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Chapter 4

Chapter 4.odt

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University of Toronto Mississauga
Fiona Rawle

Chapter 4 Key Concepts • Nucleotides are monomers that consist of a sugar, a phosphate group, and a nitrogen-containing base. Ribonucleotides polymerize to form RNA. Deoxyribonucleotides polymerize to form DNA. • DNAand RNA’s primary structure consists of a sequence of nitrogen-containing bases, which contain information in the form of a molecular code. • DNA’s secondary structure consists of two DNAstrands running in opposite directions, held together by complementary base pairing, and twisted into a double helix. RNA’s secondary structure includes short double helices and structures called hairpins. What Is a NucleicAcid? • Anucleic acid is a polymer of nucleotides that 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) and Pyrimidines (cytosine, uracil, and thymine). • U is found only in ribonucleotides, and T is found only in deoxyribonucleotides. 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. Nucleotides Polymerize to Form NucleicAcids • Method: Formation of a phosphodiester bond between the phosphate group on the 5′ carbon of one nucleotide and the –OH group on the 3′ carbon of another through a condensation reaction. • Types of nucleotides involved: 1. Ribonucleotides, which contains the sugar ribose and forms RNA 2. Deoxyribonucleotides, which contains the sugar deoxyribose and forms DNA • In cells, enzyme-catalyzed polymerization of nucleotides require the addition of two extra phosphate groups. 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. 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 an
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