in 2001, a great deal of publicity was generated by the announcement of the human genome being deciphered. Those who followed the announcement closely noticed that the claim was somewhat premature, as what had been accomplished was just a draft of the genome sequence. In other words, more than a few loose ends needed to be tidied up. A step in that tidying up process has taken place now. The Human Genome Sequencing Consortium led by the Sanger Institute near Cambridge, the uk, has recently finished sequencing yet another human chromosome, number 20. This chromosome joins numbers 21 and 22 that were sequenced earlier. It is the largest chromosome yet to be unravelled. It constitutes some two per cent of the human genome, which comprises 23 pairs of chromosomes. The instructions (genes) the human body needs to function are packaged in these chromosomes.

The genes of the chromosome 20 vary enormously in size: the smallest is 339 bases in length while the largest is more than one million bases long. Secondly, at least 57 per cent of the chromosome seems to be incapable of coding for any protein at all, a finding that raises questions regarding the occurrence of dna sequences that are