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Active Lecture PowerPoint® Presentation for
Essentials of Genetics
Seventh Edition
Klug, Cummings, Spencer, Palladino
Chapter 12
The Genetic Code and
Transcription
Copyright © 2010 Pearson Education, Inc.
Outline
• Overview of gene expression
• How is genetic information encoded?
• How is information transferred from DNA to RNA
• Differences between Prokaryotes & Eukaryotes
• Summary (animation)
Gene Expression
DNA
Transcription
Step 1
mRNA
Translation
Step 2
Gene Expression
Protein
Gene
Expression
Step 1
Transcription
Step 2
Translation
Gene Expression
• How is genetic information encoded?
• The Genetic code
• How does the information transferred
from DNA to RNA?
• Transcription
The Genetic Code
• Written in linear form
• Uses ribonucleotide bases that compose
mRNA molecules as “letters”
• Sequence of RNA is derived from the
complementary bases in the template
strand of DNA
The
Genetic
Code
Figure 13-7
Copyright © 2006 Pearson Prentice Hall, Inc.
The Genetic Code
• In mRNA, triplet codons specify one
amino acid
• Code contains “start” and “stop” signals
• Code is unambiguous, degenerate,
commaless, nonoverlapping, and nearly
universal
The Genetic Code
• The initial amino acid incorporated into all
proteins is methionine or a modified form of
methionine (fmet)
• AUG is the only codon to encode for
methionine
• When AUG appears internally in mRNA, an
unformylated methionine is inserted into the
protein
The Genetic Code
• The degenerate code: 64 codons to
specify the 20 amino acids
• The triplet nature of the code was
revealed by frameshift mutations
DNA Problem 1:
• Following is a sequence of a nontemplate strand
of DNA
5’ ATGCGAATTAGTCCGCAT 3’
Assuming that transcription begins with the first
nucleotide and ends with the last, write the
sequence of the transcript (mRNA) in the
conventional form
DNA Problem 2:
• Using the genetic code, translate the transcript
(mRNA sequence) in problem 1 into amino acid
sequence
nontemplate
template
5’ ATGCGAATTAGTCCGCAT 3’
3’ TACGCTTAATCAGGCGTA 5’
mRNA
amino acid
5’ AUGCGAAUUAGUCCGCAU 3’
... ... ... ... ... ...
Effect of
Frame-shift
mutations
Transcription
• RNA serves as the intermediate molecule
between DNA and proteins
• RNA is synthesized on a DNA template
during transcription
• Transcription selectively copies only certain
parts of the genome. Many copies of the
transcript of one gene region is made
RNA Polymerase Directs RNA
Synthesis
• RNA polymerase directs the synthesis of
RNA using a DNA template
• No primer is required for initiation. RNA
polymerase can initiate transcription de
novo
• RNA polymerase uses ribonucleotides
(rATP,rCTP, rGTP & rUTP)
Transcription in E. coli
• RNA polymerase from E. coli contains the
subunits 2a, b, b', and s
• Transcription begins by RNA polymerase
binding to template at the promoter
• The s subunit is responsible for promoter
recognition
Transcription in E. coli
• E. coli promoters have two
consensus sequences upstream of
transcription initiation site:
1. TATAAT positioned at –10
1. TTGACA positioned at –35
Prokaryotic Promoters
Steps in Transcription
1. Initiation
2. Elongation
3. Termination
Transcription
Initaition
• Transcription begins when RNA Polymerase
binds to a region of gene known as a
Promoter
Elongation
• Transcription proceeds in 5’ to 3’ direction
Termination
• Transcription stops when it reaches a region
in the gene known as Terminator
RNA Polymerase &
DNA binding
Transcription
Initiation
Transcription Elongation in E. Coli
• Once initiation completed with synthesis of
first 8–9 nucleotides, sigma (s) dissociates
and elongation proceeds with the core
enzyme
• Core enzyme (α2 β β’) elongates RNA chain by
moving along the DNA template and adding
ribonucleotides at the 3’end by forming
phosphodiester bonds
Transcription Termination in E. coli
• Transcription is terminated by signals within
the DNA sequence at the end of the gene
• Hairpin formation in RNA destabilizes the
DNA/RNA hybrid and releases RNA
transcript
• In some cases, termination depends on the
rho () termination factor
Transcription in Eukaryotes
• Occurs in the nucleus
• Is not coupled to translation
• Requires chromatin remodeling
Table 13-7
Copyright © 2006 Pearson Prentice Hall, Inc.
Eukaryotic Promoters
• TATA box (-35): a core promoter element;
transcription factors bind to them and
determines start site of transcription
• CAAT box (-80): highly conserved DNA
sequence found within promoter of many
genes; recognized by transcription factors
• Enhancers can be upstream, within, or
downstream of the gene; can modulate
transcription from a distance
Post-transcriptional Editing of
Eukaryotic mRNA
1. Addition of a 5’ cap
2. Addition of 3’ poly A tail
3. Splice out introns
Introns in Various Eukaryotic Genes
Alternative Splicing
• Introns present in pre-mRNAs derived
from the same gene can be spiced in
more than one way
• Yields group of mRNAs that, upon
translation, results in a series of related
proteins
Alternative Genome
• Read article on Alternative Genome
Simultaneous Transcription & Translation