The 22 pairs of human chromosomes, together with the X and Y chromosomes that determine gender, were unravelled in a long-term sequencing project that ended in April 2003.

 

The task since then has been to sift through the raw data to analyse each of the chromosomes, notably seeking out genes and genetic variations blamed for inherited disease.

So far, a dozen chromosomes have been sequenced and analysed in this way.

 

In their study, which appears on Thursday in the British journal Nature, scientists led by Jeremy Schmutz of the Stanford Human Genome Centre in California said on Wednesday, Chromosome 5 measures 177.7 million base pairs -- the "rungs" of matched chemical compounds that make up a spiral of DNA.

 

This makes it one of the longest chromosomes in the genome but it is also one of the poorest in genes, the DNA stretches that code for proteins.

 

The chromosome has only 923 genes, including the SMN1 and SMN2 genes, variations of which have been associated with forms of the debilitating disease called spinal muscular atrophy.

SMN stands for Survival Motor Neuron, the nerve-system cells which control the voluntary muscles needed for crawling, walking, head and neck control and swallowing.

 

Deciphering chromosomes and pinpointing their genes are the first step towards developing diagnostic tools that can predict whether a person has an inherited susceptibility to a disease.

 

The knowledge can also help in the quest for medicines to block or reverse the disease.