Understanding the Cell Cycle and Mitosis

Posted on Aug 23, 2024 in Biology

The Cell Cycle

The first stage of cell division is interphase, which is divided into three phases: G1, S, and G2. During interphase:

  • DNA is present as uncondensed chromatin (not visible under a microscope).
  • DNA is contained within a clearly defined nucleus.
  • Centrosomes and other organelles have been duplicated.
  • The cell is enlarged in preparation for division.

Phases of Interphase

  1. G1 (Gap phase 1): The cell grows larger.
  2. S (Synthesis): The genome is replicated.
  3. G2 (Gap phase 2): The second growth phase, separating the newly replicated genome and marking the end of interphase.

Mitosis

The fourth stage of the cell cycle is mitosis, which is divided into prophase, metaphase, anaphase, and telophase. During mitosis, the spindle fibers attach to the chromosomes and pull sister chromatids apart. This stage separates the two daughter genomes.

Stages of Mitosis

Prophase

  • The spindle microtubules grow and extend from each pole to the equator.
  • Chromosomes supercoil and become short and bulky.
  • The nuclear envelope breaks down.

Metaphase

  • The chromatids move to the equator.
  • The spindle microtubules from each pole attach to each centromere on opposite sides.

Anaphase

  • The spindle microtubules pull the sister chromatids apart, splitting the centromeres.
  • Each identical chromosome is pulled to opposite poles.

Telophase

  • The spindle microtubules break down.
  • The chromosomes uncoil and are no longer individually visible.
  • The nuclear membrane reforms.

Cytokinesis

Cytokinesis is the last stage of the cell cycle, during which the cytoplasm divides to create two daughter cells.

  • In animal cells: The cell is pinched in two.
  • In plant cells: A plate forms between the dividing cells.

Detailed Breakdown of Mitosis

Prophase

  • DNA supercoils and chromosomes condense (becoming visible under a microscope).
  • Chromosomes are comprised of genetically identical sister chromatids (joined at a centromere).
  • Paired centrosomes move to the opposite poles of the cell and form microtubule spindle fibers.
  • The nuclear membrane breaks down, and the nucleus dissolves.

Metaphase

  • Microtubule spindle fibers from both centrosomes connect to the centromere of each chromosome.
  • Microtubule depolymerization causes spindle fibers to shorten and contract, causing chromosomes to align along the center of the cell (equatorial plane or metaphase plate).

Anaphase

  • Continued contraction of the spindle fibers causes genetically identical sister chromatids to separate.
  • Once separated, the chromatids are considered individual chromosomes.
  • The genetically identical chromosomes move to the opposite poles of the cell.

Telophase

  • Once the two chromosome sets arrive at the poles, spindle fibers dissolve.
  • Chromosomes decondense (no longer visible under a light microscope).
  • Nuclear membranes reform around each chromosome set.
  • Cytokinesis occurs concurrently, splitting the cell into two.

Sister Chromatids

At metaphase, the chromatids move to the equator. The sister chromatids are two DNA molecules formed by DNA replication and are, therefore, identical. These sister chromatids are then separated in anaphase as the spindle microtubules attach to the centromere and pull the sister chromatids to opposite poles. As the sister chromatids separate, they are called chromosomes. This means that each pole has the same chromosomes (same genetic material). Finally, the microtubules break down, the chromosomes uncoil, and the nuclear membrane reforms. The cell then divides into two daughter cells with genetically identical nuclei.

Cytokinesis in Detail

Cytokinesis is the process of cytoplasmic division, whereby the cell splits into two identical daughter cells.

Animal Cells

  • Microtubule filaments form a concentric ring around the center of the cell.
  • Microfilaments constrict to form a cleavage furrow, which deepens from the periphery towards the center.
  • The furrow meets in the center, the cell becomes completely pinched off, and two cells are formed.
  • Separation occurs from the outside and moves towards the center, so it is described as centripetal.

Plant Cells

  • Carbohydrate-rich vesicles form in a row at the center of the cell (equatorial plane).
  • Vesicles fuse together, and an early cell plate begins to form within the middle of the cell.
  • The cell plate extends outwards and fuses with the cell wall, dividing the cell into two distinct daughter cells.
  • Separation originates in the center and moves laterally, so it is described as centrifugal.