With the help of suitable diagrams, explain the five stages of prophase-I of meiosis.

Answer: Prophase-I of meiosis is long and is divided into five sub-stages: leptotene, zygotene, pachytene, diplotene and diakinesis.

Most of the events that make meiosis different from mitosis (pairing of homologous chromosomes and crossing over) take place in prophase-I. What each diagram shows is given with each stage.

1. Leptotene: Chromatin condenses into long, thin, thread-like chromosomes that become visible; their ends (telomeres) are attached to the nuclear membrane. Diagram: nucleus with nuclear membrane and nucleolus, containing long thin single threads.
2. Zygotene: Homologous chromosomes come close and pair up along their length (synapsis) with the help of the synaptonemal complex. Each pair is called a bivalent. Diagram: pairs of similar threads lying side by side, zipped together.
3. Pachytene: Chromosomes become shorter and thicker; each bivalent shows four chromatids (tetrad). Crossing over (exchange of segments) takes place between non-sister chromatids of homologous chromosomes. Diagram: thick paired chromosomes showing four chromatids, with non-sister chromatids crossing each other.
4. Diplotene: The synaptonemal complex dissolves and homologous chromosomes begin to separate, but remain joined at the points of crossing over called chiasmata. Diagram: homologues pulling apart, held together at X-shaped chiasmata.
5. Diakinesis: Chromosomes are fully condensed; chiasmata move towards the ends (terminalisation); the nucleolus and the nuclear membrane disappear and spindle fibres begin to form. Diagram: short, thick bivalents scattered in the cell, no nuclear membrane, spindle starting to form.