Gentics 3.3

Chromosome Structure and Function

  • Visibility of Chromosomes
    • During most of a cell's life (interphase), chromosomes are not visible as distinct structures.
    • Chromosomes are traditionally represented in a stylized depiction.
    • Interphase is characterized by the presence of DNA, which is represented as chromatin rather than individual chromosomes.

Chromatin Composition

  • Definition of Chromatin
    • Chromatin is made up of DNA, proteins, and some RNA.
    • Forms of Chromatin:
    • Exists in two main forms in both prokaryotes and eukaryotes.
    • Prokaryotic Chromatin:
      • Composed of circular DNA, typically found in a compact form.
    • Eukaryotic Chromatin:
      • Comprised of linear chromosomes that consist of chromatin.

Chromatin Structure and Function

  • Structural Importance:

    • Chromatin condenses the DNA to prevent tangling during various essential processes:
    • DNA replication
    • DNA repair
    • Gene expression

  • The relaxed state of chromatin is about two nanometers wide, while its compacted form facilitates organization within the cell nucleus.

Process of Chromosome Condensation

  • Steps of DNA Compaction:
    1. DNA wraps around proteins known as nucleosomes, which are comprised of histones.
    2. Each DNA strand undergoes approximately two loops around the nucleosomes.
    3. Nucleosomes further wrap around each other to form larger fibers and loops, allowing DNA to be compacted into a structured form, analogous to a tightly wound spring.

Karyotyping and Chromosome Structure

  • Karyotype Definition:
    • A karyotype is an image representation of an individual's chromosomes taken from stained cells, typically arranged to show the chromosome set.
  • Sister Chromatids and Centromeres:
    • A chromosome is visible as a duplicated structure containing two sister chromatids joined by a centromere.
    • Chromatids may share homologous regions known as pseudoautosomal regions, facilitating their alignment.

Mitosis and Meiosis Overview

  • Definitions:
    • Mitosis:
    • Responsible for generating somatic (non-reproductive) cells.
    • The mechanism of nuclear division allowing DNA replication.
    • Meiosis:
    • Leads to the formation of gametes (sex cells) from diploid cells, resulting in haploid cells.

Meiosis vs. Mitosis Comparisons

  • Cell Cycle Stages:
    • Cells undergo similar phases: G1 (Gap 1), S (Synthesis), and G2 (Gap 2).
    • The primary difference is that meiosis consists of two rounds of division: Meiosis I and Meiosis II.

Karyokinesis and Cytokinesis

  • Karyokinesis:
    • Refers to the division of the nucleus and genetic material happens before cytokinesis.
  • Cytokinesis:
    • Division of the cytoplasm occurs after karyokinesis, completing the cell division.

Phases of Mitosis

  • Prophase:
    • The nuclear envelope breaks down, and chromatin begins to condense into distinct chromosomes.
  • Metaphase:
    • Chromosomes align at the metaphase plate.
  • Anaphase:
    • Sister chromatids are pulled apart to opposite ends of the cell as microtubules retract.
  • Telophase:
    • Nuclear membrane begins to reform; chromosomes start to decondense.
    • Cleavage furrow formation occurs during this phase, leading to division of the cytoplasm.

Meiosis Specifics

  • Meiosis I and II:
    • Meiosis I involves homologous chromosomes pairing and separating, creating haploid cells.
    • Meiosis II resembles mitosis with sister chromatids