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The egg is a haploid cell. After fertilisation, it fuses with another haploid cell to form a single diploid cell. The quantity of DNA has doubled.View attachment 54806
The answer is D
The next doubling of DNA content occurs during the S phase of the cell cycle. The DNA undergoes semi-conservative replication. It may not be noticeable under a microscope until Prophase, when we see each chromosome consisting of two chromatids, but we know the chromatin form duplicates before during the S phase.
The reduction of DNA content next, which is what the question is asking about, happens at mitosis, when two new cells are formed from the diploid zygote, so the DNA material is shared. Each get one chromatid from the one chromosome.
This process of increase and decrease in DNA material will continue, each corresponding to S phase and mitosis.
All 4 options given in the question are a part of mitosis.
The point at which we consider the single cell to have truly split into two is when mitosis is complete.
This is at Telophase, after which cytokenesis will complete the separation.