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22 Biology -- Cell Division

10. mention the types of prophase in mitosis and explain them briefly.

10. mention the types of prophase in mitosis and explain them briefly.

There are two types of prophase in meiosis, they are : Prophase I and Prophase II 

Prophase I happens after interphase in meiosis I whereas Prophase II is the first step in meiosis II and takes place directly after telophase 1.

During Prophase 1, the chromosomes become distinct and visible. Synapsis i.e. the pairing of homologous chromosomes prior to their segregation occurs, then there is the formation of bivalent tetrad. Crossing over, ie. the process of exchange of genetic materials between the chromosomes also occurs in this stage which allows for the formation of two genetically different gametes. Later, in this stage we find that the nuclear matrix and nucleolus disappear and the spindle fibers are moved to the center which connect to the kinetochores of the chromosomes. 

During Prophase II , there is the replication of the centrioles i.e. the daughter centrioles divide perpendicular to the mother centrioles. There is also the breakdown of the nuclear envelope which was formed in telophase 1. In this phase, the cells contain half of the amount of chromosomes than in the Prophase 1. 

In mitosis, the prophase is further understood by dividing it into the given sub-stages: 

1.Leptotene :

 In this stage the nucleus enlarges in size in the chromosome. The chromosomes appear thin, thread-like and single-stranded in this stage. They have swollen or beaded structures along their length and their ends appear converged towards one side of the nucleus called bouquet. 

2.Zygotene:

In this stage, the identical chromosomes come together and form bivalent or homologous pairs. Further, synapsis occurs which is referred to as the process of pairing up of homologous chromosomes. 

3.Pachytene

Due to the continuous pairing thickening and coiling the chromosomes split into 4 chromatids. Crossing Over i.e. the exchange of genetic material between the two non-sister chromatids of each tetrads take place in this stage which is in turn accompanied by chiasma. 

4.Diplotene:

In this stage, uncoiling of homologous chromosomes can be seen which remain attached at the chiasma despite of the uncoiling.

5. Diakinesis

In this stage, chromosomes become thicker and more specific. The chiasmata start to move towards the opposite ends of the chromosomes as well which is a a type of movement termed as terminalization of chiasma. By the end of this stage, the the nuclear envelope start disappearing the spindle fibers start to reappear .

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