Does UV light nick DNA?

Clinton MacDonald cbbccm2 at
Tue Jan 21 15:38:50 EST 1997


   Does UV light "nick" or cut DNA? What effect does ethidium bromide have
on this?

   A colleague and I are hosting a high school student's science project.
The student was interested in the effects of various chemicals on DNA so
we hatched up a scheme looking at the effects of UV light on recA+ and
recA- bacteria carrying a lambda lysogen. This part of the project (mostly
led by my colleague, a bacteriologist) is working fine.

   So that I could feel like I was contributing, too, I suggested we look
at effects of high doses of UV light on supercoiled plasmid DNA in vitro.
My idea was to irradiate supercoiled plasmid DNA with UV light (plus or
minus ethidium bromide), then run the DNA on an agarose gel. I reasoned
that the UV would nick the plasmid and that this would show up as more
"form II" DNA -- relaxed circles. I have always been told that UV light
would nick DNA in the presence of ethidium ("Don't hold your CsCl
gradients in the UV too long, or it will nick the DNA.")

   My colleague (a radiation biologist by training) says this is hogwash
-- the only effect of UV on DNA is to make thymine dimers. He backs off a
little on the effects of ethidium bromide, but believes it to be of no

   Who is right? Can you supply references? A high school student's first
experience in "real" science rides on the answer!

   Thanks for your comments and help!


\ Dr. Clinton C. MacDonald   \ cbbccm2 at
 \ Cell Biology & Biochemistry\ 806/743-2703
  \ Texas Tech University      \ 806/743-2990 (FAX)
   \ Lubbock, TX 79430          \

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