"Tijmen Tieleman"  wrote
>
> I have always been told that DNA describes exactly how to build a (say
> human) body.
>
We think that this is more or less the case. The zygote (joined sperm and
egg) certainly contains all the instructions somewhere.
>
> Now to describe such a big and complicated thing, I
> suppose one needs a powerful language. One that allows us to say
> 'build this and this structure, and then repeat it N times.' Or 'build
> the following 1000 structures:  as the basis for each one of
> them, and then  and   to complete
> them.' So that one doesn't have to write the basis a 1000 times. And
> some other tricks that we also use in our natural languages.
>
> Do many codes in DNA occur many times, or does DNA have a way of
> preventing such seemingly needless repetition? How complicated is the
> language in which DNA describes bodies? What sort of a language is it?
>
I wish I could answer this. Are you familiar with the genetic code for
proteins? That bit of the process is relatively well understood. What we
don't really understand is how the cells know where to migrate and how to
differentiate into the right type. Progress is being made, for instance we
know that homeobox genes are very important in determining the basic segment
that is being built, and that the linear position of these genes on the
chromosome corresponds to the position of the segment they are expressed in.
>
> Also I have some side questions:
> 1: How much data does DNA of a human and that of a bacterium contain?
>
The human genome is 3 gigabase pairs long. However about 98% of this is
thought to have no function. Bacteria vary, ranging from about 0.5 to 10
Mega base pairs . Bacteria don't contain much junk DNA, however.
>
> 2: How much of that data is needed to describe the human brain? Its
> different sections, its individual neurons, the connections between
> the sections, and all that. One would expect this to be a lot. After
> all, in AI (my field) we are continually impressed by the power and
> supposed complexity of the brain, and no computer program so far, it
> seems, has gotten anywhere near.
>
About half the genes in the body are expressed in the brain. However it is
impossible that all the connections are determined genetically, even all the
cells cannot be under direct control. What we don't know is exactly how the
brain organises itself.
>
> But my main question is about the language that DNA uses to describe.
>
We just don't know. Genes contain "promoters" which regulate transcription,
and sometimes the promoters are bound by the prducts of other genes. One of
the few systems that has been described in detail is the "lac" (lactose
induced) operon in E. coli. Lots of research on development is needed before
we can have good answers to these questions.
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