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What happens to the number of DNA molecules and chromosomes during meiosis?

The situation here is also similar to mitosis at initial stages.
Points to remember:
  • The number of chromosome = count the number of functional centromere
  • The number of DNA molecule  = count the number of chromatids.
When the DNA molecule number change?
  • The number of DNA molecules increase only when DNA replicates that is only in S phase or synthetic phase.
  • The number of DNA molecules decreases only when the cell divides.
Now when the chromosome number change?
The number of chromosomes increases only when sister chromatids separated during anaphase of mitosis and anaphase II of meiosis. Think, in anaphase I of meiosis, only homologous chromosomes are separated. Only in anaphase II sister chromatids are separated.
  • As in the case of DNA molecule, reduction in chromosome number takes place only by cell division.
Re read these 4 points. Imagine and make it yours. Now you can answer any question regarding this.
Now let us work out the question with the above points in mind.


At G1 phase:
  •  No. of DNA molecule =4
  • No. of chromosome=4 and each chromosome has a single chromatid
At G2 phase:
In S phase DNA replicates. 
  • No. of DNA molecule in G2 will be =8
  • No. of chromosome=4 and as two DNA molecules are held at a common centromere. Or in other words each chromosome has two chromatids.
During Meiosis, there is no change in no. of DNA molecule and chromosomes in prophase and metaphase.
At metaphase:
  • No. of DNA molecule =8
  • No. of chromosome=4
During Anaphase I: homologous chromosomes separate but the no.of chromosome remains 8. 
After cytokinesis at the end of meiosis I :the resulting 4 chromosomes are distributed between two cells. Now each cell has 2 chromosomes with 4 chromatids 
  • No. of DNA molecule =4 
  • No. of chromosome=2
No change in number occurs till metaphase II



Meiosis I
Meiosis I


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Meiosis II
Meiosis II
At metaphase II: 
  • No. of DNA molecule =4 
  • No. of chromosome=2
During Anaphase II: 
sister chromatids separate and the 
  • No. of chromosome increases temporarily to 4 
  • No. of DNA molecules remain 4. 
After cytokinesis at the end of meiosis II: the resulting 4 chromosomes are distributed between two cells. Now each cell has 2 chromosomes with 2 chromatids.
  • No. of DNA molecule =2 
  • No of chromosomes per cell is 2. 
Now the four cells formed have half the number of chromosome to that of the parent cell, and that is the result of meiosis.

In the figure initial chromosome number is 4 (2n=4). Then the changes in no. of DNA molecules and chromosomes during mitosis and meiosis is summarised in the table. 

Correlate this with figure  

Cell cycle Stages
Number of DNA molecules /cell
Number of Chromosomes/cell
G1
4
4
G2
8
4
In Mitotic Stages
Metaphase
8
4
Anaphase
8
8
After cytokinesis of Mitosis
4
4
In Meiotic Stages
Metaphase I
8
4
Metaphase II
4
2
After cytokinesis of Meiosis II
2
2


Here is the table showing the summary of variation in no. of chromosomes and DNA molecule (chromatid) in human cell during mitosis and meiosis.

Characteristic  in a human cell

Mitosis
Meiosis I
Meiosis II
Chromosome number in a parent cell before cell division begins
46

46
Chromatid number in a parent cell before cell division begins
92
92
Crossing over at prophase
No
Yes
No
Chromosome arrangement on metaphase plate
Chromosome line up
Homologues pair
Chromosome line up
No. of chromosomes in each daughter nucleus
46
23
No. of chromatids in each daughter nucleus
46
46
23
No. of daughter cells at the end of division
2
2
4
Chromosome notation for daughter cells
2n
n
Genome notation for daughter cells
Diploid
Haploid
Purpose of division
Cell replacement, asexual reproduction, Organism growth
Sexual reproduction
Genetics of daughter cells
Genetically identical
Genetically variable
Types of cells where cell division occurs
Somatic cells
Reproductive cells
(testes, anthers, ovaries)
Types of cells produced
Somatic cells
Gametes, sperms, eggs, pollen


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