Overview

• Duration and Complexity
  • Longer and more complex compared to mitotic prophase.
  • Subdivided into five phases: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.

Leptotene

• Chromosome Visibility
  • Chromosomes gradually become visible under the light microscope.
  • Continuous compaction of chromosomes throughout leptotene.

Zygotene

• Chromosome Pairing (Synapsis)
  • Chromosomes start pairing, synapsis occurs.
  • Paired chromosomes referred to as homologous chromosomes.
  • Electron micrographs show the formation of the synaptonemal complex.
  • Complex formed by synapsed homologous chromosomes is called a bivalent or tetrad.

Pachytene

• Distinct Chromatids and Recombination Nodules
  • Four chromatids of each bivalent become distinct, forming tetrads.
  • Appearance of recombination nodules, sites of crossing over.
  • Crossing over: Exchange of genetic material between homologous chromosomes.
  • Enzyme-mediated process involving recombinase.
  • Recombination between homologous chromosomes completes by the end of pachytene.

Diplotene

• Dissolution of Synaptonemal Complex
  • Recognition by dissolution of synaptonemal complex.
  • Recombined homologous chromosomes tend to separate except at crossover sites.
  • X-shaped structures formed at crossover sites called chiasmata.
  • Diplotene duration can extend for months or years in oocytes of some vertebrates.

Diakinesis

• Terminalisation of Chiasmata and Preparation for Separation
  • Marked by terminalisation of chiasmata.
  • Chromosomes fully condensed, meiotic spindle assembled.
  • Nucleolus disappears, nuclear envelope breaks down.
  • Transition to metaphase.