Scientists have determined the DNA sequence of the entire human genome (a fact you may already know from the various news stories that accompanied the completion of the Human Genome Project). So it may come as a surprise that scientists still don't know exactly how many genes humans have. Are they lazy or just unmotivated? As it turns out, neither.
It's sometimes tricky to tell exactly when a sequence at some particular locus in human genome contains instructions used for making a person and when it contains junk. And because the human genome is more than 95 percent junk, small variations in how scientists go about deciding what is a gene and what isn't can make a big difference in the total number of genes they think humans have.
As researchers keep refining their techniques for identifying genes in this big sea of DNA, they end up revising their numbers progressively downward. Currently, they think they have a good handle on the situation and are reasonably certain that the human genome contains about 25,000 genes. Is that a lot? "A lot" is a relative thing. Common intestinal bacteria have about 5,000 genes; yeast has about 6,000; the common laboratory roundworm has around 18,000; and the fruit fly Drosophila has 14,000. And many plants seem to have as many genes as human do, and some have far more.
What's up with all this junk?
The human genome is not the only one that scientists have sequenced in its entirety. Initial genome sequencing projects concentrated on smaller creatures, such as bacteria, and it turns out that very little of those genomes is junk. Although researchers can't say with certainty why humans have so much junk DNA, it's not hard for them to come up with reasons why smaller, rapidly dividing organisms do not. For example:
it They're little, so all that junk DNA won't fit.
I They're in a hurry, because competing bacteria are sucking up nutrients and reproducing as fast as they can. They don't have time to replicate an enormous genome that's mostly junk without falling behind and being overrun by all the other bacteria.
Humans, however, are not in such a hurry all the time, and the energy it takes to copy human DNA is a very small part of our total energy budget.
When scientists look at a sequence of DNA and identify genes, they have techniques for determining that a particular piece of DNA makes something and can use the genetic code to determine the amino-acid sequence of the protein that piece of DNA makes. But they still don't have a very good way of looking at a protein sequence and figuring out what the protein actually does. If the protein looks like something else whose function scientists understand, they have some clues. But if the protein isn't similar to something scientists already know about, they're often in the dark about what particular genes do. Thus, researchers don't yet understand exactly why organisms have the numbers of genes that they have.
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