Nucleic Acids

Lecture Six

Nucleic Acids
Dr. Khalidah Merzah

Nucleic Acids Are Essential For Information Transfer in Cells

•Information encoded in a DNA molecule is transcribed via synthesis of an RNA molecule
•The sequence of the RNA molecule is "read" and is translated into the sequence of amino acids in a protein.

First discovered in 1869 by Miescher.

•Found as a precipitate that formed when extracts from nuclei were treated with acid.
•Compound contained C, N, O, and high amount of P.
●1944 Oswald, Avery, MacLeod and McCarty demonstrated that DNA is the molecule that carrier genetic information.
●1953 Watson and Crick proposed the double helix model for the structure of DNA
Nucleic acids are long polymers of nucleotides.
Nucleotides contain a 5 carbon sugar, a weakly basic nitrogenous compound (base) and one or more phosphate groups.
Nucleosides are similar to nucleotides but have no phosphate groups.

Nucleoside 5'-triphosphates are carriers of energy

Bases serve as recognition units
Cyclic nucleotides are signal molecules and regulators of cellular metabolism and reproduction
ATP is central to energy metabolism
GTP drives protein synthesis
CTP drives lipid synthesis
UTP drives carbohydrate metabolism
Other Functions of Nucleotides

Nucleic acid backbone takes on extended conformation.

Nucleotide residues are all oriented in the same direction (5’ to 3’) giving the polymer directionality.
The sequence of DNA molecules is always read in the 5’ to 3’ direction

Properties of DNA Double Helix

Distance between the 2 sugar-phosphate backbones is always the same, give DNA molecule a regular shape.
Plane of bases are oriented perpendicular to backbone
Hydrophillic sugar phosphate backbone winds around outside of helix
Noncovalent interactions between upper and lower surfaces of base-pairs (stacking) forms a closely packed hydrophobic interior.
Hydrophobic environment makes H-bonding between bases stronger (no competition with water)
Cause the sugar-phosphate backbone to twist.








Factors stabilizing DNA double Helix
Hydrophobic interactions – burying hydrophobic purine and pyrimidine rings in interior
Stacking interactions – van der Waals interactions between stacked bases.
Hydrogen Bonding – H-bonding between bases
Charge-Charge Interactions – Electrostatic repulsions of negatively charged phosphate groups are minimized by interaction with cations (e.g. Mg2+)