Growth, repair and human reproduction

Mitosis: growth and repair

In topic 2C you saw that plant cells divide by mitosis. Human cells do exactly the same. Before a cell divides it copies all 46 of its chromosomes (the long strands of DNA that carry your genetic instructions). Then it splits so that each new cell gets a complete, identical set of 46.

Your body uses mitosis for three jobs:

• Growth. From zygote to adult, all the extra cells are made by mitosis.
• Repair. When you graze your knee, skin cells around the wound divide to fill the gap. A broken bone heals the same way.
• Replacement. Many cells wear out and are replaced constantly. You lose and replace the cells lining your gut every few days and your outer skin layer every month or so.

Red blood cells show how big that job can be. They have no nucleus, which makes room for more haemoglobin, but it also means they can't divide and only live about 120 days. Your bone marrow has to make about 2 million new red blood cells every second to keep up. Red blood cells make up about 84% of all the cells in your body by number (they are tiny, so they are only a small part of your mass).

Specialising: stem cells

The zygote can become every one of the more than 200 types of cell in your body. As an embryo develops, cells switch on some instructions and switch off others, and become specialised for one job: nerve cells, muscle cells, red blood cells and so on. This is called differentiation. Many specialised cells, like most nerve and heart muscle cells, lose the ability to divide.

Stem cells are unspecialised cells that can keep dividing and can turn into other types of cell. Adults still have some, in the bone marrow (making all the blood cells), skin, gut lining and elsewhere. They are why your body can repair itself. Bone marrow transplants, which give a patient healthy blood stem cells, have been used since the 1960s to treat leukaemia. Scientists are researching ways to use stem cells to repair damaged hearts, spinal cords and eyes.

Meiosis: making gametes

Every human body cell has 46 chromosomes, in 23 pairs. One chromosome of each pair came from your mother and the other from your father. If a sperm and an egg each had 46 chromosomes, the baby would have 92, then 184 in the next generation, and so on. That can't work.

So gametes (sex cells) are made by a different kind of cell division called meiosis. Meiosis halves the chromosome number: each gamete gets just one chromosome from each pair, 23 in total. When a sperm (23) fertilises an egg (23), the zygote has 46 again.

Which chromosome of each pair goes into a gamete is random. With 23 pairs, each person can make more than 8 million different combinations (2²³), and that's before counting the swapping of pieces between chromosomes that also happens in meiosis. That's why brothers and sisters look alike but different.

Sperm and eggs

A sperm is built to travel. Its head holds the nucleus (23 chromosomes) and a cap of enzymes to get through the egg's outer layer. The mid-piece is packed with mitochondria to release energy, and the tail whips to swim. An egg (ovum) is one of the largest cells in the body, just visible without a microscope. It can't move by itself, but it contains a store of food to support the early embryo.

The reproductive systems

• Ovaries contain the eggs (a baby girl is born with about a million immature eggs, all she will ever have) and make the hormones oestrogen and progesterone.
• Fallopian tubes (oviducts) carry an egg from the ovary to the uterus. Fertilisation usually happens here.
• The uterus (womb) is a muscular organ where a baby develops. Its lining, the endometrium, thickens each month ready to receive an embryo.
• The cervix is a ring of muscle at the base of the uterus, which stays closed during pregnancy.
• The vagina is a muscular tube leading to the outside of the body; it is also the birth canal.

• Testes make sperm, around a thousand every second from puberty onwards, and the hormone testosterone. They sit outside the body in the scrotum, which keeps them about 2 to 3 °C cooler than body temperature, because sperm develop best slightly cool.
• The epididymis is a coiled tube where sperm are stored and mature.
• The vas deferens (sperm ducts) carry sperm towards the urethra.
• The seminal vesicles and prostate gland add fluid containing sugar for energy. Sperm plus this fluid is called semen.
• The urethra, running through the penis, carries semen (and, at other times, urine) out of the body.

Puberty and the menstrual cycle

The reproductive organs are present from birth but only mature during puberty, which usually starts between about 8 and 14 and is triggered by hormones. Both sexes have a growth spurt and grow body hair. In females the breasts develop, the hips widen and periods begin. In males the voice deepens, muscle mass increases and facial hair grows. Everyone goes through puberty at a different time and pace, and that's completely normal.

From puberty until menopause (usually around age 50), the female body goes through the menstrual cycle, which lasts about 28 days on average, although anything from about 21 to 35 days is normal.

1. Days 1–5: menstruation (a period). The lining of the uterus breaks down and passes out through the vagina with some blood.
2. Days 6–13: the lining rebuilds and thickens, and an egg matures in an ovary.
3. About day 14: ovulation. An egg is released into a fallopian tube.
4. Days 15–28: the lining stays thick, with a rich blood supply, ready for an embryo. If no embryo implants, hormone levels drop and the cycle starts again with a new period.

Fertilisation, pregnancy and birth

During sexual intercourse, sperm are deposited in the vagina. Millions of sperm swim up through the uterus into the fallopian tubes, but only a few hundred get near the egg and only one can fertilise it. When its nucleus fuses with the egg's nucleus, a zygote with 46 chromosomes forms. The egg's outer layer immediately changes so no other sperm can get in.

The zygote divides by mitosis as it travels down the tube. After about a week, the ball of cells, now an embryo, sinks into the thick uterus lining. This is implantation and the start of pregnancy. The placenta develops where the embryo meets the uterus wall. Here the mother's blood and the baby's blood flow very close together without mixing, so oxygen and nutrients diffuse to the baby and carbon dioxide and wastes diffuse back. The baby is linked to the placenta by the umbilical cord and is cushioned by fluid in the amniotic sac.

From about 8 weeks, when all the main organs have started to form, the embryo is called a foetus. Pregnancy lasts about 40 weeks (counted from the first day of the last period). At birth, the muscular wall of the uterus contracts strongly, the cervix widens, and the baby is pushed out through the vagina.

Harmful substances can cross the placenta too. Alcohol, nicotine and many drugs reach the baby, which is why pregnant women are advised not to drink alcohol or smoke at all.