Lecture on Chromosomes by Professor Dave

Overview of DNA and Chromosomes

• DNA forms a double helix with millions of base pairs.
• DNA is not free-floating; it is coiled to save space.

Chromosomal Structure

• Nucleosomes: DNA wrapped around histones.
• Further supercoiling forms recognizable chromosomes.
• In diploid human cells, there are 46 chromosomes (23 pairs, one maternal and one paternal).
• Homologous pairs: contain the same genes but different alleles from each parent.
• Chromosomes duplicate to form sister chromatids, separated during mitosis.

Mendelian Genetics and Chromosome Theory

• Mendel’s abstract concept of genes was confirmed with chromosome theory of inheritance.
• Genes are stretches of DNA coding for proteins, located on chromosomes.
• Law of Segregation: Homologous chromosomes are separated in meiosis.
• Law of Independent Assortment: Random arrangement of homologous pairs during meiosis.

The Genome

• Genes compose only 1-1.5% of the genome.
• Noncoding DNA: Transcribes RNAs, regulates gene expression, includes centromeres and telomeres.
  • Telomeres: Repeated sequences protecting chromosome ends from erosion during replication.
  • Transposons: DNA sequences that can move within the genome.

Sex Chromosomes and Inheritance

• Males have XY chromosomes; females have XX.
• Sex determination depends on whether sperm carries X or Y chromosome.
• X-linked Genes: Present only on the X chromosome.
  • Males express recessive phenotypes if they inherit a recessive X-linked allele.
  • Disorders like color-blindness and hemophilia are X-linked.
• X Inactivation: One X chromosome is largely inactive in females, chosen randomly.
  • Example: Female cat fur coloration due to active X alleles from different parents.

Chromosome Structure and Gene Expression

• Chromosomes consist of looped domains around a scaffold.
• Nucleosomes are formed by DNA wrapping around histones.
• Zoom in to see DNA double helix, nitrogenous bases, and atoms.
• Genes in chromosomes express to produce proteins.

Conclusion

• Understanding chromosome structure connects biology, biochemistry, and chemistry.
• Further study on gene expression and regulation follows.