1.3 Preparing for conception
1.3.1 Why are cells different?
Now let us go on with our story and assume that we have decided the time is right to have a baby. The primary requirement for conception is that healthy gametes should be produced. We shall therefore look first at how gametes are made, and then examine some of the factors affecting their quality. But we must start with an explanation of what gametes are, and what sets them apart from other kinds of cell. In other words, what makes gametes special? Gametes are the cells that fuse to form a new individual, as you saw above, but why would the fusion of any two cells not do the job just as well? In fact, experiments have been done in which pairs of body cells have been fused together artificially, but in no case has an embryo, far less a living person, ever been produced. Perhaps we need to think about different sorts of cells.
All the cells in your body are ‘your cells’ but, if you could look at them using a microscope you would see that there are many obvious differences in their appearance. The shape of the cell, position of the nucleus, the number of mitochondria and the presence or absence of various cytoplasmic structures are all clues to the cell's function. Some such differences are visible by microscopy, but there are many others which are apparent only at the level of the molecules within the cells.
The molecules within cells can be classified broadly as carbohydrates, lipids or proteins. Many of the characteristic differences shown by different types of cell are due to the different proteins found in these cells. This is an important concept, which you should remember.
Q From general knowledge, can you name any specific proteins found in the body?
A Well-known proteins include collagen, which enhances the stretchiness of skin, keratin, the protein that forms hair and nails, and insulin, the hormone involved in controlling the level of blood glucose. Perhaps you thought of others: many are listed on the labels of cosmetics or food items, or in cookery books.
Every cell in the body contains thousands of proteins, but different types of cell contain different proteins. For example, collagen is made by several different types of cell, including skin cells, keratin is made by skin cells only, and insulin is made by a particular group of cells in an organ called the pancreas. All cells make proteins by assembling amino acids, which are obtained from food. There are only 20 amino acids used in the body, so differences between proteins (each of which can be hundreds of amino acids long) arise from differences in the order in which the amino acids are joined together.
Q How does a cell ‘know’ how to assemble amino acids in a particular order?
A From the genetic code – the instructions carried by the DNA making up the chromosomes in the cell nucleus.
The chromosomes contain all the information needed to produce the molecules out of which an individual is made: not just about which proteins out of the whole lot to make, but also when to make them and where (in which cells of the developing embryo) they are made.
Q If all cells of an individual carry the same chromosomes, and therefore the same instructions, how can different types of cell make different proteins?
A They use different subsets of information from the chromosomes (i.e. different sets of genes).
Because the chromosomes carry all the information needed for the molecular composition of a new individual, they are very important structures.
Q Where do a cell's chromosomes come from?
A Ultimately, all an individual's cells are derived from the one original cell that resulted from the fusion of an egg with a sperm: the fertilized egg. So the cell's chromosomes came from the gametes’ chromosomes.
Q Each cell of a particular species contains the same number of chromosomes (23 pairs, i.e. 46, in humans). What would you predict would be the number present in a cell resulting from the fusion of two others?
A It would have twice as many. 46 chromosomes in each of the fusing cells would give a fertilized egg containing 92.
But since each cell contains the same number (46), this cannot be what happens. We can resolve this paradox by considering our original question: what makes gametes special? The answer (at long last!) is that human gametes contain only half the normal number of chromosomes, i.e. 23 – only one member of each pair. How this is accomplished we shall see below.