Final answer:
The first phase of the cell cycle is the G1 phase, where the cell grows and prepares for DNA replication. It is followed by the S phase, where DNA duplication occurs, and G2 phase, where final preparations for cell division are made before entering mitosis.
Step-by-step explanation:
The first phase of the cell cycle is the G1 phase. During the G1 phase, the cell undergoes normal growth processes, preparing for subsequent phases. This phase is characterized by cell growth and the accumulation of the building blocks of chromosomal DNA and associated proteins. After the G1 phase, the cell enters the S phase, where DNA replication occurs, followed by the G2 phase, where the cell prepares for mitosis or meiosis.
The cell cycle is composed of interphase and the mitotic phase. Interphase itself includes G1, S, and G2 phases, and it is during this time that a cell grows, duplicates its DNA, and makes final preparations for cell division, leading to the mitotic phase. The G1 phase, being the first part of interphase, is the correct answer to which phase of the cell cycle occurs first.