The cell cycle

The cell cycle is a precisely controlled sequence of events that all body cells from multicellular organisms use to grow and divide. It describes the period from when a cell is produced by cell division until it divides to produce two identical daughter cells.

The cell cycle typically 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 and can be subdivided into three phases:

  • 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. During this phase, each chromosome produces two identical copies called sister chromatids, which remain joined by a centromere. Protein molecules called histones are also synthesized and attach to the DNA.

  • G2 phase (Gap Phase 2): The cell continues to grow, and proteins needed for cell division are made. The new DNA synthesized during the S phase is checked for errors and usually repaired. Preparations for cell division, such as the production of tubulin protein for microtubules of the mitotic spindle, are also made.

• Mitosis (M phase): This is the phase where the nucleus divides. It is a continuous process but is typically described in four main stages:

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

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

  • Anaphase: The centromeres divide, separating each pair of sister chromatids. The spindle fibers contract, pulling the now individual chromatids to opposite poles of the spindle, centromere first. This makes the chromatids appear v-shaped.

  • Telophase: The chromatids reach the opposite poles and uncoil, becoming long and thin again (now called chromosomes again). A nuclear envelope reforms around each group of chromosomes, forming two nuclei. Cytokinesis usually finishes during this stage.

• Cytokinesis: This is the division of the cytoplasm. In animal cells, it involves the constriction of the cytoplasm, while in plant cells, a new cell wall forms between the two new nuclei.

Functions of the Cell Cycle and Mitosis: Mitosis produces two genetically identical daughter cells from a parent cell. Its primary functions include:

• 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. For example, cells in the skin and gut lining are rapidly replaced.

• 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 during the immune response 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 discussed in previous conversation): While mitosis maintains the diploid chromosome number and produces genetically identical cells, meiosis is a reductive division. Meiosis produces four genetically different haploid cells (gametes) from a single diploid parent cell. Key differences arise because mitosis involves one division that separates sister chromatids, whereas meiosis involves two divisions (Meiosis I separates homologous pairs, and Meiosis II separates sister chromatids). Meiosis also introduces genetic variation through processes like crossing over and independent assortment, which do not occur in mitosis.