The diplotene stage, also called diplonema, is a key substage of prophase I in meiosis where genetic recombination solidifies. This MCQ tests understanding of chromosome dynamics after crossing over, essential for biology and genetics learners.

Correct Answer: 2. Separation of the paired chromosomes except at the chiasmata

During diplotene, the synaptonemal complex dissolves, causing homologous chromosomes to desynapse or separate. They remain physically linked at chiasmata—X-shaped crossover points—ensuring proper segregation in meiosis I. This stage often prolongs in oocytes, visible as distinct bivalents under microscopy.

Explanation of All Options

Each option ties to a specific prophase I stage; here’s why others don’t fit diplotene:

• 1. Reduction in the number of chiasmata
  Chiasmata persist and may terminalize (move toward ends) in diplotene and diakinesis, but reduction happens later during anaphase I when homologues separate fully.

• 3. Crossing-over and recombination between homologous chromatids
  This occurs earlier in pachytene, where double-strand breaks and strand exchange form chiasmata; diplotene reveals these structures post-recombination.​

• 4. Pairing of homologous chromosomes and synaptonemal complex formation
  Synapsis and synaptonemal complex assembly define zygotene; by diplotene, pairing ends as the complex disassembles.​

Key Diplotene Features

Homologues repel each other post-pachytene, repuckering chromosomes into thread-like loops held by chiasmata. This visible separation aids recombination studies and explains genetic diversity. Errors here link to aneuploidy disorders like Down syndrome. For students, memorize: leptotene (condensation), zygotene (pairing), pachytene (crossing over), diplotene (separation), diakinesis (condensation).