Friday 17 April 2015

Light-Years of DNA

Light-year, and DNA. Not two scientific terms you expect to see on the same page, but over your lifetime your body will produce around one light-year of DNA! That is about one trillion kilometres. Don't believe me? Let's do some maths:

Every cell in your body has two copies of your genome, held in 23 pairs of chromosomes. The human genome is approximately three billion (3×109) base pairs of DNA.

The famous double helix of DNA has about 10 base pairs per twist, and each twist is 3.4 nanometers long (3.4×10-9 metres, the same as roughly 20 carbon-carbon bonds).

This means that the total length of DNA contained in every cell of your body is approximately 2 meters (3×109 base pairs multiplied by 0.34×10-9 metres per base pair, doubled because of the two copies).

Your body has about ten trillion (1×1013) cells (excluding red blood cells), and this remains roughly constant through your life. There is a huge turnover of these cells though, as your body replaces cells to maintain itself.

Every time a cell is replaced its 2 metres of DNA must be produced. In most tissues the cells are replaced in a couple of months, and in many they are replaced in just a couple of days. Even cells in bones are replaced every few years.

The average lifetime of a cell is probably one or two months, so if you live to 80 then your cells are replaced about 500 times throughout the course of your life.

This means that the total length of DNA your body produces in your lifetime is approximately 1×1016 metres (2 metres multiplied by 1×1013 cells, multiplied by 500 replacements). 1×1016 metres (ten thousand trillion metres) is about one light-year (0.946×1016 metres)! Most amazingly it would not be a light-year of random DNA sequence, but ten thousand trillion identical copies of your DNA, faithfully replicated by your cells.

An estimation of the number of cells in the human body
How quickly do different cells in the body replace themselves?
Thanks to Rob Phillips for making me think about this!


  1. Ten thousand trillion mostly* identical copies

  2. Also, cell turnover greatly decreases with increasing age.

  3. An average human genome x average no: of human cells ≈ 1 light year?


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