🚀 go-pugleaf

kmuldrew@acs.ucalgary.ca (Ken Muldrew) wrote:

>toms@kaylor.ncifcrf.gov (Tom Schneider) wrote:

>>Ken Muldrew (kmuldrew@acs.ucalgary.ca) wrote:
>>: toms@kaylor.ncifcrf.gov (Tom Schneider) wrote:
>>: >Ken Muldrew (kmuldrew@acs.ucalgary.ca) wrote:


>>: Just to give people an idea of how fragile DNA can be, a
>>: [Caucasian] without sunscreen, standing in the noon-day sun of the
>>: tropics, will suffer about 50 000 single strand breaks in every skin
>>: cell that's exposed per hour (just about the maximal rate of repair,
>>: coincidence?).

>>Fascinating!

>>Perhaps you have references you can share with us for both numbers?

>Oh boy! I knew I was going to be pulled over sooner or later. Well,
>the numbers come from a graduate course in Oncology that I took about
>11 years ago, the lecturer being a chap named Paterson at the
>University of Alberta. I'll take a look tonight to see if I still have
>the course notes. A quick look in Kornberg (DNA Replication, 2nd ed.)
>didn't reveal any numbers for SS breaks, but there is a number for
>missing base mutations--about 10 000 purines and a few hundred
>pyrimidines are lost in each cell per day (section 21-1). Anyone who
>doesn't believe that DNA is fragile should take a flip through chapter
>21, you'll soon be convinced.

Well, I looked it up and my memory was faulty (sorry). The numbers for
single strand breaks should have read 2300/hr/cell with maximal rate
of repair being 20 000. UV light principally induces pyrimidine
dimerization (an intra-strand cross-link) which occurs in the above
scenario at the rate of 50 000 dimers/hr/cell which is just equal to
the maximal rate of repair. This is from the lecture notes. A
reference which might contain these numbers, or should at least point
to a source, is:

Cancer Predisposition, Carcinogen Hypersensitivity, and Aberrant DNA
Metabolism.  Paterson, MC, Gentner NE, Middlestatd MV & Weinfeld M.
J. Cell. Physiol. supp. 3: 45-62.  1984.

Sorry for the mistake.


Ken Muldrew
kmuldrew@acs.ucalgary.ca