How many bases in a heical turn?

Hans Beernink hbeernin at
Tue Aug 15 08:17:40 EST 1995

Billy Li (billyli at wrote:

: Sorry for an easy question.  I have read from Kornberg that there are 10 
: bases in a helical turn.  My supervisor insists that it is a bit more than 
: 10, perhapseven 11.  My guess is that for different types of DNA (A, B 
: and Z) the numbers are slightly different.  Anyone has the exact 
: numbers please.

: Please email or post.

: Thanks in advance.

: --
: Billy Li,  email: billyli at
: Assistant Lecturer,
: Department of Statistics,
: University of Hong Kong.
: Tel: (852) 28591987     Fax: (852) 28589041

both you and your supervisor are right:

It depends on the type of DNA, and how the structure was determined 
(e.g. x-ray or NMR).  In addition, it depends on the base sequence.  In 
general, average values are used for most purposes.  These are as follows:


bases/turn		~11		~10.4		~12
Rotation/bp(deg)	32.7		34.6		30
Rise			0.23nm		0.33nm		0.38nm

These values were taken from Dickerson, Science 216:475-485 (1982).
For a general class, these values are fine.  If you are doing structural 
modeling or determination, however, you may want something a bit more 

Best regards, 

"The worst monotonous drone coming from a lectern or the most eye-splitting
textbook written in turgid English is nothing in comparison to the 
psychological Sahara that starts right in your bedroom and spurns the 

	-Joseph Brodsky, from "In praise of Boredom"
	 delivered as a commencement address at Dartmouth College.

Hans T.H. Beernink, Department of Biochemistry, University of Vermont
hbeernin at 

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