Cell division: mitosis and meiosis

A small cut heals in a few days, and hair that falls out grows back. Behind both is the same trick: worn out, dead or damaged cells are swapped for fresh ones, made when living cells grow and divide.

Growth works the same way. A cell can only get so big, so a growing body adds more cells. And the swapping never stops. Of all the cells in your body, about 1 per cent are renewed each day: hundreds of billions of new cells, every single day.

Prokaryotic cells such as bacteria divide too. What sets eukaryotic cells apart is tight control: they move through a fixed, ordered series of steps known as the cell cycle, whose stages you will meet in higher classes.

Rest an onion on a jar of water so that only its base touches the water, and wait five or six days for fresh roots. The tips are fixed in aceto-alcohol, softened with a drop of dilute hydrochloric acid, stained with aceto-carmine, warmed gently and squashed under a coverslip so the cells spread out. Do the acid and warming steps only with your teacher.

Under the microscope the cells do not all look alike. Root tips divide all the time, so each cell is caught at a different moment of division. Try it on the virtual slide below.

Every new cell comes from a cell that was already there, and the process that makes it is cell division. Three things depend on it: growth, the repair of damaged tissue, and reproduction. Where cells are lost all day long, as in the skin, cells keep dividing without a break to make up the loss.

Division comes in two kinds, each with its own purpose. Mitosis serves the body itself: growth, repair, upkeep, and asexual reproduction. Meiosis serves sexual reproduction, and it is the source of genetic variety.

You began as a single fertilised egg. That one cell divided again and again to make the trillions of cells in your body, and the most common kind of division that did it is mitosis.

Step through it below. Watch the chromosomes: each one is copied, they line up, and the copies are pulled apart so each new cell gets one of every kind.

Mitosis gives two daughter cells that are genetically identical to the parent. Nothing is lost or added: each copy carries the parent's full DNA and its full chromosome count, so the genetic information stays largely the same from one body cell to the next.

Meiosis is the division that makes gametes, so you find it only in reproductive organs. Animals, including humans, make sperm by meiosis in the testes and eggs by meiosis in the ovaries. Plants use it in two places: the male parts, the anthers, make pollen grains with it (each later gives rise to sperm cells), and the female parts, the ovaries, make egg cells.

Think of meiosis as two rounds, back to back, so the parent cell divides twice. Round one: one cell becomes two, and each keeps only half the parent's chromosomes. Round two: both of those split again, much as in mitosis. The count at the end is four cells from one, each carrying half the parent's chromosome number.

When two gametes from two parents join in fertilisation, the full chromosome number comes back. No two gametes are quite alike, which is why children take after their parents without ever being exact copies. You will study this in Chapter 11.

Mitosis and meiosis have to happen properly and under control. Mistakes have real effects on the body.

Errors in mitosis can make cells divide without control. This can form tumours and leave body cells with an abnormal number of chromosomes. This is the link to cancer, which you will see again with cell theory.

Errors in meiosis show up in the next generation. A baby may be born with a genetic disorder, sometimes seen as problems in development or as distinctive physical features. A pregnancy may end early. Or it may simply become harder to have children at all.