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From DNA to genes, an instruction manual for the human body

30/06/2015

From DNA to genes, an instruction manual for the human body

Genetic research has come a long way since DNA was discovered in the 1950s.

It allows doctors to offer tests that can detect cancer very early on. Thanks to developments in genetic testing, patients who develop cancer can be treated in the very early stages of the disease and can also benefit from a personalized medicine approach that provides them with the most appropriate therapeutic solution.

But all of these discoveries in genetics rely on an understanding of the gene. What exactly does the word “gene” refer to?

A gene is a portion of DNA, short for deoxyribonucleic acid, which is the chemical molecule that carries an individual’s genetic information. As carriers of genetic information, most genes define a particular characteristic of a person, for example eye color or blood group.

 

DNA: a map for the development of living organisms

 

The human body is made up of different components, such as organs like the brain, heart, and muscles.

Closer examination of these organs reveals that each is made up of tissues that are themselves made up of connected cells. These cells are generally alike and fulfill similar roles within the tissue to ensure the organ functions properly. In the heart, for example, the cells contract in a synchronized manner within the tissue, enabling the heart to beat and fulfill its role in pumping blood.

All of these cells have one thing in common: they all contain a very large molecule of DNA in their nucleus. This molecule is made up of 4 smaller molecules (adenine, thymine, guanine, and cytosine) reproduced billions of times.

With a few exceptions, a complete, identical copy of DNA is found in all the cells of the human body, whether they be skin cells, muscle cells, or other types of cells. Insofar as it carries genetic information, DNA acts like a fact sheet, which, thanks to the particular configuration of these 4 molecules, is unique to each person on the planet.

But DNA’s unique sequencing allows it to do more than just identify each individual. It can also be used to encode messages and operating commands, like an instruction manual that tells cells how to function.

 

The gene: an information unit

 

The role of DNA is not only to inform and describe (since it holds all the information about an individual: eye color, sex, etc); it also has a functional role similar to that of an instruction manual.

It holds all the information that enables cells to carry out their functions. If a bone cell needs to produce bone, the DNA contains all the information needed to carry out this operation. In fact, the bone cell needs only a tiny portion of the information found in the DNA in order to function; it does not, for example, need information about eye color. Only small sections of DNA that are specific to a particular function or designation “express themselves” in order to be of use to the cell. It is these small sections that we call genes, and the total genetic material of each human cell (called the genome) contains around 20 000 such genes.

In short, most genes are carriers of a characteristic, mechanism, or cell function. For example, some are responsible for eye color, others enable bone cells to produce bone, and others again enable muscle cells to produce contractile proteins. At a functional level, genes or DNA segments are instruction manuals that cells read, understand, and interpret throughout their life.

These genes are passed on during fertilization. This means that the child receives a copy of its mother’s genes and a copy of its father’s genes.

However, some of these inherited genes can be altered and may trigger a disease because the functional information (the instruction manual) that they contain is corrupted. The cell that normally needs the gene in question in order to fulfill its function (to contract, for example) will no longer be able to function, completely or partially, which in turn triggers organ malfunction and leads to illness.

Having multiple versions of the same gene is therefore an advantage. If one gene passed on by a parent is altered, the other gene can function normally and in some cases the individual will not develop the disease. In the case of Duchenne muscular dystrophy, for example, the altered gene that is responsible for the disease is found on the X chromosome passed on by the mother. Women possess 2 copies of the X chromosome. If one of a woman’s two X chromosomes contains the altered gene and the other a functional gene, muscular dystrophy is not triggered. In contrast, boys only have one copy of the X chromosome. So if the X chromosome passed on to the boy carries the altered gene, there is no healthy gene available on another chromosome to prevent the disease from being triggered.

By studying these genes, their roles, and their alterations (whether these are passed on by parents or appear during an individual’s lifetime, eg, in cancer), genetic research makes a considerable contribution to the fight against certain diseases. Identifying the particular genes responsible for a disease means that in some areas it is now possible to conceive of new therapeutic approaches and develop medicines capable of affecting these genes before the disease even appears. Ultimately, a better understanding of the genes that cause diseases will allow doctors to evaluate the risk of a disease being triggered even before the first clinical signs appear and to take preventative measures.

Research is now focusing on the development of treatments capable of modifying, repairing, or even replacing these defective genes so as to restore altered function. The advent of personalized medicine, particularly in oncology, is also a very promising development. By identifying the genes that influence the efficacy of a treatment, doctors will gain a much clearer understanding of how a patient will respond to a particular treatment, which will enable them to choose the most suitable treatment based on the patient’s genetic makeup, avoiding the risk of treatment failure or potential side effects.

Current and future advances therefore rely on an ever increasing understanding of DNA and the gene.

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