Mutations

A cell. One of the building blocks of life. Here is the cell nucleus, and inside we find the chromosomes. The chromosomes consist of long molecules, DNA molecules. And in the DNA, spread out along this long molecule, we find the genes, the ‘recipes’ that control the characteristics of the individual.

This mushroom, this leopard, this apple tree. They look and function the way they do, thanks to the genes that they have. Do you remember how genes work? The DNA molecule is like a twisted rope ladder. The rungs on the ladder differ from one another.

A, T, G, A, C, C… The letters are four different substances attached in pairs, base pairs, along the ladder. The order of the base pairs are protein recipes. If the base pairs are combined in a different order, it gives different proteins and different characteristics. Back to the cell. Now it divides.

This is how cells reproduce. When the cell divides, the chromosomes are copied, and the genes. Check! A, T, G, A, G, T ... And here, in the new cell: A, T, G, A, G, T … Yes.

Seems right. The same genes in the new cell, compared with the mother cell. Now the cells divide again. And again, the chromosomes are copied. And the genes are copied too.

Perfect copies. But wait a second. Something is not right. A, T, G, A, G ... G!

You see? This copy is not totally perfect. The chromosome is slightly different, compared to the mother cell’s. There has been an alteration in the genome, a mutation. So, when the chromosome is copied, the result is not completely perfect.

It becomes almost perfect. When cells divide, there is an error in about one base pair out of 100,000! But since a human being has several billions of base pairs in the DNA, mutations occur in your body every second. The cell has a protection against that. It has a mechanism that can repair incorrect copies in almost all cases, but sometimes an odd incorrection might slip through.

We just saw how the base pair T-A was lost in the copy and was swapped to the base pair C-G. The base pair was replaced during the mutation. But mutations can occur in other ways too. Watch! Now a base pair vanishes during the copy, and...

is not replaced. The mutation occurs because a rung on the ladder disappears. Now the cell divides again. DNA is copied, and now … Another base pair shows up. One that was not there in the first place.

These are three types of mutation that can occur. All three have the consequence that the order of base pairs in the DNA molecule is just a tiny bit changed, compared to the mother cell. This means that the recipe for proteins might be just a tiny bit changed. Does it matter? For the most part, the DNA molecule does not consist of genes.

And neither does it contain any protein recipe. And if the mutation has occurred in any of those parts, nothing in particular happens. None of the recipes are affected, and hence the cell keeps building proteins as usual. But, if the mutation happens in a gene, that is in any of the letters telling the cell how to build the protein, then the cell might start to produce new proteins, that give the cell somewhat new characteristics. It is rare, but it happens.

Usually what happens is that the cell stops functioning and dies. If you are unlucky the cell can develop into a cancer cell, that starts to divide and attack the body. So, mutations usually have no significance. In rare cases they might cause illnesses. And in really rare cases, if the mutation occurs… ...in a gene… ...and in a cell that is the start of a new individual, for instance, an egg or sperm cell, ..then a tiny, tiny change has happened in the genome.

A change that is passed on to the next generation, and might affect its characteristics.