Meiosis

Prophase I of meiosis is a complex stage involving the pairing of homologous chromosomes and genetic exchange. It is divided into several sub-stages (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis), during which the key events occur in a specific order.

Let's analyze the sequence of the major events listed:

1. Synapsis: This is the process where homologous chromosomes pair up side-by-side. This pairing begins during the Zygotene stage and is completed by the Pachytene stage. It forms a structure called a bivalent or tetrad. Synapsis must occur first to bring the homologous chromosomes close together.
2. Crossing Over: This is the physical exchange of genetic material between non-sister chromatids of the paired homologous chromosomes. This crucial event happens during the Pachytene stage, after synapsis has occurred, while the chromosomes are intimately paired.
3. Chiasmata Formation/Visibility: Chiasmata (singular: chiasma) are the X-shaped points where crossing over has occurred. While crossing over happens in Pachytene, the chiasmata become visibly distinct during the Diplotene stage, as the homologous chromosomes begin to separate slightly but remain held together at the points of exchange. So, chiasmata become apparent after crossing over.
4. Terminalisation: This is the process where the chiasmata appear to move towards the ends (termini) of the chromatids. This occurs during late Diplotene and Diakinesis, as chromosome condensation continues and the homologous chromosomes prepare to separate further. Terminalisation happens after chiasmata are formed.

Therefore, the correct chronological sequence of these major events in Prophase I is:

Synapsis → Crossing over → Chiasmata (visibility) → Terminalisation

Comparing this sequence with the given options:

• Synapsis → Crossing over → Chiasmata → Terminalisation. (Correct)
• Crossing over → Synapsis → Chiasmata → Terminalisation. (Incorrect)
• Chiasmata → Synapsis → Crossing over → Terminalisation. (Incorrect)
• Chiasmata → Crossing over → Synapsis → Terminalisation. (Incorrect)

The correct sequence is given in the first option.