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Nucleic Acids

Nucleic Acids Polynucleotides whose primary structure

contains a code or set of directions by which they can duplicate themselves and guide the synthesis of proteins.

2 types of nucleic acids:

1. DNA found mainly in the nucleus, contains genetic codes to make RNA

2. RNA found in the cytoplasm although synthesized in the nucleus, contains codes to make proteins

What are the monomers of polynucleotides?

Polynucleotides are composed of nucleotides.

PHOSPATE SUGAR

Ribose or

Deoxyribose

NUCLEOTIDE

BASE

PURINES PYRIMIDINES

Adenine (A)

Guanine(G)

Cytocine (C)

Thymine (T)

Uracil (U)

The sugar: Ribose for RNA & 2-deoxyribose for DNA

RIBOSE DEOXYRIBOSE

CH2OH

H

OH

C

C

OH OH

C

O

H HH

C

CH2OH

H

OH

C

C

OH H

C

O

H HH

C

The nucleobases are purine or pyrimidine derivatives

Some unusual nucleobases (found principally in tRNA)

Nucleoside = base + sugar

Purine Nucleotides

Pyrimidine Nucleotides

How do nucleotides combine to give nucleic acids?

By formation of an ester bond from one nucleotide to the next. Result: 2 ester bonds (3 of one nucleoside and 5 of the next nucleoside), a 3,5-phosphodiester bond.

The Structure of DNA In terms of amounts, A=T and G=C (Chargaffs

Rule, 1950, Austrian chemist who taught in Columbia U.). This led to the conclusion that the DNA structure is composed of 2 polynucleotides forming a helix (Watson & Crick, 1952).

What stabilizes the helix? H-bonds between bases in opposite chains & base stacking

Russian chemist Phoebus Levene: discovered the phosphate-sugar-base order in nucleotide, discovered the sugar (ribose) component of RNA, discovered the sugar (deoxyribose) component of DNA

The Structure of DNA Chargaffs 2nd Rule

The Structure of DNA Base Pairing

Are there other conformations of the double helix?

Interactions of bases: base stacking

In standard B-DNA, each base pair is rotated 32 wrt the preceding base pair: optimal for maximal base pairing, not optimal for maximal overlap of bases, H2O gets into minor groove

Propeller-twist: optimal for maximal base overlap, less optimal for maximal base pairing, H2O is eliminated from minor groove

Supercoiling in Prokaryotes

Supercoiling in Eukaryotes

Denaturation of DNA Involves breaking of H-bonds between base

pairs and the disruption of stacking interactions.

Usually carried out by heating the DNA in solution.

Greater amount of G-C pairs, higher melting temperature of a DNA molecule.

G-C pairs are also more hydrophobic, so they stack better, so higher melting temp.

The principal kinds of RNA