Lecture on Chromosomes

Overview

• Importance of nucleic acids, focus on DNA structure
• Transition from DNA to understanding chromosomes
• Chromosomal structure and arrangement

DNA Structure

• DNA is not free-floating in the nucleus
• Coiling of DNA for space efficiency
  • DNA-Histone Complexes: Nucleosomes
  • Supercoiling leads to recognizable chromosome shape

Chromosomes in Human Cells

• Human diploid cells: two versions of each chromosome (maternal and paternal)
  • Total: 46 chromosomes, 23 pairs
• Chromosomal duplication:
  • Through DNA replication, results in two identical sister chromatids
• Homologous Chromosomes:
  • Contain same genes, different alleles (one from each parent)
  • Not identical

Mendelian Genetics & Chromosomal Theory

• Mendel’s abstract concept of genes
• Chromosome theory developed through microscopy
• Genes: stretches of DNA that code for proteins
• Meiosis:
  • Homologous pairs segregate (law of segregation)
  • Random arrangement (law of independent assortment)

Genes and Non-Coding DNA

• Chromosomes have hundreds/thousands of genes
• Non-coding DNA:
  • Comprised majority of genome
  • Functions: RNA transcription (other than mRNA), replication origins, gene regulation, centromeres, telomeres

Telomeres and Replication

• Telomeres:
  • Repeated sequences (TTAGGG in humans)
  • Prevent loss of genetic info
  • Enzyme: Telomerase
• Transposons: DNA sequences that can move within the genome

Sex Chromosomes

• Male vs. Female Chromosomes:
  • Females: XX
  • Males: XY
• Meiosis:
  • Egg cells: X
  • Sperm cells: X or Y
  • Sex determination: random (coin flip)

X-linked Genes

• Males have one allele for X-linked genes
  • Can express recessive phenotype if allele is recessive
  • Disorders: colorblindness, hemophilia
• X-chromosome inactivation in females
  • Leads to phenotypic variation e.g., fur color in cats

Chromosome Structure

• Tiered structure of chromosomes
  • Replicated chromosomes: two sister chromatids
  • Looped domains on scaffold
  • Nucleosomes (DNA wrapped around histones)
  • Double helical DNA
• Genes and protein expression

Conclusion

• Chromosome structure understanding leads to gene expression insights
• Upcoming topics: gene expression regulation
• Encouragement to follow, support, and contact for more information