|MadSci Network: Genetics|
Hi Millie! Sounds like your interested in the basic DNA secondary structure discovered by Watson and Crick (with the help of Franklin and Wilkins) in 1953. Before 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 be 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) bases 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 between individual nucleotides along a given strand, one can conclude that there is approximately 10.5 nucleotides per complete turn (However, obviously there 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 mind 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 wider. 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, NY (1997). Luke. 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.
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