1.2 In-depth Exploration of Chromosome Structure

Introduction to Chromosomes

  • Highly organized structures of DNA and proteins (histones).
  • Found within the nucleus of eukaryotic cells.
  • Carriers of genetic material, passed from generation to generation.

Chromatin: Building Material

  • Complex of DNA and proteins (histones).
  • Types:
    • Euchromatin: Less condensed, actively transcribed, lightly stained.
    • Heterochromatin: More condensed, transcriptionally inactive, darkly stained.

DNA Structure: The Blueprint

  • Double Helix: Two antiparallel polynucleotide strands.
  • Components: Deoxyribose sugar, phosphate group, nitrogenous base.
  • Nitrogenous Bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T).
  • Base Pairing: A-T, C-G (held by hydrogen bonds).

Gene: Functional Unit of Heredity

  • Specific sequence of nucleotides in DNA.
  • Carries instructions for synthesizing proteins or functional RNA.
  • Determines a particular trait or characteristic.

Chromosome Compaction

  • Hierarchical coiling to fit 2m of DNA into the nucleus.
  1. Nucleosomes: DNA wrapped around histone proteins ("beads on a string").
  2. Solenoid Fiber: Nucleosomes coiled into a 30 nm fiber.
  3. Loop Domains: Solenoid fiber organized into loops.
  4. Metaphase Chromosome: Highly condensed structure visible during cell division.

Structure of a Metaphase Chromosome

  • Sister Chromatids: Two identical parallel DNA strands, joined at the centromere.
  • Centromere: Constricted region, attachment point for kinetochore and spindle fibers.
  • Telomeres: Repetitive sequences at chromosome ends, protect against degradation.

Types of Chromosomes (by Centromere Position)

  • Metacentric: Centromere in the middle (equal arms).
  • Submetacentric: Centromere slightly off-center (one shorter, one longer arm).
  • Acrocentric: Centromere near one end (very short p arm, long q arm).
  • Telocentric: Centromere at the very end (not in humans).

Functional Classification of Chromosomes

  • Autosomes: Do not determine sex (22 pairs in humans).
  • Sex Chromosomes (Allosomes): Determine sex (X and Y in humans).