Meiosis in Animal & Plant Cells

Meiosis I (Reductional Division)

• Prophase I: Chromosomes condense and homologous pairs undergo synapsis. This is the only stage where crossing over occurs.
• Metaphase I: Homologous pairs (bivalents) align at the equator. Spindle fibers attach to the centromeres of each whole chromosome.
• Anaphase I: Homologous chromosomes are pulled to opposite poles. Crucially, the centromeres do not divide here; whole chromosomes move, reducing the ploidy from $2n$ to $n$.

Meiosis II (Equational Division)

• This stage is functionally similar to mitosis but occurs in haploid cells. There is no DNA replication between Meiosis I and II.
• Anaphase II: Centromeres finally divide, and sister chromatids are pulled to opposite poles. This results in four haploid cells, each with single-chromatid chromosomes.

• Spindle Formation: In animal cells, the spindle is organized by centrioles located within centrosomes. Most higher plants lack centrioles and organize their spindles through alternative microtubule-organizing centers.

• Cytokinesis Mechanism: Animal cells divide by forming a cleavage furrow, where a contractile ring of actin and myosin pinches the cell membrane inward. Plant cells, constrained by a rigid cell wall, form a cell plate (phragmoplast) that grows outward from the center to divide the cytoplasm.

• Identify the Stage: If you see homologous pairs (bivalents) lined up, it is Metaphase I. If you see single chromosomes in a line, it is Metaphase II. If you see four distinct nuclei forming, it is Telophase II.

• Count Centromeres: To determine the number of chromosomes, always count the centromeres. In Meiosis I, the number of centromeres is halved during the first division, which is why it is the reductional phase.

• Check for Recombination: In diagrams, recombinant chromosomes are often shown with different colored segments. This is a visual cue that Meiosis I (specifically Prophase I) has already occurred.

• Verify the Cell Type: Look for a cell wall or centrioles to determine if the question is asking about plant or animal meiosis, as this affects the description of cytokinesis.