Mitosis

Mitosis is a type of nuclear division that results in the production of two genetically identical daughter nuclei from a parent cell. These daughter cells contain an exact copy of the parent cell's DNA.

The Cell Cycle and Mitosis

Mitosis is a precisely controlled process that occurs as part of the cell cycle. The cell cycle describes the sequence of events from when a cell is produced until it divides into two identical cells.

The cell cycle consists of three main stages:

• Interphase: This is a period of cell growth and DNA replication. It is the longest part of the cell cycle.

  • G1 phase (Gap Phase 1): The cell grows, and new organelles and proteins are made. At the end of G1, the cell commits to dividing or not dividing.

  • S phase (Synthesis): The cell replicates its DNA, ready to divide by mitosis. Each chromosome produces two identical copies called sister chromatids, which remain joined by a centromere. Histone proteins are also synthesized and attach to the DNA.

  • G2 phase (Gap Phase 2): The cell continues to grow, and proteins needed for cell division (like tubulin for spindle microtubules) are made. DNA is checked for errors.

• Mitosis (M phase): This is the phase where the nucleus divides. Cell growth temporarily stops during mitosis.

• Cytokinesis: This is the division of the cytoplasm, which typically occurs after mitosis. In animal cells, it involves the constriction of the cytoplasm, while in plant cells, a new cell wall forms.

Stages of Mitosis

Mitosis is a continuous process but is described in four main stages for convenience:

1. Prophase: Chromosomes condense, becoming shorter and fatter. Centrioles (in animal cells) move to opposite ends of the cell, forming the spindle, a network of protein fibers. The nuclear envelope breaks down, and chromosomes lie free in the cytoplasm.

2. Metaphase: Chromosomes (each with two chromatids) line up along the equator (middle) of the cell and attach to the spindle fibers at their centromeres.

3. Anaphase: Centromeres divide, separating sister chromatids. Spindle fibers contract, pulling chromatids (now considered individual chromosomes) to opposite poles. This makes the chromatids appear v-shaped.

4. Telophase: Chromatids reach poles, uncoil, and nuclear envelopes reform around each group, forming two new nuclei. Cytokinesis usually finishes during this stage, resulting in two genetically identical daughter cells.

Functions of Mitosis

Mitosis is essential for several biological processes, as it produces genetically identical daughter cells:

• Growth of multicellular organisms: From a single-celled zygote, repeated cell divisions allow an organism to grow into a multicellular adult.

• Replacement of damaged or dead cells and tissue repair: Cells are constantly dying and are replaced by identical cells produced through mitosis.

• Asexual reproduction: New individuals are produced from a single parent, resulting in genetically identical offspring (clones).

• Immune response: The cloning of B- and T-lymphocytes is dependent on mitosis.

Control of the Cell Cycle and Cancer

The cell cycle and mitosis are controlled by genes. Normally, cell division stops when enough new cells have been produced. However, a mutation in a gene that controls cell division can lead to cells growing out of control. These cells keep dividing to form more and more cells, which coalesce into a tumour. Cancer is defined as a tumour that invades surrounding tissue. Cancer treatments are often designed to control the rate of cell division in tumour cells by disrupting the cell cycle.

Comparison with Meiosis

As previously discussed, mitosis produces two genetically identical daughter cells with the same (diploid, 2n) chromosome number. In contrast, meiosis is a reductive nuclear division that produces four genetically different haploid (n) cells (gametes). Meiosis involves two divisions (Meiosis I and Meiosis II) but only one round of DNA replication, unlike mitosis which has one division. Meiosis introduces genetic variation through processes like crossing over and independent assortment, which do not occur in mitosis.

Observing Mitosis

Mitosis can be observed and investigated using stained preparations of root tip cells (e.g., onion or broad bean) viewed under an optical microscope. This allows for the recognition and analysis of cells at different stages of the cell cycle.