The cell cycle: growing, copying DNA and dividing

0:00Your body makes new cells all the time. How does one cell become two?

0:06The nucleus holds DNA, the cell's instructions, in long threads called chromosomes.

0:13A cell's life goes round a loop like a clock, called the cell cycle.

0:18Most of the time, the cell is in interphase: it grows and does its normal job.

0:24Then it copies its DNA, so each chromosome now has two identical copies.

0:31Next comes mitosis, when the nucleus divides.

0:35The nuclear membrane breaks down, and the chromosomes line up across the middle.

0:40Fibres pull the two copies of each chromosome to opposite ends.

0:45A new nucleus forms around each set, giving two identical nuclei.

0:51Finally, in cytokinesis, the cytoplasm and cell membrane divide.

0:59Each new cell has the same chromosomes as the first, and can start the cycle again.

1:04So we grow mainly by making more cells, not because our cells get bigger.

1:10Cells divide for growth, for repair, such as healing a cut, and to replace worn-out cells.

1:17Some single-celled organisms, like an amoeba, divide to reproduce. This is asexual reproduction.

1:26Checkpoints act like gates: they check the cell and its DNA before it moves on.

1:32If checkpoints fail, cells can divide out of control and form a lump called a tumour, as in cancer.

1:40So: in interphase the cell grows and copies its DNA, then mitosis and cytokinesis make two identical cells.