DNA

DNA is the code of life, the means by which every living organism on Earth stores its genetic information.

You get half of your DNA from each of your biological parents, and you will pass on a selection of half of it to any child you might conceive. DNA, which stands for deoxyribonucleic acid, is curled up and stored as chromosomes in the nucleus of every one of our cells. We also have some DNA inside the mitochondria that power our cells, while plants have extra DNA within the chloroplasts that enable them to photosynthesise.

Double helix structure

The 1953 discovery of the shape of DNA, known as a double helix, is mainly credited to Francis Crick, James Watson, Rosalind Franklin and Maurice Wilkins. It is rather like a spiral staircase or twisted ladder in which every rung is a bond between matching “bases” on its two strands. But it was the work of many researchers throughout the decades that followed that determined what DNA codes for, how it is read, and how it is copied and passed on to new cells and future generations.

The order of DNA’s chemical bases form the genetic code. These come in four types: adenine (A), guanine (G), cytosine (C) and thymine (T). The bases always pair up with the same complementary compound on the other strand of DNA: A with T, and C with G.

Three bases in a row together code for a specific amino acid, the basic building blocks of proteins. ACT, for instance, tells cells to make an amino acid called threonine. In this way, each gene tells the cell’s machinery how to make a vast array of proteins.

There is a lot of DNA packed in to every human cell. If you stretched it out, it would be almost two metres long. So your three billion bases, which are more than 99 per cent the same as everyone else’s, need to be packaged up neatly. The coiled strands of your DNA are thus organised into chromosomes. Humans usually have 46 of these in each cell, 23 from each parent. The number varies in other animals: fruit flies have only eight and the black mulberry plant has 308, for example. Mitochondrial DNA is entirely inherited from an organism’s mother.

What makes DNA so amazing is that it can copy itself, which allows all known organisms to function, grow and reproduce. Each strand of DNA in the double helix can serve as a template for duplicating its sequence of bases, enabling new cells to be exact copies of existing ones – although mutations often occur as a result of small errors in this process.

DNA testing

Because everyone’s DNA is unique – except for identical twins – it can be used to identify people, which is why forensic scientists collect samples of blood, saliva or hair and the like at crime scenes.

Aside from encoding your physical features, DNA can also reveal some of your risk for certain medical conditions. For example, mutations in the BRCA1 and BRCA2 genes increase the risk of breast and ovarian cancer. However, other factors like your diet and habits also affect the risk, and we are a long way from definitively tying most diseases to precise genes, and from having tests that conclusively indicate your risk.

In recent years, at-home DNA testing services like 23andMe, Orig3n and FamilyTreeDNA have enabled people to carry out genetic “spit tests”, the results of which can tell them more about their ancestry, or their risks of passing on certain genetic diseases to their children. 23andMe also offers a test that assesses a person’s genetic risk for Alzheimer’s, Parkinson’s and coeliac disease, and seven other disorders, though experts have expressed concerns over the way users will interpret and react to the results of such tests. New Scientist has found that many companies offering these tests sell user data to drug developers for profit (once the users have agreed to participate in research).

In recent years, a technology called CRISPR has allowed us to edit DNA, giving us the potential to fix genes that cause disease and perhaps enhance future generations of humans. Researchers are also trying to repurpose the power of the molecule to make bacterial computers, neural networks and robots. Chris Simms