Chromosomes: DNA Structure

Chromosomes: DNA Structure

Organization of DNA in Chromosomes

• Chromosomes are made up of a long DNA molecule wrapped around histone proteins (H2A, H2B, H3, and H4)
• This complex of DNA and histone proteins is called chromatin
• Chromatin is further condensed into nucleosomes, which are repeating units of chromatin

Structure of a Nucleosome

• A nucleosome consists of:
  • 147 base pairs of DNA wrapped around a histone octamer (2 copies of each histone protein: H2A, H2B, H3, and H4)
  • Linker DNA (20-80 base pairs) that connects nucleosomes
  • Histone H1 that binds to the linker DNA and helps stabilize the chromatin structure

Levels of Chromatin Condensation

• Interphase chromatin: relatively open and accessible for gene expression
• Metaphase chromatin: highly condensed and visible as distinct chromosomes during cell division
• Heterochromatin: densely packed, gene-poor regions of chromatin
• Euchromatin: less compact, gene-rich regions of chromatin

Chromosome Structure

• A chromosome consists of:
  • Centromere: a specialized region that plays a crucial role in chromosome segregation during cell division
  • Telomeres: repetitive DNA sequences (TTAGGG in humans) that cap the ends of chromosomes
  • Origins of replication: specific regions where DNA replication is initiated
  • Genes: coding regions that are transcribed into RNA and translated into proteins