Biochemistry Lecture No. 23: Introduction To DNA & RNA (Cont’d)
Thursday November 1 , 2012
-Erwin Chargaff analyzed DNA composition in the late 1940s and developed two basic rules that hold
true for all DNA molecules: Base composition of DNA varies among species and within a species, base
composition is the same regardless of tissue, age, or environment (%A = %T and %G = %C). This was the
first piece of evidence for the double helical DNA model.
-The diffraction pattern suggests a double-helix with a periodicity of 3.4 nm containing 10 repeating
units. This was done by taking DNA and crystallizing them (shooting X rays through them). This second
piece of evidence determined the DNA double helix to have 10 repeating units within each 3.4 nm.
Correct Tautomeric Forms Of Bases:
-The third piece of evidence was that there are two different forms or states that bases can adopt. This
was influential as Watson and Crick were not sure as to which form of the bases occurred naturally in
DNA. In the figure above, the left tautomer is the predominant form of bases in life. Different tautomers
have different hydrogen bonding capabilities.
Double-Helix – B-DNA:
-B-DNA is the most common conformation of DNA. Some features of B-DNA include: It is antiparallel (5’
end of one strand is next to the 3’ strand of the other strand), the phosphates and bases are on the
outside and inside respectively (forms the double helical structure), and it is right-handed (one can
distinguish between the right-handed and left-handed helix spiral). Also, the phosphates are negatively-
charged and are far apart from each other.
Forces Stabilizing The Double Helix:
-The two forces stabilizing the double helix are hydrophobic interactions and base-pairing. With
hydrophobic interactions: Planar bases stack on each other (nitrogenous bases are fairly planar and are
thus able to stack on top of each other), water is excluded (no room for water to get in; favourable
because bases are relatively hydrophobic) and there is no sequence specificity (DNA mimics
hydrophobic core model of protein folding; occurs regardless of which types of bases are present).
-With base pairing: there are hydrogen bonds between nitrogenous bases, water cannot interfere (it is
excluded from the helix) and there is sequence specificity (base pairing is exclusive). Bases are also
perpendicular to the axis (flat). Watson-Crick Bas