# How much DNA do humans have?

A normal human body has approximately 3 billion base pairs. Given that humans have 10 trillion cells, how is that the number of base pairs of DNA is less than the number of cells? It can happen only if most of these cells don't have DNA, or have I misunderstand the topic?

Can you give an account of why the number of cells exceeds the number of DNA base pairs significantly? Isn't it logical to have more DNA base pairs than the number of cells?

• What research did you do before posting this question? Your conclusion was clearly absurd, so an attempt to identify the flaw in your argument was called for. I am voting to close this question as extremely low quality. Commented Jul 23, 2017 at 8:46
• No, number of base pairs is not at all related to number of cells. The correct statement should be 'a normal human body has approximatrly 3 billion base pairs of DNA in every cell'. I suggest you keep studying. Commented Jul 23, 2017 at 10:39
• 1 base would code for one cell? That would make for either very simple cells, or very information dense DNA.
– AliceD
Commented Aug 5, 2017 at 14:09

How much DNA is there in our cell nuclei?

In humans, there are about 6 billion base pairs in the nucleus of each cell. Why 6 billions and not 3? There are 3 billions base pairs per haploid genome (see ploidy) and therefore 6 billions base pairs for the whole genome. This number may actually further double during specific phases of the cell cycle. Each single cell contains a copy of the whole genome (with a few exception such as red blood cells for example).

So the total number of base pairs in a 10 trillion cells individuals is about $$6\cdot 10^9 \cdot 10^{13} = 6\cdot 10^{22}$$ base pairs.

# Further information for fun

How much DNA in our cells outside of nuclei

Now, in cells there is DNA outside of nuclei. In animals (incl. humans), we have DNA in mitochondria as well. there is of the order of 104104 base pairs per mitochondria (16,569 sites to be accurate) and there are of the order of 104104 mitochondria per cell, that is 108108 base pairs total per cell which is only one order of magnitude below the number of base pairs in the nucleus per cell.

There is therefore an extra $$3\cdot 10^8 \cdot 10^{13} = 3\cdot 10^{21}$$ base pairs in cells outside the nucleus.

How much DNA in bacteria living in the human body?

Note that by "in the human body", I also include "in the gut" even though the inside of the guts are typically considered as being the outside our body.

There are about 100 trillions of bacteria in every one of us (this number is a vague approximation; see wiki > Human microbiota). Yes, most of the cells living in us are not our cells but are bacterial cells!

Bacterial genome vary widely but as a gross and almost insulting approximation, let's consider that the average bacterial genome from our microbiome has a genome size of $$10^6$$ base pairs (note bacteria don't have nuclei or mitochondria).

That means that in our body we have to add to the above numbers $$10^6 \cdot 10^{14} = 10^{20}$$ base pairs (which is only a hundredth of $$10^{22}$$).

How much DNA in the viruses in our body?

Let's not forget our virome. Unfortunately, I could not find estimates of the number of viruses (and viroids) in our body.

How much DNA is outside cells and viruses in our body?

Of course, there's plenty of DNA in our body. There's DNA in everything we eat and this DNA end up in our blood stream and in our cells. I failed to find an estimate of how much DNA does that represent.

• The addition of mitochondrial base pairs would not even show up on that measurement anyway, mitochondria only have a few genes.
– John
Commented Jul 23, 2017 at 1:20
• @John Actually, there is of the order of $10^4$ base pairs per mitochondria (16,569 sites to be accurate) and there are of the order of $10^4$ mitochondria per cell, that is $10^8$ base pairs total per cell which is almost of the same order of magnitude than the number of base pairs in the nucleus. So... it may show up on these calculations. It would show up for sur if we consider an extra significant digit. Commented Jul 23, 2017 at 1:29