Junk DNA has traditionally been defined by what it isn't. It's the bits of the genome that don't code for the strings of amino acids that form proteins. It was dismissed for a long time as having no function. Which is a pretty classic example of the surprisingly common phenomenon in biology where, if we don't know something, we assume there is nothing to know. Whoops.
How Much Junk DNA Is There In The Human Genome?
Lots. It forms about 98% of all the DNA in every cell. That's a lot of material to maintain if it has no function, don't you think?
What Does Junk DNA Do?
A lot of things. It maintains the integrity of our chromosomes; regulates the ways the protein-coding genes are expressed; influences how we age and generally introduces incredible degrees of subtlety and flexibility into how we use the relatively small numbers of genes that code for proteins.
Does Any Of This Matter?
Yes indeed. Junk DNA contributes to all sorts of situations, from the correct control of gene expression in female cells to the regulation of pathways that drive cancer. From Ernest Hemingway's mutant cat to exoneration of the innocent through DNA fingerprinting, junk DNA impacts on an astonishing range of biological phenomena.
Can I Learn More?
As luck would have it, my new book is coming out in the UK and USA on March 5th 2015. It's called Junk DNA: A Journey Through The Dark Matter Of The Genome. Go on, you know you want to.