Mouse genome to resolve gene count issue. Go Celera!
Reading the book of genes
DNA catalog draft close to completion
05/15/2000
By Sue Goetinck Ambrose / The Dallas Morning News
Scientists are swiftly reading through the complete book of human genes. It's a strange book, because scientists won't really know when they've reached the last page.
But they do know they are nearing the end.
The book of human genes, or genome, is a tough read. It's about 3 billion DNA letters long, and it contains some sentences that are practically illegible with today's technology.
Even though the genome may never be read down to the very last letter, it's still a gold mine for scientists.
"We mine it all the time," said Skip Garner, a researcher at the University of Texas Southwestern Medical Center at Dallas.
Scientists are reading the lengthy genome because it's one of the most interesting "books" on Earth. The human genome contains all the genetic information it takes to make a human.
The genome isn't much to look at on first glance - the alphabet of DNA contains only four letters: A, G, C and T. These letters are actually abbreviations for the four important chemicals that make up DNA. The precise sequence of the letters holds instructions to guide the development of a fertilized egg into a fully formed person. Every cell of the body (except for red blood cells) has its own copy of the genome tucked inside its chromosomes, a ready reference for any challenge that comes along.
When a stomach cell needs to divide to replace one that has died, the cell turns to the genome for guidance. When it's time for a developing embryo to grow a heart, the secret to doing it right is in the genome.
By knowing every letter of the genome, researchers will have their best picture yet of how the body operates. Scientists have already used passages of the genome to find the roots of a variety of inherited diseases.
Interesting passages are being found all over the genome because many pieces of the genome book are being read at the same time. With each read of a passage, geneticists get closer to knowing the precise order of DNA letters.
A group of government-funded scientists around the world are continually releasing the order of the DNA letters into databases available to anyone via the World Wide Web. In mid-June a "draft" of the full human genome will be done, researchers announced last week. A Maryland biotechnology company, Celera Genomics, is also reading the human genome and says it will make its reading of the entire genome freely accessible by the end of the year.
Having a draft in hand is equivalent to skimming a book or a newspaper that contains some spelling errors, said Eric Lander, a genome scientist at the Whitehead Institute for Biomedical Research in Cambridge, Mass.
"If you skimmed all the articles in The Dallas Morning News, you would have completed draft coverage," he said.
Skimming a newspaper - even if it has errors in it - can give you a good idea of what's going on in the world, even though some details might be missed.
For instance, if you read the words "Gorge W. Bush," you'd know right away that the article was referring to the presidential candidate.
Some mistakes, on the other hand, might lead to more confusion. If you read the words "George Bush." you might wonder if it's the father or the son. You might be able to figure it out from the rest of the sentence, but maybe not.
These mysteries ultimately will be solved, Dr. Lander said.
Some parts of the genome, such as chromosomes No. 21 and 22, are essentially finished. Eventually, the genome will have only 1 error for every 10,000 DNA letters - or about one typographical error for an article twice as long as this one. But some especially cryptic passages in the genome may never be read perfectly, at least with today's techniques.
Nevertheless, scientists say they will have the vast majority of the genome deciphered. And then they'll be able to answer a seemingly simple question: How many genes does it take to make a human?
Genes are special passages embedded in the genome amid stretches of DNA letters with no apparent purpose. Each individual gene serves as the instructions for building an important molecule - such as a protein molecule - that helps carry out cell functions. The simplest bacterium has only several hundred genes. Complicated organisms have more: A fruit fly has 13,000 genes and a tiny roundworm has about 19,000.
Estimates for the number of human genes change all the time. The latest guesses range from 40,000 to 140,000.
"I think we'll really know within a year," said Richard Gibbs, a genome scientist at the Baylor College of Medicine in Houston. By then, scientists will know the DNA letters in much more detail, and also know much more about the genome of humans' distant evolutionary cousin, the mouse.
The mouse genome is different from the human genome, of course, or people would look just like mice. But both mice and humans are mammals, and their genomes are known to have many similarities.
Even very rough readings of the mouse genome can be matched up with the human genome. The areas that match the best are likely to be genes.
"We did that and found genes that we couldn't find otherwise," Dr. Gibbs said.
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