base pairs | a bonobo humanity?

Posts Tagged ‘base pairs’

a DNA dialogue 4: purines, mostly

Canto: So what’s a pyrimidine, molecularly speaking, and why does it differ from a purine, and why does it matter?

Jacinta: They’re two different types of nitrogenous bases, dummy, which are a subset, maybe, of nucleotide bases. All of which is largely gobbledygook at present.

Canto: Ok, we know there are four different nitrogenous bases in DNA. Two of them, A & G, adenine and guanine, are purines, which structurally are two-carbon nitrogen ring bases. The other two, thymine and cytosine, T & C, are pyrimidines, which are one-carbon nitrogen ring bases. Uracil, in RNA, is also a pyrimidine. It replaces the thymine used in DNA.

Jacinta: That’s right, now we know that in DNA these nitrogenous bases are connected across the double helix, in pairs, in a particular way. A (a purine) always connects with T (a pyrimidine), and similarly C is always bonded to G. So why is this?

Canto: Why is it so? Well, put simply, the molecular structure of purines, which you’ll note have a two-carbon ring structure and so are larger than pyrimidines, doesn’t allow them to bond within the group, that’s to say with other purines, and the same goes with pyrimidines. It’s essentially due to the difference between hydrogen bond donors and acceptors for these groups.

Jacinta: So, looking at purines first, considering that they’re one of the building blocks of life, it’s not surprising that we find them in lots of the food we eat, especially in meat, mostly in organs like kidneys or liver. Structurally they’re heterocyclic aromatic organic compounds – as are pyrimidines. Heterocyclic simply means they have a ring structure composed of more than one element – in this case carbon and nitrogen. An aromatic compound isn’t quite what you think – structurally it means that it’s strong and stable, due to resonance bonds, which we won’t go into here. Below is a model of a purine molecule, which has the chemical formula C₅H₄N₄ – the black globes are carbon atoms, the nitrogens are blue and the hydrogens white.

Purines and pyrimidines are both self-inhibiting and activating, so they actively bond with each other but inhibit self-bonding, so that they maintain a more or less equal amount as each other within the cell.

Canto: So that’s purines in general, but in DNA there are two purines, adenine and guanine, which must differ structurally – and are there any others?

Jacinta: Oh yes, caffeine is a purine, as well as uric acid…

Canto: Definitely aromatic.

Jacinta: And there are many others. Purines are very important molecules, used throughout the body for a variety of purposes, as components of ATP, cyclic AMP, NADH and coenzyme A, for example.

Canto: I’ve heard of some of those…

Jacinta: As to the difference between adenine and guanine, here’s how it’s described in this Research Gate article, which I’m sure is reliable:

The main difference between adenine and guanine is that adenine contains an amine group on C-6, and an additional double bond between N-1 and C-6 in its pyrimidine ring, whereas guanine contains an amine group on C-2 and a carbonyl group on C-6 in its pyrimidine ring

Canto: Shit, that explanation needs to be explained, please.

Jacinta: Haha well let’s look at more diagrammatic structures, but first – an amine group, also called an amino group, is a derivative of NH₃ (ammonia), consisting of a nitrogen atom bonded to hydrogen atoms, at its simplest. This gives adenine the formula ‎C₅H₅N₅. Guanine has, in addition to the amine group, a carbonyl group, which is a carbon double bonded to an oxygen, C=O. This gives guanine the formula C₅H₅N₅O. Anyway, it’ll all become clear over the next dozen or so years…