Re: How can so few genes produce so many proteins?

Dear Ross,

thank you for your question! In general, each gene in our genome does code 
for one protein.  While there are millions of protein molecules at work 
in our cells, many of these molecules are present in multiple copies and are 
encoded by the same gene.  

There are a few exceptions to the "one gene: one protein" rule.  Some genes 
can actually code for more than one protein via a mechanism known as 
"alternative splicing".  When the information in any gene sequence is 
transcribed into messenger RNA, some portions of the code are cut out before 
the message is translated into the amino acid sequence that makes up a 
protein.  This cutting and splicing happens to all messenger RNAs, but a few 
genes can actually vary which bits get cut out.  For example, if the gene 
sequence reads as follows:

...region A  region B  region C region D region E....

a gene that's alternatively spliced could allow for regions B and D being 
cut out of the mRNA in some cases, thus producing a protein based on the 
code of regions A, C, and E. But under other circumstances, regions A and D 
could be cut out, resulting in a protein synthesized from the code in 
regions B, C, and E. So, it is possible to obtain two functionally different 
proteins from one gene.  

Another exception (sort of) to the one gene: one protein rule is the fact 
that although each gene encodes the recipe for making a polypeptide chain, 
or protein, many of these molecules can't act alone, but must join with 
other proteins in order to perform their designated function.

One more thing to keep in mind is that although the researchers at work on 
the human genome project can look at the sequence and predict how many genes 
there will be, and thus estimate the number of proteins obtainable from 
these genes, it is only an educated guess. 
Some sequences may appear to be bona-fide genes, but in fact don't ever get 
transcribed in real life.  We probably won't know exactly how many active 
genes--or how many proteins--are at work within the human cell for many more 
years to come, but the human genome project is a very exciting step toward 
this knowledge.  If you'd like to find out more about the progress of the 
HGP, I've listed a very good website at the bottom of this message.

I hope this information is helpful. Please don't hesitate to write again if 
you have further questions.

Warm Regards, 

Jen
 http://
www.ornl.gov/hgmis/research/research.html