What is DNA?
DNA stands for deoxyribonucleic acid. The molecule is sometimes called the ‘blueprint of life’ or the ‘molecule of life’ because it contains all the information needed for an organism to grow and survive. The DNA molecule is a long twisted helical structure, looking something like a twisted ladder. The two backbones of this ladder are the DNA strands, and the rungs are the four chemical bases adenine (A) thymine (T) guanine (G) and cytosine (C) . The bases face each other across the strand and are always paired in the following way: A partners T, and G partners C.
When DNA is replicated, the molecule unzips and each strand with its bases becomes the template for the formation of another DNA molecule.
What is a Gene?
There used to be a simple answer for this, and that was that a gene is a length of DNA that codes for a protein. It’s what Francis Crick the co-discoverer of the structure of DNA termed the ‘central dogma of molecular biology.’ A section of DNA is copied into RNA which is then whisked out of the nucleus to the ribosomes - the protein making factories of the cell. However, there are several discoveries that although they haven’t completely overturned this idea, are certainly giving scientists reason to pause and consider deeply when asked what a gene is. For example, part of Crick’s dogma was based on the idea that ‘junk DNA’ was just that, cellular rubbish. However, it turns out that these regions are highly conserved and do serve some functions, although not a lot is known about that right now. Some scientists argue that a gene’s role may be to produce RNA which itself has many functions, one of which is to help produce proteins.
Where is DNA Found?
In the human genome DNA can be found in several locations. In human cells it’s found inside the nucleus and also in mitochondria. In viruses and bacteria it can be free-floating and in the case of the latter it’s also housed in structures known as plasmids.
How are Genes Inherited?
Our cells contain 23 pairs of chromosomes. We inherit one pair from each of our parents. This means that sperm and eggs must have half the number of chromosomes, otherwise when they combine to form an embryo there would be far too many. Sperm and eggs receive their 23 chromosomes through a complex process of cell division known as meiosis.
Is Gene Therapy Dangerous?
Gene therapy is a promising technology that aims to replace faulty genes that cause diseases with fully functioning versions. One of the ways of getting the new genes into a body is by using a virus. Virus are adept at getting inside bodies and dropping their genetic payload. So scientists have hijacked the viral genetic machinery for our benefit. However, whilst the virulent factors are ‘made safe’ there have been several deaths during gene therapy trials. In 1999 Jesse Gelsinger became the first human victim of a gene therapy experiment. Whilst these deaths are obviously a tragedy advances in medical technology are sadly not risk-free. That’s as true for genetics as it is for other fields such as developments in brain surgery and organ transplants.
Why Isn’t Gene Therapy A Common Procedure Yet?
Gene therapy is technically very difficult and this field of human genetics is still very much in its infancy. It is extremely difficult to put a gene in a precise inside a cell, and to have it work in the way you want it to. Plus, this must all be achieved without disturbing the function of other genes in the genome. For some genetic disorders such as Muscular dystrophy there’s another hurdle - the correct genes must get to every single muscle cell and this has proved to be impossible. Although new research using dog models has come up with a way of getting the genes to all muscle types.
Those impatient by the ‘slow’ progress of gene therapy may like to know that it’s not the only game in town. There’s another technology that’s being developed and it’s been given the nickname of ‘DNA scissors.’ Proteins called zinc finger nucleases can cut DNA at specific locations, say where a bad gene is, allowing it to be repaired. The cell heals the broken strand using copies of a replacement gene, such as a good copy of the faulty gene you want to eliminate. It’s still in the early stages of development, but it may well give gene therapy a much needed boost.