Mitosis and the Cell Cycle: Stages

0:35The cell cycle loop

Cells that can divide go round a repeating loop, called the cell cycle. Not every cell in your body keeps dividing. Many specialised cells, like nerve cells, have left the cycle for good. Most of the cycle is interphase, when the cell grows and carries out its normal jobs. It makes new proteins, and it makes new organelles, so that there will be enough for two cells.

Mitosis is the short part where the nucleus divides. Cytokinesis, the division of the cytoplasm, finishes the job.

1:07Interphase: DNA replication

Here is the key idea. DNA replication happens in interphase, not in mitosis. Before a cell divides, every chromosome is copied. Each chromosome now consists of two identical copies, called chromatids, joined at a region called the centromere. Because they are exact copies, the two chromatids carry identical DNA.

That is why mitosis can make daughter cells that are genetically identical to the parent. So by the time mitosis starts, the DNA has already been doubled. Mitosis just shares it out.

1:43Prophase

Stage one is prophase. The chromosomes condense. They coil up, getting shorter and thicker, so they become visible. That coiling matters, because short, tight chromosomes are much easier to move around without tangling or breaking. The nuclear envelope breaks down, and spindle fibres start to form from the two poles of the cell.

2:06Metaphase

Stage two is metaphase. Metaphase is the stage that is easiest to spot down a microscope, because the chromosomes make a neat line. The spindle fibres attach to the centromeres, and the chromosomes line up across the middle of the cell, called the equator. Each chromosome is still a pair of chromatids at this point. Hold on to that for the exam question.

2:29Anaphase

Stage three is anaphase. The spindle fibres shorten and pull. The centromeres divide, and the chromatids are separated and dragged to opposite poles. The centromeres lead the way, so the chromatids look V shaped, with their arms trailing behind. This is why a picture of anaphase shows V shapes pointing towards the poles.

Examiners love that detail, so say it when a question asks you to explain the appearance. Once separated, each chromatid counts as a chromosome in its own right.

3:02Telophase and cytokinesis

Stage four is telophase. The chromosomes reach the poles and uncoil. A new nuclear envelope forms round each group, and the spindle breaks down. Then cytokinesis. The cytoplasm divides, and the cell pinches into two. In animal cells, the cell surface membrane pinches inwards. In plant cells, a new cell wall forms across the middle instead.

Either way, each new cell gets a nucleus and some cytoplasm. The result is two daughter cells, each with identical DNA to the parent cell.

4:16Examiner trap and recap

The trap. Chromosomes do not split in prophase, and DNA does not replicate in prophase. DNA is copied in interphase, and the chromatids are pulled apart in anaphase.

Recap. Copy in interphase, line up in metaphase, pull apart in anaphase, two cells at the end.

Next time, we ask what happens when this loop is not controlled, which is cancer.