genetic code

1. Copyright © 2009 Pearson Education, Inc.
PowerPoint® Lecture Presentation for
Concepts of Genetics
Ninth Edition
Klug, Cummings, Spencer, Palladino
Chapter 14
The Genetic Code and Transcription
Lectures by David Kass with contributions from
John C. Osterman.
Copyright © 2009 Pearson Education, Inc.

2. Copyright © 2009 Pearson Education, Inc.
• The genetic code is:
• written in linear form – composed
of mRNA
• RNA derived from
complementary bases in DNA
• In mRNA, triplet codons specify
1 amino acid
• code contains “start” and “stop”
signals
• unambiguous
• degenerate
• commaless
• nonoverlapping
• nearly universal
Section 14.1

3. Copyright © 2009 Pearson Education, Inc.
• Genetic code is
degenerate,
w/many amino
acids specified
by more than
one codon.
• Only tryptophan
and methionine
are encoded by
a single codon.
Section 14.4

4. Copyright © 2009 Pearson Education, Inc.
• Wobble
hypothesis
predicts that
hydrogen bonding
between the codon
and anticodon at
the third position is
subject to modified
base-pairing rules.
Section 14.4

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• The genetic code shows order in that
chemically similar amino acids often share
one or two middle bases in the triplets
encoding them.
Section 14.4

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• The initial amino acid incorporated into all
proteins is a modified form of
methionine—N-formylmethionine (fmet).
(in bacteria)
• AUG is the only codon to encode for
methionine.
• Initiator codon
• When AUG appears internally in mRNA,
an unformylated methionine is inserted
into the protein.
Section 14.4

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• Three codons (UAG, UAA, and UGA)
serve as termination codons and do not
code for any amino acid.
Section 14.4

8. Copyright © 2009 Pearson Education, Inc.
• The Genetic Code Is Nearly Universal
• Mitochondrial DNA revealed some
exceptions to the universal genetic code.
Section 14.6

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• In some viruses, overlapping genes have
been identified in which initiation at different
AUG positions out of frame with one another
leads to distinct polypeptides.
Section 14.7

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• mRNA serves as the intermediate
molecule between DNA and proteins.
• mRNA is synthesized on a DNA template
during transcription.
Section 14.8
http://ichristianschool.org/images/mrna.gif

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• RNA polymerase directs the synthesis of
RNA using a DNA template.
• No primer is required for initiation
• The enzyme uses ribonucleotides instead
of deoxyribonucleotides.
Section 14.10

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• Transcription
begins with
template
binding by
RNA
polymerase at a
promoter.
• The s subunit
is responsible
for promoter
recognition (in
bacteria).
Section 14.10

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• Transcription begins at the transcription
start site, where the DNA double helix is
unwound to make the template strand
accessible.
Section 14.10

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• E. coli promoters have two consensus
sequences, TTGACA and TATAAT,
positioned at –35 and –10 with respect to
the transcription initiation site.
Section 14.10

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• Once initiation
has been
completed with
the synthesis of
the first 8–9
nucleotides,
sigma (s)
dissociates and
elongation
proceeds with the
core enzyme.
Section 14.10

16. Copyright © 2009 Pearson Education, Inc.
• At the end of the gene, transcription
terminates due to hairpin formation in the
RNA.
• In some cases, termination depends on
the rho () termination factor.
Section 14.10

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• Transcription in Eukaryotes Differs from
Prokaryotic Transcription in Several Ways
• Occurs in nucleus and is not coupled to
translation.
• Requires chromatin remodeling.
• In addition to promoters, enhancers also
influence transcription regulation.
• Eukaryotic mRNAs require processing to
produce mature mRNAs.
Section 14.11

18. Copyright © 2009 Pearson Education, Inc. Table 14.7
Eukaryotes possess three forms of RNA
polymerase, each of which transcribes different
types of genes.

19. Copyright © 2009 Pearson Education, Inc.
• RNA polymerase II (RNP II) promoters
have a core promoter element and
promoter and enhancer elements.
• The TATA box is a core promoter element
that binds the TATA-binding protein (TBP)
of transcription factor TFIID and
determines the start site of transcription.
• CAAT box
Section 14.11

20. Copyright © 2009 Pearson Education, Inc.
• General transcription factors are
required for all RNP II mediated
transcription and help RNA polymerase II
bind to the promoter and initiate
transcription.
Section 14.11

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• Heterogeneous
nuclear RNA
(hnRNA) is
posttranscriptionally
processed by the
addition of a 5' cap
and a poly-A tail.
• Introns are removed
by splicing.
• Exons spliced
together.
Section 14.11

22. Copyright © 2009 Pearson Education, Inc.
• Introns (intervening sequences) are
regions of the initial RNA transcript that
are not expressed in the amino acid
sequence of the protein.
• Introns are removed by splicing and the
exons (expressed) are joined together in
the mature mRNA.
• The size of the mature mRNA is usually
much smaller than that of the initial RNA.
Section 14.12

23. Copyright © 2009 Pearson Education, Inc. Figure 14.12

24. Copyright © 2009 Pearson Education, Inc. Table 14.8

25. Copyright © 2009 Pearson Education, Inc. Figure 14.13
Group 1 Introns

26. Copyright © 2009 Pearson Education, Inc.
• Pre-mRNA introns
are spliced out by
the spliceosome
in a reaction
involving the
formation of a
lariat structure.
Section 14.12

27. Copyright © 2009 Pearson Education, Inc. Figure 14.15
14.13 Transcription Has Been Visualized by
Electron Microscopy