DNA molecule winding out of a chromosome. Uncoiled and stretched out, all the DNA packed into one of your cells would measure about 6 feet long!
Illustration courtesy of National Human Genome Research Institute/Darryl Leja
With the help of new techniques and powerful computers, scientists have finally pieced together in order the entire human genome. This means that they have strung together in the correct order all three billion (that’s 3,000,000,000) or so biochemical rungs of our spiral ladder-shaped DNA molecule. What we now have is the entire book of life for making a human being.
This is a hugely big deal! Why? Well, much of what happens in our bodies is the result of molecules called proteins doing their thing. And proteins are made from recipes called genes that are contained in our DNA. (The sum total of all the genes in a living creature is called its genome <gee-nome>.) By having all the genes spelled out in the right order, researchers will now have an easier time figuring out which genes make what proteins. This in turn will help in figuring out which genes are responsible for or have an affect on different diseases when they get messed up. That may lead to better ways of tackling some diseases. Also, knowing the human genome sequence may help scientists figure out just what makes humans "human."
The news articles discuss the highlights of what scientists have learned so far from looking at the human book of life. Among the findings and surprises are that little of our DNA is actually genes that make us "us" and that microbes have had a big impact on our genomes.
As the news articles note, very little of our total DNA—only about 5%—spells out genes. Scientists had expected to find lots of genes in our genome, as many as 100,000. They now estimate that there may be only about 30,000 genes that provide the blueprints for humans. That’s less than three times the number of genes a simple, itty-bitty fruit fly has. It’s just a few hundred more than mice have.
What gives? Aren’t we humans amazingly more complex than mice or flies? Then why don’t we have lots more genes? This article helps explain some of the answers:
Representation of a fragmented gene. The parts labeled "exon" are the parts needed to code for a protein. The parts labeled "intron" are the non-gene sequences breaking up the gene. Note: most genes are much, much longer than shown in this simple illustration.
Illustration courtesy of National Human Genome Research Institute/Darryl Leja
In short, we might have more genes. Genes in creatures more complex than bacteria are trickier for scientists to find because in many cases the genes don’t just start at a particular point on the DNA ladder and then go on without break until they stop at a specific point. Instead, many genes of higher creatures are broken up into several pieces separated by strings of non-gene DNA. It’s not only like finding a needle in a haystack, but finding a needle that’s been broken into a dozen different pieces scattered about in the hay. And while the segments that make up the genes of creatures such as fruit flies or worms tend to be all about the same length, the gene fragments in humans vary wildly in length.
But then again, we might not have more than 30,000 or so genes. The fragmentation of human genes may explain why we don’t need to have a whole lot more separate genes. While in simple creatures, like fruit flies, each gene tends to code for just one protein, many human genes can code for two, five or more proteins by combining their pieces-parts in different ways. Our genes are flexible. With just a few more genes than mice or fruit flies, we can make many, many more proteins.
The news articles I’ve listed here also talk about the effects microbes have had on our genomes over the years. For one thing, the genome decoders reported that about 200 of our 30,000 genes appear to have been pretty much cribbed directly from bacteria that infected our proto-cells millions of years ago. That was pretty amazing news. In fact, some scientists thought that this sounded like way too high a number. It would seem to suggest that direct transfer of DNA from bacteria to mammalian cells is simpler and more common than what we see happening around us. Now, more recent reports say that the majority of these 200 genes didn't come to humans directly from bacteria at all. Rather, these genes were picked up by other simpler creatures that were early ancestors of humans and eventually then came to be part of human DNA.
Ok, so what about the fact that if we only have 30,000 genes, that leaves a whole lot of our genome full of non-gene stuff that doesn’t spell out any kind of recipes for useful proteins? Why are our genomes full of so much junk? And what is all that stuff?
A significant amount of that "junk DNA" turns out to be genetic material from early viruses and other parasites that infected our proto-cells millions of years ago, inserted themselves into our evolving DNA and stayed. As the "Journey into the Genome" article says, "The genome is a museum of the viral infections suffered by humanity and its ancestors. Viruses made us what we are."
Some of these non-gene DNA pieces in us don’t seem to do anything at all except get copied along with all the rest of the DNA when one of our cells divides. But other pieces may well affect nearby genes and how they work. Some of these non-gene elements move around through the genome, "jumping" from here to there. They can cause disease if they jump into a human gene and mess its sequence up. Muscular dystrophy <mus-que-ler dis-trophy> and haemophilia <hee-moe-phee-lee-uh> are two diseases suspected of being caused by these mobile elements. But mobile elements may have some benefits for their hosts, too. Scientists are still trying to figure all that out.
So when you think about what makes you "you," the answer may not be as simple as you thought. Not only do you need to consider that a whole bunch of the cells making up your body are actually microbial cells, such as the bacteria living in your guts, but now you also have to think about the fact that only about 5% of your DNA is human genes. Does that mean a lot of you isn’t actually "you" after all? It's an interesting thought, isn’t it?
|
 
|
