Caenorhabditis Elegans - Genome

Genome

C. elegans was the first multicellular organism to have its genome completely sequenced. The sequence was published in 1998, although a number of small gaps were present; the last gap was finished by October 2002. The C. elegans genome sequence is approximately 100 million base pairs long. The genome consists of six chromosomes (named I, II, III, IV, V and X) and a mitochondrial genome. Its gene density is about 1 gene/5kb. Introns, or non-expressed sequences, are 26% of the genome. Some large intergenic regions contain repetitive DNA sequences. Many genes are arranged in operons: polycistronic series that are transcribed together. C. elegans and other nematodes are among the few eukaryotes currently known to have operons.

The genome contains approximately 20,470 protein-coding genes. The number of known RNA genes in the genome has increased greatly due to the 2006 discovery of a new class of 21U-RNA genes, and the genome is now believed to contain more than 16,000 RNA genes, up from as few as 1,300 in 2005. Scientific curators continue to appraise the set of known genes, such that new gene predictions continue to be added and incorrect ones modified or removed.

In 2003, the genome sequence of the related nematode C. briggsae was also determined, allowing researchers to study the comparative genomics of these two organisms. Work is now ongoing to determine the genome sequences of more nematodes from the same genus, such as C. remanei, C. japonica and C. brenneri. These newer genome sequences are being determined using the whole genome shotgun technique, which means they are likely to be less complete and less accurate than that of C. elegans, which was sequenced using the "hierarchical" or clone-by-clone approach.

The official version of the C. elegans genome sequence continues to change as new evidence reveals errors in the original sequencing; DNA sequencing is not an error-free process. Most changes are minor, adding or removing only a few base pairs (bp) of DNA. For example, the WS202 release of WormBase (April 2009) added two base pairs to the genome sequence. Occasionally, more extensive changes are made, as in the WS197 release of December 2008, which added a region of over 4,600 bp to the sequence.