Nucleic Acid Structures

Learning objectivesUnderstand the molecular

structure of DNA. Understand the molecular

structure of RNA. Understand the relationships

between purines, pyrimidines, nucleosides, nucleotides, and nucleic acids.

Understand the structural differences between DNA and RNA

Nucleic acid functionsDNA –genetic information storehouseRNA

message between the genome and the ribosome for the building of proteins (messenger RNA, or mRNA),

act as the catalytic moiety in a RNA-protein enzyme complex (rRNA)

serve as the translator between mRNA sequence and protein sequence (tRNA)

RNA and DNA- recent discovery catalyze some reactions without protein involvement (ribozymes or deoxyribozymes).

DNA from archeological samples

Compared to modern DNA

Evidence of DNA stability

9000 year old “Kennewick man” skull discovered along the banks of the Columbia River in Washington state believed to have originated from Southeast Asia, Japan or Polynesia

SUGAR + BASE = NUCLEOSIDE

Nucleotide – monomeric unit in nucleic acid

Deoxythymidine 5’-monophosphate

Deoxyadenosine 5’-monophosphate

Deoxycytidine 5’-monophosphate

Deoxyguanosine 5’-monophosphate

N-glycosidic bond3’,5’-phosphodiester bond

C1’-N1= Pyr

C1’-N9 = Pur

Structural features of the DNA(secondary structure)Double helix; 2 topographic features –

major groove and minor grooveTwo strands antiparallelIn aqueous environment phosphate-

sugar backbone outside of the helix; purine and pyrimidine rings inside the structure

Strand stabilized by :H-bonds between complement basesVan der Waals and Hydrophobic interactions between stacked bases

A B Z

Conformational varieties of the secondary structure

Conformational varieties of the secondary structure

B-DNA – crystallized from water; water retained in the inner structure; predominant form under physiological conditions10 base pairs/turn of helixRight handedDistance bet base pairs 0.34 nmDiameter – 2.0 nm or 20 A

A-DNADehydrated form of B-DNARight handed helix11 base pairs/helixDiameter = 26 A

Z-DNAFound in short stretches of

native DNA and synthetic DNALeft handed helix12 base pairs/helixDiameter = 18 A

85-90 degrees as the melting temperature

Tm = midpoint of each curve

At normal temp Increased tem, DNA denatures

More light is absorbedSample becomes more turbid

Tertiary structure of the DNACircular (relaxed) – in E.coli; simian

virus 40; bacteriophage; certain animal species

Supercoiled DNA – extra twisting in the linear duplex; allows DNA to be more compact in the cell; regulatory role in replicationTopoisomerase – change topology of the DNA

Quadruplex DNA – four stranded; in protozoan DNA; occur in G-rich regions; regulating and stabilizing telomeres and regulation of gene expression