Biology – Passage of information from parents to offspring | e-Consult

Meiosis is a specialized type of cell division that occurs in sexually reproducing organisms to produce gametes (sperm and egg cells in animals, spores in plants). It involves two rounds of division, meiosis I and meiosis II, resulting in four daughter cells, each with half the number of chromosomes as the parent cell. The behaviour of chromosomes during meiosis is crucial for ensuring genetic diversity and maintaining the correct chromosome number across generations.

Meiosis I: Separating Homologous Chromosomes

Meiosis I is characterised by the separation of homologous chromosomes. This process involves the following stages:

• Prophase I: Chromosomes condense and become visible. Homologous chromosomes pair up in a process called synapsis, forming tetrads (each containing two sister chromatids). Crossing over occurs during this stage, leading to genetic recombination. The nuclear envelope breaks down.
• Metaphase I: Homologous chromosome pairs align at the metaphase plate. The spindle fibres attach to the kinetochores of each chromosome.
• Anaphase I: Homologous chromosomes are separated and pulled towards opposite poles of the cell. Sister chromatids remain attached.
• Telophase I & Cytokinesis: Chromosomes arrive at the poles. The nuclear envelope may re-form around each set of chromosomes. Cytokinesis occurs, dividing the cell into two haploid daughter cells. Each daughter cell contains one chromosome from each homologous pair.

Meiosis II: Separating Sister Chromatids

Meiosis II is very similar to mitosis. It involves the separation of sister chromatids. The stages are:

• Prophase II: Chromosomes condense again (if they decondensed after Telophase I). The nuclear envelope, if reformed, breaks down.
• Metaphase II: Chromosomes align at the metaphase plate. Spindle fibres attach to the kinetochores of sister chromatids.
• Anaphase II: Sister chromatids are separated and pulled towards opposite poles of the cell.
• Telophase II & Cytokinesis: Chromosomes arrive at the poles. The nuclear envelope re-forms around each set of chromosomes. Cytokinesis occurs, resulting in four haploid daughter cells.

Role of Cellular Structures:

• Nuclear Envelope: Breaks down during prophase I and II, allowing spindle fibres access to the chromosomes. Re-forms during telophase I and II.
• Cell Surface Membrane: Involved in cytokinesis, the physical division of the cytoplasm to form two daughter cells.
• Spindle Apparatus: Composed of microtubules, it attaches to the chromosomes at the kinetochores and is responsible for separating the homologous chromosomes in meiosis I and the sister chromatids in meiosis II.

Differences between Meiosis I and Meiosis II: