Whose genome is it anyway?
The public Human Genome Project took blood and sperm samples from a dozen
anonymous donors and combined them. The project鈥檚 commercial rival, Celera
Genomics, put an ad in The Washington Post, selected 30 men and women
from a variety of ethnic backgrounds and used six of them. When reporters
recently asked Celera boss Craig Venter if his own DNA had been added to the
mix, he pointedly ignored the question.
What are all these meaningless strings of letters, such as ACTGTTACCG?
The strings show the order of the four different molecular
bases鈥攁denine (A), cytosine (C), thymine (T) and guanine (G)鈥攁long a
strand of DNA. The human genome is 3.2 billion letters long. As much as 97 per
cent of this is 鈥渏unk鈥�.
What is a gene and how many do we have?
A gene is a stretch of DNA that tells a cell how to make a protein. Each
amino acid is coded for by a three-letter sequence. The cell machinery reads
this code, grabs the appropriate amino acids, and strings them together to make
a protein. Most researchers believe we have around 100 000 genes.
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What is a chromosome?
The human genome comes in 23 pieces, packaged in proteins to form
chromosomes. Our cells have two copies of each chromosome鈥攐ne from each
parent鈥攎aking 46 in all. The 23rd pair consists of the sex
chromosomes.
How do you sequence the genome?
Researchers take a short piece of a chromosome and make thousands of
incomplete copies鈥攁ll starting from one end. They attach a fluorescent
label to the other end, using different colours for different letters. These
fragments are separated according to size, allowing the sequence to be read out.
Then the short sequences are strung together to give the chromosome鈥檚 sequence.
Sequencing machines have vastly accelerated the pace: the yeast genome took ten
years to decipher, but modern machines could do it in a day.
What is the 鈥渟hotgun鈥� sequencing technique?
This is the quick technique that Celera Genomics is using. Instead of
sequencing adjacent segments of chromosomes, an entire genome is chopped into
little pieces and sequenced. Then, like an intricate puzzle, the random bits are
put together by computers that look for overlapping fragments. Earlier this
year, Venter showed the technique works even for large genomes, by sequencing
the fruit fly genome.
What is a mutation?
A mutation is a misspelling in the code for a protein that can be passed from
one generation to the next鈥攁n A instead of a G, say. The result can be a
disease, such as cystic fibrosis.
What if the genome were a book?
It has been estimated that it would take over nine years to read aloud. If
written out, the genome would fill about 200 volumes, each the size of a
telephone directory. Happy reading.