Sinh học - Chapter 19 (part 1): Nucleic acids

Chapter 19 (part 1)Nucleic AcidsInformation 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. Nucleic Acids Are Essential For Information Transfer in CellsCentral Dogma of BiologyNucleic AcidsFirst 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.Was an acid compound found in nuclei therefore named nucleic acidNucleic Acids1944 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 DNANucleic AcidsNucleic acids are long polymers of nucleotides.Nucleotides contain a 5 carbon sugar, a weakly basic nitrogenous compound (base), one or more phosphate groups.Nucleosides are similar to nucleotides but have no phosphate groups.Pentoses of NucleotidesD-ribose (in RNA) 2-deoxy-D-ribose (in DNA) The difference - 2'-OH vs 2'-H This difference affects secondary structure and stability Nitrogenous BasesBases are attached by b-N-glycosidic linkages to 1 carbon of pentose sugar – (Nucleoside)NucleosidesBase is linked via a b-N-glycosidic bond The carbon of the glycosidic bond is anomeric Named by adding -idine to the root name of a pyrimidine or -osine to the root name of a purine Conformation can be syn or anti Sugars make nucleosides more water-soluble than free basesAnti- conformation predominates in nucleic acid polymersNucleotidesPhosphate ester of nucleosidesThe plane of the base is oriented perpendicular to the plane of the pentose groupOther Functions of NucleotidesNucleoside 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 metabolismNucleotide monomers are joined by 3’-5’ phosphodiester linkages to form nucleic acid (polynucleotide) polymersNucleic 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 Nucleic AcidsBases from two adjacent DNA strands can hydrogen bondGuanine pairs with cytosineAdenine pairs with thymineBase pairing evident in DNA compositionsH-bonding of adjacent antiparallel DNA strands form double helix structureProperties of DNA Double HelixDistance between the 2 sugar-phosphate backbones is always the same, give DNA molecule a regular shape.Plane of bases are oriented perpendicular to backboneHydrophillic sugar phosphate backbone winds around outside of helixNoncovalent 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.View down the Double HelixSugar-phosphatebackboneHydrophobic Interior with base pair stackingStructure of DNA Double HelixRight handed helixRise = 0.33 nm/nucleotidePitch = 3.4 nm / turn10.4 nucleotides per turnTwo groves – major and minorWithin groves, functional groups on the edge of base pairs exposed to exteriorinvolved in interaction with proteins.Factors stabilizing DNA double HelixHydrophobic interactions – burying hydrophobic purine and pyrimidine rings in interiorStacking interactions – van der Waals interactions between stacked bases.Hydrogen Bonding – H-bonding between basesCharge-Charge Interactions – Electrostatic repulsions of negatively charged phosphate groups are minimized by interaction with cations (e.g. Mg2+)