How much DNA do you have in your body? A DNA base pair has a length of a little less than one nanometer (a billionth of a meter). There are approximately three billion DNA base pairs in the human genome, and since we are diploid organisms, each cell actually contains six billion base pairs. Therefore, if you took all the DNA in a human cell and stretched it out, it would reach a few meters. Now, there are about 10 trillion cells in the human body. So, all the DNA in your body, if laid end to end, would reach from the Earth to the Sun a hundred times! Yes, you read that correctly. Yes, that is a long freaking way!
Now, how much DNA does a salamander have?
Let’s take the mudpuppy, Necturus maculosus. Mudpuppies are aquatic salamanders from eastern North America that retain their external gills throughout life. I’ve caught them in Cayuga Lake in upstate New York by wading through the shallows near shore at night while shining a flashlight down into the water. Mudpuppies grow to be over 40 centimeters long. These little guys have about 30 times as much DNA in every cell as a person has: about 180 billion DNA base pairs. I don’t really know how many cells a mudpuppy has, but I estimate that they weigh about a pound. Cells with large genomes tend to be larger, so so the number of cells per pound is probably less in a mudpuppy than in a human. Given their weight and cell density, let’s say that a mudpuppy has about 1/300 as many cells as an adult human. This is a very rough estimate, but it will do for our purposes. So, if you stretched out this salamander’s entire DNA library, it would also reach to the sun multiple times, but probably less than your DNA library (more like 10 times).
Let’s try another salamander example. The Japanese giant salamander, Andrias japonicus, can weigh over 100 pounds. It has about half the DNA per cell that a mudpuppy has: “only” about 90 billion DNA base pairs. But since it’s so large, and since its cells are intermediate in size between those of humans and those of mudpuppys, a giant salamander probably has nearly as many cells as an adult human. If so, the sum total of this critter’s DNA would reach from the Earth to the sun more times than your DNA would… maybe as many as 1000 times!
Why do we have so much DNA? And why do salamanders have bigger genomes than we do? After all, humans appear more complex than salamanders, so it would seem that we would need more genes… but even humans shouldn’t need genomes so large that they can be measured in astromical units. For years, biologists were puzzled by this conundrum, called the “C-value paradox.” The solution seems to be that most of the genome is junk. Both salamanders and humans have lots of DNA, but most of that DNA doesn’t actually do anything, and you could build a highly organized animal using much less DNA. Of course, this still doesn’t explain why the junk DNA exists at all. This question is not yet entirely resolved. One idea is that junk DNA is very slightly deleterious; in species with large population sizes, natural selection is able to weed out the junk, but in species with small population sizes, natural selection isn’t strong enough to stop junk from accumulating. Both salamanders and primates have relatively small effective population sizes compared to, say, most species of bacteria. Junk DNA has built up in vertebrates over millions of years, and perhaps salamanders have just ended up with more of it due to chance.