Re: Number of chromosome repairs

[Mike Grissom <mikeg@SLAC.Stanford.EDU>]

Bernie, et al.,

Just a reminder from the radiation exposure point-of-view...

The following quotes are from an ancient edition of
"Radiobiology for the Radiologist" by Eric Hall (1973,
Harper and Row, Publishers, Inc.) with my annotations
in square brakets []:

	"Three types of aberrations ...:  the ring and
	the dicentric, which are chromosome aberrations,
	and the anaphase bridge, which as described as
	a chromatid aberration.  All three represent
	gross distortions and are, consequently, clearly
	visible [at metaphase]."

	"The formation of a ring...break is induced by
	radiation in each arm of a single chromatid early
	in the cell cycle...The fragment has no centromere
	and probably will be lost at mitosis..."

	"A dicentric...result will be a grossly distorted
	chromosome having two centromeres (a dicentric).
	There will also be a fragment which has no centromere
	(acentric fragment) [which probably will be lost
	at mitosis]..."

	"An anaphase bridge...results from breaks which occur
	late in the cell cycle (in G<sub>2<sub>) after the
	chromosomes have replicated...At anaphase...the section
	of chromatin between the two centromeres is stretched
	across the cell between the poles...hinders the separation
	into two new daughter cells.  This is called an anaphase
	bridge.  The two fragments may join as shown, but since
	there is no centromere it will probably be lost at the
	first mitosis."

Hence, double-breaks of chromosomes tend to be (at the cellular
level) fatal to one or both daughter cells post mitosis and not
repaired.  This is genetic information that may or may not be
detrimental to the whole organism (depending upon what organism,
how many cells were impacted, and or degree an organ system was
impacted).

This is a link to radiation sources, but then natural background
radiation likely plays a role in the natural occurrence of
breaks requiring repair.  Of course, very high doses of
radiation will produce increased numbers of double-strand breaks
which is the basis of blood cytogenetic analysis and dose
estimation post accident exposure levels of medical concern,
such as some of the irradiator incidents mentioned on the Web
recently that Dr. Ricks, et al., investigated and developed
records on at REAC/TS in Oak Ridge.

I would agree that a firmer 'background DNA repair' rate
number or well-defined range (allowing for great biological
diversity) would be very useful in studying such effects
as chemical carcinogenicity and testing the LNT!  Particularly
for the human genome...

S.,

MikeG.


At 08:52 AM 1/28/98 -0600, you wrote:
>	According to Myron Pollycove, who is an expert in this area,
>chemical and spontaneous processes cause an average of 150,000 single
>strand breaks and 1.0 double strand breaks in each cell every day.
>Multiply this by the trillion cells in the body to get the total number.
>A large fraction of these are presumably repaired. Do you consider repair
>of the DNA molecule to be "chromosome repair"?
>
>Bernard L. Cohen
>Physics Dept.
>University of Pittsburgh