16.1.4—Chromosome behaviour during meiosis

Identify a meiosis image by combining chromosome behaviour, spindle arrangement, nuclear-envelope change and cell division evidence. The key distinction is whether homologous pairs are present and whether whole homologues or sister chromatids are separating.

1. Decide whether the image shows meiosis I or meiosis II. Paired homologues indicate meiosis I; single chromosomes and the developing cell count provide supporting evidence. Two new cells indicate the first division, while four products indicate the second division.
2. Assign the PMAT stage only after checking more than one feature. In prophase, chromosomes condense; in metaphase, they align at the spindle equator; in anaphase, chromosomes or chromatids move towards opposite poles; in telophase, nuclei reform and cytokinesis begins.
3. For meiosis I, check for homologous pairs at prophase/metaphase and whole chromosomes moving with centromeres intact at anaphase. For meiosis II, check single-file chromosomes and centromere division followed by chromatid separation.
4. Cross-check the label against spindle direction, nuclear membranes and cell-surface division. Do not assign a stage from one shape or position alone.

The most diagnostic evidence is pairing versus single chromosomes, the alignment pattern, centromere behaviour and the number of forming cells. Uneven staining, rotation or a crowded photomicrograph can hide one clue, so the final identification should be justified by converging visible features.

A pair of dark chromosome shapes is not automatically a homologous pair. This card teaches image/diagram identification of meiosis stages; it does not explain crossing over, independent assortment or other sources of genetic variation.