2.1.6(a)-(b) - Cell cycle phases and checkpoint regulation

The cell cycle as an ordered sequence

The cell cycle is the ordered sequence of events that a dividing eukaryotic cell passes through from one cell division to the next. For this lesson, the sequence is:

1. Interphase, made of G1, S and G2.
2. Mitosis, where the copied genetic information is separated into two nuclei.
3. Cytokinesis, where the cytoplasm divides and two daughter cells are formed.

The point of the sequence is accuracy as well as division. DNA must be copied before it is separated, and the copied chromosomes must be separated before the cell is finally split. If these events happen successfully, each daughter cell receives the same genetic information as the original cell.

Now check the distinction between the two division processes.

Interphase prepares the cell

Interphase is not a resting phase. It is the long preparation period before cell division, and OCR expects the three named parts: G1, S and G2.

In G1, the cell grows and carries out normal metabolism. It makes proteins and other cell contents needed for continued function and later division. A cell that is too small or short of resources is not a good candidate for DNA replication.

In S phase, the DNA is replicated. Each chromosome now consists of two sister chromatids. DNA content doubles during S phase, but the chromosome count does not double at that stage. The sister chromatids can later be separated to daughter nuclei. This is why S phase is central to producing genetically identical cells: without accurate DNA replication, the daughters could not inherit the same genetic information.

In G2, the cell continues to grow and prepares for mitosis. It makes proteins needed for chromosome movement and division, and it must be ready to separate the copied genetic information.

Use phase names precisely. Interphase includes G1, S and G2; S phase is where DNA replication happens. Mitosis is not the whole cell cycle.

Use that phase language to reason about what would go wrong if the sequence were rushed.

Mitosis separates and cytokinesis splits

After interphase, the cell enters the division part of the cycle. In this lesson, keep the boundary clear: the detailed stages of mitosis are taught separately. Here, the essential idea is that mitosis separates the copied genetic information so that two nuclei form with the same genetic information.

Cytokinesis divides the cytoplasm. It can begin during late mitosis and is completed as the daughter cells separate after nuclear division. This produces two daughter cells. In animal cells, cytokinesis usually involves the cell surface membrane pinching in. In plant cells, new cell wall material forms between the two daughter cells. You do not need the detailed plant-cell mechanism here; the key point is that cytokinesis completes physical separation of the cell contents after nuclear division.

"Genetically identical" does not mean the two cells must be exactly the same size or contain the same number of every organelle. It means they inherit the same genetic information. A small difference in cytoplasm at the instant of division is not the same as genetic difference.