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Cell Division Part 3

Cell Division Part – 3

Meiosis:
It occurs during formation of reproductive cells and the numbers of chromosomes are reduced to half the number of chromosomes in the parent cell.
During meiosis, the genetic material is replicated once, whereas the cell divides twice.
It is divided into two parts


1. Meiotic Division - I:
·  Chromosomes are reduced to half and distributed in two cells
·  Also called as reductional division


2. Meiotic Division – II:
·  The number of chromosomes remains same in new cells
·  Also called as equational division

Meiosis follows interphase as described earlier in cell division part – 1


Meiosis - I:
It is further divided into four phases:
  1. Prophase I
  2. Metaphase I
  3. Anaphase I
  4. Telophase I

Prophase – I:
It is a beginning phase and lasts for long time and further divides into five sub stages

1.  Leptotene:
·  Chromosomes begin to condense and appear as filamentous
·  Both the end of chromosomes remain linked to the nuclear membrane
·  Each chromosome is made up of two chromatids and a centromere joining them


2.  Zygotene:
·  Two member chromosomes of each pair of homologous chromosomes move towards each other and start lateral union along their length. This process is called synopsis
·  As the process progress, chromatids of homologous chromosomes twins around one another at some places and the allelic genes on two chromosomes become arranged opposite to one another.
·  Two homologous chromosomes have a ladder like appearance and each group of chromosomes appears bivalent.


3.  Pachytene:
·  It begins at the end of synopsis and condensation of chromosomes get completed
·  Each group of chromosomes now appears tetravalent.
·  Exchange of genes occurs at various places along their lengths and this process is called as crossing over.


4.  Diplotene:
·  The member of each pair of homologous chromosomes start moving away from one another but their union is maintained at the points of crossing over
·  Chiasmata appear along the length of homologous pair of chromosomes and the number of chiasmata depends on the length of chromosomes.
·  Gene exchange occurs at the location of chiasmata


5.  Dikinesis:
·  Condensation of chromosome becomes completed and they move towards nuclear membrane.
·  Chromatids become separated even at the site of chiasmata
·  Each chromosome is made up of two chromatids and a centromere holding them together
·  The size of nucleolus decreases at the beginning of prophase and finally it disappears at the end of prophase and nuclear membrane is disintegrated
·  Bipolar spindle is formed and each chromosome attached through its centromere


Metaphase – I:
·  Pair of homologous chromosomes arranged at the equatorial plane of the cell. Centromere of each chromosome is arranged towards two poles and chromatids are arranged freely.

Anaphase – I:
·  The two members of each pair of chromosomes move away towards their respective poles and thus the number of chromosomes is halved.

Telophase – I:
·  Nuclear membrane and nucleolus are reformed and bipolar spindle is disintegrated
·  At the end of their phase, two nuclei formed and each nucleus contains half the number of chromosomes as were present in the parent cell.
·  Each chromosome is made up of two chromatids and a centromere holding them together
·  Cytokinesis occurs and forms two new cells.



Self study:
Example-1: Exchange of genes at various places of chromosomes occurs is called as _____________

a)  Crossing over
b)  Crossing down
c)  Cross connection
d)  Crossing

Answer: Exchange of genes at various places of chromosomes occurs is called as crossing over


Example-2: Match the following:

A B
Leptogene Synopsis process
Zygotene Chromosomes condense
Pachytene Gene exchange at chiasmata
Diplotene Chromatids of chromosomes are twined around one another


Answer:

A B
Leptogene Chromosomes condense
Zygotene Synopsis process
Pachytene Chromatids of chromosomes are twined around one another
Diplotene Gene exchange at chiasmata

Example-3: Meiosis is also called as a ______________
a)  Cytokinesis
b)  Synopsis

c)  Reductional division
d)  Interphase

Answer: Meiosis is also called as a Reductional division
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