Chromosomal structure

Chromatin Basics - DNA's Zip File

• Nucleosome: The fundamental unit of DNA packaging. Consists of DNA wrapped around a core of eight histone proteins, resembling "beads on a string."
• Histones: Positively charged proteins (rich in lysine and arginine) that bind tightly to negatively charged DNA.
  • H1 histone acts as a linker, binding the nucleosome and the entry/exit DNA strands.

• Two Types of Chromatin:
  • Euchromatin: Less condensed, transcriptionally active, and accessible to enzymes. 📌 Euchromatin is truly transcribed.
  • Heterochromatin: Highly condensed, transcriptionally inactive. 📌 Heterochromatin is highly condensed.

⭐ Euchromatin is genetically active and appears light on electron microscopy, whereas heterochromatin is mostly inactive and appears dense/dark.

Nucleosomes - Beads on a String

• The fundamental subunit of chromatin, visualized as "beads on a string."
• Each "bead" is a nucleosome core: DNA wrapped around a histone octamer.
  • Histone Octamer: Contains two copies each of core histones H2A, H2B, H3, and H4.
  • H1 Linker Histone: Binds to the linker DNA and the octamer, stabilizing the structure.
• Electrostatic Binding:
  • Histones are rich in positively charged amino acids (Lysine and Arginine).
  • This positive charge attracts the negatively charged phosphate backbone of DNA.

⭐ Histone acetylation, via histone acetyltransferases (HATs), neutralizes lysine's positive charge. This relaxes DNA coiling (euchromatin) and ↑ transcription. 📌 Histone Acetylation makes DNA Active.

Higher-Order Packing - From String to Structure

• Nucleosomes coil further, stabilized by Histone H1, to form a solenoid or 30-nm fiber.
• This fiber is then arranged into radial loops that attach to a central protein scaffold.
• Progressive compaction and folding lead to the highly condensed structure of a chromosome.

⭐ The most condensed form of chromatin is the metaphase chromosome, which allows for the safe segregation of the genome during mitosis.

Special Regions - Caps & Connectors

• Centromere:

  • Constricted region holding sister chromatids together.
  • Essential for chromosome segregation during cell division; acts as the attachment site for spindle fibers.

• Telomeres:

  • Protective caps at the ends of linear chromosomes, preventing DNA degradation.
  • Sequence: Repetitive TTAGGG repeats. 📌 Telomeres Get Shorter (TGS) with each cell division.
  • Maintained by telomerase, a reverse transcriptase that lengthens telomeres.
  • Progressive shortening contributes to the Hayflick limit and cellular aging.

⭐ Telomerase, a reverse transcriptase, is highly active in stem cells and cancer cells, preventing telomere shortening and enabling cellular immortality.

High‑Yield Points - ⚡ Biggest Takeaways

• Euchromatin is transcriptionally active and less condensed ("Eu-" = true, "truly transcribed").
• Heterochromatin is highly condensed and transcriptionally inactive.
• Histones are rich in positive amino acids (Lysine, Arginine) that bind to negative DNA.
• DNA methylation at CpG islands typically represses transcription.
• Histone acetylation activates transcription by relaxing chromatin coiling.
• Barr bodies, inactive X chromosomes in females, are classic examples of facultative heterochromatin.