How long would the DNA be from a single human if all of the molecules were lined up end to end?
Our genome is a little over three billion base pairs (the A,T,G,C sequence of molecular “letters” in our DNA) long (IHGSC 2004). We have two copies per cell, so that puts us at over six billion base pairs of DNA sequence. One complication is the XX versus XY difference between females and males. Women have slightly more DNA than men because the Y chromosome is shorter. This works out to an average of 6.32 billion base pairs per human in our cell’s nucleus (Piovesan et al. 2019). (Of course, all of our chromosomes are of slightly different lengths among people because of natural variation, but this is a minimal effect in our calculation.)
We also have mitochondria outside of our cell’s nucleus with their own 16,569 bp chromosome (Anderson et al. 1981). This is tiny compared to our nuclear genome. However, there are 100s to 1000s of mitochondria per cell, and each contains from one to 10 copies of the mitochondrial chromosome (Cole 2016). These numbers vary widely between cells and over time. Let’s take 
as a rough working estimate of the number of mitochondrial chromosomes per cell. This comes out to 
additional base pairs per cell, which brings our average up slightly to approximately 6.36 billion base pairs per cell.
There are a lot of bacteria (and other things such as fungi and viruses) living on and in our bodies that we have coevolved with, and they have a direct effect on our health in both positive and negative ways. The mitochondria originated as a symbiotic bacteria that has now been incorporated into our cells. Here I am not including these additional organisms, our microbiome, because it is highly variable, there are a lot of unknowns, and we generally don’t feel like they are a part of us in terms of DNA sequence.
Next is the length of a base pair in a DNA strand, which is about 3.4 Å (an Ångstron is 
meters, about the width of a hydrogen atom). This is
![\[3.4 \times 10^{-10} \times 6.36 \times 10^9 \approx 2.2 \mbox{m}\]](https://floydsplace.us/wpress/wp-content/ql-cache/quicklatex.com-35377a8615d5ec563bba7a19b8efe928_l3.png "Rendered by QuickLaTeX.com")
or 7 feet 2 inches of DNA per cell. This is longer than the height or maximum arm span of the average human.
It is amazing that a cell is able to pack all of that DNA into such as small space and still function. The longest chromosome, our Chromosome 1, is about 250 million base pairs of DNA long (Piovesan et al. 2019). That’s a single molecule that is over eight centimeters or over three inches long!
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That’s just for one cell. How long would the DNA be for all of your cells? How many cells that are in an average human body is hard to answer. They vary widely in size among different tissues and this is a tricky question to answer. However, a reasonable estimate is about 
cells (30 trillion with a “t”), which incidentally is about the same as the number of bacterial cells in our microbiome (Sender et al. 2016). However, approximately 70% of these are red blood cells, which neither have a nucleus with large chromosomes nor mitochondria, so we need to reduce this number to 30% of the total to estimate the number of DNA-containing cells. (A further complication is that some of our cells, such as some in the liver or heart muscle, have more than two copies of each chromosome, but this is the minority of cells, so we will ignore it in this calculation.) So,
![\[0.3 \times 3.0 \times 10^{13} \times 2.2 \mbox{m} \approx 2 \times 10^{13} \mbox{m.}\]](https://floydsplace.us/wpress/wp-content/ql-cache/quicklatex.com-694656c80293c36f9f0d669ca71582f7_l3.png "Rendered by QuickLaTeX.com")
or 20,000,000,000 km!
kilometers, or over 12 billion miles, is a longer distance than we are used to in our daily lives. The Earth’s circumference is a little over 40,000 km. Your DNA would wrap around the Earth 500,000 times. The Moon is 384,400 km away. Your DNA would go to the Moon and back 26,000 times. The Sun is 150,000,000 km away. Your DNA would go to the Sun and back 67 times. However, we don’t want to fold and bend the DNA; we want to stretch it out in a line to see how long it is.
km. On the right side are the closest neighboring stars a few lightyears away.The diameter of Neptune’s orbit (the planetary solar system) is nine billion kilometers, and your DNA could easily cover twice that distance. The Heliopause, which defines the Heliosphere “bubble” around the Sun, the boundary between the Solar System and interstellar space, is about 17 billion kilometers from the Sun (on its closest edge). Your DNA should be able to just reach that distance to interstellar space. That’s a long way!
That’s only for one person. Recently we passed eight billion people on Earth. At this point, we are moving to a scale of millions of light years, which would encompass not only our own galaxy but several of our neighboring galaxies, not just interstellar but intergalactic distances! And then there are all of the other species on earth with a huge range of genome sizes and numbers of individuals. Conservative estimates would put it well beyond the diameter of the observable Universe, over 90 billion light years. I honestly don’t know how even to attempt to estimate this more precisely. Landenmark et al. (2015) have tried and came up with 
base pairs of DNA in Earth’s biosphere with more than 
operations per second in terms of DNA information processing speed. Numbers this large are hard to think about intuitively. For example, there are only about 
protons in the observable Universe (known as the Eddington number), and since most atoms are hydrogen with one proton, this is close to the number of atoms in the observable Universe. If we view DNA as a way to encode information, it is clear that there is a tremendous amount of biological information on the surface of this planet.
References
- Anderson, S., Bankier, A. T., Barrell, B. G., de Bruijn, M. H., Coulson, A. R., Drouin, J., … & Young, I. G. (1981). Sequence and organization of the human mitochondrial genome. Nature, 290(5806), 457-465. https://www.nature.com/articles/290457a0
- Cole, L. W. (2016). The evolution of per-cell organelle number. Frontiers in Cell and Developmental Biology, 4, 85. https://www.frontiersin.org/articles/10.3389/fcell.2016.00085/full
- IHGSC (International Human Genome Sequencing Consortium). (2004). Finishing the euchromatic sequence of the human genome. Nature, 431(7011), 931-945. https://www.nature.com/articles/nature03001
- Landenmark, H. K., Forgan, D. H., & Cockell, C. S. (2015). An estimate of the total DNA in the biosphere. PLoS Biology, 13(6), e1002168. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002168
- Piovesan, A., Pelleri, M. C., Antonaros, F., Strippoli, P., Caracausi, M., & Vitale, L. (2019). On the length, weight and GC content of the human genome. BMC Research Notes, 12(1), 1-7. https://bmcresnotes.biomedcentral.com/articles/10.1186/s13104-019-4137-z
- Sender, R., Fuchs, S., & Milo, R. (2016). Revised estimates for the number of human and bacteria cells in the body. PLoS Biology, 14(8), e1002533. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002533
- Wikipedia article on the Eddington number, https://en.wikipedia.org/wiki/Eddington_number
Media
- Structure of DNA, https://commons.wikimedia.org/wiki/File:DNA-structure-and-bases.png
- Human chromosomes, https://en.wikipedia.org/wiki/File:Human_karyotype_with_bands_and_sub-bands.png
- Yardstick on a map, https://commons.wikimedia.org/wiki/File:Ein_Yard_(Kantholz).jpg
- Earth from Apollo 17, https://en.wikipedia.org/wiki/The_Blue_Marble
- Distance from Sun, https://commons.wikimedia.org/wiki/File:PIA17046_-_Voyager_1_Goes_Interstellar.jpg
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