nucleic acid structures
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Nucleic Acid Structures
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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
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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).
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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
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SUGAR + BASE = NUCLEOSIDE
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Nucleotide – monomeric unit in nucleic acid
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Deoxythymidine 5’-monophosphate
Deoxyadenosine 5’-monophosphate
Deoxycytidine 5’-monophosphate
Deoxyguanosine 5’-monophosphate
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N-glycosidic bond3’,5’-phosphodiester bond
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C1’-N1= Pyr
C1’-N9 = Pur
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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
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A B Z
Conformational varieties of the secondary structure
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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
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A-DNADehydrated form of B-DNARight handed helix11 base pairs/helixDiameter = 26 A
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Z-DNAFound in short stretches of
native DNA and synthetic DNALeft handed helix12 base pairs/helixDiameter = 18 A
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85-90 degrees as the melting temperature
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Tm = midpoint of each curve
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At normal temp Increased tem, DNA denatures
More light is absorbedSample becomes more turbid
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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
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