© 2011 pearson education, inc. key concepts nucleotides consist of a sugar, phosphate group, and...
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© 2011 Pearson Education, Inc.
Key Concepts
Nucleotides consist of a sugar, phosphate group, and nitrogen-containing base. Ribonucleotides polymerize to form RNA. Deoxyribonucleotides polymerize to form DNA.
DNA’s primary structure consists of a sequence of nitrogen-containing 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.
© 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.
© 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.
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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.
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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.
© 2011 Pearson Education, Inc.
© 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.
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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 helix
2. 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.
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RNA Structure and Function
Like 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 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.
© 2011 Pearson Education, Inc.
RNA’s Versatility
• RNA is structurally, chemically, and functionally intermediate between DNA and proteins.
• 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
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.