What is DNA?

The instruction book inside every cell

Your body is made of tiny living units called cells. Inside most of them is a control centre called the nucleus, and inside the nucleus is DNA, which is short for deoxyribonucleic acid. DNA is a set of instructions for building and running a living thing. Nearly every cell in your body has the same copy of those instructions. So does nearly every cell in a koala, a gum tree or a mushroom, because DNA is the hereditary material in humans and almost all other living things. 'Hereditary' means it can be passed from parents to their young.

DNA is very long and very thin. If you could stretch out the DNA from just one of your cells, it would be about two metres long. That is taller than most grown-ups, yet it fits inside a nucleus that is far too small to see without a microscope. It fits because it is wound up tightly around proteins and packed into thread-like bundles called chromosomes. Most human cells have 46 chromosomes, arranged in 23 pairs.

A twisted ladder with four letters

DNA is made of two long strands that wind around each other, a bit like a twisted ladder. This shape is called a double helix. The two sides of the ladder are made of sugar and phosphate molecules. The rungs are made of pairs of chemicals called bases.

There are four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). They pair up in a fixed way: A always goes with T, and C always goes with G. Each rung is called a base pair. The order of the bases along the ladder is a code, and it works a bit like letters in a sentence. Change the order and you change the message. One set of human DNA has about 3 billion bases. If you read them out at one letter a second, without stopping to sleep, it would take about 95 years.

The pairing rule also explains how DNA copies itself. Before a cell divides, the two strands of the ladder pull apart. Each strand acts as a pattern for building a new partner strand, because A can only join T and C can only join G. The result is two identical copies of the DNA, so each new cell gets a full set of instructions.

Genes: recipes for proteins

A gene is a stretch of DNA that works like a recipe. Many genes hold the instructions for making a protein. Proteins are large molecules that do most of the work in your cells and help build your body's tissues and organs. Humans have about 19,900 genes that are used to make proteins. Some genes are tiny, only a few hundred base pairs long, and others are more than 2 million. Many genes do not make a protein at all. Instead they help to switch other genes on and off.

Every kind of living thing has its own DNA. In 2018, a team of 54 scientists from 29 organisations in seven countries finished reading the whole genome of the koala. The project was led by researchers at the Australian Museum Research Institute and the University of Sydney. A genome is a living thing's complete set of DNA. The koala's has more than 3.4 billion base pairs, a little more than a human's. Koalas live on eucalyptus leaves, which have chemicals that are harmful to many animals. The scientists found that the koala has extra copies of a family of genes that may help it break down those chemicals. They also found proteins in koala milk that protect joeys in the pouch. This kind of information can help to protect koalas, which face threats such as habitat loss and disease.

Why you are like your family, but not the same

You get your DNA from your parents. Chromosomes come in pairs, and in each pair one chromosome came from your mum and one from your dad. So you inherit half of your chromosomes from each of them. Usually you have two copies of each gene, one from each parent. This is why you may share features with both. Brothers and sisters usually get different mixes of their parents' DNA, so they can look alike without being the same.

Humans are far more alike than different. The DNA of any two people is 99.9 per cent the same. The tiny difference, about one letter in every thousand, is part of what makes each person unique. Different versions of the same gene, with small differences in their base order, are called alleles.

A trait is a feature of a person, such as eye colour or height. Traits can be shaped by genes, by the environment, or by both. Eye colour mostly comes down to genes that control how much of a pigment called melanin is in the iris. Height is mostly inherited. Scientists estimate that somewhere between 60 and 80 per cent of the differences in height between people come from the DNA they inherit. The rest comes mainly from things like the food you eat. Many things about you are not written in your DNA at all. Which language you speak, or whether you can ride a bike, is learned.

Who worked out DNA's shape?

DNA was first found in 1869 by a Swiss scientist called Friedrich Miescher, who pulled a substance he named 'nuclein' out of the nuclei of white blood cells. Nobody yet understood how important it was. In the 1950s scientists wanted to know DNA's shape, because the shape might explain how DNA could store and copy information. At King's College London, the chemist Rosalind Franklin and her student Raymond Gosling shot X-rays at thin fibres of DNA. In May 1952 they took a very clear picture of a damp form of DNA, called Photograph 51. Its cross-shaped pattern of spots is what you would expect from a helix. Franklin also made careful measurements of DNA's size and structure.

At Cambridge, James Watson and Francis Crick were building models of DNA out of metal and cardboard. In January 1953 Maurice Wilkins, a colleague of Franklin's, showed Photograph 51 to Watson, without Franklin knowing. Watson and Crick also saw an unpublished report that included some of her measurements. Their model of a double helix with base pairs fitted the evidence. It also suggested how DNA could copy itself. On 25 April 1953, the journal Nature published three papers together: one by Watson and Crick, one by Wilkins and his co-workers, and one by Franklin and Gosling.

Who deserves the credit is still discussed. Watson and Crick did not ask permission to use the King's College data. In a longer paper in 1954 they said that without Franklin's data their structure would have been very unlikely, perhaps impossible. Historians still debate how much the photograph itself mattered compared with her other measurements. Some historians now argue that Franklin was an equal partner in solving DNA's shape. She died in 1958, aged 37. In 1962 the Nobel Prize in Physiology or Medicine went to Crick, Watson and Wilkins. A Nobel Prize cannot be awarded to someone who has already died, so Franklin could not share in it.

An Australian has a Nobel Prize for DNA research too. Elizabeth Blackburn, who was born in Hobart, shared the 2009 prize for discovering how the ends of chromosomes are protected by caps called telomeres.

📚 Words to know

DNA

Short for deoxyribonucleic acid. A long molecule in cells that carries the instructions for building and running a living thing.

Base

One of the four chemicals in DNA's code: A, T, C and G. A pairs with T, and C pairs with G.

Gene

A stretch of DNA that works like a recipe. Many genes hold the instructions for making a protein.

Chromosome

A thread-like bundle of tightly wound DNA in a cell's nucleus. Humans usually have 46, in 23 pairs.

Double helix

The shape of DNA: two strands twisted around each other like a spiral ladder.

🧪 Try this: pull DNA out of a strawberry

1. Ask an adult to help. You will need 3 or 4 strawberries (fresh, or frozen and thawed), a zip-top plastic bag, a teaspoon of dishwashing liquid, a pinch of salt (about a quarter of a teaspoon), a quarter of a cup of water, a coffee filter or a fine sieve, a clear glass, a toothpick, and rubbing alcohol (isopropyl alcohol) from a chemist or hardware store. Put the alcohol in the freezer for 30 minutes first. Safety: rubbing alcohol is poisonous and catches fire easily. Only an adult should handle it, away from flames and heaters, and nobody should ever taste or drink it.
2. Take the green tops off the strawberries and put the berries in the bag. Squeeze the air out, seal it, and squish the berries with your fingers for about two minutes, until they are mushy. You are pulling the cells apart.
3. Mix the dishwashing liquid, salt and water in a cup, add it to the bag, and squish gently for one more minute. The soap breaks open the fatty membranes around the cells, and the salt helps the DNA clump together. Pour the mush through the filter into the glass.
4. Tilt the glass and slowly pour the cold alcohol down the side until you have about as much alcohol as strawberry liquid. It floats on top. Do not stir. Watch where the two layers meet. White, stringy clumps will start to rise into the alcohol. That is DNA, thousands of strands tangled together. Lift some out with the toothpick.
5. Wash your hands when you finish. Challenge: try the same thing with a banana or a handful of spinach. Which one gives you the most DNA? Why do you think different living things give different amounts?