DNA and Chromosomes

Key Concepts

• DNA vs. Chromosomes:
  • A chromosome is a DNA molecule.
  • In prokaryotes (e.g., bacteria), DNA and chromosomes are essentially the same.
  • In eukaryotes, DNA is complexed with proteins (histones) to form chromatin.
  • Chromatin is crucial for organizing DNA and is involved in cell division and gene expression.

Haploid and Diploid

• Haploid:
  • One of each type of chromosome.
  • Example: Human gametes have a haploid number of 23.
• Diploid:
  • Two of each type of chromosome.
  • One homolog is paternal, the other is maternal.
• Example: Human somatic cells have a diploid number (2n).

Life Cycle of Sexually Reproducing Organisms

• Zygote:
  • A single-cell, diploid (2n) organism.
• Mitosis:
  • Cell division where DNA is copied; chromosome number remains constant.
• Meiosis:
  • Reduces chromosome number by half to produce gametes.
  • Two stages: Meiosis I and Meiosis II.

Cell Cycle

• Cell Cycle Phases:
  • S Phase: DNA synthesis/replication.
  • G1 and G2 Phases: Gaps in cell cycle.
  • M Phase: Mitosis or Meiosis.
• Interphase: Includes all phases except mitosis.

Meiosis Details

• Meiosis I:
  • DNA is replicated before division.
  • Results in two daughter cells with 2n chromosomes.
• Meiosis II:
  • Further division without DNA replication.
  • Produces four haploid gametes.
• Terminology:
  • Sister Chromatids: Copied chromosomes.
  • Diad: Pair of sister chromatids.
  • Synaptonemal Complex: Holds diads together.
  • Bivalent/Tetrad: Four chromatids.

Mendel’s Law of Equal Segregation

• Meiosis results in equal segregation of chromosomes.
• Equal distribution of maternal and paternal chromosomes into gametes.