MadSci Network: Genetics

Re: How many amino acids pass before the DNA makes a full twist?

Date: Thu Mar 21 01:33:13 2002
Posted By: Luke Collyer, Graduate Student, Biochemistry & Molecular Biology, Monash University
Area of science: Genetics
ID: 1016402970.Ge

Hi Millie!

Sounds like your interested in the basic DNA secondary structure 
by Watson and Crick (with the help of Franklin and Wilkins) in 1953. 
we begin, don't forget that DNA is a nucleic acid, not a protein, and is 
therefore composed of nucleotides, not amino acids. 

Using a technique called X-ray diffraction, the deduced the following:

- DNA has two strands
- The strands are arranged in an antiparallel manor
- Adenine (A) must be paired with Thymine (T), whilst Guanine (G) must 
paired with Cytosine (C)
- The two strands form a right-handed helical-type structure
- hydrophilic (describes molecules that like water) backbones of 
deoxyribose (the sugars) and negatively charged phosphate groups are 
on the 
outside of the helix (to interact with surrounding water)
- Purine (Adenine and Guanine) and Pyrimidine (Thymine and Cytosine) 
are located on the inside of the helix

The results also provided distances between nucleotides within the DNA 
helix (0.34nm) and the length of a complete turn within the helix 
(originally thought to be about 3.4nm, now considered 3.6nm). Therefore, 
considering the length of a complete turn (3.6nm) and the distance 
individual nucleotides along a given strand, one can conclude that there is 
 approximately 10.5 nucleotides per complete turn (However, obviously 
can't be half a nucleotide - this measurement is just useful for picturing 
the structure). 

So there you go. Hope this helps with your model. You should keep in 
that the structure of DNA can vary significantly from the structure 
proposed by Watson and Crick. For example, in solutions devoid of water, 
DNA can contain 11 nucleotides per complete turn, making it slightly 
Some DNA may also taken on a structure even more different - Some 
DNA can 
take on a left-handed rotation, with 12 bases per turn. The latter 
structure takes on a zigzag appearance. I wouldn't worry about these too 
much, but just remember that the classical DNA secondary structure 
originally discovered by Watson and Crick is not the ONLY structure that 
DNA can form. 

You may find the following text-book quite useful - Lehninger, AL, Nelson, 
DL, and Cox, MM. Principles of Biochemistry, 2nd Ed., Worth Publishers, 


Admin note:

If that twist were in the coding region of a gene, then 10.5 bases would 
correspond to just over 3 codons (there are 3 bases in a codon).  If this 
DNA were transcribed into RNA, and the RNA translated into protein, one 
twist of the orginal DNA would correspond to just over 3 amino acids in 
the protein product. 

Current Queue | Current Queue for Genetics | Genetics archives

Try the links in the MadSci Library for more information on Genetics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2002. All rights reserved.