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Chromosome and chromatid count in humans during mitosis


   chromatid and chromosome

How to count the number of chromosomes and chromatids in meiosis or mitosis? I found this in many past question papers. In order to answer such questions we need to have some basic concepts regarding chromosomes and chromatids. Rest is easy.

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,
In this figure, chromosome before DNA replication  has a single chromatid. After DNA replication in S phase,  two chromatids  are held at a  common centromere representing a single chromosome.

Let us take the case of ours to explain this. You just imagine what happens at each stage. the number of chromosome in each cell is 46 
Human cell [2n=46] 
G1 Phase
• Chromosome #: 46
• Chromatid #: 46 
G2 Phase
DNA replication has already occurred [during S phase]. The number of DNA molecule increases only when DNA replicates that is in the S phase of the cell cycle.
That is, 2 chromatids attached to a common centromere region = 1 chromosome
Therefore Chromosome #: 46 
                   Chromatid #: 92 
Mitosis


Prophase
o Chromosome #: 46
o Chromatid #: 92

Metaphase
o Chromosome #: 46
o Chromatid #: 92

Anaphase: separation of sister chromatids
o Chromosome #: 92
o Chromatid #: 92

Telophase/Cytokinesis
o Cell= 2n [diploid]
o Chromosome #: 46
o Chromatid #: 46

The purpose of mitosis is to make two genetically identical cell. A the beginning of G1, the chromosome and chromatid number is 46. After mitosis, each daughter cell has 46 chromosomes.
Hope everything is clear.
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