G1
Summary
G1 (Gap 1) is the first phase of interphase, after mitosis and before S phase. The cell grows and makes proteins and organelles. The testable point is the G1 checkpoint (restriction point): p53 and Rb normally block entry into S phase if DNA is damaged or growth signals are absent.
Detail
Meaning: G1 is the stretch of interphase between the end of M phase and the start of S phase. The cell grows, synthesizes proteins and organelles, and carries out its normal functions. Each chromosome is still a single, unreplicated chromatid. Near the end of G1 the cell passes the G1 checkpoint (restriction point), which assesses cell size, nutrients, growth factors and DNA integrity. Cyclin–CDK complexes (such as cyclin D–CDK4/6 and cyclin E–CDK2) phosphorylate Rb, which releases E2F to turn on S-phase genes. If DNA is damaged, p53 raises p21, which inhibits the CDKs and halts the cycle. A cell that stops dividing leaves G1 for G0.
On the MCAT: Expect a flow cytometry graph or DNA-content plot, where G1 cells sit at the lower peak (2n DNA amount) and G2/M cells at the higher peak (4n). Another common setup is a mutation or drug: loss of p53 or Rb function removes the brake, so cells with damaged DNA can enter S phase, which is linked to cancer. Removing growth factors tends to arrest cells in G1 or G0.
Common confusion: DNA replication happens in S phase, not G1. Chromosome number stays the same from G1 to G2, but DNA amount doubles in S. G0 is a separate resting state: some cells stay there permanently, while others can re-enter G1 given the right signals.
Sources
- Molecular Biology of the Cell
- Campbell Biology
- Molecular Biology of the Gene
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.