Q.Describe the following:

These three terms all describe events in prophase I of meiosis:

• (a) Synapsis — the pairing of homologous chromosomes, in which each chromosome lines up alongside its partner. It occurs in the zygotene stage and is accompanied by a protein scaffold called the synaptonemal complex.
• (b) Bivalent — a pair of synapsed homologous chromosomes held together; because it contains four chromatids it is also called a tetrad.
• (c) Chiasmata — the X-shaped points where recombined homologous chromosomes remain joined at the sites of crossing over, seen during diplotene. …

All three terms belong to prophase I of meiosis, the long and elaborate stage during which homologous chromosomes come together and exchange material.

1. Synapsis. During the zygotene sub-stage the chromosomes begin to come together in pairs, each chromosome lining up alongside its partner. This close pairing is called synapsis, and the two members that pair in this way are the homologous chromosomes, one inherited from each parent. The pairing is accompanied by the assembly of an elaborate protein structure between the partners, the synaptonemal complex.
2. Bivalent. A pair of synapsed homologous chromosomes held together is called a bivalent. Because each homologous chromosome consists of two chromatids, the paired structure contains four chromatids in all, and for this reason a bivalent is also called a tetrad. Bivalents become especially clear at the pachytene stage.
3. Chiasmata. During diplotene the synaptonemal complex breaks down and the recombined homologous chromosomes begin to move apart, but they cannot separate completely because they remain tied together at the places where crossing over occurred. These persisting connections appear as X-shaped structures called chiasmata. …

Step 1 — Recognise all three terms belong to prophase I's timeline and place them in the order they occur: synapsis (zygotene) → bivalent (visible from zygotene, clearest at pachytene) → chiasmata (diplotene onward).

Step 2 — Define each term precisely:

• Synapsis: pairing of homologous chromosomes.
• Bivalent (tetrad): the paired four-chromatid unit that results from synapsis.
• Chiasmata: the X-shaped points where the paired homologues stay joined after crossing over. …