BIOLOGY MOD 6

Mutations definition

• a permanent change that occurs in our DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors.

• the new source of alleles in organisms

Mutagen defintion

-An environmental agent which can permanently damage DNA.

3 different types of mutagens and examples

-Naturally occurring mutagens (e.g. HPV (STD), transposons)

-Ionising electromagnetic radiation (e.g. UV rays, X-rays)

-Chemicals (e.g. asbestos, barbecuing food causes mutagenic chemicals)

What mutagens do

-Mutagens dramatically increase the rate of mutations and may be natural or artificial.

-In some cases, mutagens act as carcinogens (cancer causing agents).

Different types of mutations:

-Neutral: No effect on survival

-Beneficial: Increases likelihood of survival

-Harmful: Decreased likelihood of survival.

How mutations are repaired

• They are detected and repaired by enzymes.

Effect of mutations

-Mutations can have no effect on the polypeptide or a significant effect.

-If the amino acid is greatly changed and it is important in the protein, it can affect the individual greatly.

How does ionising radiation cause mutations?

Ionising radiation causes mutations by removing electrons from atoms/molecules, thus forming ions.

What type of ionising radiation is bad?

Short term, high exposure

How does UV radiation cause a mutation?

-a thymine dimer is produced

-The dimer is detected and the surrounding DNA opens to form a bubble.

-Enzymes cut out the damaged region from the bubble.

-DNA polymerase replaces the cut out and ligase seals it.

-If the dimer is not cut properly, problems like skin cancer can occur.

How do radioactive elements cause mutations?

-Radioactive elements like Uranium-236 are another source of EM radiation.

-These elements release gamma rays as they decay

What are chemical mutagens?

Chemical mutations are a substance that can cause mutation (change to DNA)

Different types of chemical mutagens:

intercalating agents, base analogues and/or DNA reactive chemicals.

How do intercalating agents cause chemical mutations?

• a chemical mutagen inserts itself into the bonds between the base pairs and alter the shape of DNA.

• can cause deletion or insertion which can lead to subsequent errors in DNA replication.

Specific example of an intercalating agent:

Ethidium Bromide- a labraotory stain used to visualise DNA

What is a base analogue

A chemical mutagen where the substance is structurally similar to the nitrogenous base in the DNA.

How do base analogues cause chemical DNA mutation?

• it incorporates into the DNA sequence instead of the usual bases. ➡ ️ The DNA no longer functions.

Example of a base analogue:

5-Bromouracil

What is a naturally occuring mutagen?

A naturally occurring (biological) factor that can cause a mutation (a change in DNA).

How do viruses cause (naturally occuring) DNA mutation?

• they insert their genetic information into the chromosomes of the host cell.

Example of a (natrually occuring) virus which causes DNA mutation:

HPV (AKA genital warts) is responsible for the majority of cervical cancers

What are transponsons and how do they cause DNA mutation (naturally occuring)

-Short DNA sequences that can move around the genome.

-They are extremely common in eukaryotic genomes (over 45% of the genome.)

Define mutation

A mutation is a permanent change in the sequence of DNA that can be beneficial, neutral or harmful due to mistakes when the DNA is copied or from the environment.

What is a point mutation?

Point mutations are a change in the DNA or RNA sequence that alters, adds or removes only one or very few nucleotides.

What is a silent mutation? (point mutation)

When a new codon still codes for the same amino acid.

-Neutral mutation

-No effect on the polypeptide

What is a missense mutation? (point mutation)

When the new codon results in a replacement amino acid.

-Polypeptide chain has a new amino acid

-Polypeptide chain is still formed

-Effect of the altered polypeptide chain depends on the importance of the amino acid that was replaced.

What is a nonsense mutation (point mutation)

When the new codon results in the creation of a stop codon.

-Polypeptide chain terminates due to stop codon.

-Polypeptide chain is incomplete and the protein is non-functional.

What is a frameshift mutation? (point mutation)

When one or two nucleotides are added or removed from the DNA sequence, which alters every codon from that point onwards.

-Polypeptide chain has significant changes as every codon is changed.

-Loss of functional protein and different polypeptides is formed.

-A stop codon may form, which shortens the polypeptide.

What are the 2 types of frameshift mutation?

1. Nucleotide insertion: one or two nucleotides are added to the DNA sequence.

2. Nucleotide deletion: one or two nucleotides are removed from the DNA sequence.

What is a chromosomal mutation?

The change in structure or number of chromosomes in a cell, which can significantly affect many genes at once.

What is a duplication mutation? (chromosomal mutation)

A replication of a section of a chromosome.

-Multiple copies of the same genes on the chromosome.

-Often leads to increased gene expression.

-Can be harmful depending on the gene involved.

What is an inversion mutation? (chromosomal mutation)

A section of a sequence breaking off the chromosome, rotating 180° and reattaching to the same chromosome.

-May involve as few as two bases or several genres.

What is a deletion mutation? (chromosomal mutation)

Removal of sections of a chromosome.

-Deletions can cause disrupted or missing genes, which can have serious effects on growth and development.

-Chromosomal deletions are often fatal

What is an insertion mutation? (chromosomal mutation)

A section of one chromosome breaks off and attaches to a different chromosome.

What is a translocation chromosome? (chromosomal mutation)

A whole chromosome or a segment of a chromosome becomes attached to or exchanged with another chromosome of segment.

What is Aneuploidy? (chromosomal mutation)

-The presence of an abnormal number of a particular chromosome

-E.g. an extra chromosome (trisomy) or a missing chromosome.

What is a polyploidy? (chromosomal mutation)

The presence of more than the usual set of 2 chromosomes

-E.g. triploid syndrome

What are somatic cells?

Somatic cells are body cells (non-reproductive cells) which contain two sets of chromosomes.

-These cells are not inherited

What are germ line cells?

Germ-line cells are gametes (reproductive cells) produced in the gonads.

-Any mutations in these gametes can be inherited.

What are the stages of the cell cycle?

G0- cell cycle resets

G1- Cellular contents, excluding chromosomes, are duplicated.

S- Each of the 46 chromosomes are duplicated by the cell.

G2- The cell double checks duplicated chromosomes for error, and makes repairs.

Mitosis

Cytokenesis

What are somatic mutations?

-These mutations are often due to replication errors prior to mitosis.

-Somatic mutations may have no effect on the individual at all, depending on the gene that is changed.

E.g. if the gene that is changed is a tumor suppressor gene, then it may result in cancer.

-Mutations in some somatic cells will not be passed onto the next generation and therefore do not affect the gene pool, only the individual.

When do spontaneous mutations occur?

These occur in the S phase of the cell cycle, and if it's not repaired during proofreading during the G2 phase, then it will be passed on to the daughter cells.

When the mutated cell continues to divide by mitosis, the mutation is amplified within the tissue- which may result in phenotypic changes.

Example of a phenotypic change in organs:

Cancer

How do germline mutations affect the gene pool?

-When a gamete carrying a mutation fuses with another gamete, an embryo forms.

-The mutation is replicated in every cell of the embryo as it undergoes mitosis and grows into a child.

-For example: cystic fibrosis.

What is coding DNA? (exons)

the sequence of bases used for the production of polypeptides or proteins.

What is non-coding DNA?

the sequence of bases that are not used in the production of polypeptides or proteins.

What does coding DNA affect?

Mutations in coding DNA usually affect the type or sequence of amino acids in the polypeptide or protein produced, and therefore the phenotype of the individual.

• Example: mutations in tumor suppressor genes can result in cancer.

What does non-coding DNA affect?

-affect gene expression and cell functioning.

-some have no effect.

-'switch on' genes or 'switch off' genes.

-Coding for end products, e.g. rRNA and RNA.

-linked to birth defects

-associated with having a predisposition to a disease, or developmental abnormalities.

What are the 3 causes of genetic variation?

fertilisation, meiosis and mutation.

How does fertilisation increase genetic variation?

-Fertilisation creates genetic variation by bringing together two unique sets of genetic material from different parents.

-Random fusion of gametes (each gamete carries a random combination of alleles due to meiosis)

-Genetic mixing: (offspring inherit half their genes from each parent)

How does meiosis increase genetic variation?

-Independent assortment: The random separation of homologous chromosome pairs during meiosis I leads to millions of possible chromosome combinations in gametes.

-Crossing over (recombination): Homologous chromosomes exchange genetic material, creating new allele combinations that are different from the parents.

How does mutation increase genetic variation?

-by altering DNA sequences

-Spontaneous DNA replication errors: creates new alleles.

-Environmental factors: Cause mutations, e.g. electromagnetic radiation or chemicals.

-Chromosomal mutations: Large DNA segments are deleted, duplicated or rearranged, further increasing genetic diversity.

Advantages of genetic variability:

-Greater variability improves the ability of the population to adapt to changes in the environment.

-Little or no variability means a population is more likely to be wiped out.

How can mutations affect a gene pool

-Mutations are the original source of introducing genetic variations to a gene pool

-They can confer advantageous traits that enhance an organisms fitness

-An accumulation of beneficial mutations over generations can lead to evolutionary adaptations

-Overall imact of mutations on the gene pool is gradual, as mutation rates are low

What is gene flow

Gene flow, or migration, involves the transfer of alleles between populations, through the movement of individuals or their gametes.

How does gene flow affect a population?

-This process can introduce new genetic material into a population's gene pool, increasing genetic diversity.

-For example, when individuals from one population migrate and breed with members of another, they can introduce new alleles.

-Gene flow tends to homogenise (make similar) differences between populations, reducing the likelihood of divergence and speciation.

-Restricted gene flow can lead to increased differentiation and potentially the formation of a new species.

What is genetic drift?

random fluctuations in allele frequencies within a population

-significant in smaller populations

What causes genetic drift? (and the effects)

-Natural disasters (bottleneck effect)

-The establishment of a new population by a small number of individuals (founder effect)

How does genetic drift affect a gene pool?

-Genetic drift can lead to the loss of genetic variation, as alleles may be randomly fixed or lost over time.

-This reduction in genetic diversity can decrease a population's ability to adapt to environmental changes and increase the risk of extinction.

-Unlike natural selection, genetic drift does not necessarily favour alleles that confer a selective advantage.