Mapping the Human Genome

Your 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.

Some 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 pairs,
•  545 protein-coding genes
•  134 "fake" genes (sequences like active genes but which don't seem to make any protein)

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.