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

Passage of Information from Parents to Offspring – Meiosis

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 to be covered are:

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

Prerequisite Knowledge (Cambridge 9700 – AS‑Level Outcomes)

Overview of Meiosis

• One S‑phase (DNA replication) followed by two successive nuclear divisions – Meiosis I and Meiosis II.
• Result: four genetically distinct haploid cells (gametes in animals; spores in many plants).
• Key features required for exam answers:

  • Synapsis and crossing‑over in Prophase I (synaptonemal complex formation).
  • Homologous chromosomes separate in Anaphase I; sister chromatids stay together.
  • Sister chromatids separate in Anaphase II.
  • DNA replication occurs once only before Meiosis I; no DNA synthesis** between the two divisions.
  • Spindle‑assembly checkpoint (SAC) operates in both divisions to prevent premature separation.

Detailed Behaviour at Each Main Stage

Prophase I

• Chromosomes: Condense; each consists of two sister chromatids.
• Synapsis: Homologous chromosomes pair along their lengths, held together by the synaptonemal complex, forming a tetrad (bivalent).
• Cross‑over: Non‑sister chromatids exchange DNA at chiasmata. This creates recombinant chromosomes and is the main source of genetic variation.
• Nuclear envelope: Disintegrates, allowing spindle microtubules to contact kinetochores.
• Spindle: Microtubules nucleated from centrosomes (animal) or dispersed MTOCs (plant) form a bipolar spindle.
• Checkpoint: SAC ensures all kinetochores are attached before progression.

Metaphase I

• Chromosomes: Tetrads line up on the metaphase plate; homologues orient randomly (independent assortment).
• Nuclear envelope: Absent.
• Spindle: Each homologue is attached to spindle fibres from opposite poles via its kinetochores.

Anaphase I

• Chromosomes: Homologous chromosomes are pulled to opposite poles; sister chromatids remain together.
• Nuclear envelope: Still absent.
• Spindle: Shortening of kinetochore microtubules separates the homologues.

Telophase I

• Chromosomes: Reach the poles; each still consists of two sister chromatids.
• Nuclear envelope: Re‑forms around each set of chromosomes.
• Cell‑surface membrane:

  • Animal cells: Contractile actin‑myosin ring may begin to constrict (cleavage furrow) – cytokinesis is often incomplete.
  • Plant cells: Vesicles accumulate at the centre forming a cell plate that may start to appear.

• Inter‑kinesis: Short resting period; no DNA replication takes place.

Prophase II

• Chromosomes: Condense again; each chromosome now consists of a single chromatid.
• Nuclear envelope: Breaks down a second time.
• Spindle: A new bipolar spindle assembles.

Metaphase II

• Chromosomes: Align singly on the metaphase plate (no tetrads).
• Nuclear envelope: Absent.
• Spindle: Kinetochores of each chromatid attach to opposite poles.

Anaphase II

• Chromosomes: Sister chromatids separate and move to opposite poles, becoming individual chromosomes.
• Spindle: Shortening of kinetochore fibres pulls the chromatids apart.

Telophase II

• Chromosomes: Arrive at poles; de‑condense.
• Nuclear envelope: Re‑forms around each haploid set of chromosomes.
• Cell‑surface membrane:

  • Animal cells: Cleavage furrow deepens, completing cytokinesis.
  • Plant cells: Cell plate fuses with the existing cell wall, producing four separate cells.

Link to Inheritance (Mendelian & Modern Genetics)

• Law of Segregation:

  • Meiosis I separates homologous chromosomes (each carries one allele of a gene).
  • Meiosis II separates sister chromatids, ensuring each gamete receives a single allele.

• Law of Independent Assortment: Random orientation of each homologous pair at Metaphase I leads to independent segregation of different chromosome pairs.
• Recombination frequency:

  Recombination % = (number of recombinant gametes ÷ total gametes) × 100.

  Useful for AO2 mapping questions – e.g., if 30 % of offspring are recombinants between genes A and B, the map distance is 30 cM.

• Genetic linkage map example (pea, Pisum sativum)

  Gene	Map position (cM)
  R (round seed)	0
  Y (yellow seed)	15
  A (axial flower)	27
  B (bushy plant)	42

  Cross‑overs between R–Y (15 cM) occur in 15 % of meioses, etc.

• Punnett‑square illustration of independent assortment (two chromosomes, each with two alleles):

  Parent 1:   A a   |   B b
  Parent 2:   A a   |   B b
  Gamete combinations:  AB, Ab, aB, ab  (4 possibilities)
  

  This demonstrates the 9:3:3:1 phenotypic ratio for a dihybrid cross.

Meiotic Errors and Their Consequences

Quantitative Example (AO2)

Question: In a species with 2n = 12, a researcher records the following chromosome numbers in the four products of Meiosis II: 6, 6, 6, 5. Calculate the probability that a randomly chosen gamete will be aneuploid and state the type of error that most likely occurred.

Solution:

1. Total gametes = 4.
2. Aneuploid gametes = 1 (the one with 5 chromosomes).
3. Probability = 1/4 = 0.25 → 25 %.
4. Most likely error: non‑disjunction in Anaphase II (sister chromatids failed to separate).

Comparison of Meiosis in Plant vs. Animal Cells

Key Points to Remember

• Meiosis reduces chromosome number from diploid (2n) to haploid (n) in two successive divisions.
• Cross‑over in Prophase I creates chiasmata, generates recombinant chromosomes and underpins recombination maps.
• DNA replication occurs once only before Meiosis I; there is no DNA synthesis** between Meiosis I and II.
• The nuclear envelope breaks down twice and reforms twice.
• Animal cells use a contractile ring; plant cells construct a new cell plate.
• Spindle dynamics (attachment, tension, SAC) are essential for accurate segregation in both kingdoms.
• Meiotic errors (non‑disjunction, Robertsonian translocation, premature chromatid separation) lead to aneuploidy and are frequent A‑Level exam topics.
• Meiosis provides the cellular basis for Mendelian segregation, independent assortment, recombination frequency and modern concepts of genetic linkage.