Mythconception: We share half our DNA with bananas

This week Lewis Thomson has been going bananas over this slippery science…

Lewis - All life on Earth shares the same basic code: DNA. And because all living things on Earth share a common ancestor, the DNA code in different organisms is much more similar than you might expect. It’s often said that we share 50% of our DNA with bananas! 

But as similar as our DNA is, it’s not that similar. The only organisms you share 50% of your DNA with are your parents and your children. When you were conceived, half of your father’s DNA joined with half of your mother’s DNA to make your DNA.

So where does this banana statistic come from? Is it just complete nonsense? Well, no. We do in fact share about 50% of our genes with plants – including bananas.
So what’s the difference between sharing 50% of our DNA with something, and sharing 50% of our genes with something? Well, rather surprisingly, genes – the regions of DNA that code for proteins – only make up about 2% of your DNA. So sharing 50% of our genes with bananas means we only actually share 1% of our DNA with them – not 50. So what’s the rest of our DNA for if only 2% makes proteins? Well, about 8% of our DNA is made up of gene regulatory regions – these act like switches to control when and where the genes are turned on and off. But the remaining 90% of our DNA is thought to be mostly non-functional – it’s often called ‘junk DNA’.

Some of this junk DNA is what we call ‘dead genes’. These are regions of DNA which used to be functioning genes, but because of mutations in evolution they no longer make proteins. For example, humans – along with many other mammals - have a dead vitamin C gene. Whereas some animals can produce their own vitamin C, we have to get it from our diet by eating fruit! There are also regions called ‘jumping genes’. When a virus infects a cell, it injects its own DNA into that cell, and this DNA replicates itself as much as it can. When this happens in sperm cells or egg cells, the offspring will have that virus DNA integrated into its own. Because viruses are everywhere, this has happened a lot in evolution – and so about 50% of our DNA is made up of these jumping genes. It’s like we have millions of sleeping viruses inside us! They’re called jumping genes because they can replicate and jump around, inserting themselves into random places. Thankfully, we have lots of genes dedicated to stopping these sleeping viruses from waking up, so they don’t do this.

But let’s get back to bananas. Even though genes only make up 2% of our DNA, it’s still surprising that half of the genes we have are also found in bananas. But animals and plants share a common ancestor – a single-celled life form which probably lived about 1.6 billion years ago. The genes that we share with bananas would have been present in that ancestor, and have been passed down to all animals and plants alive today. And the reason that we’ve kept these genes, is that they’re involved in fundamental cell processes – like making energy and repairing damage. Just like that single-celled ancestor, and our banana relatives, we need these processes to survive – and so we share half of our genes, but not half of our DNA with bananas.