A gene refers to a sequence of DNA that codes for a particular protein (made up of a string of amino acids). Active genes generally are delimited by a 'start' and a 'stop' codon (a short DNA sequence which is recognized by the cell as a point either to start or to stop translation of the gene) according to the genetic code. Much of the chromosome in many organisms contains DNA of no apparent function (sometimes referred to as '[junk DNA]?'). In many cases, this probably represents genes which have become inactive due to DNA rearrangement or mutation at the delimiting codon sites. An interesting field of exploration is the attempt to 're-activate' such "lost" genes, producing such things as real hen's? teeth?, for instance. Other junk DNA is thought to serve structural roles in the chromosome, such as the regions of heterochromatin? near the chromosome's centrosome? or the telomere?s at the ends. Finally, some junk DNA serves a regulatory role, providing binding sites for the many signal proteins that affect gene transcription.
Typical numbers of genes in an organism:
| organism | # of genes | base pairs |
|---|---|---|
| Plants | <50000 | <1011 |
| Humans | 35000 | 3x108 |
| Flies? | 12000 | 1.6x108 |
| Fungi | 6000 | 1.3x107 |
| Bacteria | 4000 | 107 |
| Mycoplasma Genitalium | 500 | 106 |
| DNA viruses | 10-300 | 5000-200.000 |
| RNA viruses | 1-25 | 1000-23.000 |
| Viroids | 0-1 | ~500 |
| Scrapie?/BSE | 0 | 0 |
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