Mitosis and stem cells

Last time

Before we start: last time, in Meiosis, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

From one cell to trillions

You began as one cell. Now you have about thirty trillion, and almost every one has exactly the same chromosomes. The type of cell division that makes this possible is mitosis. Today: what mitosis does, why it matters, and the special cells that can become anything, stem cells.

Mitosis

Mitosis is nuclear division giving rise to genetically identical cells. Each new cell has the same number of chromosomes as the parent cell. In humans, a body cell with forty-six chromosomes makes two cells, each with forty-six.

Before it divides

Before mitosis, each chromosome is copied exactly. The two copies stay joined for a while, so each chromosome looks like an X. Then the copies are separated, one going to each new cell.

Watch mitosis

Here are the stages. The chromosomes shorten and become visible, line up across the middle, the copies are pulled to opposite ends, and two new nuclei form. Then the cell divides into two identical cells.

Why mitosis matters

Growth: making new cells as an organism gets bigger. Repair of damaged tissues, such as healing a cut. Replacement of worn-out cells, such as skin and red blood cells. And asexual reproduction, for example in plants and yeast.

Cancer

Normally, mitosis is controlled, so cells divide only when needed. A mutation can cause cells to divide uncontrollably, forming a tumour. This is cancer.

Stem cells

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions. Embryonic stem cells can become almost any type of cell. Adult stem cells, such as those in bone marrow, can become only a few types, such as blood cells.

Using stem cells

Stem cells could be used to replace damaged cells, for example in diabetes or after spinal injury. Bone marrow transplants already use stem cells to treat leukaemia. Some people object to using cells from human embryos.