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Solution


Q.

In the 'S' phase of the cell cycle:  [2014]
1 amount of DNA doubles in each cell  
2 amount of DNA remains the same in each cell  
3 chromosome number is increased  
4 amount of DNA is reduced to half in each cell  

Ans.

(1)

In S-phase (synthesis phase) of cell cycle, the chromosomes replicate. For this, their DNA molecules function as templates and form carbon copies. The DNA content doubles, i.e., 1C to 2C for haploid cells and 2C to 4C for diploid cells. As a result, duplicate sets of genes are formed. Along with replication of DNA, new chromatin fibres are formed which, however, remain attached in pairs and the number of chromosomes does not increase. As chromatin fibres are elongated chromosomes, each chromosome comes to have two chromatin threads or sister chromatids which remain attached at a common point called centromere."