Describe the behaviour of chromosomes in plant and animal cells during meiosis and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle (names of the main stages of meiosis, but not the sub-divisions of prophase I, are expected: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II and telophase II)

SouravNovember 1, 2024

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Meiosis is a complex process of cell division that reduces the chromosome number by half, resulting in the formation of haploid gametes. This process occurs in both plant and animal cells, and although there are some differences, the overall behavior of chromosomes and the associated cellular structures during meiosis is quite similar. Here’s a detailed description of the behavior of chromosomes and the associated changes in the nuclear envelope, cell surface membrane, and spindle apparatus during each stage of meiosis:

1. Meiosis Overview

Meiosis consists of two sequential divisions: meiosis I and meiosis II. Each division includes distinct stages that contribute to the proper segregation of chromosomes.

2. Stages of Meiosis

Meiosis I

1. Prophase I:
   • Chromosome Behavior: Chromosomes condense and become visible under a microscope. Each chromosome consists of two sister chromatids joined at the centromere. Homologous chromosomes pair up in a process called synapsis, forming structures known as tetrads or bivalents.
   • Nuclear Envelope: The nuclear envelope begins to disintegrate.
   • Cell Surface Membrane: The cell surface membrane remains intact.
   • Spindle Apparatus: The spindle apparatus starts to form, with microtubules emerging from the centrosomes.
2. Metaphase I:
   • Chromosome Behavior: The tetrads align at the metaphase plate (equatorial plane) of the cell, with each homologous chromosome facing opposite poles.
   • Nuclear Envelope: The nuclear envelope is completely disintegrated.
   • Cell Surface Membrane: The cell surface membrane remains unchanged.
   • Spindle Apparatus: Spindle fibers attach to the centromeres of the homologous chromosomes, ensuring proper alignment.
3. Anaphase I:
   • Chromosome Behavior: Homologous chromosomes are pulled apart and move toward opposite poles of the cell. The sister chromatids remain attached at their centromeres.
   • Nuclear Envelope: The nuclear envelope is still absent.
   • Cell Surface Membrane: The cell surface membrane remains unchanged.
   • Spindle Apparatus: The spindle fibers shorten, pulling the homologous chromosomes toward the poles.
4. Telophase I:
   • Chromosome Behavior: The separated homologous chromosomes reach the opposite poles and begin to de-condense. In some species, this is followed by cytokinesis, resulting in two haploid daughter cells.
   • Nuclear Envelope: The nuclear envelope may reform around each set of chromosomes, but in many cases, it remains absent as cells proceed directly to meiosis II.
   • Cell Surface Membrane: If cytokinesis occurs, the cell surface membrane pinches in to separate the two daughter cells.
   • Spindle Apparatus: The spindle apparatus disassembles.

Meiosis II

5. Prophase II:
   • Chromosome Behavior: Chromosomes condense again if they de-condensed during telophase I. Each chromosome consists of two sister chromatids.
   • Nuclear Envelope: If it reformed in telophase I, the nuclear envelope disintegrates again.
   • Cell Surface Membrane: The cell surface membrane remains unchanged.
   • Spindle Apparatus: A new spindle apparatus forms in each haploid cell.
6. Metaphase II:
   • Chromosome Behavior: Chromosomes align at the metaphase plate. Each chromosome is still composed of two sister chromatids.
   • Nuclear Envelope: The nuclear envelope is absent.
   • Cell Surface Membrane: The cell surface membrane remains unchanged.
   • Spindle Apparatus: Spindle fibers attach to the centromeres of the sister chromatids.
7. Anaphase II:
   • Chromosome Behavior: The sister chromatids are pulled apart and move toward opposite poles of the cell.
   • Nuclear Envelope: The nuclear envelope remains absent.
   • Cell Surface Membrane: The cell surface membrane remains unchanged.
   • Spindle Apparatus: The spindle fibers shorten, separating the sister chromatids.
8. Telophase II:
   • Chromosome Behavior: The separated sister chromatids reach the poles and begin to de-condense, resulting in four genetically diverse haploid nuclei.
   • Nuclear Envelope: The nuclear envelope reforms around each set of chromosomes.
   • Cell Surface Membrane: If cytokinesis occurs, the cell surface membrane pinches in to separate the two cells, leading to four haploid gametes.
   • Spindle Apparatus: The spindle apparatus disassembles.