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Not to get too up-close and personal here, but right now, there are 3 billion pairs of DNA molecules inside each of your cells. That’s how many base pairs make up the human genome.

Together, they code for every single thing that makes us who we are, as members of our species, and as individuals. If you think of each pair of molecules as a byte of computer memory, it’s a little like fitting an entire person on less hard drive space than you’d need for an HD movie. Turns out 3 billion base pairs is actually a pretty typical genome size for a eukaryote, an organism with a nucleus in its cells.

But there are a few plants and animals with genomes so huge they completely blow this number out of the water. And we’re not totally sure why. So far, we’ve found 10 species with genomes of over 100 billion base pairs, including a few lungfish, salamanders, flowering plants, and a fern.

Based on the giant genomes we know about, it looks like there’s an upper size limit of around 150 billion base pairs, and for good reason. Beyond that, there are just too many problems involved in maintaining all that

DNA: the energy cost of making it, the need for really big cells to store it in, and the fact that cell duplication takes a really long time. But even 100 billion base pairs takes an enormous amount of maintenance. And we’re still figuring out exactly how and why these species ended up with so much DNA. You might think a larger genome would allow for a more complex organism.

I mean, that’s usually how it works with computer data. But the organisms with the biggest genomes aren’t especially complex. As cool as it is that lungfish have both lungs and gills, they don’t need a genome 33 times the size of ours to code for that.

In fact, it seems like more DNA doesn’t mean more unique genes. Sequencing an entire giant genome is really hard, so we haven’t been able to do a lot of analysis so far. But researchers have found that huge genomes contain a lot of repetitive sequences and non-coding DNA.

One way genomes balloon in size is polyploidy, the accidental duplication of an organism’s entire set of chromosomes during reproduction. Most organisms, us included, are diploid, meaning we have two sets of chromosomes. Duplication beyond this usually messes up animal genomes beyond repair.

But plants that end up with extra copies of all of their chromosomes can often manage just fine. They can carry right on with the mutation, pass it on to their offspring, and continue to thrive. Maybe because their structures are more flexible than those of animals.

An extra branch here or there isn’t the catastrophe that an extra arm could be. Some of these plants with especially giant genomes are actually octoploid. That’s eight sets of chromosomes!

Even though polyploidy isn’t really a thing in animals, our genomes do have sequences called transposable elements that frequently duplicate themselves and jump from one spot in the genome to another. Animals with huge genomes, like those weird salamanders and lungfish, have probably built up a lot of sequences like this over time. They eventually get deactivated if they’re not being used, but they still sit there and add to the total genome length.

Usually, genome expansion is balanced out by other forces that encourage downsizing, like when a chunk of DNA is swapped out for a shorter segment from another chromosome. In fact, even though a tiny minority of flowering plants have enormous genomes, some scientists think that pruning the size of their genomes is what allowed flowering plants to take over the world. It made their cells smaller, which let them pack more equipment for photosynthesis into their leaves and outcompete other types of plants.

So, rather than giving them an advantage, maybe these few species have unusually enormous genomes because they just didn’t have any particular reason to prune them. For example, one group of plants that tends toward larger genomes, the geophytes, store up energy in bulbs or tubers. Think of a sweet potato or an onion, or a flower that grows from a bulb, like a daffodil.

Those energy reserves might be what enables them to synthesize lots of DNA without too much trouble. As for what the deal is with those salamanders and lungfish… well, scientists are still working on it. We know some of the how, they build up DNA faster than they lose it, and have lots of the kind of sequences that jump around and copy themselves, but we’re still figuring out the why.

Some researchers think salamanders built up a larger genome early on in their evolution, and some of that DNA was later incorporated into the genetic code for their more unique traits, like the ability to regrow limbs. Which means it may have been worth it for them to keep their larger genomes around, despite the energy cost. But we have no idea if this is actually what happened.

Clearly, we still have a lot to learn about the species that have ended up with more than their fair share of DNA. But one thing’s for sure: when it comes to genomes, bigger isn’t necessarily better, or more complex. So, the next time you see an innocent-looking fern or an onion, just remember: they’re bigger, and a lot weirder, on the inside.

And speaking of coding for every single thing that makes a person a person or an onion an onion, you want to learn more about coding? Right now Skillshare is offering SciShow viewers two months of unlimited access to over 20,000 classes for free. And you can take classes like Creative

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Aga explains everything you need to code for your own SVG animations. We like partnering with Skillshare because they’re all about making learning fun and engaging and Aga’s teaching style really embodies that. We’ll link to this class and the offer for two free months of Skillshare in the description.

Check it out and if you like it, she has a longer, more in depth class on coding your own website. Thank you. [♪ OUTRO].