Question

Describe the structure of the DNA molecule. In your description, be sure to include the names of the components that comprise the molecule, as well as the complementary base pairing rules.

Answer 1

Rosalind Franklin used X-ray crystallography to understand the structure of DNA.
The building blocks of DNA are nucleotides, which are made up of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base .
Each DNA molecule is actually composed of two single strands held together along their length with hydrogen bonds between the bases.
DNA is made up of two strands that are twisted around each other to form a right-handed helix or double helix.
The diameter of the DNA double helix is uniform throughout because a purine (two rings) always pairs with a pyrimidine (one ring) and their combined lengths are always equal
There are four types of nitrogenous bases in DNA. Adenine (A) and guanine (G) are double-ringed purines, and cytosine (C) and thymine (T) are smaller, single-ringed pyrimidines. Base-pairing takes place between a purine and pyrimidine: namely, A pairs with T, and G pairs with C. So, Adenine and thymine are complementary base pairs, and cytosine and guanine are also complementary base pairs. This is the basis for Chargaff’s rule.
Chargaff had shown that of the four kinds of monomers (nucleotides) present in a DNA molecule, two types were always present in equal amounts and the remaining two types were also always present in equal amounts. This meant they were always paired in some way.
The phosphate group of one nucleotide bonds covalently with the sugar molecule of the next nucleotide, and so on, forming a long polymer of nucleotide monomers. The sugar–phosphate form a  “backbone” for each single strand of DNA, and the nucleotide bases stick out from this backbone. The carbon atoms of the five-carbon sugar are numbered clockwise from the oxygen as 1′, 2′, 3′, 4′, and 5′
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