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.