Download Ch. 17 From Gene to Protein

Chapter 17
From Gene to Protein
Protein Synthesis
 The information content of DNA
 Is in the form of specific sequences of nucleotides along the
DNA strands
 The DNA inherited by an organism
 Leads to specific traits by dictating the synthesis of proteins
 The process by which DNA directs protein synthesis, gene
expression
 Includes 2 stages, called transcription and translation
 http://www.youtube.com/watch?v=41_Ne5mS2ls
 http://www.youtube.com/watch?v=983lhh20rGY&feature=relate
d
Protein Synthesis Overview
 Transcription
 Is the synthesis of RNA under the direction of DNA
 Produces messenger RNA (mRNA)
 http://www.youtube.com/watch?v=vJSmZ3DsntU&feature=relat
ed
 Translation
 Is the actual synthesis of a polypeptide, which occurs under the
direction of mRNA
 Occurs on ribosomes
 Involves mRNA, tRNA and rRNA
 http://www.youtube.com/watch?v=D5vH4Q_tAkY&feature=rela
ted
Protein Synthesis Overview
Nuclear
envelope
In eukaryotes
 RNA transcripts are
modified before
becoming true mRNA
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
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The Genetic Code
How many bases correspond to an amino acid?
 Genetic information
 Is encoded as a sequence of nonoverlapping base
triplets, or codons
 4 bases allow for 64 (= 43) different codons
 Codons must be read in the correct reading frame
 For the specified polypeptide to be produced
Converting DNA to Amino Acids
During transcription
 The gene determines the sequence of bases
along the length of an mRNA molecule
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand 3
5
A C C A A A C C G A G T
(template)
TRANSCRIPTION
mRNA
5
U G G U U U G G C U C A
Codon
TRANSLATION
Protein
Trp
Amino acid
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Phe
Gly
Ser
3
Universal Genetic Code
CUU
CUC
C
CUA
CUG
CCU
CCC
Leu CCA
CCG
Pro
AUU
AUC
A
AUA
AUG
ACU
ACC
ACA
ACG
Thr
GUU
G GUC
GUA
GUG
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lle
Met or
start
GCU
GCC
Val
GCA
GCG
Ala
G
U
UGU
Cys
UGC
C
UGA Stop A
UGG Trp G
U
CAU
CGU
His
CAC
CGC
C
Arg
CAA
CGA
A
Gln
CAG
CGG
G
U
AAU
AGU
Asn
Ser
C
AAC
AGC
A
AAA
AGA
Lys
Arg
G
AAG
AGG
U
GAU
GGU
Asp
C
GAC
GGC
Gly
GAA
GGA
A
Glu
GAG
GGG
G
Third mRNA base (3 end)
 Is either
translated into
an amino acid
or serves as a
stop signal
First mRNA base (5 end)
 A codon in mRNA
Second mRNA base
U
C
A
UAU
UUU
UCU
Tyr
Phe
UAC
UUC
UCC
U
UUA
UCA Ser UAA Stop
UAG Stop
UUG Leu UCG
Mutations
 Mutations
 Are changes in the genetic material of a cell
 Point mutations
 Are changes in just one base pair of a gene
 Can be divided into two general categories
 Base-pair substitutions
 Base-pair insertions or deletions
Mutations
 A base-pair substitution
Wild type
mRNA
 Is the replacement of
one nucleotide and
its partner with
another pair of
nucleotides
 Can cause
missense or
nonsense
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A U G
A A G U U U G G C U A A
5
Protein
Lys
Met
3
Phe
Gly
Amino end
Stop
Carboxyl end
Base-pair substitution
No effect on amino acid sequence
U instead of C
A U G A A G U U U G G U U
Lys
Met
Missense
Phe
Gly
A A
Stop
A instead of G
A U G
A A G U U U
Lys
Met
Phe
A G U U
Ser
A A
Stop
Nonsense
U instead of A
A U G U
Met
A G U U U G G C U A
Stop
A
Mutations
Wild type
mRNA
5
Protein
 Insertions and deletions
 Are additions or losses
of nucleotide pairs in a
gene
 May produce frameshift
mutations
A U G A
Met
A
G U U U
Lys
G G C U A A
Gly
Phe
Amino end
Base-pair insertion or deletion
Carboxyl end
Frameshift causing immediate nonsense
Extra U
A U G U
Met
A
A G U U
U
Missing
A G U U G G C U A A
G A
Met
Lys
Leu
Ala
Insertion or deletion of 3 nucleotides:
no frameshift but extra or missing amino acid
A
Met
U A
Stop
Frameshift causing
extensive missense
A U
U G G C
A G
Missing
A U G U U U G G C U
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Stop
Phe
Gly
A A
Stop
3
Consequences of a Point Mutation
The change of a
single nucleotide
in the DNA’s
template strand
 Leads to the
production of
an abnormal
protein
Wild-type hemoglobin DNA
Mutant hemoglobin DNA
3
5 3
C T T
C A T
mRNA
mRNA
G A A
5
G U A
3 5
3
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
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5
Transcription: Some Details to
Understand
RNA synthesis
 Is catalyzed by RNA polymerase, which pries the
DNA strands apart and hooks together the RNA
nucleotides
 Follows the same base-pairing rules as DNA, except
that in RNA, uracil substitutes for thymine
Synthesis of an mRNA Transcript
Promoter
Transcription unit
5
3
3
5
Start point
RNA polymerase
 The stages of
transcription
are
 Initiation
DNA
Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
1
5
3
Unwound
DNA
3
5
Template strand of
DNA
transcript
2 Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5  3 . In the wake of
transcription, the DNA strands re-form a double helix.
Rewound
RNA
RNA
5
3
3
5
3
5
RNA
 Elongation
 Termination
transcript
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
5
3
3
5
5
Completed RNA
transcript
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3
Non-template
strand of DNA
Elongation
RNA nucleotides
RNA
polymerase
Detailed View of
Synthesis of a
mRNA transcript
A
T
C
C
A
A
3
3 end
U
5
5
Newly made
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RNA
A
E
G
C
A
T
A
G
G
T
T
Direction of transcription
(“downstream”)
Template
strand of DNA
Processing of pre-mRNA
 Each end of a pre-mRNA molecule is modified
in a particular way
 The 5 end receives a modified nucleotide cap
 The 3 end gets a poly-A tail
A modified guanine nucleotide
added to the 5 end
TRANSCRIPTION
RNA PROCESSING
50 to 250 adenine nucleotides
added to the 3 end
DNA
Pre-mRNA
5
mRNA
Protein-coding segment
Polyadenylation signal
3
G P P P
AAUAAA
AAA…AAA
Ribosome
TRANSLATION
5 Cap
5 UTR
Start codon Stop codon
Polypeptide
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3 UTR
Poly-A tail
More pre-mRNA Processing
RNA splicing
 Removes introns and joins exons
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
5 Exon Intron
Pre-mRNA 5 Cap
30
31
1
Coding
segment
mRNA
Ribosome
Intron
Exon
Exon
3
Poly-A tail
104
105
146
Introns cut out and
exons spliced together
TRANSLATION
Polypeptide
mRNA 5 Cap
1
3 UTR
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Poly-A tail
146
3 UTR
Translation: Some Details to
Understand
 A cell translates an mRNA message into
protein
 With the help of transfer RNA (tRNA)
 Molecules of tRNA are not all identical
 Each carries a specific amino acid on one end
 Each has an anticodon on the other end
Structure of tRNA
5
3
A tRNA molecule
Amino acid
attachment site
Hydrogen
bonds
Consists of a single RNA
strand that is only about
80 nucleotides long
A AG
3
Anticodon
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5
Anticodon
Ribosomes: Some Details to
Understand
Ribosomes
 Facilitate the specific coupling of tRNA anticodons
with mRNA codons during protein synthesis
 Ribosomes are made of 2 subunits, one large and
one small
 Constructed of proteins and RNA molecules named
ribosomal RNA or rRNA
 http://www.youtube.com/watch?v=Jml8CFBWcDs
Ribosome Structure
 The ribosome
has three
binding sites
for tRNA
 The P site
P site (Peptidyl-tRNA
binding site)
A site (AminoacyltRNA binding site)
E site
(Exit site)
Large
subunit
E
mRNA
binding site
 The A site
 The E site
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P
A
Small
subunit
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Overview of
Translation
Amino
acids
Polypeptide
tRNA with
amino acid
Ribosome attached
Gly
tRNA
Anticodon
A A A
U G G U U U G G C
Codons
5
mRNA
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3
Initiation of Translation
The initiation stage of translation
 Brings together mRNA, tRNA bearing the first
amino acid of the polypeptide, and two
subunits of a ribosome
P site
3 U A C 5
5 A U G 3
Initiator tRNA
Large
ribosomal
subunit
GTP
GDP
E
A
mRNA
5
Start codon
mRNA binding site
5
3
Small
ribosomal
subunit
1
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3
Translation initiation complex
2
Elongation of the Polypeptide Chain
TRANSCRIPTION
Amino end
of polypeptide
DNA
mRNA
Ribosome
TRANSLATION
In the elongation
stage of
translation
 Amino acids
are added
one by one to
the preceding
amino acid
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Polypeptide
mRNA
E
3
P A
site site
5
2 GTP
2 GDP
E
E
P
P
A
GDP
GTP
E
P
A
A
Termination of Translation
 The final stage of translation is termination
 When the ribosome reaches a stop codon in
the mRNA
Release
factor
Free
polypeptide
5
3
3
5
5
Stop codon
(UAG, UAA, or UGA)
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3
Key Points of Chapter 17
 Genes specify proteins via transcription and
translation
 Transcription is the DNA directed synthesis of
RNA
 Eukaryotic cells modify RNA after transcription
 Translation is the RNA directed synthesis of a
polypeptide
 Mutations can affect protein structure and function