Question

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.

Answer 1

Model of DNA

Step-by-step explanation:

1) DNA base composition vary from species to species

• 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