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Last Updated: Wednesday, 15 September 2004, 17:02 GMT 18:02 UK
Human genome hits halfway mark
Human chromosomes: Cracking the human code has been a bit like painting a picture
Four years after publishing a draft of the human genetic sequence, researchers have hit the halfway mark in producing the "gold standard" version.

They have just published a detailed run-down of a 12th chromosome - known as chromosome five - which means there are just 12 left to complete.

The bundle of DNA is the largest so far, with 923 recorded genes, of which 66 are involved in human disease.

The chromosome, which was sequenced by US scientists, is detailed in Nature.

It is the second of three chromosomes that the Department of Energy Joint Genome Institute (JGI) has finalised in collaboration with colleagues at the Stanford Human Genome Center (SHGC).

Code breakers

Cracking the human genetic make-up has been a bit like painting a picture. First comes a rough sketch followed by a slightly fuller version before, finally, the minute detail is added.

When the draft version of the human genome was unveiled in June 2000, 97% of the "book of life" had been read. Then, last year, scientists announced the decoding was almost 100% complete.

Now, several institutions around the world have divided up the 24 human distinct chromosomes - the cellular structures into which DNA is wound - and are going through them with a fine-tooth comb for a final time, to fill gaps and correct errors.

This extremely accurate sequence will be a powerful tool for scientists trying to understand human disease
Spencer Abraham, Secretary of Energy
They are, as it were, dotting the I's and crossing the T's and giving the whole sequence a thorough spell-check.

"It is about getting everything in the right order," commented Dr Tim Hubbard, of the Human Genetics group at the Sanger Institute in Cambridge, UK.

"In the draft version there were 100,000 gaps in the whole genome. It was a small percentage of the sequence, but it meant you were uncertain about the order of the pieces.

"It is important for doing experiments to have the complete sequence - to have no gaps at all."

Giant chromosome

According to researchers at the JGI and SHGC, the landmark chromosome five is a genetic behemoth, containing key disease genes and a wealth of information about how humans evolved.

"This extremely accurate sequence will be a powerful tool for scientists trying to understand human disease," said US Secretary of Energy, Spencer Abraham.

Graphic, BBC
The double-stranded DNA molecule is held together by four 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"; there are about 2.9 billion 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 starting templates to make proteins; these sophisticated molecules build and maintain our bodies
The giant chromosome is made up of 180.9 million "letters" - the A's, T's, G's and C's that group together to make up the genetic code.

Of the 923 genes that sit on chromosome five, 66 are known to be linked to disease when they go wrong. Another 14 diseases seem to be connected to chromosome five genes, but they have not been linked to specific genes yet.

Having a detailed picture of chromosome five will be an immense help to researchers investigating these illnesses.

"It is very useful to have a base sequence which you can then compare individuals to," Dr Hubbard told BBC News Online.

"Then you can look for key differences between people that do have the disease and people that don't have the disease."

Another feature of chromosome five will pique the interest of scientists studying the difference between humans and chimpanzees.

Despite great similarities between the genomes of the two species, there are some key structural variations.

In particular, one large section of chromosome five is flipped backwards in humans compared with chimps.

Such an inversion makes it impossible for the two chromosomes to pair up during reproduction, which could have driven a wedge between the evolving ancestral populations.

'Junk' DNA

It is not just the genes in chromosome five that the scientists are interested in. Volumes of genetic material lie in between the genes, which for a long time were dismissed as "junk" by researchers.

But on closer inspection, it seems this judgement was premature. The fact that sequences of junk were conserved for hundreds of generations suggests they have a function worth holding on to.

"Important genetic motifs gleaned from vast stretches of non-coding sequence have been found on chromosome five," said Eddy Rubin, JGI's director.

"Comparative studies conducted by our scientists of the vast gene desert... have shown these regions, conserved across many mammals, actually have a powerful regulatory influence."

Over the next few months, the remaining 12 human chromosomes should be completed to a final gold standard of accuracy.

Dr Hubbard concluded: "Several groups are working on the remaining chromosomes - tidying them up - and they should all be complete by the end of the year."

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