Lecture Notes: Chromosomes and Genetic Inheritance

Introduction to Chromosomes

• DNA Structure Review
  • DNA forms a double helix with millions of base pairs.
  • DNA is coiled to save space, wrapped around proteins called histones, forming nucleosomes.
  • Further coiling leads to chromosome formation.

Chromosomal Structure

• Diploid Cells

  • Human diploid cells have 46 chromosomes (23 pairs, one maternal, one paternal).
  • Each chromosome duplicates into two identical sister chromatids.

• Homologous Chromosomes

  • Contain the same genes but different alleles.
  • Two identical sister chromatids are pulled apart during mitosis.

Mendelian Genetics and Chromosome Theory

• Mendel's Observations

  • Genes are stretches of DNA coding for proteins, located on specific chromosomes.
  • Homologous pairs have genes at the same loci.

• Laws of Segregation and Independent Assortment

  • Separation of homologous pairs during meiosis accounts for the law of segregation.
  • Random arrangement of homologous pairs during meiosis explains independent assortment.

Genome Composition

• Genes and Noncoding DNA

  • Genes constitute 1-1.5% of the genome; the rest is noncoding DNA.
  • Noncoding DNA functions: RNA transcription, gene expression regulation, centromeres, telomeres.

• Telomeres

  • Found at chromosome ends, sequence TTAGGG repeated.
  • Protects genetic information during DNA replication.
  • Telomerase enzyme extends telomeres in some cells.

• Transposons

  • Sequences that can move within the genome.

Sex Chromosomes and Inheritance

• Sex Chromosomes

  • Females have two X chromosomes; males have one X and one Y chromosome.
  • During meiosis, egg cells get X; sperm may get X or Y.
  • Sex determination depends on X or Y in sperm.

• X-linked Genes

  • Present only on the X chromosome; males express recessive phenotype if allele is recessive.
  • Disorders related to X-linked genes: color-blindness, hemophilia.

• X-inactivation in Females

  • One X chromosome inactivated randomly in each cell.
  • Results in phenotypic variations, e.g., two colors of fur in female cats.

Chromosome Structure Details

• Hierarchical Structure
  • Chromosome consists of looped domains wrapped around a scaffold.
  • Nucleosomes are DNA wrapped around histones.
  • DNA is a double helix with nitrogenous bases.

Conclusion

• Chromosomes contain genes that produce proteins in the body.
• Upcoming topic: Gene expression and regulation.