Lorry collecting rubbish or 'junk'

More than genes

The genome is far from just a collection of genes

Genes take up only a small part of the genome: protein-coding sequences account for only around 1.5 per cent of human DNA.

Biologists used to regard the other regions, which don’t code for proteins, as ‘junk’. Now, though, these regions are slowly yielding their secrets. Some have regulatory roles and determine the activities of the protein-coding sections. Others are probably parasitic bits and pieces that are along for the evolutionary ride.

At least 10 per cent, and perhaps a lot more, of the whole genome is transcribed into RNA at some time. Non-coding RNA sequences are often short-lived, and some are much smaller than the messenger RNAs (mRNAs) that are translated to make proteins. The function of non-coding RNA is still being teased out, but its versatility is giving it a starring role in the workings of the cell. Small RNAs that bind to mRNA or DNA can, for example, tag mRNA for disposal or stop a protein being made.

Scientists are still working to understand how much of the genome is functional – but there are different ways of defining ‘functional’. For example, in 2012, scientists working on the US-led Encyclopedia of DNA Element (ENCODE) project published papers claiming that more than 80 per cent of the genome is “functional”. However, there is debate among scientists about how this project differentiated between activities that are biologically important and useful for humans and those that are not.

By contrast, a study published in 2014 found that just 8 per cent of the DNA in humans is functional. In this study, functional DNA was defined as DNA that has evolved unexpectedly slowly and is therefore thought to have an important role in the body. However, other scientists say that, for DNA, being functional is more than just being conserved evolutionarily, arguing that some non-conserved parts of the genome have a role in certain diseases.

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About this resource

This resource was first published in ‘Genes, Genomes and Health’ in January 2010 and reviewed and updated in December 2014.

Genetics and genomics
Genes, Genomes and Health
Education levels:
16–19, Continuing professional development