How long would all the DNA in your body stretch?
— Over 40 times the distance to the Sun. But "hundreds of round trips" was an overcount
The DNA inside a single cell, stretched straight, is about 2 metres long. Join together the DNA from every cell in your body and you get a length more than 40 times the distance from Earth to the Sun. That's an astonishing number — but the well-known claim of "hundreds of round trips to the Sun" is actually about 10 times too big. That's because most of the cells in your body don't have any DNA at all.
Have you ever heard this in a science class or on a TV documentary? "If you joined up all the DNA in your body, it would be long enough to travel to the Sun and back hundreds of times."
It's a striking thing to hear. But think about it for a moment and it gets strange. How can a 2-metre-long thread fit inside a cell too small to see?
And is that "hundreds of round trips" figure even true? In this article, we'll do the maths ourselves and check.
Only two things decide the real number
Counted by number, the most common cells in your body are red blood cells. As they mature, red blood cells discard their nucleus, so they have no DNA. Cells that do have DNA make up only about a tenth of the total.
The nucleus that holds the DNA is a ball less than a hundredth of a millimetre across. The DNA is wound around spools, then folded up in many more layers, packing it into a ball hundreds of thousands of times smaller than its length.
First we'll recount "how long is it really", then look at "how does it fit".
Reason 1: there are far fewer cells to count than you'd think
A well-known estimate puts the number of cells in the human body at 37 trillion. In 2016, a more careful recount gave a figure of 30 trillion. Either way, it's an almost unimaginable number.
But look at the breakdown and it's surprising. Of those 30 trillion, around 25 trillion are red blood cells. To dedicate themselves purely to carrying oxygen, red blood cells push their nucleus out as they mature. That's why they have no DNA.
Platelets, which help blood clot, also have no nucleus. That leaves roughly 3 trillion cells with a nucleus and DNA. Look at the top bar in Figure 1: the cells with DNA are only the thin slice at the right-hand end.
Multiply those 3 trillion cells by about 2.2 metres each, and you get roughly 6.6 billion kilometres. Earth to the Sun is about 150 million kilometres. One-way, that's 44 trips to the Sun.
Pluto, out at the edge of the solar system, is on average about 5.9 billion kilometres away. The DNA in your body works out slightly longer than that. Even without "hundreds of round trips," it's still a staggering number.
Reason 2: the 2-metre thread is wound up in many layers to fit
The nucleus is about 6 micrometres across (6 thousandths of a millimetre). Inside it sits about 2.2 metres of DNA. That's a length-to-width ratio of about 370,000 to 1.
Let's scale the nucleus up to the size of a tennis ball (6.5 cm across) and keep that same ratio. The DNA inside becomes a thread about 24 kilometres long and about 0.02 mm thick. Try the slider in Figure 2 to change the scale.
A thread this long and thin only avoids tangling because of how cleverly it's wound. DNA first wraps around small protein balls called histones, about twice around each — like thread wound onto a bobbin.
These bobbins link up like beads on a string, which then bundle further and fold into loops. When a cell divides, this reaches its most tightly packed form: the "chromosome."
The important part is that it isn't just crammed in. Sections being used right now are loosely unwound, while unused sections stay tightly packed. The way it's wound is itself part of the mechanism that decides which genes get switched on.
Every time a cell divides, all 2 metres of its DNA get copied in full. It's thought that hundreds of billions of new cells are made in the body every day. The machinery that corrects copying mistakes is working non-stop too.
Summary
The DNA in a single cell is about 2.2 metres long. But red blood cells, which make up over 80% of the body's cells, have no DNA at all. Counting only the roughly 3 trillion cells that do have DNA, the total comes to about 6.6 billion kilometres — 44 times the distance to the Sun, and about as far as Pluto. And each strand is wound up in many layers to fit inside a nucleus 370,000 times smaller than its length.
"Hundreds of round trips" was an overcount.
Even so, your body holds a thread long enough to reach Pluto.
For what red blood cells do as they travel around your blood, see How did we find out that blood circulates around the body?, and for how DNA gets damaged by UV light, see Why do you get sunburnt even on a cloudy day?
- Mash up broccoli florets (or a banana) and mix them into 100ml of water with a teaspoon of salt and a few drops of washing-up liquid, stirring well. The detergent breaks open the cell membranes.
- Strain the mixture through a tea strainer into a glass, keeping just the liquid. Pour in well-chilled rubbing alcohol gently down the side of the glass, about the same amount as the liquid.
- Wait a few minutes, and cloudy white threads will rise up at the boundary between the layers. That's a clump of DNA gathered from hundreds of millions of cells.
Alcohol is flammable, so do this away from any flame, with an adult present. What you're seeing isn't a single strand but a tangled bundle of countless DNA molecules.
Want to know more? — terms, formulas, and textbook linksEach section is labelled with the level it belongs to, from middle-school science to university-level courses
- MSCovered in middle-school science
- HSCovered in high-school "Biology Basics / Biology"
- HS+Advanced high-school content, or textbook sidebar material
- Univ.Not covered in high school — university-level content (molecular biology, cell biology)
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has names
- DNA (deoxyribonucleic acid): a long double-helix molecule that records genetic information as a sequence of four kinds of building blocks (bases).
- Base pair: a pair of bases linking the two strands of the double helix. Also used as the unit for measuring DNA length.
- Nucleus: the compartment inside a cell that stores DNA. Red blood cells and platelets have none.
- Chromosome: the most tightly folded form DNA takes during cell division. Humans have 46.
MSHSChecking with a formula: the total length of all DNA in the body
We want the total length L of all the DNA in the body joined together. Multiply three things: the number of cells, the number of base pairs per cell, and the length of one base pair.
| In symbols | L = N × b × a |
| In words | Total length = number of cells with DNA × base pairs per cell × length of one base pair |
| Where it comes from | DNA is a "ladder" with base pairs spaced at regular intervals, so its length is proportional to the number of rungs. Summing that over every cell gives this counting formula |
| L | Total length of all DNA in the body (metres) |
| N | Number of cells with DNA (cells) |
| b | Base pairs per cell (count) |
| a | Spacing between neighbouring base pairs (nanometres) |
| Base pairs per cell b (one set from each parent) | estimated at about 6.4 billion (6.4 × 10⁹) |
| Base pair spacing a | about 0.34 nanometres (0.34 × 10⁻⁹ m) |
| Number of cells with DNA, N | estimated at about 3 trillion (out of ~30 trillion total, minus red blood cells and platelets) |
| Earth to the Sun | about 150 million kilometres |
| Sun to Pluto (average) | about 5.9 billion kilometres |
| Length of one cell's DNA, b × a (the 10⁹ and 10⁻⁹ cancel out) | 6.4 × 0.34 ≈ 2.2 (metres) |
| Total length L = N × 2.2 m (in trillion metres) | 3 × 2.2 = 6.6 |
| 6.6 trillion metres is 6.6 billion kilometres. How many trips to the Sun? | 6.6 ÷ 0.15 = 44 |
| Compared with the distance to Pluto | 6.6 ÷ 5.9 ≈ 1.1 |
| If you count all 37 trillion cells, including red blood cells (trillion metres) | 37 × 2.2 = 81.4 |
| Overcount factor | 81.4 ÷ 6.6 ≈ 12 |
The famous "hundreds of round trips to the Sun" figure counted cells with no DNA too, making it about 12 times too large. Counted correctly, it's still long enough to reach Pluto.
| Nucleus diameter | about 6 micrometres (6 × 10⁻⁶ m) |
| Length ÷ diameter (in units of 10,000x) | 220 ÷ 6 ≈ 37 |
| Length when scaled up to a 0.065 m tennis ball (in units of 10,000 m) | 0.065 × 37 ≈ 2.4 |
So a ball 6.5 cm across holds a thread about 24 kilometres long — the same figure Figure 2 starts with.
HSHS+Layers of packing, and how cells are counted
HSIn Biology Basics, you learn that DNA wraps around histones to form nucleosomes, which then link up into chromatin. The genome is about 3 billion base pairs, and since body cells carry one set from each parent, that's about 6.4 billion base pairs in total.
HS+Each nucleosome has about 147 base pairs wrapped around it roughly 1.7 times. This alone shrinks the length several-fold. Cell counts are worked out organ by organ, from each organ's volume and the size of a single cell, then added up. The gap between the 37 trillion (2013) and 30 trillion (2016) estimates comes mainly from differences in how red blood cells were counted and which body size was used as the baseline.
Univ.Higher-order chromatin structure and loop extrusion
Textbooks have long described strings of nucleosomes as coiling into a "30-nanometre fibre." But a growing number of studies report that no clear fibre of that kind is visible inside living cells. The current mainstream view explains folding inside the nucleus using a "loop extrusion model," where proteins such as cohesin pull out loops of DNA, together with "topologically associating domains" measured by a technique called Hi-C.
📖 For the derivation of the formulas and further detail: Nucleosome (Japanese Wikipedia) / Chromatin (Japanese Wikipedia)
ResearchWhat isn't fully understood yet
- How is DNA arranged inside the nucleus? We know each chromosome occupies roughly its own "territory," but the finer details of folding differ from cell to cell and over time, and the full picture hasn't been worked out yet.
- Why doesn't it tangle? Every round of copying or division creates twists and tangles, and research is still ongoing into exactly what order the untangling enzymes act in.
- The exact number of cells in the body. Estimates vary, as with the 30-trillion and 37-trillion figures. The number also changes with age, body size and sex, and no one has ever directly counted an individual's cells.
In other words, everything in this article is "the best explanation given what we currently know." If the cell-count estimate changes, the total length changes by the same proportion.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science Year 2 "How living things are built" / Year 3 "Rules of heredity and genes" | Cells and nuclei, DNA and chromosomes |
| HS | Biology Basics "Genes and how they work" | Base pairs, genome size, histones |
| HS+ | Biology "Expression and regulation of genetic information" | Nucleosomes, cell-count estimates |
| Univ. | Molecular biology / cell biology | Higher-order chromatin structure, loop extrusion |
| Research | 3D genome structure inside the nucleus | Chromosome territories, range of cell-count estimates |
| — | Connection to daily life | The habit of checking a scientific figure against what it actually counted |
- Sender R., Fuchs S., Milo R. (2016) Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLOS Biology
- Bianconi E. et al. (2013) An estimation of the number of cells in the human body. Annals of Human Biology
- Alberts B. et al. Molecular Biology of the Cell (『細胞の分子生物学』), Newton Press (ニュートンプレス)
- Watson J. D., Crick F. H. C. (1953) Molecular structure of nucleic acids. Nature 171
※ This article is a general-audience science explainer. The figures given are approximations meant to aid understanding. Estimates of cell numbers and base-pair counts vary between sources.