Secondary structure of DNA
The secondary structure of DNA is the arrangement in space of two nucleotide chains or strands in a double helix, with the nitrogenous bases facing each other and linked by hydrogen bonds. This structure was deduced from the following experimental data:
It was observed that the density and viscosity of the DNA in aqueous dispersion were higher than expected due to its chemical composition and molecular weight, so it was assumed that there must be hydrogen bonds between the groups -NH 2, -CO-, and = NH.
Chargaff found that all DNAs had the same number of adenine (A) and thymine (T) molecules, and as many cytosine (C) as guanine (G).
n° adenine molecules / n° thymine molecules = 1
n° cytosine molecules / n° guanine molecules = 1
From this it was deduced that hydrogen bonds were formed between A and T and between C and G.
With X-rays it was discovered that DNA has a structure of 20 Å (0.2 nm) in diameter , in which certain units were repeated every 3.4 Å (0.34 nm), and that there was another greater repetition every 34 Å (3.4 nm).
1 nm (nanometer) = 10 Å (angstrom) = 1 × 10-9 meters = 1 × 10-6 millimeters
With all these data, J. Watson and F. Crick devised, in 1953, the model of the double helix of DNA, which says that DNA is made up of two antiparallel chains of polynucleotides, with the 5 '→3' bonds oriented in different sense, complementary and wound one on top of the other in a double helix or plectonemic form (for the two chains to separate, they must be unwound).
The fact that they are complementary chains implies that if there is adenine in a chain, in its complement, there will be the nitrogenous base thymine. And if there is guanine in one, cytosine in the other.
As in proteins, in the secondary structure of DNA, the hydrophobic groups -CH 3 and -CH= of the nitrogenous bases are oriented towards the interior of the molecule, establishing hydrophobic interactions between lipophilic groups, which collaborate with hydrogen bonds in give stability to the macromolecule. The pentoses and the phosphate groups are on the outside.
By Deneapol [Public domain], via Wikimedia Commons