Question

What happens in the synthesis phase during cell cycle?
A. DNA synthesis
B. Chromosome number becomes double
C. Formation of two nuclei
D. Synthesis of proteins & RNA

Answer 1

Hint: A cell cycle is a set of events that occur in a cell as it divides and grows. A cell spends most of its time in interphase on which is when it grows, duplicates its chromosomes and prepares to divide. The cell exits interphase after that, goes through mitosis and completes its division. The daughter cells that result go through their own interphase and start a new round of the cell cycle.

Complete answer:
Option A DNA synthesis : During interphase, before mitosis or meiosis, the S phase of the cell cycle is responsible for DNA synthesis or replication. Before entering mitosis or meiosis, a cell's genetic material is doubled in this fashion, providing enough DNA to be split into daughter cells.
Since DNA synthesis occurs in the synthesis phase of cell cycle, option A is the correct answer.
Option B Chromosome number becomes double : The number of chromatids in chromosomes doubles after replication, but the nuclei remain diploid due to the same number of centromeres and chromosomes. As a result, the number of chromosomes in the nucleus, which defines ploidy, remains constant during the S phase.
Hence option B is not correct.
Option C Formation of two nuclei : The newly divided chromosomes reach the mitotic spindle during telophase, and a nuclear membrane forms around each set of chromosomes, resulting in two distinct nuclei within the same cell.
Since this happens in telophase and not the synthesis phase, option C is not correct.
Option D Synthesis of proteins & RNA : The synthesis of proteins and RNA does not take place in synthesis phase or S phase of cell cycle.
Hence option D is not correct.

Option A. DNA synthesis is the correct answer.

Note:
When DNA synthesis begins, the S phase begins; when it is finished, all of the chromosomes have been replicated, i.e., each chromosome is made up of two sister chromatids. As a result, the amount of DNA in the cell has doubled throughout this phase, yet the ploidy and number of chromosomes remain the same. RNA transcription and protein synthesis rates are extremely low during this stage.