Genetics and genetic testing - part 1

This is the first of a three part series on genetics and genetic testing. The aim is to introduce basic concepts of genetics, genetic-based diseases and genetic testing options.

Most of us are familiar with the term “gene” and sayings such as “it runs in the genes” – but many don’t necessarily understand the concept in a medical or scientific way. These articles aim to provide a more in-depth understanding of this complex topic.

Proteins, beginning at the end

Proteins are the building blocks of  our cells and tissues and are made up of specific sequences of amino acids. Functional proteins, called enzymes, direct the myriad of chemical reactions occurring throughout our bodies every second and are essential to sustaining life.

The link between our genes and our proteins

Our genetic material (DNA) is made up of a series of four nucleotides called adenine (A), guanine (G), cytosine (C) and thymine (T); strung together in long strands. Each gene is made up of a specific sequence of nucleotides arranged in groups of three; called codons (e.g. ATC, TAG). Each individual codon codes for a specific amino acid. Thus the combination and order of the letters determine the sequence of amino acids that are added together to form a protein. The body ensures separate proteins are made by separating active parts of our DNA, each coding for a different protein, with inactive “spacers”. In order to replace old and damaged cells, new cells are continuously made through a process called cell division. During cell division, our DNA is replicated and amazingly, the body makes very few mistakes during this process (in fact only one in every billion letters it reads), thereby ensuring that every new cell is an exact copy of the old cell. This is because some proteins function specifically to “check” the accuracy of the replication process. When a mistake is sensed, repair proteins are called upon to fix the broken sequence of DNA.

Watch this video of how DNA is read to produce a protein:


Sequence abnormalities

Many diseases with a genetic basis are the result of a change in the order or combination of nucleotides, or the deletion of a nucleotide - an event called a mutation. In some cases, a single letter change can result in disease, as in the case of cystic fibrosis. The alteration in the sequence results in the production of a protein which is either non-functional, or over- functional and which then results in the specific disease, such as cancer.

Watch this video of how mutations affect the protein sequence:


Chromosome abnormalities

In humans, strands of DNA are packaged into 46 chromosomes consisting of 2 pairs of 23, one of each pair is of paternal origin (the sperm cell) and one of maternal (from the egg) origin. The different chromosomes contain specific genes coding for specific proteins and functions. These are the same in all of our genes. Some genetic conditions are caused by a baby inheriting either too few or too many chromosomes when the sperm and egg combine. Perhaps the most well-known example is Down syndrome, in which the baby inherits 2 copies of chromosome 21 from one parent, and 1 copy from the other, resulting in 3 copies of chromosome 21 (called a trisomy).

Watch this video of how abnormalities in the number of chromosomes can occur:


Mutations – acquired vs inherited

Mutations in the sequence of our genes may be inherited, meaning we were born with one copy already mutated (if one of our parents carried that mutation) or it can be acquired, meaning we were born with 2 perfect copies and developed a mutation in one or both during our lifetime. As we age, the checking and repairing apparatus previously mentioned becomes less effective, and the chance of single base changes or mutations increases. This is why cancer is more common in older people. Certain chemical agents and smoking increase the risk of mutations occurring and are thus also risk factors for cancer or birth defects. These phenomena also account for the fact that people with inherited cancer syndromes generally develop cancer at a much younger age, as one copy was already mutated at birth and they only needed to acquire a mutation in one copy for both to be knocked out.

It is important to note that only 5-10 % of cancers are inherited, the rest are caused by mutations acquired within the individual’s lifetime.

What to look forward to in part 2

In part 2 we will look at trends in genetic testing, what you should consider when deciding on genetic testing, pro’s and con’s thereof and where you can access these services; with a special emphasis on Breast Cancer and Down syndrome.

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