Is it true that active genes make up only 3% of the DNA in our chromosomes? And if so, what does the other 97% do?

14 July 2012

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Question

Is it true that active genes make up only 3% of the DNA in our chromosomes? And if so, what does the other 97% do?

Answer

Robin:: Only about 1.5% of the human genome are regions that encode proteins and the rest of DNA, all the remaining 98.5%, is non-coding. Genes come in two sorts. The first sort are indeed those that direct the synthesis of protein and the second sort of gene are those that don't make a protein, but have some regulatory function as an RNA molecule. So these are non-coding RNAs and there's thousands of them in our genome. Now, all these genes whether protein coding or not, need to be controlled, switched on or off in the jargon, and that's done via non-coding DNA. The major controlling stretch is usually just before the start, just upstream of the gene itself and that's called a promoter. And then there are sequence motifs that are much more distant from the coding region and those are called enhancers. But even allowing for RNA molecules that don't make protein still leaves an awful lot of unemployed DNA. Some of that comes as telomeres. These are caps that sit on the end of each chromosome. They're repetitive stretches of DNA and essentially, they protect the end of the chromosome from deterioration or indeed from fusion with neighbouring chromosomes.

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