Cell Division

The cell cycle is the series of events that take place in a cell leading to its division and duplication. It consists of two main phases: Interphase and the M (Mitotic/Meiotic) phase. Interphase: This is the longest phase, during which the cell grows and prepares for division. It is subdivided into three stages: * G1 Phase (First Gap): The cell grows, synthesises proteins, and carries out normal metabolic functions. * S Phase (Synthesis): DNA replication occurs, resulting in each chromosome consisting of two identical sister chromatids. * G2 Phase (Second Gap): The cell continues to grow, synthesises proteins and organelles necessary for cell division, and prepares for the M phase. M Phase: This phase involves nuclear division (mitosis or meiosis) and cytoplasmic division (cytokinesis).

Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus. It is essential for growth, repair of tissues, and asexual reproduction. Stages of Mitosis (PMAT): * Prophase: Chromatin condenses into visible chromosomes. Each chromosome consists of two identical sister chromatids joined at the centromere. The nuclear envelope begins to break down, and the spindle fibres (made of microtubules) begin to form from the centrosomes. * Metaphase: The chromosomes align individually along the metaphase plate (equator) of the cell. The centromeres of all chromosomes are precisely aligned on this plane, and the spindle fibres attach to the kinetochores (protein structures on the centromeres) of each sister chromatid. * Anaphase: The sister chromatids separate at the centromere and are pulled apart by the shortening spindle fibres towards opposite poles of the cell. Once separated, each chromatid is considered an individual chromosome. * Telophase: The separated chromosomes arrive at the opposite poles of the cell and begin to decondense. New nuclear envelopes form around each set of chromosomes, and the spindle fibres disappear. Two new nuclei are formed. Cytokinesis: This is the division of the cytoplasm, which usually overlaps with telophase. In animal cells, a cleavage furrow forms and pinches the cell in two. In plant cells, a cell plate forms in the middle and grows outwards to divide the cell.

Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four haploid daughter cells (gametes). It is crucial for sexual reproduction and introduces genetic variation. Meiosis consists of two successive divisions: Meiosis I (reductional division) and Meiosis II (equational division). Meiosis I: * Prophase I: Chromosomes condense. Homologous chromosomes (one from each parent) pair up side-by-side in a process called synapsis, forming bivalents (or tetrads). Crossing over occurs, where non-sister chromatids exchange genetic material, leading to genetic recombination. The nuclear envelope breaks down, and spindle fibres form. * Metaphase I: Homologous pairs (bivalents) align at the metaphase plate. The orientation of each homologous pair is random (independent assortment), further contributing to genetic variation. * Anaphase I: Homologous chromosomes separate and are pulled to opposite poles of the cell. Sister chromatids remain attached at their centromeres. * Telophase I: The chromosomes arrive at the poles. Each pole now has a haploid set of chromosomes, but each chromosome still consists of two sister chromatids. Nuclear envelopes may reform, and cytokinesis usually occurs, resulting in two haploid daughter cells. Meiosis II: * Prophase II: If nuclear envelopes reformed, they break down again. Chromosomes condense, and new spindle fibres form in each of the two haploid cells. * Metaphase II: Chromosomes (each still composed of two sister chromatids) align individually at the metaphase plate, similar to mitosis. * Anaphase II: Sister chromatids separate at the centromere and are pulled to opposite poles, becoming individual chromosomes. * Telophase II: The separated chromosomes arrive at the poles, decondense, and nuclear envelopes reform. Cytokinesis occurs, resulting in a total of four genetically unique haploid daughter cells (gametes).

The cell cycle is tightly regulated by a complex system of molecular controls that ensure cells divide only when appropriate and that all steps are completed accurately. This control system relies on internal and external signals. Key Regulatory Elements: * Checkpoints: Critical control points where the cell cycle can be halted until conditions are favourable or errors are corrected. Major checkpoints include: * G1 Checkpoint: Checks for cell size, nutrients, growth factors, and DNA damage. If conditions are not met, the cell may enter a non-dividing state (G0 phase). * G2 Checkpoint: Checks for DNA replication completion and DNA damage. Ensures all chromosomes are replicated and undamaged before mitosis. * M Checkpoint (Spindle Checkpoint): Occurs during metaphase and checks if all sister chromatids are correctly attached to the spindle fibres before anaphase begins. * Regulatory Molecules: The cell cycle is driven by a family of proteins called cyclins and enzymes called cyclin-dependent kinases (CDKs). Cyclins activate CDKs by binding to them, forming cyclin-CDK complexes. These complexes then phosphorylate target proteins, triggering specific events in the cell cycle. Consequences of Uncontrolled Cell Division: When the cell cycle control system malfunctions, often due to mutations in genes that regulate cell growth (e.g., proto-oncogenes and tumour suppressor genes), cells can divide uncontrollably. This uncontrolled proliferation is a hallmark of cancer. Cancer cells ignore checkpoints, grow and divide excessively, and can invade other tissues (metastasis).