DNA unknown

In between the well-structured genes are large sections of DNA for which no function has yet been identified. These areas have been called ‘junk DNA’ or 'non-coding DNA' and make up a large proportion of the genomes of both plants and animals.

But is it junk at all?

We don’t really know. This DNA appears to act as a filler in between genes and a number of ideas are starting to emerge about what role it plays. This is a mystery to be solved in the next couple of decades.

Some of the ideas are:

  • it is where defective genes, or pseudogenes, are dumped
  • it is the accumulated DNA of viruses that have infected the body and failed to take over the cell
  • it acts as a protective buffer against genetic damage and harmful mutations, because the area is irrelevant to the metabolic and developmental processes (if a random change occurs in the sequence, there is no effect on the body)
  • it acts as a reservoir of sequences from which potentially advantageous new genes can emerge

Researchers believe that this unknown DNA probably plays some role in regulating the 'coding DNA' and therefore cellular processes. But there is currently very little knowledge about the relationship between non-coding DNA and the DNA of genes.

Onions contain 12 times more DNA per cell than humans. A pufferfish’s genome is only about one tenth the size of the human, yet seems to have about the same number of genes. The ratio of functional DNA to ‘in-between filler’ DNA of unknown function differs widely per species.

Chickens have a similar number of genes to humans: 20,000 to 23,000 for chickens and 25,000 to 30,000 for humans. But their genome is much smaller - they have 1 billion DNA bases, compared to about 3 billion in humans. The chicken genome appears to contain less repetitive non-coding DNA than the human genome.