3.3: DNA structure
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Define a nucleotide.
A nucleotide is a molecular structure consisting of a phosphate group, a pentose sugar and a nitrogenous base. It is the basic unit of DNA and RNA.
What is meant by a double helix and how does it apply to the DNA molecule?
A double helix is a structure where two spirals coil around each other keeping a constant diameter of the coil. The DNA molecule has a double helix structure where the sugar-phosphates form a backbone of two polynucleotide chains that coil around each other. The DNA double helix is a right-handed spiral polymer.
If a solution of DNA is heated to 90 degrees Celsius for two minutes, dissociation occurs. What is meant by dissociation of a DNA double helix?
Dissociation of a DNA double helix means that the helical strands of the DNA molecule separate to form two single chains of DNA. The heating breaks they hydrogen bonds between the complementary nitrogenous bases but not the strong sugar-phosphate bonds that join the nucleotides together to form the backbone chain.
Outline how Watson and Crick were able to work out the distance between atoms, the angles between bonds and the size of atoms to construct a three-dimensional model of DNA.
Watson and Crick based the double helix structure on X-ray diffraction photographs of DNA taken by Rosalind Franklin. Maurice Wilkins was Franklin's colleague and showed her X-ray photographs to Watson. From the photographs the 3-D model was constructed by Watson and Crick.
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