Question

What will be the number of DNA AND CHROMOSOME respectively after S phase if number of chromosome in root tip cells are 20 -

1)20&20

2)20 & 40

3)40&40

4)40&20

Answer 1

S or synthesis phase

• S or synthesis phase == DNA synthesis or replication takes place.
• During this time the amount of DNA per cell doubles.
• If the initial amount of DNA is denoted as 2C then it increases to 4C.
• However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1, even after s phase the number of chromosomes remains the same, i.e., 2n.

Therefore DNA number becomes 40 and Chromosomes number remains the same.

Option (4)