be more compact. In order to fit within the nucleus, DNA is packed into tightly coiled structures called chromatin. Chromatin condenses to form chromosomes during cell division. Prior to DNA replication, the chromatin loosens giving cell replication machinery access to the DNA strands.

Preparation for Replication

Step 1: Replication Fork Formation

Before DNA can be replicated, the double stranded molecule must be "unzipped" into two single strands. DNA has four bases called adenine (A), thymine (T), cytosine (C) and guanine (G) that form pairs between the two strands. Adenine only pairs with thymine and cytosine only binds with guanine. In order to unwind DNA, these interactions between base pairs must be broken. This is performed by an enzyme known as DNA helicase. DNA helicase disrupts the hydrogen bonding between base pairs to separate the strands into a Y shape known as the replication fork. This area will be the template for replication to begin.

DNA is directional in both strands, signified by a 5' and 3' end. This notation signifies which side group is attached the DNA backbone. The 5' end has a phosphate (P) group attached, while the 3' end has a hydroxyl (OH) group attached. This directionality is important for replication as it only progresses in the 5' to 3' direction. However, the replication fork is bi-directional; one strand is oriented in the 3' to 5' direction (leading strand) while the other is oriented 5' to 3' (lagging strand). The two sides are therefore replicated with two different processes to accommodate the directional difference.

Replication Begins

Step 2: Primer Binding

The leading strand is the simplest to replicate. Once the DNA strands have been separated, a short piece of RNA called a primer binds to the 3' end of the strand. The primer always binds as the starting point for replication. Primers are generated by the enzyme DNA primase.

DNA Replication: Elongation

Step 3: Elongation

Enzymes known as DNA polymerases are responsible creating the new strand by a process called elongation. There are five different known types of DNA polymerases in bacteria and human cells. In bacteria such as E. coli, polymerase III is the main replication enzyme, while polymerase I, II, IV and V are responsible for error checking and repair. DNA polymerase III binds to the strand at the site of the primer and begins adding new base pairs complementary to the strand during replication. In eukaryotic cells, polymerases alpha, delta, and epsilon are the primary polymerases involved in DNA replication. Because replication proceeds in the 5' to 3' direction on the leading strand, the newly formed strand is continuous.

The lagging strand begins replication by binding with multiple primers. Each primer is only several bases apart. DNA polymerase then adds pieces of DNA, called Okazaki fragments, to the strand between primers. This process of replication is discontinuous as the newly created fragments are disjointed.

Step 4: Termination

Once both the continuous and discontinuous strands are formed, an enzyme called exonuclease removes all RNA primers from the original strands. These primers are then replaced with appropriate bases. Another exonuclease "proofreads" the newly formed DNA to check, remove and replace any errors. Another enzyme called DNA ligase joins Okazaki fragments together forming a single unified strand. The ends of the linear DNA present a problem as DNA polymerase can only add nucleotides in the 5′ to 3′ direction. The ends of the parent strands consist of repeated DNA sequences called telomeres. Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing. A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA. Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape. In the end, replication produces two DNA molecules, each with one strand from the parent molecule and one new strand.

Replication Enzymes

DNA replication would not occur without enzymes that catalyze various steps in the process. Enzymes that participate in the eukaryotic DNA replication process include:

DNA helicase - unwinds and separates double stranded DNA as it moves along the DNA. It forms the replication fork by breaking hydrogen bonds between nucleotide pairs in DNA.

DNA primase - a type of RNA polymerase that generates RNA primers. Primers are short RNA molecules that act as templates for the starting point of DNA replication.

DNA polymerases - synthesize new DNA molecules by adding nucleotides to leading and lagging DNA strands.

Topoisomerase or DNA Gyrase - unwinds and rewinds DNA strands to prevent the DNA from becoming tangled or supercoiled.

Exonucleases - group of enzymes that remove nucleotide bases from the end of a DNA chain.

DNA ligase - joins DNA fragments together by forming phosphodiester bonds between nucleotides.

DNA Replication Summary

DNA replication is the production of identical DNA helices from a single double-stranded DNA molecule. Each molecule consists of a strand from the original molecule and a newly formed strand. Prior to replication, the DNA uncoils and strands separate. A replication fork is formed which serves as a template for replication. Primers bind to the DNA and DNA polymerases add new nucleotide sequences in the 5′ to 3′ direction.

This addition is continuous in the leading strand and fragmented in the lagging strand. Once elongation of the DNA strands is complete, the strands are checked for errors, repairs are made, and telomere sequences are added to the ends of the DNA.

Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.

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DNA replication is the process of copying the DNA within our cells. This process involves RNA and several enzymes, including DNA polymerase and primase.

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3D Animation

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Silent, Missense, and Nonsense Mutations

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Silent, Missense, and Nonsense Mutations

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Genes are segments of DNA located on chromosomes. A gene mutation is defined as an alteration in the sequence of nucleotides in DNA. This change can affect a single nucleotide pair or larger gene segments of a chromosome. DNA consists of a polymer of nucleotides joined together. During protein synthesis, DNA is transcribed into RNA and then translated to produce proteins. Altering nucleotide sequences most often results in nonfunctioning proteins. Mutations cause changes in the genetic code that lead to genetic variation and the potential to develop the disease. Gene mutations can be generally categorized into two types: point mutations and base-pair insertions or deletions.

Point Mutations

Point mutations are the most common type of gene mutation. Also called a base-pair substitution, this type of mutation changes a single nucleotide base pair. Point mutations can be categorized into three types:

Silent Mutation: Although a change in the DNA sequence occurs, this type of mutation does not change the protein that is to be produced. This is because multiple genetic codons can encode for the same amino acid. Amino acids are coded for by three-nucleotide sets called codons. For example, the amino acid arginine is coded for by several DNA codons including CGT, CGC, CGA, and CGG (A = adenine, T = thymine, G = guanine, and C = cytosine). If the DNA sequence CGC is changed to CGA, the amino acid arginine will still be produced.

Missense Mutation: This type of mutation alters the nucleotide sequence so that different amino acid is produced. This change alters the resulting protein. The change may not have much effect on the protein, may be beneficial to protein function, or may be dangerous. Using our previous example, if the codon for arginine ​CGC is changed to GGC, the amino acid glycine will be produced instead of arginine.

Nonsense Mutation: This type of mutation alters the nucleotide sequence so that a stop codon is coded for in place of amino acid. A stop codon signals the end of the translation process and stops protein production. If this process is ended too soon, the amino acid sequence is cut short and the resulting protein is most always nonfunctional.

Base-Pair Insertions and Deletions

Mutations can also occur in which nucleotide base pairs are inserted into or deleted from the original gene sequence. This type of gene mutation is dangerous because it alters the template from which amino acids are read. Insertions and deletions can cause frame-shift mutations when base pairs that are not a multiple of three are added to or deleted from the sequence. Since the nucleotide sequences are read in groupings of three, this will cause a shift in the reading frame. For example, if the original, transcribed DNA sequence is CGA CCA ACG GCG..., and two base pairs (GA) are inserted between the second and third groupings, the reading frame will be shifted.

Original Sequence: CGA-CCA-ACG-GCG...

Amino Acids Produced: Arginine/Proline/Threonine/Alanine...

Inserted Base Pairs (GA): CGA-CCA-GAA-CGG-CG...

Amino Acids Produced: Arginine/Proline/Glutamic Acid/Arginine...

The insertion shifts the reading frame by two and changes the amino acids that are produced after the insertion. The insertion can code for a stop codon too soon or too late in the translation process. The resulting proteins will be either too short or too long. These proteins are for the most part defunct.

Causes of Gene Mutation

Gene mutations are most commonly caused as a result of two types of occurrences. Environmental factors such as chemicals, radiation, and ultraviolet light from the sun can cause mutations. These mutagens alter DNA by changing nucleotide bases and can even change the shape of DNA. These changes result in errors in DNA replication and transcription.

Other mutations are caused by errors made during mitosis and meiosis. Common errors that occur during cell division can result in point mutations and frameshift mutations. Mutations during cell division can lead to replication errors which can result in the deletion of genes, translocation of portions of chromosomes, missing chromosomes, and extra copies of chromosomes.

Genetic Disorders

According to the National Human Genome Institute, almost all disease have some sort of genetic factor. These disorders can be caused by a mutation in a single gene, multiple gene mutations, combined gene mutation, and environmental factors, or by chromosome mutation or damage. Gene mutations have been identified as the cause of several disorders including sickle cell anemia, cystic fibrosis, Tay-Sachs disease, Huntington disease, hemophilia, and some cancers.

Watch Now: Study Shows That One Genetic Mutation Results in a 25-Point Drop in Intelligence

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Genes are segments of DNA located on chromosomes. A gene mutation is defined as an alteration in the sequence of nucleotides in DNA. This change can affect a single nucleotide pair or larger gene segments of a chromosome. DNA consists of a polymer of nucleotides joined together.

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Duplications and breakages of chromosomes are responsible for a type of chromosome mutation that alters chromosome structure. These changes affect protein production by changing the genes on the chromosome. Chromosome structure changes are often harmful to an individual leading to developmental difficulties and even death. Some changes are not as harmful and may have no significant effect on an individual. There are several types of chromosome structure changes that can occur. Some of them include:

Translocation: The joining of a fragmented chromosome to a non-homologous chromosome is a translocation. The piece of chromosome detaches from one chromosome and moves to a new position on another chromosome.

Deletion: This mutation results from the breakage of a chromosome in which the genetic material becomes lost during cell division. The genetic material can break off from anywhere on the chromosome.

Duplication: Duplications are produced when extra copies of genes are generated on a chromosome.

Inversion: In an inversion, the broken chromosome segment is reversed and inserted back into the chromosome. If the inversion encompasses the centromere of the chromosome, it is called a pericentric inversion. If it involves the long or short arm of the chromosome and does not include the centromere, it is called a paracentric inversion.

Isochromosome: This type of chromosome is produced by the improper division of the centromere. Isochromosomes contain either two short arms or two long arms. A typical chromosome contains one short arm and one long arm.

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Duplications and breakages of chromosomes are responsible for a type of chromosome mutation that alters chromosome structure. These changes affect protein production by changing the genes on the chromosome. Chromosome structure changes are often harmful to an individual leading to developmental difficulties and even death.

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https://www.thoughtco.com/dna-mutations-1224595

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DNA mutations happen when there are changes in the nucleotide sequence that makes up the strand of DNA. This can be caused by random mistakes in DNA replication or even an environmental influence like UV rays or chemicals. The changes at the nucleotide level then influence the transcription and translation from gene to protein expression. Changing even just one nitrogen base in a sequence can change the amino acid that is expressed by that DNA codon which can lead to a completely different protein being expressed. These mutations range from being non-harmful all the way up to causing death.

A point mutation is usually the least harmful of the types of DNA mutations. It is the change of a single nitrogen base in a DNA sequence. Depending on the placement of that nitrogen base in the codon, it can cause no effect to the protein. Codons are a sequence of three nitrogen bases in a row that is "read" by messenger RNA during transcription and then that messenger RNA codon is translated into an amino acid that goes on to make a protein that will be expressed by the organism. Since there are only 20 amino acids and a total of 64 possible combinations of codons, some amino acids are coded for by more than one codon. Often, if the third nitrogen base in the codon is changed, it will not change the amino acid. This is called the wobble effect. If the point mutation occurs in the third nitrogen base in a codon, then it causes no effect on the amino acid or subsequent protein and the mutation does not change the organism.

At most, a point mutation will cause a single amino acid in a protein to change. While this usually is not a deadly mutation, it can cause issues with that protein's folding pattern and the tertiary and quaternary structures of the protein.

One example of a point mutation is sickle cell anemia. A point mutation caused a single nitrogen base in a codon for one amino acid in the protein called glutamic acid to instead code for the amino acid valine. This single small change causes a normally round red blood cell to instead be sickle-shaped.

Frameshift mutations are much more serious and deadly than point mutations. Even though only one nitrogen base is affected just like in point mutations, this time the single base is either completely deleted or an extra one is inserted into the middle of a DNA sequence. This change in sequence causes the reading frame to shift, hence the name frameshift mutation.

A reading frame shift changes the three letter long codon sequence for messenger RNA to transcribe and translate. Not only is that amino acid changed, all subsequent amino acids are changed. This significantly changes the protein and can cause severe problems and even possibly death.

One type of frameshift mutation is called an insertion. Just as the name implies, an insertion occurs when a single nitrogen base is accidentally added in the middle of the sequence. This throws off the reading frame of the DNA and the wrong amino acid is translated. It also pushes the entire sequence down by one letter, changing all codons that come after the insertion and therefore completely altering the protein.

Even though inserting a nitrogen base makes the overall sequence longer, that does not necessarily mean the amino acid chain length will increase. In fact, it could seriously shorten the amino acid chain. If the insertion causes a shift in the codons to create a stop signal, a protein may never be made. Otherwise, an incorrect protein will be made. If the protein that has been changed was essential for life, then most likely the organism will die.

The other type of frameshift mutation is called a deletion. This happens when a nitrogen base is taken out of the sequence. Again, this causes the entire reading frame to change. It changes the codon and will also affect all amino acids that are coded for after the deletion. Nonsense and stop codons may also appear in the wrong places, much like an insertion.

Much like reading text, the DNA sequence is "read" by messenger RNA to produce a "story" or an amino acid chain that will be used to make a protein. Since each codon is 3 letters long, let's see what happens when a "mutation" occurs in a sentence that uses only three letter words.

THE RED CAT ATE THE RAT.

If there was a point mutation, the sentence would change to:

THC RED CAT ATE THE RAT.

The "e" in the word "the" mutated into the letter "c". While the first word in the sentence is no longer the same, the rest of the words still make sense and are what they are supposed to be.

If an insertion were to mutate the above sentence, then it might read:

THE CRE DCA TAT ETH ERA T.

The insertion of the letter "c" after the word "the" completely changes the rest of the sentence. Not only is the second word no longer readable, neither are any words after it. The entire sentence has changed into nonsense.

A deletion would do something similar to the sentence:

THE EDC ATA TET HER AT.

In the example above, the "r" that should have come after the word "the" has been deleted. Again, it changes the entire sentence. Even though in this example, some of the subsequent words are readable, the meaning of the sentence has completely changed. This shows that even if codons are changed into something that isn't nonsense, it still completely changes the protein into something that is no longer functional.

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DNA mutations happen when there are changes in the nucleotide sequence that makes up the strand of DNA. This can be caused by random mistakes in DNA replication or even an environmental influence like UV rays or chemicals. The changes at the nucleotide level then influence the transcription and translation from gene to protein expression.

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Crash Course Biology

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DNA Structure and Replication: Crash Course Biology #10

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Structure & Function

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What are Nucleic Acids? Nucleic Acid Structure & Function

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https://tinyurl.com/how-do-we-compare

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https://tinyurl.com/meiosis-reduction-division

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Cell Division: Cell Cycle and Mitosis - Worksheet

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Cell Division: Cell Cycle and Mitosis - Worksheet

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https://tinyurl.com/we-are-dividing

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Overview of Mitosis and Cancer - Worksheet

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Overview of Mitosis and Cancer - Worksheet

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https://tinyurl.com/yeah-cancer

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Image Comparing Phases of Mitosis and Meiosis - Diploid (2n) vs. Haploid (n)

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Image Comparing Phases of Mitosis and Meiosis - Diploid (2n) vs. Haploid (n)

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Mitosis Image with Brief Summary of Each Phase

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Mitosis Image with Brief Summary of Each Phase

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https://tinyurl.com/the-differnces