Chapter 4
Nucleic Acids
and the RNA
World
Lectures by Cheryl Ingram-Smith
Biological Science, Third Edition
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.
• DNA and 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 DNA strands running
What Is a Nucleic Acid?
• A nucleic 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).
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
Nucleotides Polymerize to Form Nucleic Acids
• 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
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
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.
Watson and Crick determined:
(1) Antiparallel DNA strands form a double helix—hydrophilic sugar-phosphate backbone faces the exterior, and purine– pyrimidine pairs of nitrogenous bases face the interior.
(2) DNA strands form complementary base pairs A-T and G-C. (3) The DNA double helix has two types of grooves that differ in
DNA Is a Double Helix
DNA’s Secondary Structure
Major groove
Minor groove Length of one
complete turn of helix (10 rungs per turn) 3.4 nm
Distance between bases 0.34 nm
How Does DNA Replicate?
• Complementary base pairing provides a simple mechanism for
DNA replication—each strand can serve as a template for formation of a new complementary strand.
• DNA replication requires two steps:
(1) Separation of the double helix.
(2) Hydrogen bonding of deoxyribonucleotides with
Making a Copy of DNA
DNA FORMS A TEMPLATE FOR ITS OWN SYNTHESIS.
1. If the hydrogen bonds between complementary base pairs are broken,
the DNA helix can separate. 2. Each strand of DNA can serve as a template for the formation of a new strand. Free nucleotides attach
according to complementary base pairing.
3. When the new strands polymerize to form a sugar-phosphate backbone,
secondary structure is restored.
New
New Old
Old
Making a Copy of DNA
DNA FORMS A TEMPLATE FOR ITS OWN SYNTHESIS.
1. If the hydrogen bonds between complementary base pairs are broken,
the DNA helix can separate. 2. Each strand of DNA can serve as a template for the formation of a new strand. Free nucleotides attach
Making a Copy of DNA
DNA FORMS A TEMPLATE FOR ITS OWN SYNTHESIS.
2. Each strand of DNA can serve as a template for the formation of a new strand. Free nucleotides attach
according to complementary base pairing.
3. When the new strands polymerize to form a sugar-phosphate backbone,
secondary structure is restored.
New
Old
Old
Conclusion: The original molecule has been copied. Each copy has one strand from the original DNA molecule and one new strand.
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
RNA Structure and Function
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.
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.
The First Life-Form: RNA
• RNA can both provide a template for copying itself and catalyze
the polymerization reaction to make the complementary copy.
• RNA world hypothesis: Researchers propose that the first
life-form was a self-replicating RNA molecule.
• 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 an RNA replicase could be isolated that