S-Cool Revision Summary

S-Cool Revision Summary

Fertilisation is all about getting the gametes together.

It may not sound very romantic but that's all there is to it really. In both plants and animals, the male and female gametes meet and join. They form the zygote, the fertilised egg that becomes the new organism as it divides, grows and develops. In plants it is all about the male gamete, the pollen, getting to the female gamete the ovum.

Sperm are made in a continual process in the testes. Each testis is a series of tubes in which the sperm develop.

When they are ready they are stored in the widened start of the sperm duct. Then during sexual intercourse they are fired along the sperm duct towards the urethra - the tube that extends through the penis. This process of sperm emission is called ejaculation and is under reflex control using muscles.

As the sperm moves along into the urethra it gets mixed with the secretions from the prostate gland and the seminal vesicle. These give the sperm sugars and other chemicals to fuel them during their journey to the egg. This mixture of sperm and secretion is called semen.

Eggs are formed in the ovary. Then when they are ready they are released one at a time each month. This is called ovulation.

The egg travels down the oviduct towards the uterus. If it is not fertilised it will pass down through the uterus, past the cervix and out of the vagina.

If you study the wall of the uterus you see a roughly 28 day cycle. It begins with the start of the bleeding or menses, it is this that gives the cycle its name, the menstrual cycle.

The menstrual cycle has 4 stages to it:

  1. The lining of the uterus breaks down and the bleeding starts.
  2. Stretches from day 4 to day 14, this is when the lining is repaired.
  3. On day 14 the egg is released from the ovary.
  4. The maintenance stage when the uterus is maintained in case the egg is fertilised

The different stages of the cycle are controlled by a set of four hormones:

Follicle-stimulating hormone (FSH): stimulates the ovary to get the egg ready for release. It also gets the ovary to secrete oestrogen.

Oestrogen: causes the lining of the uterus to grow and get ready for the egg. It also helps to trigger the release of the egg.

Luteinizing hormone (LH): triggers the release of the egg from the ovary once it is ready and enough oestrogen has been produced.

Progesterone: maintains the uterus lining after the egg is released. When the level of progesterone falls the lining breaks down.

By knowing about what the hormones do, doctors have been able to help women to control their egg release. This allows fertility treatment and contraception.

The Pill is a widely used means of contraception which contains both progesterone and oestrogen. This keeps the oestrogen levels high which stops further egg release.

If FSH is given to women who have problems ovulating (producing and releasing eggs) it triggers oestrogen release in the ovary and stimulates egg release.

Fertilisation is when the sperm meets the egg. The erectile tissue within the penis is filled with blood as the man becomes sexually 'aroused'. The erect penis can then be placed into the woman's vagina.

During sexual intercourse the penis is moved back and forth until ejaculation occurs. This reflex involves muscles around the sperm duct squeezing out the semen in a series of contractions.

The semen gets fired up inside the vagina. From then on they are on their own - well all 500 million or so of them.

As the embryo grows it becomes surrounded by a bag called the amnion. Inside this bag is amniotic fluid, this cushions and protects the embryo.

The embryo is supplied with food, water and oxygen via the umbilical cord that attaches it to the placenta. Waste materials such as carbon dioxide are also removed.

The placenta is a wonderful structure that is attached to the wall of the uterus and allows a very close meeting between the baby's blood and the mother's.

The two blood streams don't mix but molecules diffuse across a thin barrier between them.

Most of the time everything from fertilisation to birth goes well, thankfully.

However sometimes things go wrong. There seem to be mistakes made in the development of cells. This can also happen in older organisms too, we call a lot of these mistakes cancer.

The changes that are seen in the genetic code are called mutations.

Mutations are the changes in the DNA sequence. Or in other words, changes in parts of genes in chromosomes. The base sequences are messed up!

Sometimes as little as one base might be missing or it could be a few. On other occasions a couple of bases might be swapped around.

It is also possible that during meiosis parts of chromosomes get damaged.

If the genetic instructions are wrong what it does will also be wrong. It might end up making an enzyme the wrong shape so that it doesn't work. Anything could go wrong!

Mutations can occur naturally. However if you are exposed to things like nuclear radiation including X rays and UV rays, mutations are much more likely.

Other 'nasties' include chemicals known to cause mutations, known as "mutagens". Cigarette and tobacco smoke contains many carcinogens, cancer-causing chemicals.

All of these things can damage your DNA. Women must be careful what they consume or take while they are pregnant. Many substances such as alcohol, bacteria, viruses and drugs can cross through the placenta to the baby. This could cause serious damage to the developing embryo.

Most mutations are harmful. In developing embryos they cause abnormal development and may cause early death.

In older tissue they can cause cells to keep on dividing uncontrollably. These cells develop into tumours, spread into other parts of the body and so become cancers.

However, rarely some mutations can be beneficial. For example a bacterial cell might mutate into a form that shows antibiotic resistance.

Bad news for us, good news for Mr. Bacterium. Or a plant might mutate so that it grows in poorer soil in which nothing else grows.

Natural selection is thought to be brought about by these rare, beneficial mutations.