DNA definition: What is DNA?


11 December 2019
A man entwined in the coils of DNA (scraperboard drawing by Bill Sanderson, 1990)
DNA analysis has become an important tool in genealogy, but what is DNA and how does it work? Simon Wills explores the science behind who we are.

DNA, or deoxyribonucleic acid, is a remarkable molecule. It carries pieces of information from our ancestors, going right back to the first primates that climbed down from the trees, and even beyond that to the earliest forms of life.

Each human being has DNA that is about 99.9% the same as anyone else, so only a tiny proportion of this giant molecule is responsible for establishing the genetic components of our individuality. Chimpanzees are not our ancestors but about 99% of their DNA is the same as ours because, far back, we had ancestors in common. 

What does DNA look like?

DNA looks like a ladder that has been twisted. This is often referred to as a helix. Each side of the ladder contains an extremely long line of molecules called bases. There are four of these bases called guanine (G), cytosine (C), adenine (A), and thymine (T).

The rungs of the ladder are formed by the bases on opposite sides linking across to one other: G will always bind to C, and A binds to T. Certain short sections of the DNA molecule are called genes, and the order in which the four bases appear at these locations determine what the gene will do. Genes control or influence what makes us the same as other people and what makes us different.

The helical DNA ladders are each tightly folded in on themselves into dense structures called chromosomes. Humans have 46 of these, arranged as 23 pairs, in the nucleus or ‘control centre’ of every cell. They are of different sizes: chromosome number 1 is the biggest and holds about 8,000 genes. 

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What does DNA do?

DNA completely controls certain things but only partially controls many others. For example, your eye colour or ear shape is completely determined by your DNA. We say that traits like these ‘run in families’ for good reason! However, not all physical attributes are completely controlled by genes. You might have a set of genes for being tall, but if you are malnourished then you may not become tall because of inadequate nutrition to make the long bones required. 

This ability of DNA to influence but not control is important. Our genes may predispose us to a certain medical condition, for example, but that does not necessarily mean we will develop it because a healthy lifestyle or other factors might prevent it appearing. 

DNA also potentially influences certain human behaviours in fairly generalised ways, which might include things like, say, having an ear for music. Our environment and upbringing, however, are extremely important in determining how we behave. Hence a musical predisposition might flourish in a family where learning to play an instrument is encouraged from an early age, but in another less favourable environment it might not.

How does DNA do it?

Sperms and eggs only carry one of each pair of chromosomes, so that when sperm and egg unite as a result of sexual intercourse a complete set of 46 chromosomes is created, with half coming from each parent. This fertilised egg contains all the instructions required for creating a new person.

Children of the same parents do not all have an identical genetic make-up because chromosomes in each pair exchange bits of DNA between them when eggs or sperms are created. This means that each child will inherit a different set of DNA from each parent compared to their siblings.

Genes operate by instructing the body to build specific molecules which, in their turn, affect the way the body looks or functions. Only one gene might be needed to make certain basic functions happen, such as making a hormone. However, more commonly, genes work together.

So there is not just one gene for making a face, but many of them operate in concert to determine face shape, hair colour, nose size and so forth. This is why an ancestor might have, say, a dimple in their chin inherited via genes from their father, but red hair from their mother’s genes.

A gene’s instructions are transmitted by an intermediary known as RNA, that can leave the vicinity of a chromosome and trigger the actions that are required.

Mitochondrial and Y Chromosome DNA

Every cell contains hundreds of mitochondria. These microscopic structures are separate from the nucleus and its chromosomes, and their function is to generate energy. Yet mitochondria each have a very small amount of their own DNA which includes 37 genes. This DNA is usually inherited only from our mothers. This means that it may not vary much from generation to generation and has been used to explore family connections in the maternal line far back in time.  Interestingly, mitochondrial DNA is associated with some medical conditions too such as the hearing loss that may occur as we get older. 

The Y chromosome is exclusive to men and it has its own DNA, which is only inherited by sons from their fathers. So it can be a way to track a paternal line via its male ancestors.   

For more on DNA in family history, get the DNA special issue of Family Tree magazine, on sale 14 January.

About the author

Dr Simon Wills is a genealogist and author with more than 25 years’ experience of researching his ancestors. He has a particular interest in maritime history and natural history and his latest books are ‘The History of Birds’ and ‘The History of Trees’. He is also author of ‘How Our Ancestors Died’ .