December 5th, 2011

ariane

Advent Science: Day Five

So. we have a genome. We can store it, and we can copy it. But how do we make everything else out of it?

Here, we meet the Central Dogma of molecular biology, as named by Francis Crick, the co-discoverer of the structure of DNA. Namely, that sequence information is transferred from DNA, to RNA (ribonucleic acid, a similar molecule), to proteins. For the purposes of Advent Science, I shall tell you that one gene codes for one protein.* This is generally used as a definition for "a gene", in fact: a length of DNA that encodes a protein.

Let me repeat that, because it's the thing that underpins the entire field of genetics. Genes code for proteins. When we say someone "has the brown-eye gene", what we mean is that they have the gene that makes the protein that makes brown pigment in the eyes.

So, first we need to get the genetic information encoded in the DNA into a usable state for the cell to read it and make proteins from it, which is called transcription. When a cell receives an instruction to express a particular gene (we'll get back to how that happens in a week or so), the relevant chromosome uncoils, the histones that that gene is wound round fall off, and another set of enzymes (you may begin to detect a common theme here) bind to the DNA, "read" it, and in a similar manner to the DNA replication discussed on Day Two, creates a strand of this other molecule called RNA. Crucially, it creates a strand of RNA that only contains that gene's information. RNA, as discussed two paragraphs ago, is a molecule with a very similar structure to DNA, with one major difference: instead of the nucleotide Thymine, it instead has a similar one named Uracil.

When the strand of RNA is formed, it will fall off its complementary strand of DNA and head out of the nucleus, where it will bind to a ribosome, which is the protein synthesis machinery found in every organism from the simplest bacteria right up to vertebrates like us.

Ribosomes "read" these strands of RNA, more properly known as "messenger RNA" and make the protein that each strand encodes. Now, I realise I have spent all this time not actually telling you what a protein *is*. That's because it's going to take me a day of digression to do so, which will happen tomorrow...

*Those of you with degree level knowledge of genetics will know that this isn't strictly true, but it serves our purposes well enough.