By Dr David Whitehouse BBC News Online science editor

Chromosome six is giving up its secrets

Scientists have completed analysing the bundle of DNA in our cells which contains many of the genes responsible for organising the body's defences.

Called chromosome six, it is the largest such structure so far fully decoded in the human genome project.

UK researchers say its 166 million "letters" of DNA contain 2,190 genes, nearly 6% of the entire genetic code that describes the human lifeform.

The scientists report their study of chromosome six in the journal Nature.

Unknown function

The international project to "read" all the DNA in human cells was finally completed in April.

Researchers are currently trying to make sense of this DNA code. Their aim is to find within, the long list of letters, the locations of the genes and to determine what they do.

DNA IN HUMAN CELLS Double-stranded DNA molecule held together by chemical components called bases Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G) Groupings of these letters form the "code of life". Estimated to be about 2.9bn base pairs in the human genome wound into 24 distinct bundles, or chromosomes Written in the DNA are about 30,000 genes which human cells use as templates to make proteins. These sophisticated molecules build and maintain our bodies

So far this has been done for six chromosomes - numbers seven, 14, 20, 21, 22 and the Y, the DNA bundle that carries the information for maleness.

Now, scientists at the Wellcome Trust Sanger Institute at Hinxton near Cambridge have unveiled a fully annotated description of chromosome six.

Of its 2,190 genes, the scientists say 633 are derelict - they are inactive genes that no longer operate.

The remaining 1,557 genes are believed to be all functional. However, less than half of them have been previously described and their roles are unknown.

Chromosome six is the site of the major histocompatability complex, a large region in the DNA that is important for our immune response.

"Chromosome six is very rich in immune genes. These are the genes that give us protection from pathogens," said Stephan Beck, the head of human sequencing at Sanger.

"They basically determine whether or not you survive if you get infected by bacteria or viruses," he said.

"These genes on chromosome six are involved in breaking down small bits of the invading pathogen and presenting it on the cell surface."

On the cell's surface, the foreign components are recognised by other cells that then realise it is infected and kill it.

Disease genes

Researchers have identified roughly 130 genes on chromosome six that may predispose humans to certain diseases.

There are genetic markers for a juvenile-onset form of Parkinson's disease.

Mutations in the so-called HFE gene cause hereditary haemochromatosis, a condition that afflicts one in 400.

People who have this disorder absorb excessive amounts of iron which can lead to organ damage.

Researchers are also interested in scrutinising a large segment of the genes responsible for making transfer RNA, which helps translate the genetic blueprint into the proteins that build and maintain our bodies.

"A finished chromosome six reveals an abundance of biological information previously buried within the draft of the human genome assembly," said Jane Grimwood, from the Stanford Human Genome Center in the US.