|MadSci Network: Cell Biology|
Ruth, After some investigation, I couldn't find a specific answer to your 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 Please don't use the above answer as a gold-standard because I have made 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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