USA Biology Olympiad (USABO) Practice Exam

Which phase of the cell cycle is characterized by cell growth and preparation for division?

1. G1 phase

2. S phase

3. G2 phase

4. M phase

The correct answer is: G1 phase

The G1 phase, or Gap 1 phase, is a crucial part of the cell cycle characterized primarily by cell growth and preparation for DNA synthesis. During this phase, the cell increases in size, synthesizes a variety of proteins, and generates the necessary organelles and resources required for DNA replication. The G1 phase is essential for ensuring that the cell is adequately prepared for the subsequent S phase, where DNA synthesis occurs. It involves various checkpoints that assess the cell's conditions, such as size and nutrient availability, which help decide whether the cell will continue to the next phase. If conditions are not favorable, the cell may enter a quiescent state known as G0. In contrast, the S phase is where DNA replication takes place, and the G2 phase is focused on further growth and preparation for mitosis, including the synthesis of proteins necessary for mitosis. The M phase, or mitosis, is the phase where the actual division of the cell occurs, leading to the formation of two daughter cells. Hence, the defining feature of the G1 phase as a preparatory period emphasizes its importance in the cell cycle.