Your body, unwound
All the DNA in you, laid end to end: where would it end?
Guess first. Then watch the thread that is leaving you right now, while you read.
Start with one cellOne cell
In a single cell: 2.08 metres
Almost every cell with a nucleus carries two sets of your genome, 3.055 billion letters each: 6.11 billion letters in all. The first human genome read completely, end to end, appeared only in 2022.
In the double helix, each pair of letters sits 0.34 nanometres above the one before, a step measured with X-rays. Multiply, and the thread comes to 2.08 metres: longer than you are tall. It is split into 46 chromosomes, 4.5 centimetres each on average, wound tightly around proteins.
Guess
Where would the whole thread end?
Now lay end to end the DNA from all your cells. Pick where you think the far end lands.
Past Neptune. About 6.2 billion kilometres: 42 times the distance to the Sun, or about 21 round trips. Neptune is 30 Sun-distances out.
The sum counts only the nucleated cells of a 70-kilogram adult, each with 2.08 metres of DNA.
The cells
Only one cell in ten has DNA
A 70-kilogram adult has about 30 trillion cells. Nearly nine in ten have no nucleus: 25 trillion are red blood cells, and most of the rest are platelets. Cells with a nucleus, and so with DNA, number about 3 trillion, and only they count.
- Red blood cells, no DNA: 25
- Platelets, no DNA: ≈1
- Nucleated cells, with DNA: ≈3
An older census, from 2013, counts 37.2 trillion cells. Take away the red cells and platelets, and the thread would run to about 67 round trips. Nobody has counted your cells: both numbers are for a reference person.
The 600 myth
Where “600 times” comes from
If all the DNA in your body was put end to end, it would reach to the sun and back over 600 times (100 trillion times six feet divided by 92 million miles).
For years, textbooks and course pages said the DNA in the body would reach the Sun and back more than 600 times. That would put the end of the thread about 1,200 Sun-distances away.
Their arithmetic was right, but it started from 100 trillion cells, all with DNA. There are three times fewer cells, and nine in ten have no nucleus. The one input that was too small was the thread in a cell: six feet, 1.83 metres, against 2.08.
Right now
Your thread is not standing still
An adult body makes about 330 billion new cells every day, to replace the ones that die. Each new cell is born from a division, and before it divides, a cell copies all its DNA.
Counting one 2.08-metre copy for each new cell, your body spins out about 8,000 kilometres of new DNA every second. It is an estimate: the study counts cells, and the thread follows from them.
- Since you opened this
- 0 s
- new DNA, end to end
- 0 km
- new cells
- 0
The numbers rise and fall in proportion to weight: an approximation. The studies count a 70-kilogram adult; a child has less blood, and so fewer new cells.
Where your new thread reaches
- The Moon in under a minute
- The Sun in about five hours
- All the DNA you carry in about nine days
- As far as light goes in a day in about 38 days
Light is only about 40 times faster than your thread grows. More than two round trips to the Sun, every day.
Where it goes
Two-thirds of the copies are thrown away almost at once
Most new cells are red blood cells: over 200 billion a day, about two-thirds of the total. To be born, they too copy their DNA. Then, as they mature, they push their nucleus out, DNA and all, and a macrophage, a cell that cleans up, collects the nuclei.
So the new thread is mostly a thread thrown away. It does not mean all your DNA changes in nine days: nearly all the copies go into blood and gut cells, which are replaced quickly.
Tonight
About five hours after you opened this, the thread reaches the Sun
The DNA your body has copied in the meantime, laid end to end, would reach the Sun. Set an alarm for five hours from now. When it rings, the thread has just touched the Sun, and you felt nothing.
Game
True or false?
Eight statements about the DNA in you. Some sound like wonders and are true.
The eight statements and their answers:
- False. Most cells in the body carry DNA. About 9 in 10 cells have no nucleus: red blood cells and platelets, 26 trillion in a reference adult. Only some 3 trillion cells have a nucleus and, with it, DNA. Sender, Fuchs and Milo 2016, PLOS Biology
- False. Red blood cells carry DNA. They are the body's most numerous cells, 25 trillion of them, all without a nucleus. No nucleus, no DNA. Sender, Fuchs and Milo 2016, PLOS Biology
- True. The DNA in a single cell would stretch over 2 metres. Two sets of 3.055 billion letters, each letter 0.34 nanometres long: the sum comes to 2.08 metres. T2T 2022 and the 0.34 nm step per letter
- False. All your DNA would reach the Sun and back 600 times. The old number started from 100 trillion cells, every one with DNA. With 3 trillion nucleated cells, the sum comes to 21. Sender 2016, T2T 2022, the IAU definition of the astronomical unit
- True. A typical human cell holds 46 chromosomes. Twenty-two numbered pairs, plus the XX or XY pair. That is how the US National Human Genome Research Institute's glossary counts them. NHGRI genetics glossary
- True. Egg and sperm cells carry half the usual DNA. They are the only haploid human cells: they hold a single set of chromosomes. NHGRI genetics glossary
- False. Bacteria in the body outnumber human cells ten to one. The old 10-to-1 ratio fell in 2016 too: bacteria are about as many as human cells, some 38 trillion against 30. Sender, Fuchs and Milo 2016, PLOS Biology
- True. The first complete human genome was read end to end in 2022. The telomere-to-telomere consortium published the complete 3.055-billion-letter sequence then, with some 200 million letters read for the first time. T2T Consortium 2022, Science
Sources
How we know all this
The thread you carry: 3 trillion nucleated cells (Sender, Fuchs and Milo, 2016), each with two sets of 3.055 billion letters (the complete T2T genome, 2022), at 0.34 nanometres a letter. The distance to the Sun is the astronomical unit, exactly 149,597,870,700 metres.
The new thread: (0.33 ± 0.02) trillion cells replaced a day (Sender and Milo, 2021), counted at one 2.08-metre copy each. The times use the ends of that range: the Moon in 46 to 52 seconds, the Sun in 4 hours 56 minutes to 5 hours 35 minutes. Divisions that do not make a whole new cell, such as those of the cells that shed platelets, are left out.
Left out: nobody has counted your cells; the step to your weight is proportional, so approximate; DNA in the mitochondria is not counted; in the cell, the thread is wound around proteins.
The cells
- 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. doi:10.1371/journal.pbio.1002533 — 30 trillion cells, 3 trillion of them nucleated; the bacteria-to-cell ratio
- Bianconi E et al. (2013). An estimation of the number of cells in the human body. Annals of Human Biology 40(6): 463–471. doi:10.3109/03014460.2013.807878 — 37.2 trillion cells (abstract only)
The genome and the thread
- Nurk S et al., Telomere-to-Telomere Consortium (2022). The complete sequence of a human genome. Science 376(6588): 44–53. doi:10.1126/science.abj6987 — 3.055 billion letters (abstract read)
- Anderson MS (2023). Lateral force separation of biopolymers using an atomic force microscope. Biomicrofluidics 17(3): 034102. PMC10219682 — the 0.34 nm step per letter, measured by x-ray diffraction
- Drew HR et al. (1981). Structure of a B-DNA dodecamer: conformation and dynamics. PNAS 78(4): 2179–2183. doi:10.1073/pnas.78.4.2179 — the first B-DNA crystal structure (abstract only)
- National Human Genome Research Institute, genetics glossary: “Chromosome”, “Haploid”. genome.gov — 46 chromosomes; only the egg and sperm are haploid
The new thread
- Sender R, Milo R (2021). The distribution of cellular turnover in the human body. Nature Medicine 27: 45–48. doi:10.1038/s41591-020-01182-9 — (0.33 ± 0.02) trillion cells replaced a day, close to 90% of them blood cells (abstract read)
- Sender R, Noor E, Milo R, Dor Y (2024). What fraction of cellular DNA turnover becomes cfDNA? eLife 12: RP89321. doi:10.7554/eLife.89321 — over 200 billion red cells a day, which push out their nucleus; a macrophage collects the nuclei (full text read)
- National Human Genome Research Institute, genetics glossary: “Mitosis”. genome.gov — before dividing, a cell copies its chromosomes
The sky and the myth
- NASA Jet Propulsion Laboratory, CNEOS glossary: “au (Astronomical Unit)”. cneos.jpl.nasa.gov — the astronomical unit: exactly 149,597,870,700 m
- Kenyon College, course page “DNA Practice” (S. Lonc). biology.kenyon.edu — the popular version (“600 times”) the sum overturns
- NASA Goddard, NSSDCA: Moon Fact Sheet and Planetary Fact Sheet. nssdc.gsfc.nasa.gov — the Moon: 384,400 km (semimajor axis); Neptune: 4,515 million km from the Sun
- NIST, CODATA: speed of light in vacuum. physics.nist.gov — exactly 299,792,458 m/s
The figures
- Human male karyotype: National Human Genome Research Institute, Wikimedia Commons, Public domain
- DNA structure with labelled bases and grooves: Zephyris (Richard Wheeler), Wikimedia Commons, CC BY-SA 3.0