The cell cycle, growth and replication checkpoints

Follow a representative cycling eukaryotic cell

A common eukaryotic cell-cycle description uses G1, S, G2 and M phases. G1 is the interval before nuclear DNA replication, S is the DNA-synthesis phase and G2 is the interval after it and before mitosis. M phase includes mitosis and the associated division of the cytoplasm, cytokinesis, which can overlap the later nuclear events.

Interphase comprises G1, S and G2. It is active cellular work, not merely resting between divisions. Cells make molecules, maintain organisation and respond to signals; growth is not restricted to one moment in G1. Cell growth means increasing material or size, while proliferation involves producing daughters through division. The net population size also depends on cell loss.

Phase duration and organisation vary, and some specialised or early developmental cycles omit typical gap phases. Many cells are not currently cycling. Quiescent cells can remain alive and later re-enter a cycle under suitable conditions; terminally specialised or senescent states are different. The circle describes a selected cycling route rather than the inevitable future of every cell.

Use different labels for sets and DNA amount

Ploidy describes complete chromosome sets. In our invented example, one haploid set contains two chromosome types, so n = 2. A diploid cell has two homologous copies of each type, making 2n = 4 chromosomes. Homologues are corresponding chromosomes from the two sets; they need not have identical DNA sequences.

After one complete S phase, each chromosome has two sister chromatids, each containing a DNA double helix. We count a joined sister pair as one replicated chromosome until the sisters separate. Thus the cell still has four chromosomes, now with eight chromatids and eight chromosomal DNA molecules. The same four homologous chromosome units each carry a new sister copy.

To track DNA amount separately, 1C denotes the nuclear DNA amount in one unreplicated haploid set. The model's diploid G1 cell has 2C; after complete replication it has 4C. It remains diploid before segregation. The diagram keeps chromosome count, DNA-molecule count and C labels alongside each stage; organelle DNA is excluded.

Progression is regulated, not just timed

Cyclin-dependent kinases, or CDKs, are protein kinases: enzymes that add phosphate groups to proteins. They help control cycle transitions. Cyclins and other regulators affect their activity, helping coordinate replication and division. Signals and cellular conditions influence this control. A fixed number of elapsed minutes does not by itself establish that a cell is ready for the next event.

Checkpoint pathways can delay progression when particular problems are detected. DNA-damage and replication-stress responses help prevent entry into mitosis with incompletely replicated or damaged DNA, while supporting repair or other responses. Such control is a set of molecular interactions, not a conscious inspector or a promise that every error is corrected.

A delay can be reversible, or further responses can lead to a durable arrest or cell death. A checkpoint also has a specific scope: a spindle-attachment signal in mitosis does not establish that every nucleotide was accurately copied earlier. The next lesson follows attachment and segregation, and the cell-fate lesson distinguishes possible outcomes of a prolonged problem.

Worked example

Check your understanding

1. A diploid 2n = 4 cell completes S phase. How many replicated chromosomes and sister chromatids does the model contain?

   Show reasoning

   Four replicated chromosomes and eight sister chromatids. Each joined sister pair is still counted as one chromosome before separation.

2. Does a change from 2C to 4C nuclear DNA amount necessarily make this pre-segregation cell tetraploid?

   Show reasoning

   No. DNA replication doubled the DNA amount in sister chromatids. The cell still has the same two chromosome sets before segregation.

3. Why does a checkpoint delay not establish that a cell will die or that every problem will be repaired?

   Show reasoning

   A delay is a regulatory response to a detected condition. Subsequent outcomes depend on the problem, pathways and context; accurate repair, resumed cycling, durable arrest or death are distinct possibilities.

What to carry forward

Keep growth, DNA replication and division separate, and state whether a number refers to chromosome sets, chromosome units, DNA molecules or DNA amount.