Tuesday, 22 July 2008

What is the structure of nucleic acids?

As with the carbohydrates and proteins (and to some degree, even lipids), nucleic acids are fundamentally polymers of a particular subunit. In the case of carbohydrates, the basic subunit was the monosaccharide; with nucleic acids it is the nucleotide.

There are three components to a nucleotide:
  • A nitrogenous base
  • A pentose sugar
  • A phosphate group

By 'nitrogenous base' we technically mean a substance, containing a nitrogen atom, that accepts a proton in solution. In practice, however, we only mean a particular subset of this massively large group. Basically, all the 'nitrogenous bases' used in nucleotides are rings composed largely of carbon and hydrogen (as per usual in organic chemistry), but with a larger than usual amount of nitrogen atoms thrown in for good measure. It really is as boring as that. Anyway, the nitrogenous bases are traditionally subdivided into two groups:

  • Purines - which contain adenine and guanine, used by both DNA and RNA

  • Pyrimidines - which contain cytosine, uracil and thymidine. Both DNA and RNA use cytosine, but only DNA makes use of thymidine. RNA uses uracil instead.

OK, so much for the nitrogenous bases. Next up come the pentose sugars. Once again, this name is a little opaque, at least to non-chemists, but it basically means a monosaccharide with five carbons. As with the nitrogenous bases, in practice we don't actually mean any pentose sugar - only the pentose sugar named 'ribose' will do. RNA uses ribose as it finds it, but DNA messes with the sugar a bit. It removes a hydroxyl group (OH-) from one of ribose's carbons, and exchanges it for a hydrogen. Since the net change to ribose is thus the loss of an oxygen atom, this modified form of the sugar is known as 'deoxyribose'.

Lastly, let us turn to the phosphate groups. Incidentally, if we were to forget to add a phosphate group to our pentose sugar and nitrogenous base, we would have a nucleoside (with an 's' not a 't'). But we are kind souls, and so we will do so. As with all other phosphate groups, the molecule basically consists of a phosphorus atom covalently bonded to four oxygen atoms.

And so, with all three of our ingredients waiting, we can assemble a nucleotide. The order of the ingredients is important: the pentose sugar must be sandwiched in the middle, with the nitrogenous base on one side and the phosphate group on the other. All in all, it should look a little like this:


Nucleic acids are one of the four classes of biological macromolecules, alongside carbohydrates, lipids and proteins. There are two types of nucleic acid: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

The final step is to link several nucleotides together to form a nucleic acid. The way we do this is to join the phosphate group from one nucleotide up with the pentose sugar of the next one. Once we've done so (and here I confess I'm glossing over some of the boring chemistry of the process) we end up with a macromolecule that we can proudly label 'nucleic acid'. Obviously, RNA is a polymer consisting of these nucleotides by the bucketful. DNA has even more - it is estimated that each of our (nucleated) cells contain just over 3 billion nucleotides, spread out over our 23 pairs of chromosomes.

2 comments:

  1. Hi Jeremy you have made a small mistake while writing this article. DNA contain Adenine, Guanine, Cytosine and thymine not Uracil as you have mentioned.

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    1. Urgh! You're right, of course - I've changed it now!

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