All organisms have chromosomes, but some have more than others. While human cells have a total of 46 chromosomes, the cells of a goldfish have 100, and a dog has 78. Even a garden pea cell has 14 chromosomes. But what are they and how do we know about them?
Each of your cells contains 46 long chains of deoxyribonucleic acid (DNA) which carry your genes. As old cells die, new ones are made and each new cell must have a full set of genes. To do this, the DNA is copied and packed down into small, sausage-shaped chromosomes that are then sent into each new cell.
How to see chromosomes
To see the chromosomes, cells can be isolated as they divide and then dropped onto a small sheet of glass. This is then dipped into a special chemical dye called Giemsa and viewed using a microscope. They need to be magnified 1000 times in order to see them. Scientists routinely take photos of the chromosomes and sort them by cutting and pasting them in size order, and finding pairs by matching bands. It takes very skilled eyes to get it right!
How scientists look at chromosomes
Once grouped by size and pairs, scientists usually line chromosomes up into groups as we have on our ‘Explore your genome’ magnetic set. This way of grouping chromosomes is called a karyotype. On the image below you can see only some of the dark bands, but by using different dyes you can get up to 1000 bands across the chromosomes. Remarkably this pattern of bands along a chromosome is virtually identical in all humans.
Why are chromosomes useful to look at?
Chromosomes are routinely looked at in cells from blood, tumours and from babies growing in the womb to look for changes that might be associated with disease. For example, the addition of an additional copy of chromosome 21 is found in Down’s syndrome and large rearrangements of whole chromosomes are very common in cancers and can help doctors decide on treatments.
Zooming into the DNA code…
At the level of the DNA itself, although each of our chromosomes looks the same down a microscope, the actual order of the letters along the DNA chain on each one is very different. So the 249,250,621 letters of DNA on chromosome 1 are different to the 243,199,373 letters on chromosome 2 and so on.
But remember that you have two copies of each of these chromosomes, one inherited from each parent, so do these differ as well? The answer is yes - but only slightly. About one in every 1000 letters along the length of each chromosome will be different to its pair. So, if you look at chromosome 11, as it is over 130 million letters in length, it means that the chromosome 11 you inherited from your father has over 100,000 different letters to the one you inherited from your mother. Taking all your chromosomes together, you have two full sets of your genome, with three million different letters between them.
Just as you inherited two different genomes from your parents, they inherited two from each of their parents. Many of the differences in your genes are inherited from your grandparents and their ancestors, way back into time. Your chromosomes are therefore a genetic time capsule back into the history of humankind.
How you grow cells
You can grow human cells in a laboratory by sitting them in a solution that provides glucose and other nutrients, and by keeping them at body temperature. Many cells will grow like this for short periods of time and eventually they will divide into two new cells. Scientists use a special trick that allows them to stall cells just as they are about to divide, this means that they get lots of cells where the DNA has packed itself into chromosomes. If the cells are not stopped in this way the chromosomes are separated into the two new cells by a process called mitosis.
This can be done on cells from a blood sample, cells collected from a baby early in pregnancy by sampling the membranes sat around the baby, or later in pregnancy by sampling the fluid surrounding the baby using a needle. This is a process called amniocentesis.
Passing on your genes
You have 46 chromosomes in each of your cells, one chromosome of each pair came from your mother and one of each pair from your father. With one exception, you cannot tell just by looking down a microscope which of each pair comes from which of your parents. The exception is for the case of males: the Y chromosome is inherited from the father and the X chromosome from the mother. The process of separating out and dividing the chromosomes up to make egg or sperm cells is called meiosis and happens in special cells in the ovary or the testes.
You pass on your genes by producing eggs or sperm. In each case, only one of each of your chromosomes (23 in total) will end up in each egg or sperm. If you work this out, there are actually over eight million combinations of your chromosomes that you can pass on if they are mixed up randomly. It is actually more complicated than this simple assorting process.
Each of your pairs of chromosomes will swap genetic information with the other. This is a little like cutting each pair with scissors and swapping the bits over. As each chromosome carries slightly different versions of genes, the resulting chromosomes are actually a mixture of each of your own parental chromosomes. This occurs on every one of your chromosomes that have pairs, so includes X-X in females but not X-Y in males. This means that there are many million of combinations of your genetic material that you can pass on.