Fertilisation

1. Fertilisation

2. Making the Gametes 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.

3. In animals, as you already know, the sperm and egg join to form the zygote which becomes the baby. But how do the sperm and eggs get made? The human male is rather a simple creature - well in terms of making sperm and his plumbing that is.

4. 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 spermduct. 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.

5. 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 secretionis called semen. Finally, when the sperm are ejaculated out of the penis there is about 3-5ml. of semen with the sperm only making up a small fraction of that. However there are usually about 200 - 500 million sperm in each ejaculation!

6. Making eggs 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.

7. This all seems quite simple compared to men. But things are deceptive, it is the hormone control of ovulation that is the tricky bit!

8. Female hormones 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.

9. 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

10. The different stages of the cycle are controlled by a set of four hormones: Oestrogen and progesterone are produced by the ovary. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are produced in the pituitary gland in the brain.

11. What do they do? Studying this diagram might help you.

12. 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. 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.

13. Using the hormones 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. It works well although if you give too much you can get multiple births!

14. Fertilisation Fertilisation is when the sperm meets the egg. I'm sure you know all about the things that lead up to that occasion. 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 outthe 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.

15. Meeting gametes The sperm swim upstream against a flow of fluid moving back down the oviducts and uterus. They head towards the egg but many millions never make it. Only about 100 sperm make it to the egg. All the others perish. But 'there can only be one'!

16. These 100 sperm battle away at the outer coat of the egg until finally one manages to break through. It enters the egg and immediately fertilises it. This triggers a change in the outer coat which stops any other sperm getting in.

17. So only 1 sperm fertilised the egg out of 500 million. Not great odds for a sperm! The fertilised egg continues its journey down into the uterus. During which time it divides a number of times becoming a ball of cells.

18. At this stage this zygote is renamed an embryo. It becomes embedded or 'implanted' into the lining of the uterus. Thisis where the baby will develop over the next 9 months.

19. The embryo 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.

20. 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.

21. Is it a girl or a boy? It's the first question everyone asks. But the real question is what determines the sex of the baby? Of the 23 pairs of chromosomes in the human cell's nucleus there is 1 pair that are called the 'sex chromosomes'. In women these chromosomes are the same size and are called X chromosomes. However men have one X chromosome and a smaller one, the Y chromosome.

22. So we could say that a woman has the genotype 'XX' while a man has 'XY'. (The phenotypes are what the genes actually produce, the external features). Now the tricky bit! You knew that there had to be one.

23. When the gametes are made through meiosis all the chromosome pairs are split up. But since both of the woman's sex chromosomes are X, each egg cell will contain an X. However men's sperm could either have an X or a Ychromosome. Another way to show that is in a 'Punnet square' diagram.

24. 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.

25. What are 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!

26. 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!

27. What causes mutations? 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.

28. 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.

29. What effect do mutations have? 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.

30. 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.

31. Variation

32. Once upon a time there was a pop song with the line 'Everyone is beautiful in their own way'. A nice thought I'm sure you'll agree. I guess it means that we're all different and have a beauty in the way we're put together. Some differences, or variations, cover a whole range of things and are called continuous variations. Others differences have only a very few possible options, these are called discontinuous variations.

33. Continuous variation Some of these differences vary over a whole range, for example the height, skin colour and weight of people. We call this type continuous variation. So if you lined up a hundred people you would find a whole range of heights - within sensible limits! People don't just come in set heights like shoe sizes.

34. Discontinuous variation However there are other differences where there are only a few possible forms. For example, I can't roll my tongue lengthways. Can you? Try it now in a mirror. Don't try it if there are any big, strong aggressive people about! The only options are that you can or cannot roll your tongue, you can't half roll your tongue.

35. Other examples of discontinuous variation are blood groups and eye colour in humans. An example that could apply to plants and animals would be resistance to a particular disease. But how do all the various differences arise? Where do they come from in the first place? The answers have been found only during the last one hundred years. Differences between animals or plants come about through either genetic variation or environmental variation.

36. Genetic variation We all know that we get our features and characteristics from our parents through their genes. Even with the same parents, brothers and sisters can be very different. For one thing they might be a different sex! Each one of us receives a unique combination of genes. An interesting exception to this is where you get identical twins who have developed from the same fertilised egg. Both will be the same sex and share identical genes. Identical twins are similar but can still have differences. One might end up being much stronger than the other, this due to environmental variation.

37. Environmental variation This is where the environment that you grow up in or live in has an effecton you. For example if you had an identical twin who was brought up from childhood in a much poorer environment than you, where they might not get fed as much as you. What effect might the environment have on their development? You might find that when you met up later they were perhaps lighter in weight than you, perhaps shorter in height, and they might have performed less well at school. To easily and clearly see the effects of environmental variation look at plants, as they are often susceptible to environmental effects.

38. For example: Imagine two plants grown in the same soil but one is in the shade of a large tree. The plant that is in the shade will not grow as large as the one that is not shaded.

39. The Genetic Code The genetic code is carried by an amazing molecule called Deoxyribonucleic acid, or DNA to its friends. DNA is an amazingly long and complicated molecule. Where is the genetic code? The DNA is found in the nucleus of all cells. It is formed into X-shaped bundles called chromosomes. In human cells, except for eggs and sperm, there are 46 chromosomes. These are divided into 23 pairs. Each chromosome has the appearance of two knitted sausages tied together in the middle.

40. The more scientific description would be that a chromosome is made up of two chromatids held together in the middle by a centromere. You choose which is easier to remember!

41. If you start to unpick each of the chromosomes you get down to a single thread which is the DNA.

42. What is the genetic code? This DNA strand looks a bit like a ladder twisted into a double helix. The rungs of the ladder are made up of pairs of base molecules connected to each other. It is the order of the bases (that form the rungs across it like on a ladder) that carry the actual genetic code.

43. To make things a bit easier for once, there are only 4 different types of bases. Each is usually known by the first letter of its name: • Adenine (A), • Cytosine (C), • Guanine (G), • Thymine (T). Even easier is the fact that the order the bases join up to form the 'rungs' is fixed. Adenine and Thymine always join together, and Cytosine and Guanine always join.

44. What DNA does But so what? Nice molecule, but what does it actually do? Each group of 3 bases on one side of the DNA carries the genetic code for one of the 20 different amino acid molecules. Once the whole code for one gene is read the cell can make the specific protein. Each gene codes for one complete protein. Many of these proteins are actually enzymes. So the chromosomes in each cell contain every gene needed to create a new human cell or whole body! Or a plant or animal!

45. Amazing!! It's like carrying around quite a stack of filing cabinets each stuffed full of sheets with instructions on how to build a new you! And that's in each and every cell.

46. Passing on the Code The genetic code contained in our chromosomes is of no real use unless it can be used to make new cells. The code is passed on to the new cells using either of two processes, mitosis or meiosis.

47. Mitosis Mitosis is the process used during growth to make new cells within a plant or animal. It is also used during asexual reproduction, in which an individual can clone itself to produce identical offspring. Humans don't make clones of themselves naturally like plants can. For example: strawberry plants send out runners. However our body often has to make new cells to replace damaged ones or as we grow. So human cells also go through mitosis in the same way as animal and plant cells but is for growth and repair. The offspring cells have the same number of chromosomes as the parent cells, therefore they are diploid.

48. The process of mitosis is shown below: The DNA starts off as long strands in a soup-like mush in the nucleus. The chromosomes become clear as the DNA twists up. Each double arm (chromatid) is a copy of each other.

49. Chromosomes line up along the centre line and begin to be pulled apart by fibres. Membranes form about the separated chromosomes. These become nuclei of daughter cells.

50. The chromosomes unwind back into loose threads within the daughter cells. The single-armed chromosomes within the daughter cells replicate themselves to create the double arms (chromatids) again.