Less DNA in my genome than my lunch?

The human genome contains billions of letters of DNA, but some plants and animals have billions more. The surprising difference in genome length across different species is perfectly captured by the findings of 'the onion test'. In collaboration with the Genetics Society, we've produced an infographic to highlight the scale of junk DNA.

  • burger labelled with the amount of DNA in each ingredient
    Credit: Ant Lewis

DNA is the genetic code that contains all the instructions our bodies need to grow. But how much DNA is in a person? What about in other animals, or plants? You might think that more complex organisms, like us, might need more DNA than simpler species, like plants, but that’s not always the case.

We've produced an infographic to highlight the curious nature of genomes.

The Onion Test

Consider a burger. Surely there’s more DNA in a cow (beef) or pig (bacon) than in something like lettuce, onions, or the wheat used to make bread? Not quite – this is what a burger would look like if its ingredients were sized according to how much DNA was originally in them:

Animation of a burger showing how the relative size of the ingredients would change if size was determined by the length of the genome for each ingredient

Credit: Ant Lewis

DNA is made up of long sequences of paired molecules, known as base pairs. Wheat (14.5 billion base pairs) and lettuce (3.2 billion) have longer genomes than pigs (3 billion) or cows (2.6 billion). Most surprising of all, an onion has a whopping 17.5 billion base pairs inside every cell. 

Why would an onion need 6 times more DNA than a cow? Only a fraction of the genome is actual genes – sections that directly code for proteins – and this size imbalance, known as ‘the onion test’ has been said to illustrate that much of the rest of the sequence must be unused junk. Whatever function this ‘non-coding’ DNA might have, why would an onion need so much more than a human or a cow?

Non-coding DNA: junk or func? 

The function of this ‘junk’ DNA is a hotly debated topic. There are some ideas about specific functions for non-coding DNA sequences. These include making microRNAs, little pieces of a DNA-like molecules that can regulate gene activity, and sequences that help with the process of copying the cell's DNA. However, some scientists believe that much of this non-coding DNA does not serve a specific purpose at all and is just a product of mutations which have arisen in the genome over time. 

It is hard to think of a logical reason why an onion, which is a much simpler organism than a human, would need more functional DNA. The need to find functions for these mysterious non-coding DNA sequences may just be driven by our desire to believe that humans are somehow special or unique compared to the rest of the biological world – or at least compared to the contents of our lunch. 

DNA sequence length across different organisms

Onions aren’t entirely unusual in having a very large genome. Birds, fish and mammals vary significantly in the number of base pairs that make up their DNA. Some, like the marbled lungfish, have genomes many times longer than the onion. But remember – total DNA length doesn’t necessarily correlate with the number of actual genes. The small pufferfish genome contains a similar number of genes to the cat’s much longer DNA sequence.

Graph showing total genome length for different organisms. The marbled lungfish has the longest genome, and the pufferfish has the smallest

Credit: Ant Lewis

You can learn more about non-coding DNA and the implications of the onion test by tuning into the Genetics Society podcast with Kat Arney. This infographic and blog post were created in collaboration with The Genetics Society to celebrate their 100 year anniversary. More exciting genetics content from this collaboration wlll be available over the coming weeks.

This is a re-upload of the blog post that was originally uploaded on the 12th February 2020. This re-upload corrects an error from the original blog post which suggested that the genome length of a yeast is 12.1 billion base pairs, rather than the actual figure of 12.1 million base pairs. 

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