the Human Genome
international research effort has been recently completed by two
research teams. The contents of the human genome are now completely
determined, although we don't know nearly as much about the proteins that most genes make.
genes differ from those of a monkey or chimp by only about 1 or
2%. You differ from other people by far less than that. How then,
can we be so different from each other and from chimpanzees?
The key lies in knowing
that what is important is not so much that we share so many of the
same genes, but that we differ so greatly in which genes GET
EXPRESSED. That is, a gene does nothing if it is not "turned on"
so that it can transcribe its code into RNA.
Click here to see how DNA is "unzipped" so that it can
transcribe its code.
Recent research has discovered a small set of genes that make
proteins that control expression of other genes. These
gene-expression control genes make proteins that stick to the DNA
helix and determine which segments can open up for transcription
to occur. The gene-expression control genes are called "zinc
finger" genes, because the proteins that they make have little
"fingers" of amino acids, held in that shape by electrical
interaction of certain amino acids with zinc atoms. These fingers
insert themselves into the DNA and determine which genes can get
expressed and which are shut down.
Comparison of zinc-finger genes across species indicates that it
is THIS PORTION of the genome that varies most greatly among
species. Thus, these zinc-finger genes seem to be more susceptible
to natural selection forces than the rest of the genome.
facts about human genes
Although the complete
sequence of the A-T and C-G pairs have been determined, it is still
somewhat of a guess as to how many genes there actually are. The smallest
human chromosome, number 22, has about:
- 33.5 million base
- 545 protein-coding
- 134 "fake" genes
(sequences like active genes but which don't seem to make any
Segments of DNA that don't do anything
may be left over from our primitive ancestors. But many segments of
DNA could be active but just don't have clear signs
saying "I am a gene!" Another thing to remember is that many body
structures and functions are controlled by more than one gene. Yet
another point to consider is that genes may be present but
turned off only temporarily.
Chromosome 21 was among the
first to be sequenced because it contains genes that cause such
terrible diseases as Down's syndrome, Alzheimer's disease, certain
forms of epilepsy, hearing loss, and leukemia. Sequencing
this chromosome was only the first step to discovering how it is
involved in these diseases.
Some intriguing facts about
chromosome 21 are that it has
- 225 protein-coding
segments of DNA
- 59 fake genes
- duplications (one
93-base sequence has 10 copies and a six-base sequence that
repeats 17 times)
- "nonsense" DNA that
doesn't code for anything. Of the total sequence, about 1/3
doesn't seem to code for anything. One seven-million base
stretch contains just one protein-coding gene.