Chromosomes and Chromatid Count in Humans during Meiosis

In the last post, we discussed about How to count the number of chromosomes and chromatids in mitosis ? The concept is same in the case of meiosis also. These are the essential prerequisites for meiosis.

1) DNA should have replicated before meiosis
2) Inorder for crossing over in meiosis I of prophase I to occur, 99.7% of DNA replication and 75% of histone synthesis must occur by prophase I.

 The key points are

• The number of chromosomes=count the number of centromeres
• The number of DNA molecule = count the number of chromatids
• The number of DNA molecule increases only when DNA replicates that is in the S phase of the cell cycle
• The number of DNA molecules decreases only when the cell divides,

Meiosis I [first cell division]
Prophase I
o Chromosome #: 46
o Chromatid #: 92

Metaphase I
o Chromosome #: 46
o Chromatid #: 92

Anaphase I (Remember, homologous chromosomes are separated therefore chromosome number and chromatid number remains the same at this stage)
o Chromosome #: 46
o Chromatid #: 92

Telophase I
o Chromosome #: 46 
o Chromatid #: 92

End of meiosis I (in daughter cells)
o Chromosome #: 23 ( That is why Meiosis I is called as reduction division, reduction in chromosome number to half)

o Chromatid #: 46

Meiosis II [2nd Cell Division] is similar to Mitosis
Prophase II
o Chromosome #: 23
o Chromatid #: 46

Metaphase II
o Chromosome #: 23
o Chromatid #: 46 

Anaphase II (sister chromatids are separated similar to mitosis)
o Chromosome #: 46
o Chromatid #: 46 

Telophase II before Cytokinesis
o Chromosome #: 46
o Chromatid #: 46

End of meiosis II (in daughter cells)
o Cell = n [haploid]
o Chromosome #: 23

o Chromatid #: 23

Meiosis is reduction division that occurs only in germ cells where gametes are produced with half the chromosome number to that of the parent cell.Gametes are always haploid. Gametes should be haploid for maintaining the chromosome number of the species.

This is achieved by meiosis, the reduction division in germ cells.

The purpose of meiosis is the production of 4 cells with half the number of chromosomes. Here at the beginning of meiosis,  chromosome number is 46 (diploid). After meiosis, the resulting cells or gametes has 23 chromosomes (haploid).

Hope you got the point.

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