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

Published by Patrick Mutisya · 8 days ago

Cambridge A-Level Biology – Passage of Information from Parents to Offspring

Passage of Information from Parents to Offspring

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 and telophase II.

Overview of Meiosis

Meiosis consists of two successive nuclear divisions (Meiosis I and Meiosis II) that reduce the chromosome number from diploid (2n) to haploid (n) while preserving the genetic complement of each chromosome.

Key Structures Involved

  • Chromosomes – DNA‑protein complexes that become visible as distinct entities after condensation.

  • Nuclear envelope – double membrane that surrounds the nucleus; disassembles and re‑forms during specific stages.

  • Cell surface membrane – plasma membrane that may undergo cytokinesis after telophase.

  • Spindle apparatus – microtubule network that originates from centrosomes (animal cells) or microtubule‑organising centres (plant cells) and attaches to kinetochores.

Behaviour of Structures During Each Stage

StageChromosome behaviourNuclear envelopeCell surface membraneSpindle apparatus
Prophase IChromosomes condense; homologous chromosomes pair (synapsis) forming bivalents; crossing‑over may occur.Breaks down (disassembles).Cell membrane remains intact; cortex begins to polarise.Spindle fibers start to form; in animal cells centrosomes migrate to opposite poles; in plant cells microtubule‑organising centres nucleate spindle microtubules.
Metaphase IBivalents align on the metaphase plate; homologues are oriented toward opposite poles.Absent.Cell membrane still intact; cortical tension increases.Spindle microtubules attach to kinetochores of each homologue; tension aligns bivalents.
Anaphase ISister chromatids remain together; homologous chromosomes separate and move to opposite poles.Absent.Cell membrane unchanged.Shortening of kinetochore microtubules pulls each homologue toward its pole; polar microtubules elongate to push poles apart.
Telophase IChromosomes arrive at poles; may decondense partially.Re‑forms around each set of chromosomes, creating two daughter nuclei.Cleavage furrow (animal) or cell plate (plant) begins to form.Spindle disassembles as nuclear envelopes re‑form.
Prophase IIChromosomes condense again if they had decondensed; each chromosome consists of two sister chromatids.Breaks down again.Cell membrane fully separates the two daughter cells (cytokinesis completed).New spindle apparatus forms in each haploid cell; centrosomes duplicate in animal cells.
Metaphase IIChromosomes line up singly along the metaphase plate; sister chromatids face opposite poles.Absent.Intact cell membrane of each haploid cell.Spindle fibers attach to kinetochores of sister chromatids.
Anaphase IISister chromatids separate and are pulled toward opposite poles.Absent.Intact.Kinetochore microtubules shorten, separating chromatids; polar microtubules continue to elongate.
Telophase IIChromatids reach poles and decondense into chromatin; nuclear envelopes re‑form.Re‑forms around each set of chromosomes, yielding four haploid nuclei.In animal cells, cytokinesis completes with a second cleavage furrow; in plant cells, a second cell plate forms, resulting in four genetically distinct gametophytes.Spindle disassembles in each new cell.

Comparison of Plant and Animal Meiosis

  • Centrosomes: Animal cells possess centriolar centrosomes that organise spindle poles; plant cells lack centrioles and use diffuse microtubule‑organising centres.

  • Cytokinesis: Animal cells form a contractile ring leading to a cleavage furrow; plant cells construct a cell plate from vesicles that coalesce at the centre of the cell.

  • Spindle attachment: Both cell types attach spindle microtubules to kinetochores, but the geometry of the spindle differs because of the presence/absence of centrioles.

Suggested diagram: A schematic series showing chromosome alignment and segregation through the eight stages of meiosis, with separate panels for plant and animal cells highlighting differences in spindle origin and cytokinesis.

Key Points to Remember

  1. Meiosis reduces chromosome number by half, producing four non‑identical haploid cells.

  2. Homologous chromosomes separate in Meiosis I; sister chromatids separate in Meiosis II.

  3. The nuclear envelope disassembles at the start of each division and re‑forms at the end.

  4. Spindle formation is essential for accurate chromosome movement; its origin differs between plant and animal cells.

  5. Cytokinesis mechanisms differ: cleavage furrow (animal) vs. cell plate (plant).