An Overview of the Human Genome

The past twenty years have brought many surprises as we continue to explorethe human genome. We have far fewer protein-coding genes than most peopleexpected – only around 20,000. That is no more than the 1 mm long nematodeworm Caenorhabditiselegans has, which is widely studied as one of the verysimplest multicellular organisms. Coding sequence makes up less than 2% ofour genome. The transcriptome, by contrast, is very much more complex. Notonly do most genes encode multiple transcripts (by using multiple promotersand alternative splicing), but also the majority of all the non-repetitive part ofour genome is transcribed, at least in some cells and at some times. Cells areawash with a great variety of non-coding RNA species, and for many of themwe have little idea of their function. Sophisticated gene regulation must be thekey to building a human using no more genes than a tiny nematode worm has.The very concept of ‘the gene’ is becoming harder to define. This is a veryexciting time to be working in human genetics!