Reading out DNA

Christophe Roos roos at operoni.helsinki.fi
Tue Nov 11 08:16:15 EST 1997


> Could anyone please explain to someone who is not an expert in this
> field how the information stored in the DNA sequences of the various
> chromosomes is read out/interpreted during the growth of a creature?

The chromosomes can be considered as very long strings of four letters
(to know more about genome sequencing projects, go to the URL
http://www.ornl.gov/TechResources/Human_Genome/genetics.html#organism).
The four letter code (ACGT) is read in groups of three (triplets), each
triplet being "translated" into one given amino acid. 64 different
triplets can be achieved, about three of them meaning "stop" in the
translational stage (depending on the organism and organelle under
consideration). The rest of them code for about 20 different amino
acids, meaning that several triplets have the same translation. The
amino acids (peptides) form linear chains. Once a chain (polypeptide) is
complete, it will fold into a complex three-dimensional structure, that
will give the polypeptide (protein) its function. In brief, the linear
4-letter code of the genes (discrete segments of the chromosomes) is
translated into proteins that will adopt 3-dimensional structures given 
by their amino acid sequence/order. A complex interplay of proteins and 
many other molecules as well as environmental factors will eventually 
lead to the formation of an organism.

You can view the genome (the set of all chromosomes in one cell of an
organism) as a manual or book of instructions how to make a cell (in a
community if you consider a multicellular organism).

> Is it similar to what can be done in mathematics with Iterated
> Function Theory, where complex structures are created by iterative
> application of a finite set of rules (= finite number of bases in
> DNA)? 

Partially yes, partially no. With regards to the above comparison of
the genome to a book of instructions, it is clearly an
oversimplification to say that the instructions can be viewed as a set
of iterations performed on the alphabet... The finite "set of rules" is
on a higher complexity level than the four bases (alphabet): it is
probably more apropriate to view the different modules (domains) of the
proteins as the unitary blocks (maybe the sentences in the book could
be used for the iterative process).

Historically, one interest of mathematicians for genetics lie in 
the view that all the complexity of living organisms lie in the four 
letter code. However, one remarkable difference introduced by biological 
systems is the lack of unitary "rules" at some level from DNA sequence 
to complex organisms.


ChR

_________________________________________________________________________
Christophe Roos            Dr.Sc., doc.   |   Institute of  Biotechnology
                                          |   &  Dept. of Biosciences
Phone:                 +358 9 7085 9367   |   Division   of  Genetics
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