Latest update: 29/06/2017

DNA transfer

Our somatic cells renew themselves and multiply by dividing.
The nucleus, however, cannot simply be divided in two, because the number of chromosomes (which contain all the genetic information) has to be passed on unchanged from cell to cell. They are therefore copied and consequently duplicated - during the phase of cell division that we call mitosis - and each pair is then divided between the two cells.
This happens in all our somatic cells, including those from which the embryo develops after the egg cell has been fertilised by a sperm cell.
These two, however, are the only cells in our body that come into being in a different way. Since sperm and egg cells must contain only 23 chromosomes each (rather than 23 pairs of chromosomes) they are the result of a process of reduction division (or meiosis). First the maternal and paternal chromosomes both multiply and then they join together in pairs, exchanging material in the process. We call this phenomenon recombination. After reduction division, the recombined chromosomes are shared out at random between the newly created cells.

All in all, cell division is a complex process in which the mechanisms involved in copying and recombination are responsible for variations in DNA. It is these (sometimes small and sometimes larger) variations in our DNA that make each one of us unique. Sometimes they result in 'errors' and these may form the basis for specific disorders.


Congenital or acquired

Not all genetic disorders are inherited. Some genetic mutations or chromosomal abnormalities can occur spontaneously, and we call these de-novo mutations. It is therefore possible for an inherited disorder to appear in a family out of nowhere, because once the mutation has occurred it can also be passed on to the next generation. This means that it is not always easy to estimate the likelihood that an individual person will have an inherited disorder.

The phenomenon of chromosome duplication (as in trisomy 21 or Down's syndrome) cannot always be called inherited, although it is a congenital disorder which normally occurs in every cell in the body. In other words it is genetic but not always inherited, so it does not occur until the time when the embryo is formed.

What is more, an embryo may, while developing in the uterus, undergo a genetic mutation that results in an abnormality. Although a certain genetic predisposition may cause the abnormality, in these cases it may also occur due to purely external factors, such as the mother's diet, environment or state of health during the pregnancy.

Finally there are some genetic disorders that appear later in life and do not occur in all our cells. If these cause a disease or abnormality we call them 'acquired' or 'sporadic'. Cancer is one example of a genetic disease: a specific DNA mutation transforms healthy cells in the body into cancer cells with specific characteristics. This allows them to escape from the control mechanisms that regulate the division, growth and functioning of cells in our body and they begin to grow in an uncontrolled way.

 

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