Red blood cell
25 trillion, the most numerous
It has none. It sheds its nucleus to make room for more oxygen, so the DNA strand has nowhere to sit.
Your body, unwound
All the DNA in you would reach the Sun and back. How many times?
About 21 times. The sum starts from 3 trillion nucleated cells, each holding just over 2 metres of DNA, and ends at 6.2 billion kilometres. Redo it with your own numbers below.
See the sumThe answer
Guess first, then measure
If you unwound all the DNA in your body, strand by strand, how far would you get? Move the slider to your number of round trips to the Sun, then press the button.
The counted answer
About 21 round trips. That is 6.2 billion kilometres of DNA, some 42 times the one-way distance from Earth to the Sun.
The sum counts only nucleated cells, about 3 trillion in a 70-kilogram adult, each counted at 2.08 metres of DNA. Change the assumptions in the calculator.
The counted answer: about 21 round trips to the Sun, 6.2 billion kilometres.
The cells
Only one cell in ten carries DNA
The body holds about 30 trillion cells. Most are red blood cells, and red blood cells have no nucleus. Without a nucleus, the DNA strand has nowhere to sit.
Which of these carry DNA? Choose before you learn.
Skin cell
one of many with a nucleus
It has the full set: two copies of every chromosome. That is how most nucleated cells look.
Platelet
about a trillion, from the blood
It has none. It is one of the cells without a nucleus, and the sum leaves it out with the red blood cells.
Egg cell
joins with the sperm
It has some, but half: a single set of chromosomes. The other half comes from the sperm.
The share reads another way too: 9 in 10 cells have no nucleus. The nucleated count, about 3 trillion, is the one that enters the sum above.
Calculator
The sum, with your numbers
Pick the cell census and the set you count. The result rebuilds itself, in kilometres, in Earth–Sun distances and in round trips.
- cells counted
- kilometres of DNA
- one-way Earth–Sun distances
- round trips
Scaling the 70-kilogram reference adult to your mass is a simple rule of three: a rough estimate.
Each nucleated cell enters the sum with 2.08 metres: two sets of 3.055 billion letters at a 0.34-nanometre step.
On the honest assumptions, 3 trillion nucleated cells, the sum comes to 6.2 billion kilometres: 42 one-way Earth–Sun distances, about 21 round trips.
The scale
From you down to one letter
Step down from the whole person to the step between two letters. Each step states its own size.
That is how tall the cell censuses' reference adult stands. Everything below fits inside.
The strand in a cell runs longer than the person carrying it. The length comes from 6.11 billion letters times 0.34 nanometres.
The 46 chromosomes split the strand into 46 pieces, wrapped with protein. This is the mean; real chromosomes differ.
That is how many letters one complete set holds, first read end to end in 2022. An ordinary cell keeps two sets.
Each letter adds 0.34 nanometres to the strand, as x-rays measured.
The 600 myth
Where the number 600 came from
For years, course pages and popular books wrote that the body's DNA reaches the Sun and back over 600 times. Their arithmetic was sound. The numbers they fed it came from older guesses.
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).
| Aspect | Then | Now | Why |
|---|---|---|---|
| The cells | 100 trillion, every one with DNA | 30 trillion, only 3 nucleated | The 2013 and 2016 censuses counted fewer, and 9 in 10 cells have no nucleus. |
| The strand in one cell | six feet (1.83 m) | 2.08 metres | The complete 2022 genome runs a little longer than thought; here the old number was too small. |
| The result | over 600 round trips | about 21 round trips | On the old inputs, 600 still comes out. On the measured inputs, 21 comes out. |
The course page said one more thing: read aloud at 10 letters a second, 3 billion letters would take 9.5 years. With the complete genome's 3.055 billion letters, it would take 9.7 years.
Game
True or false?
Eight statements about the DNA in you. Some sound like marvels 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 și 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 și 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 și 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 cell counts come from two published censuses: Bianconi et al. (2013), 37.2 trillion, and Sender, Fuchs and Milo (2016), 30 trillion, 3 trillion of them nucleated. The genome length comes from the complete T2T-CHM13 sequence (2022): 3.055 billion letters. The 0.34-nanometre step is the x-ray-diffraction value. The Sun's distance is the astronomical unit, defined as exactly 149,597,870,700 metres.
Five things the sources do not say stand beside the claims they touch: nobody has counted your cells, and every person differs; the 0.34-nanometre step belongs to the stretched strand, while in the cell the strand sits wrapped with protein; scaling to your mass is a rough rule of three; mitochondrial DNA stays outside the count; and the platelet slice comes from subtraction, which is why it reads “about”.
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 strand
- 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 Sun 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
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