The Nucleus and DNA packaging: Quick Review

Prokaryotic vs. Eukaryotic Cells

  • Prokaryotic Cells:
    • Divide by fission.
    • Typically have one circular chromosome.
    • DNA molecules attach to the plasma membrane during replication.
  • Eukaryotic Cells:
    • Divide by mitosis or meiosis.
    • Usually have multiple chromosomes.
    • Contain a nucleus that replicates and divides during cell division.

Nucleus Structure and Function

  • Largest organelle, containing most of the cell's DNA.
  • Site of DNA replication and initial steps of protein production.
  • Nuclear pores facilitate molecule transport (RNA, ribosomal proteins out; proteins, carbohydrates, signaling molecules, lipids in).

DNA Packaging

  • Challenge: Fitting 2 m of DNA into a 5-10 μm nucleus.
  • DNA is compacted using proteins called histones, forming chromatin.
  • Histones provide energy for DNA folding via electrostatic interactions.

Histones and Nucleosomes

  • Histones: Small, positively charged proteins (H1, H2A, H2B, H3, H4).
  • DNA is negatively charged due to phosphate groups.
  • Nucleosome: Basic chromatin unit, containing nine histone proteins and about 166 base pairs of DNA.

Chromatin Structure

  • Packaging into nucleosomes shortens DNA length sevenfold.
  • Chromatin is further coiled into a 30-nm fiber.
  • Chromatin: DNA, histones, and other proteins.
    • Heterochromatin (condensed) and euchromatin (extended).
    • Functions: DNA packaging, strengthening DNA, controlling gene expression and DNA replication.

Chromatin Accessibility

  • Transcription and replication require temporary DNA strand separation.
  • Nucleosomes and 30-nm fibers pose barriers to enzymes.
  • Mechanisms for increasing accessibility:
    • Enzymatic modification of histones.
    • Displacement of histones by chromatin remodeling complexes.
    • These processes are reversible.

Chromosomes

  • DNA packaged into thread-like structures in the nucleus is called chromosomes.
  • Each chromosome has a centromere, dividing it into two arms (p arm and q arm).
  • Humans have 46 chromosomes (22 pairs + 2 sex chromosomes).

DNA storage

  • One chromosome can have as few as 50 million base pairs or as many as 250 million base pairs.
  • Proteins compact chromosomal DNA into the microscopic space of the eukaryotic nucleus.
  • The resulting DNA-protein complex is called chromatin.

Cell Division

  • Nuclear envelope breaks down during cell division.
  • Thick, coiled chromosomes line up on the metaphase plate.
  • Spindle fibers attach to chromosomes.

Levels of DNA Packaging

  • Double-stranded helical structure of DNA (2 nm).
  • DNA complexed with histones to form nucleosomes (11 nm).
  • Nucleosomes fold to produce a 30-nm fiber.
  • 300-nm fiber forms loops.
  • Compressed and folded to produce a 250-nm-wide fiber.
  • Tight coiling produces the chromatid of a chromosome (1400 nm).