Cell Cycle and Regulation

Objective: Sequence G1, S, G2, and division and relate each to cell function.

The eukaryotic cell cycle consists of interphase and an M phase. During G1, a cell grows and carries out many ordinary functions; in S phase, its DNA is replicated; in G2, it prepares for division and checks replicated material. M phase includes nuclear division and typically cytokinesis. Some cells enter G0, a nondividing state that may be temporary or long lasting depending on the cell type. Time spent in each phase varies widely among cells. DNA replication duplicates each chromosome into sister chromatids but does not double the chromosome number when chromosomes are counted by centromeres.

Objective: Distinguish chromosomes from chromatids and homologous pairs.

A chromosome is a DNA molecule packaged with associated proteins; counting is conventionally based on centromeres. After S phase, a replicated chromosome contains two sister chromatids joined at a centromere. Homologous chromosomes are a pair carrying corresponding gene loci, often with different versions of genes; they are not identical to sister chromatids. Diploid cells have two sets of chromosomes, and haploid cells one set, with exceptions in life cycles and tissue types. The DNA amount can double in S phase while ploidy remains unchanged. Upon separation, former sister chromatids are counted as individual chromosomes.

Objective: Describe checkpoint decisions and limits of repair.

Cell-cycle progression is regulated by cyclins, cyclin-dependent kinases, signaling networks, and checkpoints. At the G1/S transition, cells assess conditions such as growth signals and DNA integrity before committing to replication. Additional control before mitosis considers DNA replication and damage; the spindle assembly checkpoint delays chromatid separation until chromosomes are properly attached to spindle microtubules. A checkpoint can pause progression and allow repair, but not every error is detected or repaired. Breakdown of regulation can contribute to uncontrolled proliferation; cancer is a complex group of diseases involving multiple changes rather than a single universally sufficient mutation.