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

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Objective

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. The main stages of meiosis to be covered are:

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

Overview of Meiosis

Meiosis consists of two consecutive nuclear divisions (Meiosis I and Meiosis II) following a single round of DNA replication. The purpose is to halve the chromosome number, producing four genetically distinct haploid cells (gametes in animals, spores in many plants).

Behaviour of Chromosomes

During meiosis the chromosomes undergo a characteristic series of movements that differ between the first and second divisions.

1. Prophase I – Homologous chromosomes pair (synapsis) and form tetrads. Crossing‑over may occur, creating recombinant chromosomes.
2. Metaphase I – Tetrads align on the metaphase plate with homologues opposite each other.
3. Anaphase I – Homologous chromosomes (each still consisting of two sister chromatids) are pulled to opposite poles.
4. Telophase I – Chromosomes arrive at the poles; the cell often enters a brief interphase (no DNA replication).
5. Prophase II – Chromosomes (now each a single chromatid) condense again; a new spindle forms.
6. Metaphase II – Chromosomes line up singly on the metaphase plate.
7. Anaphase II – Sister chromatids separate and move to opposite poles.
8. Telophase II – Nuclear envelopes reform around each set of chromosomes, and cytokinesis yields four haploid cells.

Behaviour of the Nuclear Envelope

The nuclear envelope follows a predictable pattern:

• Disintegrates during prophase I and prophase II, allowing spindle microtubules to contact chromosomes.
• Re‑forms around each set of chromosomes during telophase I and telophase II.
• In many plant cells, a new nuclear envelope may appear before cytokinesis, whereas in animal cells it typically forms after the cleavage furrow has begun.

Behaviour of the Cell Surface Membrane

Both plant and animal cells undergo cytokinesis, but the mechanisms differ:

• Animal cells: A contractile actin‑myosin ring forms at the cell equator, constricting the membrane to produce a cleavage furrow.
• Plant cells: A cell plate forms from vesicles that coalesce at the centre of the cell, eventually becoming a new cell wall that separates the daughter cells.

Behaviour of the Spindle Apparatus

The spindle is assembled from microtubules nucleated by centrosomes (animal cells) or microtubule‑organising centres (plant cells, which lack true centrosomes).

• During prophase I and prophase II the spindle poles migrate to opposite ends of the cell.
• In metaphase I the spindle attaches to each homologous pair via kinetochores; in metaphase II it attaches to individual chromosomes.
• Spindle fibers shorten during anaphase I (separating homologues) and anaphase II (separating sister chromatids).

Comparison of Meiosis in Plant vs. Animal Cells

Key Points to Remember

• Meiosis reduces chromosome number from diploid (2n) to haploid (n).
• Crossing‑over in prophase I creates genetic variation.
• The nuclear envelope breaks down twice (once for each division) and reforms twice.
• Animal cells use a contractile ring; plant cells build a new cell plate.
• Spindle dynamics are essential for accurate chromosome segregation in both kingdoms.