Genetic Mutations Study Notes

2.3 Genetic Mutations

Points to Know

• What are mutations?
    - Mutations are errors or changes in DNA, genes, or chromosomes that can have various consequences.

• Difference Between Spontaneous and Induced Mutations
    - Spontaneous Mutation: Occurs by chance, with no identifiable cause.
    - Induced Mutation: Results from a specific identifiable cause.

• Mutagenic Agents
    - Factors that trigger mutations:
      - Radiation: Types include ultraviolet radiation, nuclear radiation, and X-rays.
      - Chemical Substances: Examples include formalin, asbestos, tobacco, and benzene (historically used in pesticides).
      - Infectious Agents: Such as human papillomavirus (HPV).

• Difference Between Gene Mutations and Chromosomal Mutations
    - Gene Mutations: Include substitution and frameshift mutations.
    - Chromosomal Mutations: Involve alterations in the structure of chromosomes impacting multiple genes.
      - Examples of chromosomal mutations include Cri du Chat syndrome, Turner’s syndrome, and Klinefelter’s syndrome.

• Terminology:
    - Allele: Variant form of a gene.
    - Locus: Position of a gene on a chromosome.
    - Genotype: Genetic constitution of an individual.
    - Phenotype: Observable traits of an individual.
    - Homozygous: Having two identical alleles for a gene (e.g., BB or bb).
    - Heterozygous: Having two different alleles for a gene (e.g., Bb).
    - Dominant Allele: Masks the expression of a recessive allele.
    - Recessive Allele: Only expressed if two copies are present.
    - Carrier: An individual possessing one recessive allele for a trait without exhibiting that trait.

• Cell Division:
    - Mitosis and Meiosis:
      - Mitosis:
        - Produces diploid cells (2n), which are identical to the parent cell.
        - Involves one cell division.
      - Meiosis:
        - Produces haploid cells (n), used for reproduction (gametes).
        - Involves two rounds of division (Meiosis I and II).
      - Similarities: Both processes involve DNA duplication.
      - Differences: Mitosis produces genetically identical cells, while meiosis results in genetically varied cells.

Definition and Causes of Mutation

• Mutations are genetic mistakes that cause errors or changes in DNA, genes, or chromosomes.
• They can happen by chance or through identifiable causes:
    - Spontaneous Mutation: Cannot be linked to a specific cause.
    - Induced Mutation: Caused by known mutagenic agents.
• Examples of Mutagenic Agents:
    - Radiation:
      - UV radiation.
      - Nuclear radiation.
      - X-rays.
    - Chemical Substances:
      - Carcinogenic elements like asbestos and benzene.
    - Infectious Agents:
      - Pathogens like HPV, which may lead to cancerous mutations.

Types of Mutations

Gene Mutations

• Gene mutations can be categorized into two primary types:
    - Substitution Mutation:
      - Replacement of one base with another.
      - Example: In the sequence, C replaced by T, changing the amino acid from Glutamic Acid (Glu) to Lysine (Lys).
      - Only affects the codon with the altered nucleotide.
    - Frameshift Mutation:
      - Involves the insertion or deletion of one or two nucleotides.
      - Causes a shift in reading frame for the subsequent codons.
      - Example: In the sentence "THE FAT CAT SAT," if an E is inserted, it becomes "THE EFA TCA TSA T."
      - This dramatically alters the meaning as it affects the entire reading afterward.

Chromosomal Mutations

• Chromosomal mutations involve structural alterations of chromosomes, often occurring during meiosis.
• These mutations can lead to multiple gene alterations.
• Examples of Chromosomal Abnormalities:
    - Normal Karyotype:
    - Trisomy 21 (Down Syndrome): Characterized by an extra chromosome 21 (47 chromosomes instead of 46).
    - Klinefelter Syndrome:
      - Males with one extra X chromosome (XXY).
    - Turner Syndrome:
      - Females with only one X chromosome (monosomy X).

Specific Examples of Chromosomal Mutations

Cri du Chat Syndrome

• A chromosomal deletion syndrome caused by a loss of a section of chromosome 5.
• Infants affected may exhibit a high-pitched cry resembling that of a cat.

Turner’s Syndrome

• A chromosomal disorder in females characterized by the absence of one of the two X chromosomes.
• Physical Characteristics:
    - Short stature.
    - Low hairline.
    - Shield-shaped thorax.
    - Widely spaced nipples.
    - Various skeletal and genital abnormalities.

Klinefelter's Syndrome

• Affects males who have an additional X chromosome (XXY instead of XY).
• Physical Characteristics:
    - Reduced facial and body hair.
    - Possible breast development (gynaecomastia).
    - Increased risk of osteoporosis and altered fat distribution.