For the first time, scientists have mapped virtually an entire human chromosome, the blob in question. Chromosome number 22, second-smallest of the 23 human chromosomes, is one of the chains of molecules that bear the genetic recipe for human life.
The achievement, announced Wednesday, is an important step for the $3 billion Human Genome Project, which is attempting to detail the tens of thousands of genes that make us human.
"This is the first time that we've had a complete chapter in the human instruction book, and that's pretty amazing," said Francis Collins, who chairs the international project. "I think this is probably the most important scientific effort that mankind has ever mounted."
In laying out the chemical instructions for life, scientists believe they are in the early stages of revolutionizing the study of human development and medicine.
Already, researchers have begun testing several biological therapies that replace faulty genes - or at least correct their misfirings - to make cells work correctly. Such therapies, if they can be made reliable, would bring a more precise way to treat diseases without the sometimes debilitating side effects of conventional drugs.
A draft of the entire genome was expected to be done next spring, but the milestone announced Wednesday may hasten its completion. The study's details appear in the journal Nature.
"It's like seeing the surface or the landscape of a new planet for the first time," said Nature's Mark Patterson.
The human genetic pattern, or genome, is a biological map laying out the sequence of 3 billion pairs of chemicals that make up the DNA in each cell. All human DNA is contained within 23 pairs of chromosomes.
What the scientists have done is lay out the order of about 545 of the estimated 700 genes on chromosome 22, which has about 1.1 percent of the genes in the human body.
The group of scientists studying chromosome 22, which includes British, American, Japanese, Canadian and Swedish researchers, could find only 97 percent of the chromosome's genetic material. Technological limits prevented them from analyzing the remaining 3 percent - as many as 200 genes. Nevertheless, they consider the chromosome to be cracked.
Genes are arrayed along chromosomes, the rod-shaped bodies inside the nucleus of a cell.
Chromosome 22 is the second-smallest of the 23 pairs. Scientists took aim at the tiny target because its genes are densely packed and very active in a variety of diseases, including leukemia and schizophrenia.
Mutations to genes along chromosome 22 can also contribute to heart defects, immune system disorders, and mental retardation, they said.