Chromosomes and Alterations

Overview

• Review of DNA, chromosomes, heredity, mitosis, and meiosis.
• Discussion on errors during these processes, leading to genetic diversity and alterations.

Chromosome Number Alterations

• Normal somatic cells have 46 chromosomes; gametes have 23.
• Nondisjunction can occur during meiosis:
  • Meiosis I: Homologous chromosomes fail to separate, leading to gametes with N + 1 or N - 1 chromosomes.
  • Meiosis II: Sister chromatids fail to separate, producing similar abnormal gametes.
• Resulting zygotes from abnormal gametes exhibit aneuploidy (abnormal chromosome number):
  • Monosomy: One version of a chromosome.
  • Trisomy: Three versions of a chromosome.
• Consequences:
  • Miscarriages due to chromosomal abnormalities.
  • Genetic disorders arising from aneuploidy:
    • Down syndrome (trisomy 21)
    • Klinefelter syndrome (extra X in males)
    • Turner syndrome (monosomy X in females)

Polyploidy

• More than two complete sets of chromosomes:
  • Diploid: Normal state with two sets (46 total).
  • Triploidy (3n): Three sets.
  • Tetraploidy (4n): Four sets.
• Causes of polyploidy:
  • Nondisjunction leading to diploid egg fertilization.
  • Diploid zygote replicating and failing to divide.
• Examples:
  • Rare in animals (some fish and amphibians).
  • Common in plants:
    • Bananas: Triploid
    • Wheat: Hexaploid (6n)
    • Strawberries: Octoploid (8n)

Chromosomal Structural Changes

• Beyond point and frameshift mutations:
  • Chromosome breakage due to radiation or damage can result in:
    • Deletion: Loss of chromosome fragment.
    • Duplication: Fragment attached to sister chromatid.
    • Inversion: Fragment reattached in a flipped position.
    • Translocation: Fragment attached to a different chromosome.
• Deletion and duplication during crossing over can cause:
  • Missing genes if significant sections are involved in fertilization.
  • Potential for miscarriage or early childhood death due to genetic loss.
• Advances in linking chromosomal changes to specific conditions represent significant scientific progress.

Conclusion

• Understanding of chromosomes and their alterations is crucial for genetics and pathology.