Expert: John Locke - 2/24/2010

Question
Hello,

I am trying to make a model of a brain in computer and I just thought that the way DNA works might inspire me somehow. But I don't know biology or genetic.

Now, I know that gene are molecules of A-T or G-C pairs. The length of pairs can be long or short. And each length (is it chromosome?) is designed for specific function. For example, a chromosome controls how a leave grow or how a fruit's skin looks. I have seen letters on leaves or fruit skin by genetic modification.

Could you tell me how these pair do what they do? How the system of A-T G-C code works?

Thanks you

Answer
Thanks for using Allexperts. The answer to your question is deceptively simple, but underlying that simple answer is an intensely complex and rapidly developing understanding of how genes work the way that they do.

Here's the basics: as you noted, the DNA nucleotides pair up as either AT or GC. They form continuous paired strings of nucleotides called chromosomes--each chromosome consists of two continuous DNA strands that are joined in the middle by hydrogen bonds. If you started at one end of the DNA strand and worked your way toward the other end, you would find that large sections of the strand are nonfunctional DNA whose purpose we don't really understand. Some of the DNA, however, is used as a template for the manufacture of proteins whose functions are essential to the organism. That's how DNA works: it serves a template for protein synthesis.

Proteins, as you may know, are made of constitutive elements called amino acids, of which there are twenty that are essential to protein synthesis in humans. Each amino acid is assigned a corresponding three letter sequence of nucelotides (TTA, CAG, CCA, etc.); the DNA is used as a template to make an RNA strand that will in turn be used as a guide to manufacture proteins. The DNA strand (and the RNA strand made from it) can be envisioned as a long string of three letter code groups that specify different amino acids to be added in sequence to the protein being manufactured. There are 64 possible three letter code groups but only 20 amino acids, so multiple code groups may correspond to one amino acid--this is termed degeneracy--and several code groups are reserved as marking start/end points for the sequence.

Let me give you an example. Suppose a section of DNA carries this sequence:

CTATTGGGCCACGT
Enzymes that are responsible for reading this code and producing a corresponding RNA template view it in 3-letter segments:

CTA TTG GGC CCA CGT

A corresponding RNA template is produced to this DNA strand, but let's ignore that for now. The structure responsible for actually making the protein--the ribosome--will read this strand and associate it with the given amino acid that each 3-letter group represents. In this case:

Leucine Leucine Glycine Proline Arginine

Those are the amino acids that would be added, in that sequence, to the protein being made by the ribosome.

That is in short how the DNA ACTG code system works; the nucleotides are grouped into 3-letter sequences that represent amino acids to be added to the protein that each gene codes for. I said this is deceptively simple because this system, while easy to explain and relatively simple, is responsible for all the fantastic complexity that exists in all living creatures.

You may wish to use these websites as a further resource:
http://learn.genetics.utah.edu/content/begin/dna/transcribe/
http://www.ncbi.nlm.nih.gov/About/primer/genetics_genome.html