Scientists Revise Map of Human Genome

By | November 22, 2006

Scientists Revise Map of Human Genome

Scientists have revised the map of the human genome, saying human beings are genetically more complex than previously thought. The discovery has surprised experts who say it is likely to transform medical research. VOA’s Jessica Berman reports.

In 2000, the Human Genome Project unveiled a road map of the six billion chemical bases, or alphabet molecules, that make up the body’s genetic structure called DNA.

The DNA encodes for 30,000 genes or proteins which are responsible for every physical characteristic in the body, including eye and hair color. At the time, scientists said all humans could be 99.9 percent genetically identical.

But as they peered more deeply into the DNA of unrelated individuals, researchers made a startling discovery – large segments of their DNA, from thousands to millions of units, varied greatly, a phenomenon called copy number variations, or CNVs.

The discovery means that the genes of any given individual are at least 10 to 12 percent different from those of another human. “This was really quite startling and shocking,” said Stephen Scherer. “So, it adds a whole new type of variation for us to consider in disease mapping studies.”

Each person inherits 46 chromosomes, 23 from each parent, which means that genes usually come in pairs. But not always, according to Scherer.

“What we found in this study of 270 individuals worldwide is that up to 10 percent of the genes are actually present varying from the general two that we usually see,” he said. “So, in some cases there’s one. In some cases there’s three. In some cases there’s five. In some cases, there’s actually none.”

The findings, published in Nature, are part of the international HapMap project to identify and catalog similarities and differences among human beings of different racial and ethnic groups. Investigators sampled the DNA of people in Africa, Asia and Europe. – voanews

Huh? How do you get five of one gene if you only get one copy from your mother and one from your father? Ah, here is a clue:

“Ectopic recombination between locally repeated DNA sequences is of fundamental importance in the evolution of gene families, generating copy-number variation in human DNA and often leadingto pathological rearrangements. Despite its importance, little is known about the dynamics and processes of these unequal crossovers and the degree to which meiotic recombination plays a role in instability” – pnas

So what is Ectopic recombination?

“Ectopic recombination refers to when recombination occurs out of register to produce deletions, duplications and other rearrangements. This occurs because there are many identical or very similar stretches of DNA sequence in chromosomes. ” – ergito

So, very similar patterns in DNA can cause confusion, perhaps when egg and sperm cells are formed during meiosis and/or during fertilization when the DNA from egg and sperm combine. Interestingly “Mechanisms that prevent ectopic recombination seem also to exist in mammals.” The natural random mixing of genes that does happen causes extra copies of some genes and increases the genetic variation between individual humans.

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