Chapter 4What Is a Nucleic Acid?Slide 3Slide 4Slide 5Could Chemical Evolution Produce Nucleotides?Nucleotides Polymerize to Form Nucleic AcidsSlide 8The Sugar-Phosphate Backbone Is DirectionalSlide 10The Polymerization of Nucleic Acids Is EndergonicSlide 12What Is the Nature of DNA's Secondary Structure?Watson and Crick’s Model of DNA’s Secondary StructureSlide 15Slide 16Slide 17Slide 18Summary of DNA’s Secondary StructureDNA Contains Biological InformationHow Does DNA Replicate?Slide 22Is DNA a Catalytic Molecule?RNA Structure and FunctionRNA’s Secondary StructureSlide 26Slide 27RNA’s VersatilityThe First Life-Form: RNAKey ConceptsSlide 31© 2011 Pearson Education, Inc.CHAPTER 4© 2011 Pearson Education, Inc.What Is a Nucleic Acid?•A nucleic acid is a polymer of nucleotide monomers.•Nucleotides are each composed of a phosphate group, a sugar, and a nitrogenous base.–The sugar is ribose in ribonucleotides and deoxyribose in deoxyribonucleotides.•There are two groups of nitrogenous bases: –purines (adenine, guanine)–pyrimidines (cytosine, uracil, and thymine)•Uracil (U) is found only in ribonucleotides, and thymine (T) is found only in deoxyribonucleotides.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Could Chemical Evolution Produce Nucleotides?•Simulations of chemical evolution have not yet produced nucleotides. •Sugars and purines are easily made, but pyrimidines and ribose are not easily synthesized. •Ribose problem: Ribose would have had to have been dominant on ancient Earth for nucleic acids to form.© 2011 Pearson Education, Inc.Nucleotides Polymerize to Form Nucleic Acids Nucleic acids form when nucleotides polymerize.•A condensation reaction forms a phosphodiester linkage (phosphodiester bond) between the phosphate group on the 5′ carbon of one nucleotide and the –OH group on the 3′ carbon of another.•Types of nucleotides involved: –Ribonucleotides, which contain the sugar ribose and form RNA –Deoxyribonucleotides, which contain the sugar deoxyribose and form DNA© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The Sugar-Phosphate Backbone Is Directional•The sugar-phosphate backbone of a nucleic acid is directional—one end has an unlinked 5′ carbon, and the other end has an unlinked 3′ carbon. •The nucleotide sequence is written in the 5′  3′ direction. This reflects the sequence in which nucleotides are added to a growing molecule.•This nucleotide sequence comprises the nucleic acid’s primary structure.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The Polymerization of Nucleic Acids Is Endergonic•Polymerization of nucleic acids is an endergonic process catalyzed by enzymes.•Energy for polymerization comes from the phosphorylation of the nucleotides.–Phosphorylation is the transfer of one or more phosphate groups to a substrate molecule. This raises the potential energy of the substrate and enables endergonic reactions.•In nucleic acid polymerization, two phosphates are transferred, creating a nucleoside triphosphate.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.What Is the Nature of DNA's Secondary Structure? •Erwin Chargaff established two empirical rules for DNA: 1. The total number of purines and pyrimidines is the same.2. The numbers of A’s and T’s are equal and the numbers of C’s and G’s are equal.© 2011 Pearson Education, Inc.Watson and Crick’s Model of DNA’s Secondary Structure•James Watson and Francis Crick determined: 1. DNA strands run in an antiparallel configuration.2. DNA strands form a double helix.–The hydrophilic sugar-phosphate backbone faces the exterior.–Nitrogenous base pairs face the interior.3. Purines always pair with pyrimidines.–Specifically, strands form complementary base pairs A-T and G-C.–A-T have two hydrogen bonds.–C-G have three hydrogen bonds.–DNA has two different sized grooves: the major groove and the minor groove.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.file:///Users/ericarowe/Documents/UTK%20Biology/UTK%20Bio140%20Fall%202011/Chapter_04/A_PowerPoint_Lecture_Tools/04_Lecture_Outline/DNA_double_helix.html© 2011 Pearson Education, Inc.file:///Users/ericarowe/Documents/UTK%20Biology/UTK%20Bio140%20Fall%202011/Chapter_04/A_PowerPoint_Lecture_Tools/04_Lecture_Outline/StructureOfRNAandDNA.html© 2011 Pearson Education, Inc.Summary of DNA’s Secondary StructureDNA’s secondary structure consists of two antiparallel strands twisted into a double helix. The molecule is stabilized by hydrophobic interactions in its interior and by hydrogen bonding between the complementary base pairs A-T and G-C.file:///Users/ericarowe/Documents/UTK%20Biology/UTK%20Bio140%20Fall%202011/Chapter_04/A_PowerPoint_Lecture_Tools/04_Lecture_Outline/Hydrogen_bonds_DNA.html© 2011 Pearson Education, Inc.DNA Contains Biological Information •DNA can store and transmit biological information.•The language of nucleic acids is contained in the sequence of the bases.•DNA carries the information required for the growth and reproduction of all cells.© 2011 Pearson Education, Inc.How Does DNA Replicate? •Complementary base pairing provides a simple mechanism for DNA replication – each strand can serve as a template for the formation of a new complementary strand.•DNA replication requires two steps: 1. Separation of the double helix2. Hydrogen bonding of deoxyribonucleotides with complementary bases on the original template strand, followed by phosphodiester bond formation to form the complementary strand© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Is DNA a Catalytic Molecule? •DNA’s stability makes it a reliable store for genetic information – it is less reactive than RNA but more resistant to chemical degradation. Stable molecules such as DNA make poor catalysts. •Because DNA does not appear to be able to catalyze any chemical reaction, biologists think that the first life-form was made of RNA, not DNA.© 2011 Pearson Education, Inc.RNA Structure and FunctionLike DNA, RNA has a primary structure consisting of a sugar-phosphate backbone formed by phosphodiester linkages and, extending from that backbone, a sequence of four types of nitrogenous bases.•The primary structure of RNA differs from DNA in two ways:1. RNA contains uracil instead of thymine.2. RNA