Topic 18.docx

Introduction to DNA and RNA ● Deoxyribonucleic acid DNAis the molecule that holds a cell's genetic material, which is the information passed from parent to offspring and is required to produce and organism ● Ribonucleic acid (RNA) has several different functions; the basic ones being: ○ acting as a template from which proteins are produced (mRNA) ○ along with proteins, forming the ribosome and catalyzing protein synthesis (rRNA) ○ carrying amino acids to growing peptide chain during protein synthesis (tRNA) ● DNAand RNAare linear polymers composed of a monosaccharide, a nitrogenous base, and a phosphate group ○ DNAis composed to deoxyribonucleotide monomers; RNA of ribonucleotide monomers Monosaccharides ● Ribonucleotides contain the pentose ribose, deoxyribonucleotides contain deoxyribose which means that the 2’carbon does not bear a hydroxyl group Nitrogenous bases ● There are two types of nitrogenous bases: purines and pyrimidines; both are planar and hydrophobic ● Purines found in DNAand RNAare adenine (A) and guanine (G) and add via the 1’ carbon of ribose, or the 9th position nitrogen of deoxyribose ● The three types of pyrimidines are cytosine (C), thymine (T), and uracil (U) ○ C is found in both RNAand DNAbut U is only found in RNAand T only in DNA ○ Uracil is identical to thymine except that uracil lacks a methyl group ● Pyrimidines join to the 1’carbon of ribose or 1st N position of deoxyribose ● In living systems, the anomeric carbon of ribose or deoxyribose is ordinarily in the β configuration when joined to a nitrogenous base ● The sugar and base are joined via an N-glycosidic bond; a sugar joined to a nitrogenous base is called a nucleoside ○ nucleosides are: adenosine, guanosine, cytidine, thymidine, and uridine Phosphates ● Phosphates are attached to the 5’C by a phosphodiester linkage; once a phosphate is attached to that position of a nucleoside, a nucleotide is formed DNAand RNAStructure ● Mononucleotides are linked together to form an unbranched polynucleotide chain through 3’and 5’phosphodiester bonds ● The phosphate on the 5’carbon of one pentose forms a phosphodiester bond to the hydroxyl group at the 3’position on another pentose ● The ends of a polynucleotide at which the 5’carbon is not attached to another monosaccharide is called a 5’end; same goes for 3’ends ● By convention, polynucleotides are written 5’to 3’left to right; context usually makes it clear whether it is DNAor RNA ● Asingle strand of DNAis not ordinarily found in the cell, rather two polynucleotides are tightly, but noncovalently associated with each other in antiparallel fashion ○ one runs 5’to 3’and the other strand runs 3’to 5’ ● The two strands wrap around each other to form a double helix; normally the helix is right-handed (curve is in clockwise direction) ● The base pairs on the strands lie on the inside and phosphates on the outside as this minimizes charge repulsion between the phosphates and allows for salt stabilization of the negative charge on the phosphates ●