Conservation of genomic sequences between species indicates regions of functional importance, containing protein-coding gene, non-coding genes or regulatory elements. In the December 5 Nature, Emmanouil Dermitzakis and colleagues describe a thorough analysis of syntenic regions of human chromosome 21 (Nature, 420:578-582, December 5, 2002).

Dermitzakis et al compared the 33.5 Mb of chromosome 21 to sequences from mouse chromosomes 10, 16 and 17. There were 3,491 blocks that are over 100 bp long and have at least 70% sequence identity. Around two thirds of these correspond to regions with unknown function, and only a small number of these regions appear to represent potential unknown genes, and most are probably not transcribed. A three-way species sequence analysis (human, mouse and rabbit) provided evidence for broad conservation of these sequence blocks, supporting their functional significance.

This study provides an example of the power of combined computational, experimental and...

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