G1fase

G1fase, or the G1 phase, is the first stage of the eukaryotic cell cycle that follows mitosis. During G1, cells grow in size, increase metabolic activity, and synthesize RNA and proteins required for DNA replication. Chromatin remains relatively active in transcription, and the cell assesses its environment and internal conditions before committing to DNA synthesis. The phase ends with the G1/S transition, marking the point at which the cell commits to genome duplication.

Regulation in G1 is governed by growth signals and checkpoints. The G1/S checkpoint, often called the restriction point, is controlled by cyclin-dependent kinases (CDKs) and cyclins, notably cyclin D with CDK4/6 and later cyclin E with CDK2. Phosphorylation of the retinoblastoma protein (Rb) releases E2F transcription factors that drive S-phase gene expression. In response to unfavorable conditions or DNA damage, tumor suppressor pathways (for example p53 leading to p21) can enforce a G1 arrest, preventing replication. Some cells may exit the cycle entirely and enter a quiescent state known as G0.

Characteristics and variability of G1 vary across organisms and cell types. The duration can range from a few hours to days. In preparation for S phase, the cell activates DNA replication licensing by loading MCM helicases onto replication origins. Nucleotide and histone biosynthesis also increases to support genome duplication and chromatin assembly.

In summary, G1fase prepares the cell for DNA replication by supporting growth, metabolism, and environmental assessment to determine whether cell division should proceed.