04_Lecture_Outline_Bio 1107_Fall 2011 (Nucleic).ppt

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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

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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.

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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₎

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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.

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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:

1. Ribonucleotides, which contain the sugar ribose and form RNA

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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}

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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.

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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.

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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.

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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

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DNA Contains Biological Information

• DNA can store and transmit biological information.

• The language of nucleic acids is contained in the sequence of the bases.

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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

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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.

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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}

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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.

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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.

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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