Download 28_Lecture

Organic Chemistry
6th Edition
Chapter 28
Paula Yurkanis Bruice
The Chemistry of the
Nucleic Acids
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Nucleic Acids
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In nucleic acids, the phosphate group is a
phosphodiester:
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The Bases in Nucleic Acids
• Adenine, guanine, cytosine, and thymine are found in
DNA.
• Adenine, guanine, cytosine, and uracil are found in
RNA.
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Nucleosides = Base + Sugar
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Nucleotides = Base + Sugar +
Phosphate
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Different Forms of Nucleotides
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Other Important Nucleotides
Cyclic AMP and cyclic GMP serve as a link between
several hormones and certain enzymes that regulate
cellular function
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Nitric Oxide Transduction Pathway
The cGMP second messenger causes blood vessel
dilation
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Cyclic Nucleotide Messengers Are
Terminated by Phosphodiesterases
Viagra is a phosphodiesterase
inhibitor that extends the cGMP message
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Nucleic Acids Are Composed of
Nucleotide Subunits
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DNA Is Synthesized in the 5 to 3 Direction
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Complementary Base Pairing in DNA
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Base Pairing in DNA
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The sugar–phosphate backbone is on the outside,
and the bases are on the inside:
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The DNA Double Helix
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DNA Is More Stable Than RNA
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Replication of DNA
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Transcription: synthesis of RNA from a DNA blueprint
Promoter: marks the beginning of a gene
Sense strand: informational strand
Anti-sense or template: used to synthesize RNA
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Transcription of DNA
• A stretch of bases representing a portion of a gene is
called an exon.
• A stretch of bases that contains no genetic information is
called an intron.
• mRNA is spliced prior to leaving the nucleus.
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Translation: synthesis of a protein from an RNA blueprint
There are three kinds of RNA:
• Messenger RNA (mRNA) determines the sequences of
amino acids in a protein.
• Ribosomal RNA (rRNA) is a component of ribosomes,
where protein synthesis takes place.
• Transfer RNA (tRNA) is the carrier of amino acids for
protein synthesis.
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A Transfer RNA
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The Proposed Mechanism for
Aminoacyl-tRNA Synthetase
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The Substrate-Binding Site for tRNAHis
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•
•
A protein is synthesized from its N-terminal end to its Cterminal end.
Each amino acid is specified by a three-base sequence known
as the genetic code.
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Translation
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L-Hydroxyproline
Sold in health food stores to support connective
tissue. Does it work?
No. L-Hydroxyproline is not incorporated into
connective tissue, because it has no genetic code
Proline codes
L-Hydroxyproline
CCU
CCC
CCA
CCG
L-Hydroxyproline, the major amino acid in
collagen, results from the post-translational
oxidation of proline
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Why does DNA contain thymine instead of uracil
although the synthesis of thymine is energetically
expensive?
Because thymine prevents potentially lethal
mutations
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How does the presence of thymine prevent mutations?
Because cytosine can be converted to uracil…
Having T’s in place of U’s in DNA allows the U’s that are
found in DNA to be recognized as mistakes
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Large DNA can be cleaved into smaller fragments by
restriction endonucleases:
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DNA Sequencing
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Polymerase Chain Reaction (PCR)
strand
separation
base pairing
with primers
DNA
synthesis
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Laboratory Synthesis of DNA
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Oligonucleotide
Synthesis with
Phosphoramidites
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The amino groups of the bases must be protected:
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Removal of the Protecting Group by
Ammonia
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Utilization of H-phosphate
monomer to synthesize
DNA strand
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