Chromosomes

Overview of Cell Biology

Chromosomes

  • Chromosomes are structures that contain an organism's genetic material.

  • They exist in pairs in somatic (non-reproductive) cells, known as diploid cells.

  • Structure of a typical human chromosome:

    • Contains about 140 million nucleotides.

    • In the cell, DNA is coiled to fit into a smaller space.

Cell Cycle

  • The cell cycle consists of phases through which a cell goes through to divide and replicate.

  • Phases of the Cell Cycle:

    1. Interphase: Cell growth and chromosomes replicate.

    • Subdivided into:

      • G1 Phase (Gap 1): Cell growth before DNA synthesis.

      • S Phase (Synthesis): DNA replication occurs, forming sister chromatids.

      • G2 Phase (Gap 2): Preparation for division.

    1. Mitosis: Division of the nucleus.

    • Subdivided into:

      • Prophase: Chromosomes condense and spindle apparatus begins to form.

      • Metaphase: Chromosomes align at the equatorial plane.

      • Anaphase: Sister chromatids separate and move to opposite poles.

      • Telophase: Nuclear envelope reappears and chromosomes decondense.

    1. Cytokinesis: Division of the cytoplasm, resulting in two separate daughter cells.

Eukaryotic Cell Structure vs Prokaryotic

  • Cell division in eukaryotic cells is more complex than in prokaryotic cells due to:

    • Eukaryotic cells have significantly more DNA.

    • Eukaryotic DNA is organized into linear chromosomes packaged with proteins, while prokaryotic DNA is circular.

Characteristics of Chromosomes

  • Chromatin: Complex of DNA (~40%) and proteins (~60%).

    • DNA wraps around histones, forming nucleosomes (10 nm fiber).

    • Chromosomes exist in pairs in somatic cells (diploid).

    • Humans have 46 chromosomes (23 pairs).

  • Chromosomal Abnormalities:

    • Missing chromosome is termed as monosomy (usually fatal).

    • Extra chromosome known as trisomy (also often fatal, except for smaller chromosomes).

Cancer and Cell Division Control

  • Cancer arises from a failure in cell division control, leading to uncontrolled cellular growth.

    • Factors that contribute to cancer include:

    • Radiation.

    • Genetic predisposition (heredity).

    • Environmental toxins like pesticides and herbicides.

    • Viral infections leading to mutation of proto-oncogenes (becoming oncogenes) and tumor suppressor genes.

  • Characteristics of cancer cells include:

    • Abnormal nuclei.

    • Formation of tumors with the ability to undergo angiogenesis (development of new blood vessels).

    • Ability to undergo metastasis (spread to other organs).

    • Immortality (continuous division).

Checkpoints in the Cell Cycle

  • The cell cycle is regulated at three key checkpoints to ensure proper division:

    1. Spindle checkpoint: Ensures all chromosomes are attached to spindle microtubules.

    2. G2/M checkpoint: Ensures the cell is ready to enter mitosis.

    3. G1/S checkpoint: Also known as the Start or Restriction Point, it is the primary decision point for cell division.