Study Notes on Mutations and Chromosomal Abnormalities

DNA and Human Genome Changes

  • The DNA of the human genome, like that of all genomes, is not a stable entity but is subject to a series of heritable changes known as "mutations".

Mutation Overview

  • Definitions:
    • Mutation: a change in the genetic constitution of a cell.
    • Types of cells affected:
      • Somatic Cells: non-heritable mutations.
      • Germ Cells: heritable mutations.
  • Types of mutations:
    • Genomic mutations: affect entire genomes.
    • Chromosomal mutations: affect the structure or number of chromosomes.
    • Gene mutations: affect specific genes.

Mutation Statistics

  • The mutation rate varies in different gene regions.
    • Genomic DNA breakdown:
    • Non-coding: 75%
    • Non-coding (gene-related): 22%
    • Coding: 3%

Single Nucleotide Polymorphisms (SNPs)

  • Definition:
    • A single nucleotide polymorphism (SNP) is a variation in genetic material at a single nucleotide position where the polymorphic allele exists in a population in a frequency greater than 1%.

Mutation Types and Unintended Effects

  • Induced Mutations: caused by external agents such as:
    • Viruses: certain oncogenic viruses can induce tumors.
    • Physical agents:
    • Ionizing radiation (X-rays, UV rays) can cause cellular disruption leading to mutations or radical formation.
    • UV rays can create thymine dimers.
  • Replication Errors:
    • Spontaneous mutations arise from cellular duplication errors.
    • Every time a human cell divides, over three billion base pairs must be accurately replicated.
    • Example statistics:
    • Approximately $10^6$ cells replicate every second.
    • About $5.5 imes 10^3$ mutations can occur per replication cycle.
    • DNA replication takes 2-3 hours.
    • Around 100,000 base pairs are replicated per second.
  • Silent mutations: do not affect gene function.
  • Loss-of-function mutations: lead to gene expression loss.
  • Gain-of-function mutations: can result in altered-function proteins (e.g., cancer).
  • Conditional mutations: express phenotypes under specific conditions (e.g., Siamese cats).

Mutations at a Molecular Level

  • Insertion: addition of one or more base pairs can shift reading frames and alter polypeptide sequences.
  • Deletion: loss of one or more base pairs can lead to similar frame shifts or loss of amino acids.
  • Substitution: variations in DNA can result in mutation types:
    • Silent mutations: change nucleotide sequence without altering amino acids.
    • Missense mutations: cause amino acid exchanges.
    • Nonsense mutations: create stop codons leading to truncated proteins.
  • Example of an SNP related to sickle cell anemia, affecting hemoglobin leading to red blood cell shape change.

Phenotypic Effects of Mutations

  • Mutations can lead to various phenotypic expressions including:
    • Silent mutations
    • Loss-of-function mutations
    • Gain-of-function mutations
    • Conditional mutations

Chromosomal Mutations

  • Types of chromosomal mutations:
    • Deletion: loss of part of a chromosome.
    • Duplication: duplication of a chromosome segment.
    • Inversion: rearrangement of chromosome segments.
    • Translocation: exchange of chromosome segments between non-homologous chromosomes.
  • Non-disjunction can lead to aneuploidy (e.g., Down syndrome) associated with genetic abnormalities due to an extra chromosome or loss of a chromosome.

Anomalies in Chromosome Number

  • Most autosomal monosomies (missing a chromosome) are incompatible with embryonic development.
  • Trisomy (extra chromosome) of chromosome 21 leads to Down syndrome.
  • Example statistics:
    • Down syndrome occurs in about 1 in 700 births.
    • Trisomy 21 results in 47 chromosomes.
    • Characteristic phenotypes include facial traits, short stature, and increased risk for certain health conditions.

Notable Syndromes Related to Chromosomal Abnormalities

  • Edwards syndrome (trisomy 18): serious developmental delays and characteristic physical anomalies.
  • Patau syndrome (trisomy 13): severe malformations and high mortality in infancy.
  • Klinefelter syndrome (XXY): male individuals with additional X chromosome showing symptoms of infertility and varied gender characteristics.
  • Turner syndrome (45,X): female phenotypes lacking a second sex chromosome leading to developmental deficiencies.

Genetic Polymorphism and Population Genetics

  • SNPs contribute to genetic diversity in human populations and can be influenced by environmental factors or evolutionary pressures.
  • Approximately 2 out of 10 pregnancies may terminate early due to chromosomal abnormalities.