Ans. (a.) g 1/s checkpoint.
Cell cycle checkpoints are defined as control mechanisms to regulate cell divisions in eukaryotic cells. G1 (gap 1) phase represents first phase of cell division, during which cell prepares to divide by growing its size, synthesizing mRNA and proteins.
Cell cycle checkpoint that checks whether the dividing cell size is adequate or not in order to move in next cell cycle phase is G1/S checkpoint.
After passing G1/S checkpoint, cell enters to the next phase, known as S phase (synthesis phase), in which DNA replication takes place.
Thus, the correct answer is option (a).
