Infographic on the structure of DNA. DNA has a sugar-phosphate backbone with a set of four different bases holding its two strands together. DNA acts as a code for building proteins. An enzyme transcribes it into mRNA, which carries the code out of cell nuclei, then ribosome build proteins from the code.
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Today’s post crosses over into the realm of biochemistry, with a look at the chemical structure of DNA, and its role in creating proteins in our cells. Of course, it’s not just in humans that DNA is found – it’s present in the cells of every multicellular life form on Earth. This graphic provides an overview of its common structure across these life forms and a brief explanation of how it allows proteins to be generated.

DNA is found in the nucleus of cells in multicellular organisms and was first isolated in 1869, by the Swiss physician Friedrich Miescher. However, its structure was not elucidated until almost a century later, in 1953. The authors of the paper in which this structure was suggested, James Watson & Francis Crick, are now household names and won a Nobel prize for their work. This work, however, was heavily reliant on the work of another scientist, Rosalind Franklin.

Franklin herself was also investigating the structure of DNA, and it was her X-ray photograph, clearly showing the double helix structure of DNA, that greatly aided their work. She had yet to publish her findings when Watson and Crick obtained access to them, without her knowledge. However, her failure to win a Nobel prize is not an oversight, but merely a consequence of the committee’s policy that Nobel prizes cannot be awarded posthumously.

The double helix model of DNA (deoxyribonucleic acid) consists of two intertwined strands. These strands are made up of nucleotides, which themselves consist of three component parts: a sugar group, a phosphate group, and a base. The sugar and phosphate groups combined form the repeating ‘backbone’ of the DNA strands. There are four different bases that can potentially be attached to the sugar group: adenine, thymine, guanine and cytosine, given the designations A, T, G and C.

The bases are what allow the two strands of DNA to hold together. Strong intermolecular forces called hydrogen bonds between the bases on adjacent strands are responsible for this; because of the structures of the different bases, adenine (A) always forms hydrogen bonds with thymine (T), whilst guanine (G) always forms hydrogen bonds with cytosine (C). In human DNA, on average there are 150 million base pairs in a single molecule – so many more than shown here!

The cells in your body constantly divide, regenerate, and die, but for this process to occur, the DNA within the cell must be able to replicate itself. During cell division, the two strands of DNA split, and the two single strands can then be used as a template in order to construct a new version of the complementary strand. As A always pairs with T, and G always pairs with C, it’s possible to work out the sequence of bases on one strand using the opposite strand, and it’s this that allows the DNA to replicate itself. This process is carried out by a family of enzymes called DNA polymerases.

When DNA is used to create proteins, the two strands must also split. In this case, however, the DNA’s code is copied to mRNA (messenger ribonucleic acid), a process known as ‘transcription’. RNA’s structure is very similar to that of DNA but with a few key differences. Firstly, it contains a different sugar group in the sugar-phosphate backbone of the molecule: ribose instead of deoxyribose. Secondly, it still uses the bases A, G and C, but instead of the base T, it uses uracil, U. The structure of uracil is very similar to thymine, with the absence of a methyl (CH3) group being the only difference.

Once the DNA’s nucleotides have been copied, the mRNA can leave the nucleus of the cell and makes its way to the cytoplasm, where protein synthesis takes place. Here, complicated molecules called ribosomes ‘read’ the sequence of bases on the mRNA molecule. Individual amino acids, which combined make up proteins, are coded for by three-letter sections of the mRNA strand. The different possible codes, and the amino acids they code for, were summarised in a previous post that looked at amino acid structures. A different type of RNA, transfer RNA, is responsible for transporting amino acids to the mRNA, and allowing them to join together.

This process isn’t always flawless, however. Errors can occur in copying DNA’s sequence to mRNA, and these random errors are referred to as mutations. The errors can be in the form of a changed base, or even a deleted or added base. Some chemicals, and radiation, can induce these changes, but they can also happen in the absence of these external effects. They can lead to an amino acid’s code being changed to that of another, or even rendered unreadable. A number of diseases can result from mutations during DNA replication, including cystic fibrosis, and sickle-cell anaemia, but it’s worth noting that mutations can also have positive effects.

Though there are only 20 amino acids, the human body can combine them to produce a staggering figure of approximately 100,000 proteins. Their creation is a continuous process, and a single protein chain can have 10-15 amino acids added to it per second via the process outlined above. As the purpose of this post was primarily to examine the chemical structure of DNA, the discussion of replication and protein synthesis has been kept brief and relatively simplistic. If you’re interested in reading more about the subject, check out the links provided below!

Thanks goes to Liam Thompson for the help with the research for this post, and for providing an incredibly useful simple overview of the process of protein synthesis from DNA.

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References & Further Reading

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