### Re: What is the concentration of DNA in white blood cells?

Date: Fri Nov 5 14:09:29 1999
Area of science: Cell Biology
ID: 937400453.Cb
Message:
```
Ruth,
question.  Ausubel (see below for references) states that 22% of a cell's
weight is due to the combined weight of DNA, RNA, proteins, and
polysaccharides.  However, that doesn't mean we can't figure out the
concentration of DNA in a white blood cell if we use math!

To find the concentration of DNA, we need to know the weight of the DNA in
the cell and the volume of the cell (WBC).

Volume of a WBC:
A white blood cell is 10 micrometers in diamter (10*10-6m) (from
Prescott et al., 1996).  Let's assume the WBC is spherical, even though in
reality they are not definite spheres!

The volume is then 4/3*pi*radius3 or 523.6 cubic micrometers

Weight of DNA in a single cell:
As you probably know DNA is made up of single units joined together in
long, twisted chains called helices.
There are 6.6*109 base pairs per human cell and each of these is
"worth" 649 Daltons (also known as an atomic mass unit) (from Ausubel et
al., 1999).

If an atomic mass unit weighs 1.66*10-27kg, then the weight of
DNA per  cell is

6.6*109 base pairs*649 Daltons/base pair * 1.66*10-27
kg/Dalton.
All units except kg cancel to give
7.11*10-15 kg of DNA per cell

Now, convert kilograms to micrograms (ug) so our units are similar (kg are
very large compared to micrometers!)
7.11*10-15kg * 1000g/kg * 1 ug/1*10-6g = 7.11ug

Now to find the weight/volume concentration of DNA/cell, we divide the
weight by the volume and multiply by 100%
7.11ug DNA/523.6 um3=0.0135 *100% =1.36% DNA w/v

assumptions that, while enabling easier math, are not necessarily true
biologically.  For instance, white blood cells are not perfectly spherical
and some of the liquid cytoplasmic volume calculated here would be taken up
by other organelles such as mitochondria.  Also, if not packed properly,
DNA would be too long to fit in the cell so molecules called histones wind
up the DNA and pack it tightly; our calculation does not include the weight
of these histones.

The important part of the above answer is that it helps you understand the
concept of determining concentration.  This is a theoretical way to
determine DNA concentration, meaning that we haven't measured anything
experimentally.  In the lab, scientists use a machine that measures DNA
concentration for a relatively large sample, i.e. DNA from many more cells.
These spectrophotometers measure the scattering of light when wavelengths
are passed through a special container holding the sample to calculate DNA
concentration.

Thanks for writing!
-Tinsley

Ausubel et. al. 1999.  "Short Protocols in Molecular Biology". New York:
John Wiley and Sons, p.A2-1.
Prescott et al. 1996. "Microbiology".  Dubuque, IA: Wm. C. Brown
Publishers, p. A16.

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