The G2 phase, also known as the second gap phase, is an important preparatory stage within the cell cycle before a cell undergoes division. During this phase, the cell continues to grow and synthesizes proteins and organelles essential for successful cell division. It also functions as a checkpoint, ensuring the cell is ready to proceed into mitosis.
The Cell’s Preparatory Phase
Cell division begins in the G1 phase, or first gap phase. During G1, the cell grows and synthesizes cellular components, preparing for DNA replication. At this stage, each chromosome exists as a single, unduplicated DNA molecule.
Following G1, the cell enters the S phase, or synthesis phase, dedicated to DNA replication. Here, each chromosome is duplicated, resulting in two identical copies called sister chromatids. These sister chromatids remain joined at a constricted region called the centromere. While the amount of DNA within the cell effectively doubles during the S phase, the actual number of chromosomes does not change because the duplicated copies are still considered part of the original chromosome structure.
The G2 phase immediately follows the S phase. An important function of the G2 phase is to conduct checks for any DNA damage or replication errors that may have occurred during the S phase. If issues are detected, the cell can pause its progression to attempt repairs, ensuring genetic integrity before entering mitosis.
Counting Chromosomes in G2
Determining the number of chromosomes in any cell cycle phase, including G2, relies on a specific counting rule: a chromosome is defined by the presence of its centromere. Even though a chromosome in the G2 phase consists of two identical sister chromatids, it is still considered a single chromosome because these two chromatids are attached at a single centromere.
Consider a human somatic cell, which typically has 46 chromosomes in its unduplicated G1 state. After DNA replication in the S phase, and throughout the G2 phase, the cell will still possess 46 chromosomes. Each of these 46 chromosomes, however, will now be composed of two sister chromatids. This means that while the number of discrete chromosome structures remains constant, the total amount of DNA within the cell has doubled compared to the G1 phase.
Chromosome Dynamics Through the Cell Cycle
Chromosome number and DNA content change across different cell cycle stages. In G1, a human cell has 46 unduplicated chromosomes. After S phase, the cell in G2 maintains 46 chromosomes, but each is duplicated, comprising two sister chromatids. This state persists into early mitosis, such as prophase and metaphase, where the chromosomes condense and align.
The chromosome count changes during anaphase of mitosis. In this stage, the sister chromatids separate from each other. Once separated, each chromatid is considered an individual chromosome. This temporarily doubles the chromosome number (e.g., from 46 to 92 in human cells) before the cell divides into two daughter cells during cytokinesis. After cytokinesis, each new daughter cell returns to the G1 state with 46 unduplicated chromosomes, completing the cycle.