Ask A Scientist© Molecular Biology Archive

name         Saundra
status       educator
age          30s

Question -   Is it possible to obtain DNA from human blood consumed
by a mosquito, recognizing (I think) that the red blood cells have no
nuclei, and so wouldn't carry DNA, whereas white blood cells would?
Would
the ability to derive a DNA sample be dependant on the mosquito having
comsumed white blood cells? And if it is possible, would the digestive
breakdown of the blood by the mosquito render it impossible to extract
the DNA, and if so, after what period of time ie for how long after
drawing the blood would the DNA remain sufficiently constituted?

A good question, Saundra,
Jurassic Park has many people thinking about related issues! You correctly
note that red cells (RBCs) lack a nucleus, thus providing no genetic
encyclopedia of their history. However, a given volume of human blood
contains about 45% RBCs, about 1% platelets & white cells (WBCs); the rest
is plasma. Therefore for every 45 RBCs, you'll also get 1 WBC, on average.
The volume taken in by a mosquito can therefore contain LOTS of WBCs and
ample opportunity to sample the DNA. As to the issue of how long it would
take digestive enzymes present in the mosquito gut to render  the DNA
unreadable, which I think would doubtless happen sooner or later... I have
no idea. Several variables here: specificity of the enzymes, temperature,
etc. Hopefully an entomologist out there will read your question & provide
us some information about that.
Thanks,
CED
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You're correct that red blood cells contain no DNA and that white blood
cells do carry DNA.  And you're also correct that the mosquito would have
to ingest white blood cells.  However, I think it's pretty unlikely with
the tiny volume of blood that a single mosquito would consume that there'd
be enough human cells present to extract a DNA sample.  Also, I can't
imagine how you'd recover the blood that a mosquito has ingested without a
tremendous contamination with mosquito cells (therefore mosquito DNA).

If you wanted to do a weird experiment like extracting DNA from mosquitoes
to look for evidence of human blood, you'd need a lot of mosquitoes (a
hundred per DNA sample?).  You could grind them all up whole to make DNA
from the whole population.  Then by using PCR (polymerase chain reaction)
you could try to amplify a common human gene like a ribosomal gene.  I
don't know if it would work, but it might be cool to try.
---------
Christine Ticknor
Ph.D. Student
Yale University
New Haven, Connecticut
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