Genetic researchers announce big step in sequencing smallest chromosome

Chromosome 21 linked to Down Syndrome, Alzheimer's and several cancers

May 8, 2000
Web posted at: 12:29 p.m. EDT (1629 GMT)

By CNN Senior Producer Miriam Falco

ATLANTA (CNN) -- Researchers in Japan and Germany announced today a breakthrough in the human genome project -- Chromosome 21, the smallest chromosome in the human genome, has become only the second human chromosome to be fully sequenced.

The genetic information on Chromosome 21 has been linked to Down Syndrome, one form of Alzheimer's, and several types of cancer including Lou Gehrig's disease.

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In November, Chromosome 22 became the first fully sequenced chromosome. With only 225 active genes, Chromosome 21 contains fewer than half the number contained in Chromosome 22, which has 545 active genes, even though both chromosomes are nearly the same size. Despite showing that Chromosome 21 has so few genes, scientists are interested in its content because it is the only one linked to Down Syndrome.

Each human has 46 chromosomes, 23 from each parent. Each chromosome is a tightly wrapped package of gene-carrying DNA. They are the blueprint of a human, determining how a person looks, develops and which diseases they may get.

According to Dr. Robert Waterston, chairman of the department of genetics at Washington University in St. Louis, "each chromosome is like a volume of an encyclopedia." Sequencing the genome becomes particularly useful in aiding scientists to learn more about genetic disorders, such as Down Syndrome.

Down Syndrome is caused by the presence of three copies of genes on Chromosome 21 instead of two. Down Syndrome is the most prevalent genetic cause of mental retardation, occuring in 1-of-700 births. With the help of the genetic map of Chromosome 21, researchers can now look for specific genes directly linked to Down Syndrome.

The announcement was made by researchers representing the 62 scientists from Japan, Germany, France, Switzerland, Britain and the United States, who participated in the sequencing of Chromosome 21.

The scientists are part of the publicly funded Human Genome project -- an international research project to map each human gene and to completely sequence human DNA. The public Human Genome Project expects to have a rough draft of all the human chromosomes completed in a few months. Several private companies are also sequencing genes and chromosomes.

According to Waterston -- whose team contributed to the sequencing of Chromosome 22 -- the significance of deciphering Chromosome 21 is "that is came so fast" after sequencing Chromosome 22.

"It shows that the pace is picking up," he said. "In addition to 21 and 22 we have almost 85 percent of the genome in one form or the other."

The private company Celera announced last month that it expects to have sequenced the human genome in a matter of weeks. However, Celera and the public project are using different criteria for the rough draft. Also, Celera does not publish its findings, whereas the international Human Genome Project makes its findings available on the Internet within 24 hours of discovery.

The next chromosomes to be sequenced are likely to be Chromosome 20 and the male Y chromosome, according to Waterston.

The findings appear in today's edition of the journal Nature.

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Nature
Human Genome Sequencing
National Human Genome Research Institute (NHGRI)
The Human Genome Project
The Human Genome Organization (HUGO)

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