Largest Human Chromosome Unraveled

Dec. 20 — Scientists said on Wednesday they have deciphered the third human chromosome that contains a treasure trove of information about diseases ranging from obesity and eczema to dementia and cataracts.



With more than 727 genes and nearly 60 million DNA letters, chromosome 20 is the largest human chromosome to be finished so far.
Thirty-two of the genes are linked to genetic illnesses including the brain wasting Creutzfeldt-Jakob disease, severe immune disorders and illnesses such as heart disease, diabetes and dermatitis.

"This is one more completed chapter of our genomic anatomy textbook — medical research will be using this information for decades to come in its quest to tackle our common diseases," said Dr. Mike Dexter, the director of the Wellcome Trust, the world's largest medical charity.

Scientists from The Wellcome Trust Sanger Institute in Cambridge, England, led by Dr. Panos Deloukas, completed the sequence which is reported in the science journal Nature.

All of the information is freely available to scientists around the globe.

"It is a unique piece of the puzzle," Deloukas told Reuters. "We have probably annotated more than 95 percent of this chromosome."

Chromosome 20 is the first chromosome to be deciphered since scientists from Britain, China, France, Germany, Japan and the United States working on the Human Genome Project sequenced all the estimated 40,000 or more genes in humans earlier this year.

"In two years' time we should see the sequence of the remaining chromosomes being finished," Deloukas added.

Each chromosome is made up of a molecule of DNA in the shape of a double helix which is composed of four chemical bases represented by the letters A (adenine), T (thymine), G (guanine) and C (cytosine). The arrangement, or sequence, of the letters determines the cell's genetic code.

Chromosome 20 comprises about two percent of the three billion letters that make up the human genetic code. It is much bigger than chromosome 21, the smallest and first chromosome, mapped by Sanger scientists in 1999. Chromosome 22, the second smallest human chromosome, was sequenced in 2000. It has genetic components linked to 35 diseases and syndromes.

One of the interesting aspects of chromosome 20 is that scientists found an extra chunk of DNA containing at least one gene. They estimate 37 percent of Caucasians have the additional DNA chunk.

"For the moment we don't know whether this gene is truly functional in these humans and if it is functional what are the consequences for the people who have it. That was a bit of a surprising finding," he said.

Chromosome 20 also contains 30,000 SNPs — single nucleotide polymorphisms — which are the variations in human DNA that make people unique. SNPs contain clues about why some people are susceptible to diseases like cancer or diabetes, the best way to diagnose and treat them and how they will respond to drugs.

Any two humans are 99.9 percent similar. The 0.1 percent difference in DNA is what makes an individual unique.

"As with the Harry Potter series, we already know how long the complete works of the human genome will be -- 24 installments -- and we can't wait to get our hands on them all," Masahira Hattori and Todd Taylor of the RIKEN Yokohama Institute in Japan said in a commentary on the research. "We have already had a few glimpses of what's to come but there are many mysteries and secrets to be revealed."