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Why look at G+C content?

Daniel Weinreich dmw at MCZ.HARVARD.EDU
Mon Mar 11 12:33:50 EST 1996

The original poster on this thread (moths at Glue.umd.edu) is now having
problems with his news reader.  Probably because he liked my answer the
best ;-), he asked me to post this (slightly caustic) follow-up. 


Daniel M. Weinreich			email: dmw at mcz.harvard.edu
Harvard University 			usmail: 26 Oxford Street
Museum of Comparative Zoology			Cambridge, MA 02138
voice: (617) 495-1954			fax: (617) 495-5846

--- Begin Forwarded Message ----------------------

No, I didn't miss Chargaff's rule.  It's rather elementary!  It's also
quite irrelevant in this case:  phylogenetic analyses are performed on
single strands, not double strands (where obviously the amount of G in
BOTH strands will equal the amount of C in BOTH strands).  You apparently
missed the point that in a SINGLE strand of DNA, the C-content is totally
independent of the G-content.  Why state the G+C content of a single
strand when it would be so much more informative to state the C-content
and G-content separately?   You are only losing information.   After
all, the only reason for examining base content in a phylogenetic analysis
is to assess whether the transformation probabilities between the character
states (A,C,G and T,  *not*  G-C and A-T) are about equal or not.

So, back to my original question: why do people state G+C content in a
phylogenetic analysis of DNA sequence data?  I suggested this could be a
hangover from the days before DNA sequencing, when the G+C content of double
strands could be determined by other means.  Some people continue to
report base composition as % G+C, but seeing as single stranded DNA does not
undergo Watson-Crick base-pairing to itself, the G+C and A+T partition is
arbitrary.  One might as well have chosen to report G+A content or G+T
content, but there is less information in any one of these partitions
than in the individual base frequencies themselves.

Andrew Mitchell
Department of Entomology
University of Maryland

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