Mutations in Genetics

A mutation is a change in the DNA sequence.
It can affect a single base or involve the loss or gain of an entire chromosome.
Mutations are a fundamental driver of species diversity and are necessary for adaptation and evolution.
Mutations occur randomly with respect to their effects.
There is NO such thing as ‘directed evolution’.

Key Points: - Diversity in Species: Mutations underlie the diversity of species, fueling evolution. - Random Occurrence: Mutations occur without intention; their effects may vary greatly.
Source: SCIENCE, 2003.
Message: We need mutations!

Replication Errors: - Errors may occur during DNA replication. - DNA polymerase and mismatch repair enzymes typically correct these errors.
Environmental Exposure: - Chemicals and free radicals can chemically modify bases, altering their base-pairing ability. - Exposure to radiation, notably ultraviolet (UV) light from the sun, can lead to mispairing (e.g., hydrolysis of cytosine leading to mispairing with adenine). - Cosmic rays, gamma rays, and X-rays can also induce mutations.
Spontaneous Mutations: - Include processes like depurination (loss of purine base) and deamination (conversion of cytosine to uracil).

Defined as the proportion of all gametes that contain a new mutation.
New Mutations Are Rare: - In a study sequencing the genomes of two parents and their children, 70 new mutations were found for each child. - Most of these mutations were located in non-coding DNA (>98%), hence are unlikely to impact phenotype.

Fitness: The reproductive success of a genotype, measured by the number of offspring surviving to reproductive age. - Fitness of 1: Able to reproduce. - Fitness of 0: Unable to reproduce.

A mutation's impact on fitness can influence allele prevalence in populations: - Positive Effect: Alleles increase in population due to enhanced fitness. - Negative Effect: Alleles decrease in population due to decreased fitness. - Neutral Effect: Alleles experience genetic drift without significant immediate impact.

Determined by: - Type of Mutation: Point mutation vs. larger mutations. - Location in Genome: Functional (impacting a gene) vs. non-functional (not affecting a gene). - Cell Type: Somatic (non-reproductive) vs. germline (reproductive).

Mutations can create new alleles, effectively altering gene versions: - Gene to mRNA to Protein Modification: - Example: Different alleles (e.g., allele 1, allele 2) may have no effect, a better function, or a worse function compared to the original gene.

Somatic Mutations: - Occur in body cells. - Ultimate outcome: mutation may disappear if the cell dies. - Possible consequences: None, harm to the cell, oncogenic (cancerous) changes, or cell death.
Germline Mutations: - Occur in gametes (e.g., eggs, sperm). - Will be present in every successor cell if the individual reproduces, passing mutations to offspring.

Point Mutations: Single base substitutions.
Repeat Expansions: Continuous repeats of sequences in DNA.
Chromosomal Changes: Can include deletions, duplications, and translocations.

Point Mutation Mechanism: Swapping one base for another changes the mRNA and consequently the amino acid sequence.
Example:
- Original sequence: A U G G C A U G U G C A G U C G (Met-Ala-Cys-Ala-Val) - Mutation: A U G G C A U G C G C A G U C G (Met-Ala-Cys-Ala-Val; change affects open reading frame).

Synonymous Mutation: No change in amino acid.
Non-synonymous (Missense) Mutation: Alters an amino acid, potentially affecting protein function.
Nonsense Mutation: Introduces a premature stop codon, truncating the protein.
Frameshift Mutation: Results from insertions or deletions that shift the reading frame.

Sickle Cell Mutation: A single base change in codon 6 changes glutamic acid (polar) to valine (non-polar), causing hemoglobin aggregation and subsequent red blood cell deformity.
** cystic fibrosis (CF):** A common deletion of phenylalanine at position 508 (F508 del) interferes with CFTR protein processing and function.

Promoter Mutations: Can prevent transcription by disrupting transcription factor binding.
Splice Site Mutations: Affect mRNA splicing, causing retention of introns or exclusion of exons, leading to possible nonsense transcripts.

Mutations in regulatory sequences can impact: - Gene expression levels, timing, and tissue specificity.
Mutations in untranslated regions (UTR): 5' UTR and 3' UTR denote segments that can also be affected, impacting signal and RNA structure.

Repeat Expansions: Continuous runs of repeated DNA sequences, typically di- or tri-nucleotides, can vary in size due to DNA replication issues.
Larger repetitive sequences tend to lead to greater instability and are often associated with several diseases, especially neurodegenerative disorders.

Repeat expansions are causal of ~30 known human diseases, which can manifest differently across generations (Anticipation).
Types of disease mutation categorizations include normal, premutation, and full mutation alleles based on repeat sizes.

Examples of chromosomal mutations include: - 46, XX: Normal female - 46, XY: Normal male - 47, XX, +21: Down syndrome (extra chromosome 21) - 46, XX, del(2)(q12;q13): Deletion from chromosome 2 at specific loci. - 46, XY, rob(13;15)(q10;q10): Robertsonian translocation between chromosomes 13 and 15.