During the S phase of interphase, the amount of DNA per cell doubles.

Answer

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Hint: The cell division cycle involves inter-phase and M-phase. The S phase of the inter-phase is the synthesis phase. DNA replication occurs in this phase. The daughter cells after this phase have diploid number of chromosomes. The number of chromatids increased in this phase.

Complete answer:
The cell division completes mainly in two phases: Inter-phase and M – phase. The inter-phase includes various specialized biochemical processes that prepare the cell for initiation of cell division. There are mainly four phases:

G_{1}

phase, S phase,

G_{2}

phase,

G_{0}

phase (G stands for gap).
• The beginning phase of the inter-phase is

G_{1}

phase; the cell makes a variety of proteins that are needed for DNA replication.
• The S phase is the next phase after the

G_{1}

, all the chromosomes are replicated. After the replication, each chromosome now consists of two sister chromatids. The amount of the DNA in the cell has effectively doubled. The chromosomes remain diploid (2n). The number of chromatids doubles but the nuclei remains diploid as the number of centromeres and chromosomes remains unchanged. It remains the same from the beginning to the ending.
• The

G_{2}

phase is the phase in which the cell synthesizes a variety of proteins. During mitosis, most microtubules are produced.
• During the

G_{0}

phase, not all the cells are continually replicated. It includes non-replicating cells. It is called quiescent (dormant) or senescent (ageing or deteriorating).
• When the cells re-enter in

G_{1}

phase for an indeterminate period of time the cells begin dividing again under specific conditions.
• It is clear from the above information that during the S phase of interphase, the amount of DNA per cell doubles.

Note: The sequence of the processes is required in the cell cycle. There are many types of checkpoints present in the cells which control the cell’s progress through the cell cycle. It also ensures the key processes such as DNA replication and DNA damage repair are completed before entering into the next stage. They also ensure that both the daughter cells receive the same number of chromosomes.