Upload
dodat
View
214
Download
2
Embed Size (px)
Citation preview
11/18/2014
1
Copyright © 2009 Pearson Education, Inc.
CAMPBELL
BIOLOGY Reece • Urry • Cain • Wasserman • Minorsky • Jackson
© 2014 Pearson Education, Inc.
TENTH
EDITION
CAMPBELL
BIOLOGY Reece • Urry • Cain • Wasserman • Minorsky • Jackson
© 2014 Pearson Education, Inc.
TENTH
EDITION
17 Gene to Protein
Lecture Presentation by
Dr Burns
NVC Biol 120
Gene Expression
The process in which the information
coded in DNA is used to make proteins
A gene is the part of the DNA molecule
that codes for a specific protein
Gene expression, the process by which
DNA directs protein synthesis, includes
two stages: transcription and translation
Figure 17.1
Protein Synthesis Overview
Remember back to the lecture on the cell
we briefly went through the steps of
protein synthesis
11/18/2014
2
What structure assembles the polypeptide chains?
1. Nucleus
2. Rough
Endoplasmic
reticulum
3. Ribosomes
4. Lysosomes
Nucl
eus
Rough
Endop
lasm
ic ..
.
Rib
osom
es
Lys
osom
es
25% 25%25%25%
The Nature of Genes
The central dogma of molecular biology
states that information flows in one
direction:
DNA RNA protein
Transcription is the flow of information from
DNA to RNA.
Translation is the flow of information from
RNA to protein.
1.Transcription: DNA
unwinds and nucleotides
form base pairs to
produce a single strand of
mRNA
2. mRNA leaves nucleus
3. Translation: mRNA docks
with ribosomes. tRNA
brings amino acids to the
ribosome to be assembled
into polypeptide
RNA
During the transcription: part of DNA is copied
to make mRNA
mRNA is only a single strand – remember that
DNA is two strands.
RNA has same “handrail” structure with the
phosphates covalently bound to the sugars.
The sugars are bound covalently to bases
11/18/2014
3
Differences between RNA and DNA
1. RNA is single stranded, DNA is double
stranded
2. The sugar is different = ribose
3. RNA has four bases, but one base is
different from DNA: CGAU, the U is uracil
During transcription uracil is paired with adenine
Types of RNA
There are different types of RNA:
Messenger RNA (mRNA) – single strand, carries
information for making a protein from the nucleus to
the cytosol
Transfer RNA (tRNA) – single strand, folds back on
itself. Each tRNA carries one specific amino acid
and brings it to the ribosome
Ribosomal RNA (rRNA) – globular structure, forms
the catalytic part of ribosomes. Catalyzes the
formation of the peptide bonds between amino
acids
Types of RNA
There are different types of RNA:
small nuclear RNA (snRNA) are involved in
processing pre-mRNA, involved in splicing mRNA
signal recognition particle (SRP) is composed of
protein and RNA and involved in directing ribosome
to the RER
micro-RNA (miRNA) are very small and their role is
not clear yet, control gene expression, protect cells
from viral attack
Evolution of the Genetic Code
The genetic code is nearly universal, shared by
the simplest bacteria to the most complex
animals
Genes can be transcribed and translated after
being transplanted from one species to another
© 2011 Pearson Education, Inc.
Figure 17.6
(a) Tobacco plant expressing a firefly gene gene
(b) Pig expressing a jellyfish
Figure 17.4
DNA template strand
TRANSCRIPTION
mRNA
TRANSLATION
Protein
Amino acid
Codon
Trp Phe Gly
5
5
Ser
U U U U U
3
3
5 3
G
G
G G C C
T
C
A
A
A A A A A
T T T T
T
G
G G G
C C C G G
DNA molecule
Gene 1
Gene 2
Gene 3
C C
11/18/2014
4
Translation
Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6
Polypeptide
Coding strand
Transcription DNA
Template strand
mRNA
(complementary
copy of template
DNA strand)
Molecular Components of Transcription
RNA synthesis is catalyzed by RNA
polymerase, which pries the DNA strands
apart and hooks together the RNA nucleotides
The RNA is complementary to the DNA
template strand
RNA synthesis follows the same base-pairing
rules as DNA, except that uracil substitutes for
thymine
© 2011 Pearson Education, Inc.
The DNA sequence where RNA polymerase
attaches is called the promoter
The stretch of DNA that is transcribed is called
a transcription unit
© 2011 Pearson Education, Inc.
Transcription Transcription Transcription
RNA polymerase act in a similar manner as
DNA polymerase
Build the RNA molecule from 5’ end of the RNA
to the 3’ end
Nucleotides are added to build the
complementary RNA strand, use the hydrolysis
of phosphates to provide the energy to build the
new strand
RNA is antiparallel to DNA
Transcription
Upstream Downstream
Template strand: 3’-A-C-C-A-5’
Coding strand: 5’-T-G-G-T-3’
RNA 5’-U-G-G-U-3’
Figure 17.7-1 Promoter
RNA polymerase
Start point DNA
5 3
Transcription unit
3 5
11/18/2014
5
Figure 17.7-2 Promoter
RNA polymerase
Start point DNA
5 3
Transcription unit
3 5
Initiation
5 3
3 5
Coding (Nontemplate) strand of DNA
Template strand of DNA RNA transcript
Unwound DNA
1
Figure 17.7-3 Promoter
RNA polymerase
Start point DNA
5 3
Transcription unit
3 5
Elongation
5 3
3 5
Coding (Nontemplate) strand of DNA
Template strand of DNA RNA transcript
Unwound DNA
2
3 5 3
5 3
Rewound DNA
RNA transcript
5
Initiation 1
Figure 17.7-4 Promoter
RNA polymerase
Start point DNA
5 3
Transcription unit
3 5
Elongation
5 3
3 5
Nontemplate strand of DNA
Template strand of DNA RNA transcript
Unwound DNA
2
3 5 3
5 3
Rewound DNA
RNA transcript
5
Termination 3
3
5
5 Completed RNA transcript
Direction of transcription (“downstream”)
5 3
3
Initiation 1
© 2011 Pearson Education, Inc.
Animation: Transcription
Right-click slide / select “Play”
Copyright © 2009 Pearson Education, Inc.
Which DNA strand is copied to make RNA?
1 2
50%50%1. Coding
2. Template
11/18/2014
6
Copyright © 2009 Pearson Education, Inc.
Transcription
http://vcell.ndsu.nodak.edu/animations/tran
scription/index.htm
Transcription
RNA polymerase binds to a promoter region of
the DNA = initiation stage
The RNA polymerase acts to bind nucleotides
bound together to form the complementary
RNA strand = elongation stage
The RNA polymerase continues to add
nucleotides until it comes to a stop signal, a set
of three nucleotides on the DNA that signals
the end = termination stage
RNA Polymerase Binding and Initiation of
Transcription
Promoters signal the transcriptional start point and usually extend several dozen nucleotide pairs upstream of the start point
Transcription factors mediate the binding of RNA polymerase and the initiation of transcription
The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex
A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes
© 2011 Pearson Education, Inc.
Figure 17.8
Transcription initiation
complex forms
3
DNA
Promoter Nontemplate strand
5
3
5
3
5
3
Transcription
factors
RNA polymerase II
Transcription factors
5
3
5
3
5
3
RNA transcript
Transcription initiation complex
5 3
TATA box
T
T T T T T
A A A A A
A A
T
Several transcription
factors bind to DNA
2
A eukaryotic promoter 1
Start point Template strand
Elongation of the RNA Strand
As RNA polymerase moves along the DNA, it
untwists the double helix, 10 to 20 bases at a
time
Transcription progresses at a rate of 40
nucleotides per second in eukaryotes
Nucleotides are added to the 3 end of the
growing RNA molecule
© 2011 Pearson Education, Inc.
Nontemplate
strand of DNA
RNA nucleotides
RNA
polymerase
Template
strand of DNA
3
3 5
5
5
3
Newly made
RNA
Direction of transcription
A A A
A
T
T T T G
C
C C
G
C C C A A U
end
Figure 17.9
11/18/2014
7
If the DNA sequence was 3’-ATCG-5’ then the
complementary mRNA sequence would be:
1 2 3 4
25% 25%25%25%1. 3’-TAGC-5’
2. 5’-TACG-3’
3. 3’-UAGC-5’
4. 5’-UAGC-3’
Termination of Transcription
The mechanisms of termination are different in
bacteria and eukaryotes
In bacteria, the polymerase stops transcription
at the end of the terminator and the mRNA can
be translated without further modification
In eukaryotes, RNA polymerase II transcribes
the polyadenylation signal sequence; the RNA
transcript is released 10–35 nucleotides past
this polyadenylation sequence
Figure 17.10
Protein-coding
segment
Polyadenylation
signal 5 3
3 5 5 Cap UTR Start
codon
G P P P
Stop
codon UTR
AAUAAA
Poly-A tail
AAA AAA …
Protein synthesis in Prokaryotes
In bacteria the transcription and translation
steps are coupled
Remember that there is no nucleus so the
mRNA does not leave the nucleus to go to
the cytosol to bind with the ribosomes
The bacterial mRNA is not modified after it is
transcribed, it is used immediately after
transcription
Figure 17.3
DNA
mRNA
Ribosome
Polypeptide
TRANSCRIPTION
TRANSLATION
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
DNA
mRNA
Pre-mRNA
RNA PROCESSING
(a) Bacterial cell
(b) Eukaryotic cell
Nuclear
envelope
Eukaryotic cells modify RNA after transcription
Enzymes in the eukaryotic nucleus modify pre-
mRNA (RNA processing) before the genetic
messages are dispatched to the cytoplasm
During RNA processing, both ends of the
primary transcript are usually altered
Also, usually some interior parts of the
molecule are cut out, and the other parts
spliced together
© 2011 Pearson Education, Inc.
11/18/2014
8
Alteration of mRNA Ends
Each end of a pre-mRNA molecule is modified in a particular way
The 5 end receives a modified nucleotide 5 cap
The 3 end gets a poly-A tail
These modifications share several functions
They seem to facilitate the export of mRNA to the cytoplasm
They protect mRNA from hydrolytic enzymes
They help ribosomes attach to the 5 end
© 2011 Pearson Education, Inc.
Figure 17.10
Protein-coding
segment
Polyadenylation
signal 5 3
3 5 5 Cap UTR Start
codon
G P P P
Stop
codon UTR
AAUAAA
Poly-A tail
AAA AAA …
Split Genes and RNA Splicing
The mRNA contains regions that do not code
for amino acids that are found between coding
regions
Areas of the gene that are noncoding = introns
Coding areas of the gene = exons
RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence
Figure 17.11
5 Exon Intron Exon
5 Cap Pre-mRNA Codon numbers
130 31104
mRNA 5 Cap
5
Intron Exon
3 UTR
Introns cut out and
exons spliced together
3
105
146
Poly-A tail
Coding
segment
Poly-A tail
UTR 1146
11/18/2014
9
The Functional and Evolutionary Importance of
Introns
Some introns contain sequences that may
regulate gene expression
Some genes can encode more than one kind of
polypeptide, depending on which segments are
treated as exons during splicing
This is called alternative RNA splicing
Consequently, the number of different proteins an
organism can produce is much greater than its
number of genes
© 2011 Pearson Education, Inc.
Fig. 16.17
RNA splicing
In some cases, RNA splicing is carried out by
spliceosomes
Spliceosomes consist of a variety of proteins
and several small nuclear ribonucleoproteins
(snRNPs) that recognize the splice sites
© 2011 Pearson Education, Inc.
11/18/2014
10
Ribozymes
Ribozymes are catalytic RNA molecules that
function as enzymes and can splice RNA
The discovery of ribozymes rendered
obsolete the belief that all biological catalysts
were proteins
© 2011 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
mRNA modifications
http://vcell.ndsu.nodak.edu/animations/mrn
aprocessing/index.htm
Translation is the RNA-directed synthesis of
a polypeptide: a closer look
Genetic information flows from mRNA to
protein through the process of translation
© 2011 Pearson Education, Inc.
Translation
http://vcell.ndsu.nodak.edu/animations/tran
slation/index.htm
Codons
Three mRNA bases code for one amino acid
The three mRNA bases together are called a
codon
So when CCU are next to each other as a
codon then that will be read as proline
11/18/2014
11
Figure 17.5 Second mRNA base
Fir
st m
RN
A b
ase
(5 en
d o
f co
do
n)
Th
ird
mR
NA
base
(3
en
d o
f co
do
n)
UUU
UUC
UUA
CUU
CUC
CUA
CUG
Phe
Leu
Leu
Ile
UCU
UCC
UCA
UCG
Ser
CCU
CCC
CCA
CCG
UAU
UAC Tyr
Pro
Thr
UAA Stop
UAG Stop
UGA Stop
UGU
UGC Cys
UGG Trp
G C
U
U
C
A
U
U
C
C
C A
U
A
A
A
G
G
His
Gln
Asn
Lys
Asp
CAU CGU
CAC
CAA
CAG
CGC
CGA
CGG
G
AUU
AUC
AUA
ACU
ACC
ACA
AAU
AAC
AAA
AGU
AGC
AGA
Arg
Ser
Arg
Gly
ACG AUG AAG AGG
GUU
GUC
GUA
GUG
GCU
GCC
GCA
GCG
GAU
GAC
GAA
GAG
Val Ala
GGU
GGC
GGA
GGG Glu
Gly
G
U
C
A
Met or
start
UUG
G
Molecular Components of Translation
A cell translates an mRNA message into protein
with the help of transfer RNA (tRNA)
tRNAs transfer amino acids to the growing
polypeptide in a ribosome
© 2011 Pearson Education, Inc.
tRNA
mRNA codes for which amino acids go in
what order,
DNA also copied to make tRNA
tRNA (transfer RNA) brings the amino acids
to the ribosomes
One side of tRNA attaches to an amino acid,
the other side will attach to the mRNA
tRNA
Each tRNA is covalently bound to an amino
acid
tRNA molecules have an anticodon region =
complementary to the mRNA region
Figure 17.14
Polypeptide
Ribosome
tRNA with
amino acid
attached
Amino
acids
tRNA
Anticodon
Codons
U U U U G G G G C
C
G
C
G
5 3
mRNA
11/18/2014
12
The Structure and Function of Transfer RNA
Molecules of tRNA are not identical
Each carries a specific amino acid on one end
Each has an anticodon on the other end; the
anticodon base-pairs with a complementary
codon on mRNA
© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.
BioFlix: Protein Synthesis
A tRNA molecule consists of a single RNA
strand that is only about 80 nucleotides long
© 2011 Pearson Education, Inc.
The Structure and Function of Transfer RNA Figure 17.15
Amino acid
attachment
site
3
5
Hydrogen
bonds
Anticodon
(a) Two-dimensional structure (b) Three-dimensional structure (c) Symbol used
in this book
Anticodon Anticodon
3 5
Hydrogen
bonds
Amino acid
attachment
site 5
3
A A G
Because of hydrogen bonds, tRNA actually
twists and folds into a three-dimensional
molecule
tRNA is roughly L-shaped
© 2011 Pearson Education, Inc.
The Structure and Function of Transfer RNA
Accurate translation requires two steps
First: a correct match between a tRNA and an
amino acid, done by the enzyme aminoacyl-
tRNA synthetase
Second: a correct match between the tRNA
anticodon and an mRNA codon
Translation
11/18/2014
13
Flexible pairing at the third base of a codon is
called wobble and allows some tRNAs to bind to
more than one codon
Translation Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P P P Adenosine
ATP
Figure 17.16-1
Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
P P i
i
i
Adenosine
Figure 17.16-2 Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
P P i
i
i
Adenosine
tRNA
Adenosine P
tRNA
AMP
Computer model
Amino
acid
Aminoacyl-tRNA
synthetase
Figure 17.16-3
Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
P P i
i
i
Adenosine
tRNA
Adenosine P
tRNA
AMP
Computer model
Amino
acid
Aminoacyl-tRNA
synthetase
Aminoacyl tRNA
(“charged tRNA”)
Figure 17.16-4
Ribosomes
Ribosomes facilitate specific coupling of tRNA
anticodons with mRNA codons in protein
synthesis
The two ribosomal subunits (large and small)
are made of proteins and ribosomal RNA
(rRNA)
© 2011 Pearson Education, Inc.
11/18/2014
14
Ribosomes
Bacterial and eukaryotic ribosomes are
somewhat similar but have significant differences
Some antibiotic drugs specifically target bacterial
ribosomes without harming eukaryotic ribosomes
© 2011 Pearson Education, Inc.
Figure 17.17a
tRNA
molecules
Growing
polypeptide Exit tunnel
E P A
Large
subunit
Small
subunit
mRNA 5
3
(a) Computer model of functioning ribosome
Figure 17.17b
Exit tunnel
A site (Aminoacyl-
tRNA binding site)
Small
subunit
Large
subunit
P A
P site (Peptidyl-tRNA
binding site)
mRNA
binding site
(b) Schematic model showing binding sites
E site
(Exit site)
E
Figure 17.17c
Amino end
mRNA
E
(c) Schematic model with mRNA and tRNA
5 Codons
3
tRNA
Growing polypeptide
Next amino
acid to be
added to
polypeptide
chain
A ribosome has three binding sites for tRNA
The P site holds the tRNA that carries the
growing polypeptide chain
The A site holds the tRNA that carries the next
amino acid to be added to the chain
The E site is the exit site, where discharged
tRNAs leave the ribosome
© 2011 Pearson Education, Inc.
Ribosomes Building a Polypeptide
The three stages of translation
Initiation
Elongation
Termination
All three stages require protein “factors” that
aid in the translation process
© 2011 Pearson Education, Inc.
11/18/2014
15
Ribosome Association and Initiation of Translation
The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits
© 2011 Pearson Education, Inc.
Ribosome Association and Initiation of Translation
First, a small ribosomal subunit binds with mRNA and a special initiator tRNA (carrying the amino acid methionine)
Then the small subunit moves along the mRNA until it reaches the start codon (AUG)
Proteins called initiation factors bring in the large subunit that completes the translation initiation complex
© 2011 Pearson Education, Inc.
Figure 17.18
Initiator
tRNA
mRNA
5
5 3
Start codon Small
ribosomal
subunit mRNA binding site
3
Translation initiation complex
5 3
3 U
U
A A G
C
P
P site
i
GTP GDP
Large
ribosomal
subunit
E A
5
Elongation of the Polypeptide Chain
During the elongation stage, amino acids are
added one by one to the preceding amino acid
at the C-terminus of the growing chain
Each addition involves proteins called
elongation factors and occurs in three steps:
codon recognition, peptide bond formation, and
translocation
Translation proceeds along the mRNA in a 5′ to
3′ direction
© 2011 Pearson Education, Inc.
Amino end of polypeptide
mRNA
5
E
P site
A site
3
Figure 17.19-1 Amino end of polypeptide
mRNA
5
E
P site
A site
3
E
GTP
GDP P i
P A
Figure 17.19-2
11/18/2014
16
Amino end of polypeptide
mRNA
5
E
P site
A site
3
E
GTP
GDP P i
P A
E
P A
Figure 17.19-3 Amino end of polypeptide
mRNA
5
E
A site
3
E
GTP
GDP P i
P A
E
P A
GTP
GDP P i
P A
E
Ribosome ready for next aminoacyl tRNA
P site
Figure 17.19-4
Protein Synthesis
Protein synthesis proceeds from the N-
terminus to the C-terminus of the protein.
The ribosomes "read" the mRNA in the 5'
to 3' direction.
Fig. 15.18
Created by Dr. Joachim Frank
https://www.rpi.edu/dept/bcbp/molbiochem
/MBWeb/mb2/part1/translate.htm
http://cen.acs.org/articles/85/i8/Protein-
Factory-Reveals-Secrets.html
11/18/2014
17
Termination of Translation
Termination occurs when a stop codon in the
mRNA reaches the A site of the ribosome
The A site accepts a protein called a release
factor
The release factor causes the addition of a
water molecule instead of an amino acid
This reaction releases the polypeptide, and the
translation assembly then comes apart
© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.
Animation: Translation
Right-click slide / select “Play”
Figure 17.20-1
Release factor
Stop codon (UAG, UAA, or UGA)
3
5
Figure 17.20-2
Release factor
Stop codon (UAG, UAA, or UGA)
3
5
3
5
Free polypeptide
2 GTP
2 GDP 2 i
P
Figure 17.20-3
Release factor
Stop codon (UAG, UAA, or UGA)
3
5
3
5
Free polypeptide
2 GTP
5
3
2 GDP 2 i
P
Initiation
1. The mRNA binds with the small subunit of the
ribosome
2. The beginning of the coding region of mRNA is
AUG
3. The tRNA with the anticodon UAC has
methionine attached to it.
4. The tRNA with met attached binds to the P site
of the ribosomes.
5. This requires energy in the form of GTP
6. Large subunit joins the small subunit
11/18/2014
18
Elongation
This is the stage where amino acids are added to the growing polypeptide chain
7. A tRNA with the next amino acid comes into the A site, requires GTP for energy
8. The amino acids are bound by a peptide bond
9. The bond is between the carboxyl end of the P site amino acid and the amino side of the A amino acid
Elongation
10. The now ribosome moves so the free tRNA is at
the E site and the tRNA with the polypeptide
chain moves is at the P site = translocation
11. This requires GTP
12. The free tRNA exits the ribosome
Termination
13. The end of the coding region will have a
stop codon that signals the end of the
polypeptide chain
14. No tRNA binds to this codon, instead a
release factor binds, requires GTP for
energy
15. The polypeptide chain is released
What molecules are produced in
transcription?
1. Amino acids
2. Proteins
3. RNA
4. DNA
Am
ino a
cids
Pro
tein
s
RNA
DNA
25% 25%25%25%
Polyribosomes
A number of ribosomes can translate a
single mRNA simultaneously, forming a
polyribosome (or polysome)
Polyribosomes enable a cell to make many
copies of a polypeptide very quickly
© 2011 Pearson Education, Inc.
Figure 17.21 Completed
polypeptide
Incoming
ribosomal
subunits
Start of
mRNA
(5 end)
End of
mRNA
(3 end) (a)
Ribosomes
mRNA
(b) 0.1 m
Growing
polypeptides
11/18/2014
19
Completing and Targeting the Functional Protein
Often translation is not sufficient to make a
functional protein
Polypeptide chains are modified after
translation or targeted to specific sites in the
cell
© 2011 Pearson Education, Inc.
Protein Folding and Post-Translational Modifications
During and after synthesis, a polypeptide chain
spontaneously coils and folds into its three-
dimensional shape
Proteins may also require post-translational
modifications before doing their job
Some polypeptides are activated by enzymes
that cleave them
Other polypeptides come together to form the
subunits of a protein
© 2011 Pearson Education, Inc.
Post-translational Modifications
Folding, either spontaneously or with the aid of chaperone proteins
Proteolysis – some polypeptide chains are cut into smaller chains
Glycosylation – sugars are added to some proteins, some of these sugar chains are important to “address” the protein
Phosphorylation – Protein Kinases phosphorylate proteins to activate them
Methionine is often removed
Targeting Polypeptides to Specific Locations
Two populations of ribosomes are evident in cells: free ribsomes (in the cytosol) and bound ribosomes (attached to the ER)
Free ribosomes mostly synthesize proteins that function in the cytosol
Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell
Ribosomes are identical and can switch from free to bound
© 2011 Pearson Education, Inc.
Polypeptide synthesis always begins in the
cytosol
Synthesis finishes in the cytosol unless the
polypeptide signals the ribosome to attach to
the ER
Polypeptides destined for the ER or for
secretion are marked by a signal peptide
© 2011 Pearson Education, Inc.
Targeting Polypeptides to Specific Locations Targeting Polypeptide Chains
Polypeptide chains that need to be brought into
the RER will start (after MET) with a short
sequence of amino acids = signal sequence
Proteins, called signal recognition particles
(SRPs), in the cytoplasm recognize this
sequence during translation and bind to the
amino acid sequence.
11/18/2014
20
A signal-recognition particle (SRP) binds
to the signal peptide
The SRP brings the signal peptide and its
ribosome to the ER
Signal recognition particle (SRP) is composed of
protein and RNA
© 2011 Pearson Education, Inc.
Targeting Polypeptides to Specific Locations Targeting Polypeptide Chains
The signal sequence and SRP complex binds
to a receptor on the RER membrane
The complex docks with the RER
As the polypeptide chain is built, the chain is
brought into the RER
Once inside the RER the polypeptide chain can
be folded, modified
Then the protein is transported via a vesicle to
the Golgi
Figure 17.22
Ribosome
mRNA
Signal peptide
SRP
1
SRP receptor protein
Translocation complex
ER LUMEN
2
3
4 5
6
Signal peptide removed
CYTOSOL
Protein
ER membrane
Mutations of one or a few nucleotides can affect
protein structure and function
Mutations are changes in the genetic material
of a cell or virus
Point mutations are chemical changes in just
one base pair of a gene
The change of a single nucleotide in a DNA
template strand can lead to the production of
an abnormal protein
© 2011 Pearson Education, Inc.
Figure 17.23
Wild-type hemoglobin
Wild-type hemoglobin DNA
3
3
3 5
5 3
3 5
5
5 5 3
mRNA
A A G
C T T
A A G
mRNA
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
A
A
A U G
G
T
T
Sickle-cell hemoglobin
Mutant hemoglobin DNA
C
Types of Small-Scale Mutations
Point mutations within a gene can be divided into
two general categories
Nucleotide-pair substitutions
One or more nucleotide-pair insertions or
deletions
© 2011 Pearson Education, Inc.
11/18/2014
21
Substitutions
A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides
Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code
Missense mutations still code for an amino acid, but not the correct amino acid
Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein
© 2011 Pearson Education, Inc.
Wild type
DNA template strand
mRNA5
5
3
Protein
Amino end
A instead of G
(a) Nucleotide-pair substitution
3
3
5
Met Lys Phe Gly Stop
Carboxyl end
T T T T T
T T T T T A A A A A
A A A A C C
C
C
A
A A A A A
G G G G
G C C
G G G U U U U U G
(b) Nucleotide-pair insertion or deletion
Extra A
3
5
5
3
Extra U
5 3
T T T T
T T T T
A
A A A
A
A T G G G G
G A A A
A C
C C C C A
T 3 5
5 3
5 T T T T T A A A A C C A A C C
T T T T T A A A A A T G G G G
U instead of C
Stop
U A A A A A G G G U U U U U G
Met Lys Phe Gly
Silent (no effect on amino acid sequence)
T instead of C
T T T T T A A A A C C A G T C
T A T T T A A A A C C A G C C
A instead of G
C A A A A A G A G U U U U U G U A A A A G G G U U U G A C
A A U U A A U U G U G G C U A
G A U A U A A U G U G U U C G
Met Lys Phe Ser
Stop
Stop Met Lys
missing
missing
Frameshift causing immediate nonsense
(1 nucleotide-pair insertion)
Frameshift causing extensive missense
(1 nucleotide-pair deletion)
missing
T T T T T T C A A C C A A C G
A G T T T A A A A A T G G G C
Leu Ala
Missense
A instead of T
T T T T T A A A A A C G G A G
A
C A U A A A G G G U U U U U G
T T T T T A T A A A C G G G G
Met
Nonsense
Stop
U instead of A
3
5
3 5
5
3
3
5
5
3
3 5 3
Met Phe Gly
No frameshift, but one amino acid missing
(3 nucleotide-pair deletion)
missing
3
5
5
3
5 3
U
T C A A A C A T T A C G
T A G T T T G G A A T C
T T C
A A G
Met
3
T
A
Stop
3
5
5
3
5 3
Figure 17.24
Figure 17.24a
Wild type
DNA template strand
mRNA5
5
Protein
Amino end Stop Carboxyl end
3
3
3
5
Met Lys Phe Gly
A instead of G
(a) Nucleotide-pair substitution: silent
Stop Met Lys Phe Gly
U instead of C
A
A
A A
A A A A
A A T
T T T T T
T T T T
C C C C
C
C
G G G G
G
G
A
A A A A G G G U U U U U
5
3
3
5 A
A A
A A A A
A A T
T T T T T
T T T T
C C C C
G G G G
A
A
A G A A A A G G G U U U U U
T
U 3 5
Figure 17.24b
Wild type
DNA template strand
mRNA5
5
Protein
Amino end Stop Carboxyl end
3
3
3
5
Met Lys Phe Gly
T instead of C
(a) Nucleotide-pair substitution: missense
Stop Met Lys Phe Ser
A instead of G
A
A
A A
A A A A
A A T
T T T T T
T T T T
C C C C
C
C
G G G G
G
G
A
A A A A G G G U U U U U
5
3
3
5 A
A A
A A A A
A A T
T T T T T
T T T T
C C T C
G
G
G A
A G A A A A A G G U U U U U 3 5
A C
C
G
Figure 17.24c
Wild type
DNA template strand
mRNA5
5
Protein
Amino end Stop Carboxyl end
3
3
3
5
Met Lys Phe Gly
A instead of T
(a) Nucleotide-pair substitution: nonsense
Met
A
A
A A
A A A A
A A T
T T T T T
T T T T
C C C C
C
C
G G G G
G
G
A
A A A A G G G U U U U U
5
3
3
5 A
A
A A A A
A A T
T A T T T
T T T T
C C C
G
G
G A
A G U A A A G G U U U U U 3 5
C
C
G
T instead of C
C
G T
U instead of A
G
Stop
Insertions and Deletions
Insertions and deletions are additions or
losses of nucleotide pairs in a gene
These mutations have a disastrous effect on
the resulting protein more often than
substitutions do
Insertion or deletion of nucleotides may alter
the reading frame, producing a frameshift
mutation
© 2011 Pearson Education, Inc.
11/18/2014
22
Figure 17.24d
Wild type
DNA template strand
mRNA5
5
Protein
Amino end Stop Carboxyl end
3
3
3
5
Met Lys Phe Gly
A
A
A A
A A A A
A A T
T T T T T
T T T T
C C C C
C
C
G G G G
G
G
A
A A A A G G G U U U U U
(b) Nucleotide-pair insertion or deletion: frameshift causing
immediate nonsense
Extra A
Extra U
5
3
5
3
3
5
Met
1 nucleotide-pair insertion
Stop
A C A A G T T A T C T A C G
T A T A T G T C T G G A T G A
A G U A U A U G A U G U U C
A T
A
A G
Figure 17.24e
DNA template strand
mRNA5
5
Protein
Amino end Stop Carboxyl end
3
3
3
5
Met Lys Phe Gly
A
A
A A
A A A A
A A T
T T T T T
T T T T
C C C C
C
C
G G G G
G
G
A
A A A A G G G U U U U U
(b) Nucleotide-pair insertion or deletion: frameshift causing
extensive missense
Wild type
missing
missing
A
U
A A A T T T C C A T T C C G
A A T T T G G A A A T C G G
A G A A G U U U C A A G G U 3
5
3
3
5
Met Lys Leu Ala
1 nucleotide-pair deletion
5
Figure 17.24f
DNA template strand
mRNA5
5
Protein
Amino end Stop Carboxyl end
3
3
3
5
Met Lys Phe Gly
A
A
A A
A A A A
A A T
T T T T T
T T T T
C C C C
C
C
G G G G
G
G
A
A A A A G G G U U U U U
(b) Nucleotide-pair insertion or deletion: no frameshift, but one
amino acid missing
Wild type
A T C A A A A T T C C G
T T C missing
missing
Stop
5
3
3
5
3 5
Met Phe Gly
3 nucleotide-pair deletion
A G U C A A G G U U U U
T G A A A T T T T C G G
A A G
Mutagens
Spontaneous mutations can occur during DNA
replication, recombination, or repair
Mutagens are physical or chemical agents that
can cause mutations to the DNA
© 2011 Pearson Education, Inc.
While gene expression differs among the domains
of life, the concept of a gene is universal
Archaea are prokaryotes, but share many
features of gene expression with eukaryotes
© 2011 Pearson Education, Inc.
Comparing Gene Expression in Bacteria, Archaea,
and Eukarya
Bacteria and eukarya differ in their RNA polymerases, termination of transcription, and ribosomes; archaea tend to resemble eukarya in these respects
Bacteria and archaea can simultaneously transcribe and translate the same gene
In eukarya, transcription and translation are separated by the nuclear envelope
© 2011 Pearson Education, Inc.
11/18/2014
23
Figure 17.25
RNA polymerase
DNA mRNA
Polyribosome
RNA
polymerase DNA
Polyribosome
Polypeptide
(amino end)
mRNA (5 end)
Ribosome
0.25 m Direction of
transcription
If the mRNA sequence is: AUGCCCAAGUAA then the amino
acid sequence would be:
1. Start-Pro-Lys
2. Met-Pro-Lys
3. Met-Pro-Lys-Stop
4. Start-Pro-Lys-Stop
Sta
rt-P
ro-L
ys
Met
-Pro
-Lys
Met
-Pro
-Lys
-Sto
p
Sta
rt-P
ro-L
ys-S
top
25% 25%25%25%
Which of the following processes occur in the
nucleus?
1. DNA replication
transcription, and
translation
2. DNA replication and
transcription
3. DNA replication only
4. Transcription only
DNA rep
licat
ion
tran
s...
DNA rep
licat
ion
and t.
..
DNA rep
licat
ion
only
Tra
nscript
ion o
nly
25% 25%25%25%
Copyright © 2009 Pearson Education, Inc.
Which molecule is produced during
translation?
1 2 3 4
25% 25%25%25%1. Amino acids
2. Proteins
3. RNA
4. DNA
Important concepts
Know the vocabulary for this lecture
Know what monomers are bound together to make
a protein and what kind of bonds hold them
together
Differences between DNA and RNA
Know the types of RNA, their role and where they
work in the cell
Be able to determine the complementary mRNA
sequence from a DNA sequence.
Important concepts
Steps of transcription and translation, know which direction mRNA is built and which direction it is read
Be able to “read” the mRNA to make a protein, given the table of codons.
Know how the peptide bond is formed
Know the role of RNA polymerase
Know the three stages of transcription (initiation, elongation, and termination stage
Know the structure of ribosomes – what it is made of, how many subunits, what are the binding sites, which part of the ribosome is the catalytic region
11/18/2014
24
Important concepts
Know the steps of translation, what powers
translation (know which steps need the energy)
Know how the processes are different in
prokaryotes
Know the post-transcriptional and post-
translational modifications to eukaryotic mRNA
and proteins
Know how polypeptide chains are brought into
the RER
Know the types of mutations discussed in
lecture, be able to recognize examples of each