Re: What is the percentage of nucleic acids in the human body?

hi Shanera,

This is one of those questions to which there is no accurate answer. However, as we are dealing with a very large number (cells in the human body) and a very small number (mass of nucleic acid per cell), accuracy is not really an issue and we can make a fun guess!

It's estimated that there are 10¹⁴ (100 000 billion) cells in the human body. I assume this number is derived from the volume and estimated density of an average cell and the mass of an average person once you take away the non-cellular stuff, like bone. Now, you probably know that cells contain a nucleus, in which the DNA is packaged into chromosomes and that the size of the human genome is about 3x10⁹ base pairs. So, knowing that the average molecular weight of a base pair is 660 Da and remembering Avogadro's number, we can construct this formula for "weight of a genome":

m = n[1 mole/6x10²³ (bp)][660 (g)/mole] = n[1.096x10^-21 (g/bp)]

where m = mass in grams, n = genome size in bp. Don't worry about that too much - the figure for humans comes out at about 3.3x10^-12 grams (3.3 picograms) per haploid genome.

Hang on though - somatic cells are diploid, containing 2 copies of each chromosome. So we have 6.6 picograms of DNA per cell. What about gametes - sperm and ova? Well, they are haploid, but we can ignore them. A male may contain several hundred million sperm cells at any one time, but really a hundred million is pretty small compared with 100 000 billion cells in the body, so let's make life easy and pretend all the cells are somatic.

RNA is altogether trickier. Cells contain ribosomal, transfer, messenger and other small RNA molecules and the amount that they contain depends on how actively they are transcribing genes. This will vary depending on the cell type, age, metabolic rate, disease state - all kinds of things. Some genes are transcribed into hundreds or thousands of RNA copies, some into only a few copies. In short, the amount of RNA in a cell changes all the time. So let's make life extremely easy and pretend there's about the same mass of RNA as DNA in a cell.

OK, that gives us 13.2 picograms nucleic acid in a cell x 10¹⁴ cells in a body - I make that around about 1 kilogram. Average mass of an adult human? A contentious issue - let's call it 70-90 kg - so our final guess is that humans are about 1 % nucleic acids by mass.

I hope this has been helpful for you,

Moderator's note:
FYI - of those 10¹⁴ cells in the body, only 10% are really human cells, the rest beloning to the bacteria and other organisms that inhabit your gut or other bodily surfaces exposed to the environment.