Explain how the following pieces of evidence lead to the model of DNA as a double helix with complementary, antiparallel strands:

1) the amount of A always is equal to T & G is always equal to C,
2) the sugar-phosphates are on the outside of the helix, and
3) the diameter of the helix is uniform.

Nucleotide is the monomeric unit of DNA which consists of nitrogenous base, pentose sugar and phosphate group
Nitrogenous base are derivatives of two parent compounds: Purines(Adenine and Guanine) and Pyrimidines(Cytosine and Thymine)
According to Chargaff's rule, Adenine(A) = Thymine(T) and Guanine(G) = Cytosine(C)
so relationship comes out to be, Purines= Pyrimidine
A+G= C+T OR A+G/C+T= 1

2) Polymerization of nucleotide forms nucleic acids

Backbone of nucleic acid consists of alternate phosphate and sugar
Backbone of nucleic acid is is hydrophilic and negatively charged
Nitrogenous base join backbone of nucleic as a side group and always perpendicular to helical axis

3) DNA consists of two antiparallel polynucleotide chains

Coiling of polynucleotide chains create right handed helical structure
Polynucleotide chains present in DNA are complementary to each other and not identical
During coiling of two polynucleotide chains, hydrophobic nitrogenous base stacked inside between polynucleotide chain whereas hydrophobic backbone face surrounding water
Diameter of DNA double helix is 20 Angstrom (Å) and is uniform because purine always pairs with pyrimidine and their combined length is always equal

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