Transcript Protein Synthesis

The Central Dogma of Life.
replication
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 two
stages, called transcription and translation
Transcription and Translation
• Cells are governed by a cellular chain of
command
– DNA RNA protein
• Transcription
– Is the synthesis of RNA under the direction of DNA
– Produces messenger RNA (mRNA)
• Translation
– Is the actual synthesis of a polypeptide, which
occurs under the direction of mRNA
– Occurs on ribosomes
Transcription and Translation
• In prokaryotes transcription and
translation occur together
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Figure 17.3a
(a) Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
Transcription and Translation
• In a eukaryotic cell the nuclear envelope separates
transcription from translation
• Extensive RNA processing occurs in the nucleus
Nuclear
envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
(b) Eukaryotic cell. The nucleus provides a separate
compartment for transcription. The original RNA
transcript, called pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.
Transcription
• Transcription is the DNAdirected synthesis of RNA
• RNA synthesis
– Is catalyzed by RNA
polymerase, which pries the
DNA strands apart and
hooks together the RNA
nucleotides
– Follows the same basepairing rules as DNA,
except that in RNA, uracil
substitutes for thymine
RNA
• RNA is single stranded, not double stranded like DNA
• RNA is short, only 1 gene long, where DNA is very long and
contains many genes
• RNA uses the sugar ribose instead of deoxyribose in DNA
• RNA uses the base uracil (U) instead of thymine (T) in DNA.
Table 17.1
Synthesis of an RNA Transcript
• The stages of
transcription
are
– Initiation
– Elongation
– Termination
Promoter
Transcription unit
5
3
3
5
Start point
RNA polymerase
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
transcript
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
5
3
3
5
5
Completed RNA
transcript
3
Synthesis of an RNA Transcript - Initiation
• Promoters signal the
initiation of RNA
synthesis
• Transcription factors help
eukaryotic RNA
polymerase recognize
promoter sequences
1 Eukaryotic promoters
TRANSCRIPTION
DNA
RNA PROCESSING
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
Promoter
5
3
3
5
T A T A A AA
AT AT T T T
TATA box
Start point
Template
DNA strand
Several transcription
factors
2
Transcription
factors
5
3
3
5
3 Additional transcription
• A crucial promoter DNA
sequence is called a
TATA box.
factors
RNA polymerase II
5
3
Transcription factors
3
5
5
RNA transcript
Transcription initiation complex
Synthesis of an RNA Transcript - Elongation
•
•
RNA polymerase synthesizes a single strand of RNA against the DNA
template strand (anti-sense strand), adding nucleotides to the 3’ end of
the RNA chain
As RNA polymerase moves along the DNA it continues to untwist the
double helix, exposing about 10 to 20 DNA bases at a time for pairing
with RNA nucleotides
Non-template
strand of DNA
Elongation
RNA nucleotides
RNA
polymerase
A
3
T
C
C
A
A
3 end
U
5
A
E
G
C
A
T
A
G
G
T
T
Direction of transcription
(“downstream”)
5
Newly made
RNA
Template
strand of DNA
Synthesis of an RNA Transcript - Termination
•
•
Specific sequences in the DNA signal
termination of transcription
When one of these is encountered by the
polymerase, the RNA transcript is
released from the DNA and the double
helix can zip up again.
Transcription Overview
Post Termination RNA Processing
•
Most eukaryotic mRNAs aren’t ready to be translated into protein directly after being
transcribed from DNA. mRNA requires processing.
•
Transcription of RNA processing occur in the nucleus. After this, the messenger
RNA moves to the cytoplasm for translation.
•
The cell adds a protective cap to one end, and a tail of A’s to the other end. These
both function to protect the RNA from enzymes that would degrade
•
Most of the genome consists of non-coding regions called introns
•
–
Non-coding regions may have specific chromosomal functions or have regulatory purposes
–
Introns also allow for alternative RNA splicing
Thus, an RNA copy of a gene is converted into messenger RNA by doing 2 things:
–
Add protective bases to the ends
–
Cut out the introns
Alteration of mRNA Ends
• 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
Polypeptide
5 UTR
Start codon Stop codon
3 UTR
Poly-A tail
RNA Processing - Splicing
• The original transcript
from the DNA is called
pre-mRNA.
• It contains transcripts of
both introns and exons.
• The introns are removed
by a process called
splicing to produce
messenger RNA
(mRNA)
RNA Processing - Splicing
• Ribozymes are catalytic RNA molecules that
function as enzymes and can splice RNA
• RNA splicing removes introns and joins exons
5 Exon Intron
TRANSCRIPTION
DNA
Exon
3
Pre-mRNA 5 Cap
1
RNA PROCESSING
Intron
Exon
Poly-A tail
30
31
104
105
146
Pre-mRNA
Coding
segment
mRNA
Ribosome
Introns cut out and
exons spliced together
TRANSLATION
Polypeptide
mRNA 5 Cap
1
3 UTR
Figure 17.10
Poly-A tail
146
3 UTR
RNA Processing
• RNA Splicing can also be carried out by spliceosomes
RNA transcript (pre-mRNA)
5
Intron
Exon 1
Exon 2
Protein
1
Other proteins
snRNA
snRNPs
Spliceosome
2
5
Spliceosome
components
3
5
mRNA
Exon 1
Exon 2
Cut-out
intron
Alternative Splicing (of Exons)
• How is it possible that there are millions of
human antibodies when there are only about
30,000 genes?
• Alternative splicing refers to the different
ways the exons of a gene may be combined,
producing different forms of proteins within
the same gene-coding region
• Alternative pre-mRNA splicing is an important
mechanism for regulating gene expression in
higher eukaryotes
RNA Processing
• Proteins often have a modular architecture
consisting of discrete structural and functional
regions called domains
• In many cases different exons code for the
different domains in a protein
Gene
DNA
Exon 1
Exon 2
Intron
Intron
Exon 3
Transcription
RNA processing
Translation
Domain 3
Domain 2
Domain 1
Figure 17.12
Polypeptide
Translation
• Translation is the RNAdirected synthesis of a
polypeptide
• Translation involves
–
–
–
–
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Amino
acids
Polypeptide
mRNA
Ribosomes - Ribosomal RNA
Transfer RNA
Genetic coding - codons
tRNA with
amino acid
Ribosome attached
Gly
tRNA
Anticodon
A A A
U G G U U U G G C
Codons
5
mRNA
3
The Genetic Code
• Genetic information is encoded as a sequence of nonoverlapping
base triplets, or codons
• The gene determines the sequence of bases along the length of an
mRNA molecule
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand
(template)
3
A C C A A A C C G A G
T
5
TRANSCRIPTION
mRNA
5
U G G U U U G G C U C A
Codon
TRANSLATION
Protein
Trp
Amino acid
Phe
Gly
Ser
3
The Genetic Code
• Codons: 3 base code for the production of a specific amino acid,
sequence of three of the four different nucleotides
• Since there are 4 bases and 3 positions in each codon, there
are 4 x 4 x 4 = 64 possible codons
• 64 codons but only 20 amino acids, therefore most have more
than 1 codon
• 3 of the 64 codons are used as STOP signals; they are found at
the end of every gene and mark the end of the protein
• One codon is used as a START signal: it is at the start of every
protein
• Universal: in all living organisms
The Genetic Code
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
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
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
AAC
AGC Ser C
A
AAA
AGA
Lys
Arg
G
AAG
AGG
U
GAU
GGU
C
GAC Asp GGC
Gly
GAA
GGA
A
Glu
GAG
GGG
G
Third mRNA base (3 end)
First mRNA base (5 end)
• A codon in messenger RNA is either translated into an
amino acid or serves as a translational start/stop signal
Transfer RNA
• Consists of a single RNA strand that is only about 80
nucleotides long
• Each carries a specific amino acid on one end and has an
anticodon on the other end
• A special group of enzymes pairs up the proper tRNA molecules
with their corresponding amino acids.
• tRNA brings the amino acids to the ribosomes,
3
A
C
C
A 5
C G
The “anticodon” is the 3 RNA bases that
G C
C G
matches the 3 bases of the codon on the
U G
U A
mRNA molecule
A U
A U
U C
UA
C A C AG
*
G
*
U
G
U
G
C
C
*
* *
U C
*
* G AG C
(a) Two-dimensional structure. The four base-paired regions and three
G C
U A
loops are characteristic of all tRNAs, as is the base sequence of the
* G
amino acid attachment site at the 3 end. The anticodon triplet is
A
A*
C
unique to each tRNA type. (The asterisks mark bases that have been
U
*
chemically modified, a characteristic of tRNA.)
A
G
A
Amino acid
attachment site
Anticodon
C U C
G A G
A G *
*
G
A G G
Hydrogen
bonds
Transfer RNA
• 3 dimensional tRNA molecule is roughly “L” shaped
5
3
Amino acid
attachment site
Hydrogen
bonds
A AG
3
Anticodon
(b) Three-dimensional structure
Anticodon
5
(c) Symbol used
in the book
Ribosomes
• Ribosomes facilitate the specific coupling of tRNA anticodons
with mRNA codons during protein synthesis
• The 2 ribosomal subunits are constructed of proteins and RNA
molecules named ribosomal RNA or rRNA
DNA
TRANSCRIPTION
mRNA
Ribosome
TRANSLATION
Polypeptide
Growing
polypeptide
Exit tunnel
tRNA
molecules
Large
subunit
E
P A
Small
subunit
5
mRNA
3
(a) Computer model of functioning ribosome. This is a model of a bacterial
ribosome, showing its overall shape. The eukaryotic ribosome is roughly
similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules
and proteins.
Ribosome
• The ribosome has three binding sites for tRNA
– The P site
– The A site
– The E site
P site (Peptidyl-tRNA
binding site)
A site (AminoacyltRNA binding site)
E site
(Exit site)
Large
subunit
E
mRNA
binding site
P
A
Small
subunit
(b) Schematic model showing binding sites. A ribosome has an
mRNA binding site and three tRNA binding sites, known as the A, P,
and E sites. This schematic ribosome will appear in later diagrams.
Building a Polypeptide
Amino end
Growing polypeptide
Next amino acid
to be added to
polypeptide chain
tRNA
3
mRNA
5
Codons
(c) Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon basepairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A
site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged
tRNA leaves via the E site.
Building a Molecule of tRNA
• A specific enzyme called an aminoacyl-tRNA
synthetase joins each amino acid to the correct tRNA
Amino acid
P P
Aminoacyl-tRNA
synthetase (enzyme)
1 Active site binds the
amino acid and ATP.
P Adenosine
ATP
2 ATP loses two P groups
and joins amino acid as AMP.
P
Pyrophosphate
Pi
Phosphates
P
Adenosine
Pi
Pi
tRNA
3 Appropriate
tRNA covalently
Bonds to amino
Acid, displacing
AMP.
P Adenosine
AMP
4 Activated amino acid
is released by the enzyme.
Figure 17.15
Aminoacyl tRNA
(an “activated
amino acid”)
Building a Polypeptide
• We can divide translation into three stages
– Initiation
– Elongation
– Termination
• The AUG start codon is recognized by methionyl-tRNA
or Met
• Once the start codon has been identified, the ribosome
incorporates amino acids into a polypeptide chain
• RNA is decoded by tRNA (transfer RNA) molecules,
which each transport specific amino acids to the
growing chain
• Translation ends when a stop codon (UAA, UAG, UGA)
is reached
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
Large
ribosomal
subunit
P site
3 U A C 5
5 A U G 3
Initiator tRNA
GTP
GDP
E
A
mRNA
5
Start codon
mRNA binding site
1
5
3
Small
ribosomal
subunit
A small ribosomal subunit binds to a molecule of
mRNA. In a prokaryotic cell, the mRNA binding site
on this subunit recognizes a specific nucleotide
sequence on the mRNA just upstream of the start
codon. An initiator tRNA, with the anticodon UAC,
base-pairs with the start codon, AUG. This tRNA
carries the amino acid methionine (Met).
3
Translation initiation complex
2
The arrival of a large ribosomal subunit completes
the initiation complex. Proteins called initiation
factors (not shown) are required to bring all the
translation components together. GTP provides
the energy for the assembly. The initiator tRNA is
in the P site; the A site is available to the tRNA
bearing the next amino acid.
Elongation of the Polypeptide Chain
• In the elongation stage, amino acids are added one
by one to the preceding amino acid
TRANSCRIPTION
1 Codon recognition. The anticodon
of an incoming aminoacyl tRNA
base-pairs with the complementary
mRNA codon in the A site. Hydrolysis
of GTP increases the accuracy and
efficiency of this step.
Amino end
of polypeptide
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
E
mRNA
Ribosome ready for
next aminoacyl tRNA
5
3
P A
site site
2 GTP
2 GDP
E
E
P
P
A
GDP
3 Translocation. The ribosome
translocates the tRNA in the A
site to the P site. The empty tRNA
in the P site is moved to the E site,
where it is released. The mRNA
moves along with its bound tRNAs,
bringing the next codon to be
translated into the A site.
GTP
E
P
A
A
2 Peptide bond formation. An
rRNA molecule of the large
subunit catalyzes the formation
of a peptide bond between the
new amino acid in the A site and
the carboxyl end of the growing
polypeptide in the P site. This step
attaches the polypeptide to the
tRNA in the A site.
Termination of Translation
•
The final step in translation is termination. When the ribosome reaches
a STOP codon, there is no corresponding transfer RNA.
Instead, a small protein called a “release factor” attaches to the stop
codon.
The release factor causes the whole complex to fall apart: messenger
RNA, the two ribosome subunits, the new polypeptide.
The messenger RNA can be translated many times, to produce many
protein copies.
•
•
•
Release
factor
Free
polypeptide
5
3
3
5
5
3
Stop codon
(UAG, UAA, or UGA)
1 When a ribosome reaches a stop 2 The release factor hydrolyzes 3 The two ribosomal subunits
codon on mRNA, the A site of the
the bond between the tRNA in and the other components of
ribosome accepts a protein called
the P site and the last amino
the assembly dissociate.
a release factor instead of tRNA.
acid of the polypeptide chain.
The polypeptide is thus freed
from the ribosome.
Translation: Initiation
• mRNA binds to a ribosome, and the transfer RNA corresponding
to the START codon binds to this complex. Ribosomes are
composed of 2 subunits (large and small), which come together
when the messenger RNA attaches during the initiation process.
Translation: Elongation
• Elongation: the ribosome moves down the messenger RNA,
adding new amino acids to the growing polypeptide chain.
• The ribosome has 2 sites for binding transfer RNA. The first
RNA with its attached amino acid binds to the first site, and then
the transfer RNA corresponding to the second codon bind to the
second site.
Translation: Elongation
• The ribosome then removes the amino acid from the
first transfer RNA and attaches it to the second amino
acid.
• At this point, the first transfer RNA is empty: no
attached amino acid, and the second transfer RNA
has a chain of 2 amino acids attached to it.
Translation: Termination
• The elongation cycle repeats as the ribosome moves
down the messenger RNA, translating it one codon
and one amino acid at a time.
• The process repeats until a STOP codon is reached.
Polyribosomes
• A number of ribosomes can translate a single mRNA
molecule simultaneously forming a polyribosome
• Polyribosomes enable a cell to make many copies of
a polypeptide very quickly
Completed
polypeptide
Growing
polypeptides
Incoming
ribosomal
subunits
Start of
End of
mRNA
mRNA
(5 end)
(3 end)
(a) An mRNA molecule is generally translated simultaneously
by several ribosomes in clusters called polyribosomes.
Ribosomes
mRNA
0.1 µm
This micrograph shows a large polyribosome in a prokaryotic
cell (TEM).
Comparing Gene Expression In Prokaryotes And Eukaryotes
•
In a eukaryotic cell:
–
–
•
The nuclear envelope separates transcription from translation
Extensive RNA processing occurs in the nucleus
Prokaryotic cells lack a nuclear envelope, allowing translation to
begin while transcription progresses
RNA polymerase
DNA
mRNA
Polyribosome
RNA
polymerase
Direction of
transcription
0.25 mm
DNA
Polyribosome
Polypeptide
(amino end)
Ribosome
mRNA (5 end)
A summary of transcription and translation in a eukaryotic cell
DNA
TRANSCRIPTION
1RNA is transcribed
from a DNA template.
3
RNA
transcript
5
RNA
polymerase
Exon
RNA PROCESSING
2
In eukaryotes, the
RNA transcript (premRNA) is spliced and
modified to produce
mRNA, which moves
from the nucleus to the
cytoplasm.
RNA transcript
(pre-mRNA)
Intron
Aminoacyl-tRNA
synthetase
NUCLEUS
Amino
acid
FORMATION OF
INITIATION COMPLEX
CYTOPLASM
AMINO ACID ACTIVATION
tRNA
3 After leaving the
4
Each amino acid
attaches to its proper tRNA
with the help of a specific
enzyme and ATP.
nucleus, mRNA attaches
to the ribosome.
mRNA
Growing
polypeptide
Activated
amino acid
Ribosomal
subunits
5
TRANSLATION
5
E
A
A A A
U G G U U U A U G
Figure 17.26
Codon
Ribosome
Anticodon
A succession of tRNAs
add their amino acids to
the polypeptide chain
as the mRNA is moved
through the ribosome
one codon at a time.
(When completed, the
polypeptide is released
from the ribosome.)
Post-translation
• The new polypeptide is now floating loose in the
cytoplasm if translated by a free ribosome.
• Polypeptides fold spontaneously into their active
configuration, and they spontaneously join with other
polypeptides to form the final proteins.
• Often translation is not sufficient to make a functional
protein, polypeptide chains are modified after
translation
• Sometimes other molecules are also attached to the
polypeptides: sugars, lipids, phosphates, etc. All of
these have special purposes for protein function.
Targeting Polypeptides to Specific Locations
• Completed proteins are targeted to specific sites
in the cell
• 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
Targeting Polypeptides to Specific Locations
•
•
•
•
•
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
A signal-recognition particle (SRP) binds to the signal peptide
The SRP brings the signal peptide and its ribosome to the ER
Ribosomes
mRNA
Signal
peptide
Signalrecognition
particle SRP
receptor
(SRP)
protein
CYTOSOL
ER LUMEN
Translocation
complex
ER
membrane
Signal
peptide
removed
Protein
Mutation Causes and Rate
• The natural replication of DNA produces occasional
errors. DNA polymerase has an editing mechanism
that decreases the rate, but it still exists
• Typically genes incur base substitutions about once
in every 10,000 to 1,000,000 cells
• Since we have about 6 billion bases of DNA in each
cell, virtually every cell in your body contains several
mutations
• Mutations can be harmful, lethal, helpful, silent
• However, most mutations are neutral: have no effect
• Only mutations in cells that become sperm or eggs—
are passed on to future generations
• Mutations in other body cells only cause trouble when
they cause cancer or related diseases
Mutagens
• Mutagens are chemical or physical agents that interact
with DNA to cause mutations.
• Physical agents include high-energy radiation like X-rays
and ultraviolet light
• Chemical mutagens fall into several categories.
–
–
–
Chemicals that are base analogues that may be substituted into
DNA, but they pair incorrectly during DNA replication.
Interference with DNA replication by inserting into DNA and
distorting the double helix.
Chemical changes in bases that change their pairing properties.
• Tests are often used as a preliminary screen of chemicals
to identify those that may cause cancer
• Most carcinogens are mutagenic and most mutagens are
carcinogenic.
Viral Mutagens
• Scientists have recognized a number of
tumor viruses that cause cancer in various
animals, including humans
• About 15% of human cancers are caused by
viral infections that disrupt normal control of
cell division
• All tumor viruses transform cells into cancer
cells through the integration of viral nucleic
acid into host cell DNA.
Point mutations
• Point mutations involve alterations in the
structure or location of a single gene.
Generally, only one or a few base pairs are
involved.
• Point mutations can signficantly affect protein
structure and function
• Point mutations may be caused by physical
damage to the DNA from radiation or
chemicals, or may occur spontaneously
• Point mutations are often caused by mutagens
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
3
Mutant hemoglobin DNA
5
C T
T
In the DNA, the
mutant template
strand has an A where
the wild-type template
has a T.
G U A
The mutant mRNA has
a U instead of an A in
one codon.
3
5
T
C A
mRNA
mRNA
G A
A
5
3
5
3
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
The mutant (sickle-cell)
hemoglobin has a valine
(Val) instead of a glutamic
acid (Glu).
Types of Point Mutations
• Point mutations within a gene can be divided
into two general categories
– Base-pair substitutions - is the replacement of
one nucleotide and its partner with another pair
of nucleotides
– Base-pair insertions or deletions - are additions
or losses of nucleotide pairs in a gene
Base-Pair Substitutions
•
•
•
Silent - changes a codon but codes for the same amino acid
Missense - substitutions that change a codon for one amino acid into a
codon for a different amino acid
Nonsense -substitutions that change a codon for one amino acid into a
stop codon
Wild type
mRNA
Protein
A U G
5
Met
A A G U U U G G C U A A
Lys
Phe
Gly
3
Stop
Amino end
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 A A
Met
Lys
Missense
Phe
Gly
Stop
A instead of G
A U G A A G U U U A G U U A A
Met
Lys
Phe
Ser
Stop
Nonsense
U instead of A
A U G U A G U U U G G C U A A
Met
Stop
Insertions and Deletions
– Are additions or losses of nucleotide pairs in a gene
– May produce frameshift mutations that will change the
reading frame of the gene, and alter all codons
downstream from the mutation.
Wild type
mRNA
Protein
5
A U GA A GU U U G G C U A A
Met
Lys
Gly
Phe
Stop
Amino end
Carboxyl end
Base-pair insertion or deletion
Frameshift causing immediate nonsense
Extra U
AU G U A AG U U U G GC U A
Met
Stop
Frameshift causing
extensive missense
U Missing
A U G A A GU U G G C U A A
Met
Lys
Leu
Ala
Insertion or deletion of 3 nucleotides:
no frameshift but extra or missing amino acid
A A G
Missing
A U G U U U G G C U A A
Met
Phe
Gly
Stop
3