22 Biology -- Cell Division

Describe the steps of prophase-I of meiosis.

Describe the steps of prophase-I of meiosis.

It is a very complex and long phase. It is further divided into five successive substages.

  1. Leptotene or Leptonema
  2. Zygotene or Zygonema
  3. Pachytene or Pachynema
  4. Diplotene or Diplonema
  5. Diakinesis

1. Leptotene[ GK: Leptos=thin, tene= Thread ]

  • size of nucleus increases
  • chromatin thread become shortened and converted into chromosomes which are thread like and single stranded.
  • chromosomes passes beaded structure called chromomeres.

2. Zygotene[ GK: Zygo= yolk, tene= thread]

  • pairing of homologous chromosomes takes place to form bivalents. The process of passing is called synapsis.
  • bivalents are held together by ribonucleoprotein to form synaptonemal complex.

3. Pachytene[ GK: Pachys= thick, tene= thread ]

  • each homologous chromosomes split longitudinally except at the centromere region into sister chromatids. And bivalents appear as tetrads
  • non-sister chromatids of each tetrad take place. This is called crossing over which brings variation in offsprings.

4. Diplotene[ GK: Diplos= double, tene= thread ]

  • non-sister chromatids start separating but both remains united at certain point. These points are called chiasmata

5. Diakinesis[ GK: dia= across, kinesis= movement ]

  • the chiasma moves towards the ends of chromosomes.
  • the chiasmata disappear by sliding towards end of chromosomes called terminalization. 
  • nuclear member and nucleolus disappear and spindle fibres starts to appear. 





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