The document summarizes key aspects of the genetic code and transcription. It describes how the genetic code is written in mRNA using triplets of nucleotides that specify amino acids. It also explains that transcription in eukaryotes involves RNA polymerase II, promoters, and results in a pre-mRNA that undergoes splicing to remove introns and produce the mature mRNA. Visualization by electron microscopy has provided insights into the transcription process.

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

5. Copyright © 2009 Pearson Education, Inc.
• 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

7. Copyright © 2009 Pearson Education, Inc.
• 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

10. Copyright © 2009 Pearson Education, Inc.
• 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

12. Copyright © 2009 Pearson Education, Inc.
• Transcription
begins with
template
binding by
RNA
polymerase at a
promoter.
• The s subunit
is responsible
for promoter
recognition (in
bacteria).
Section 14.10

13. Copyright © 2009 Pearson Education, Inc.
• Transcription begins at the transcription
start site, where the DNA double helix is
unwound to make the template strand
accessible.
Section 14.10

14. Copyright © 2009 Pearson Education, Inc.
• 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

15. Copyright © 2009 Pearson Education, Inc.
• 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

17. Copyright © 2009 Pearson Education, Inc.
• 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

21. Copyright © 2009 Pearson Education, Inc.
• 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