How It Works

Unravelling the Mystery of DNA

In 1953, James Watson and Francis Crick discovered that the DNA molecule resembles a double helix, one of science’s most significant revelations.

Deoxyribonucleic acid, better known as DNA, is the building block of all cells. DNA not only makes the proteins that determine our biological traits, it also gets copied and passed from generation to generation. Changes in DNA over time result in the evolution of traits in a species. Although scientists had learned about DNA and suspected its genetic function since the 1890s, it’s exact structure wasn’t known until 1953. Cambridge University scientists James Watson and Francis Crick won the 1962 Nobel Prize in Medicine – along with Maurice Wilkins – for discovering that the molecule was a double helix – two ladder-like strands twisted together that resemble a spiral staircase. These long molecules are twisted, along with various proteins, into a single chromosome. While DNA structure looks complicated, it comprises just four sugars called nucleotide bases: adenine (A), thymine (T), cytosine (C) and guanine (G). These four sugars are strung together to form a sequence, similar to the way that letters of the alphabet form words, and words form sentences. Groups of three nucleotides form ‘words’ called codons, which form ‘sentences’ called genes. These genes contain information on how and when to build a protein from a combination of 20 different amino acids.

These long molecules are twisted, along with various proteins, into a single chromosome. While DNA structure looks complicated, it comprises just four sugars called nucleotide bases: adenine (A), thymine (T), cytosine (C) and guanine (G). These four sugars are strung together to form a sequence, similar to the way that letters of the alphabet form words, and words form sentences. Groups of three nucleotides form ‘words’ called codons, which form ‘sentences’ called genes. These genes contain information on how and when to build a protein from a combination of 20 different amino acids.

To build a protein, DNA is copied to a type of RNA (ribonucleic acid) called messenger RNA (mRNA). Two types of special RNA molecules, called transfer RNA (tRNA) and ribosomes (rRNA), use amino acids to build the protein using the pattern described in the mRNA. Sometimes several different proteins are made from the mRNA. This is called protein synthesis. When a cell needs to reproduce, all of its genetic information must copy over to the new cells. This means that the DNA must copy itself, or replicate. Enzymes, hormones and other chemicals in the body drive this process. Essentially the double helix zips apart and enzymes copy the codons, check the copies for accuracy, and seal up the strands. The frequency with which replication occurs depends on the type of cell in which the DNA resides. Cells in our skin, for example, are constantly dividing, so the DNA in those cells is constantly replicating itself. Sometimes there are minor changes made in the processes of DNA replication and protein synthesis. Because there are some repeater codons, these variations don’t always cause a problem. Often they result in a positive outcome, such as increased survival of certain types of diseases. However, depending on the variation, mutations can occur that can ultimately result in hereditary diseases.

DNA and Genetic Traits

When a person is conceived, they inherit one copy of each chromosome from each parent for a total of 23 pairs. There are about 200 inherited traits that are determined by these genes, including physical and behavioural. We can also inherit a predisposition towards getting a particular disease or disorder. These genetic variations are called alleles. Some are dominant, while others are recessive. While some traits are determined by a single gene, others come from multiple genes, the environment or a combination. There are multiple genes for determining eye colour, for example, but there’s no known gene for being extraordinarily good at playing a specific sport. The latter is likely a combination of genes, health, nutrition and other environmental factors.

For more amazing science and technology news, check out issue 102 of How It Works. Pick up your copy from all good retailers or from our website now. If you have a tablet or smartphone, you can also download the digital version onto your iOS or Android device. To make sure you never miss an issue of How It Works magazine, subscribe today!