Key Concepts Nucleotides consist of a sugar phosphate

Key Concepts Nucleotides consist of a sugar, phosphate group, and nitrogencontaining base. Ribonucleotides polymerize to form RNA. Deoxyribonucleotides polymerize to form DNA’s primary structure consists of a sequence of nitrogencontaining bases; its secondary structure consists of two DNA strands running in opposite directions, held together by complementary base pairing, and twisted into a double helix. DNA’s structure allows organisms to store and replicate the information needed to grow and reproduce. © 2011 Pearson Education, Inc.

Key Concepts RNA’s primary structure consists of a sequence of nitrogen-containing bases. Its secondary structure includes short regions of double helices and structures called hairpins. RNA was likely the first self-replicating molecule and a forerunner to the first life-form. Know the following scientists’ contributions to DNA’ discovery: 1. Frederick Griffith - transformation 2. Avery, Mc. Carty, Mac. Leod – DNA as the genetic material 3. Alfred Hershey & Martha Chase – DNA as the genetic material 4. Linus Pauling – Protein structure & helical shape 5. Erwin Chargaff – correct base pairing 6. Maurice Wilkins & Rosalind Franklin – confirmed helical shape 7. Watson & Crick – overall shape of DNA 8. Meselson & Stahl – DNA replication © 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.

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: 1. Ribonucleotides, which contain the sugar ribose and form RNA 2. 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 a 5′ carbon, and the other end has a 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.

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 AT and G-C. – A-T have two hydrogen bonds. – C-G have three hydrogen bonds. 4. DNA has two different sized grooves: the major groove and the minor groove. © 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

Summary of DNA’s Secondary Structure • DNA’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. © 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.

RNA Structure and Function • Like DNA, RNA has a primary structure consisting of a sugarphosphate 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 contains ribose instead of deoxyribose. – The presence of the –OH group on ribose makes RNA much more reactive and less stable than DNA. © 2011 Pearson Education, Inc.

RNA’s Secondary Structure • RNA’s secondary structure results from complementary base pairing. • The bases of RNA typically form hydrogen bonds with complementary bases on the same strand. • The RNA strand folds over, forming a hairpin structure: the bases on one side of the fold align with an antiparallel RNA segment on the other side of the fold. • RNA molecules can have tertiary and quaternary structures. © 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

RNA’s Versatility • RNA is structurally, chemically, and functionally intermediate between DNA and proteins. RNA, due to its single-strandedness, is more reactive than DNA. • Like DNA, RNA can function as an information-containing molecule, and is capable of self-replication. • RNA can function as a catalytic molecule. – Ribozymes are enzyme-like RNAs. © 2011 Pearson Education, Inc.

The First Life-Form: RNA can both provide a template for copying itself and catalyze the polymerization reactions that would link monomers into a copy of that template. Thus, most origin-of-life researchers propose that the first life-form was made of RNA. • RNA is not very stable, but might have survived long enough in the prebiotic soup to replicate itself, becoming the first life-form. • Researchers found that a ribozyme called RNA replicase could be isolated that could catalyze the addition of ribonucleotides to a complementary RNA strand. © 2011 Pearson Education, Inc.