Nucleic acid structure

Question 1

Features of B DNA

Answer 1

Double helix described by Watson and Crick
-Right handed helix
-Major and minor grooves are visible
-Bases project inwards from the backbone and are perpendicular to it
-10.5 bp per turn of the helix
-Dist between each base pair is approx 3.4 nm

Question 2

Characteristics of A-DNA

Answer 2

-Right handed helix, width 2.3nm
-Wider, more compressed structure than B-DNA bp are tilted from the axis
-11 bp/turn
-helix often formed from DNA-RNA hybrids

Question 3

When does A-DNA occur?

Answer 3

With reduced relative humidity and certain purine stretches around helix the A-form is adapted

Question 4

Characteristics of Z DNA?

Answer 4

-Left handed helix, width 1.8nm
-12 bp/turn

Question 5

When is the Z form adapted?

Answer 5

-Where there is alternating pyrimidine-purine nt Z form can be adapted

Question 6

Is DNA or RNA more stable?

Answer 6

DNA as double stranded - stacking interactions betwen base pairs in double helix structures
-hydrophobic nature

Question 7

What effect does adding an acid have on nucleic acids?

Answer 7

-Strong acid + elevated temp (100 degrees) = nucleic acids hydrolysed completely

-Dilute acid (pH 3-4) = glycosidic bond between purine and sugar destroyed which gives apurinic sites

Question 8

What effect does addition of an alkali have on i) DNA ii) RNA

Answer 8

DNA - tautomeric shift(base changes to diff structure) affects the hydrogen bonding between base pairs - double stranded structure breaks down and DNA becomes denatured

RNA - Denaturation of helical regions which can lead to RNA hydrolysis (RNA brekas down)