1. Translation and Mutation
Chapter 14
Translation and Mutation

2. You Must Know How genetic material is translated into polypeptides.
How mutations can change the amino acid sequence of a protein and be able to predict how a mutation can result in changes in gene expression.

3. Translation - overview
Amino acids
Polypeptide
tRNA with
amino acid
attached
Ribosome
Trp
Phe
Gly
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.
Each tRNA can translate a particular mRNA codon into a given amino acid.
The tRNA contains an amino acid at one end and at the other end has a nucleotide triplet that can base-pair with the complementary codon on mRNA.
tRNA C C C C
Anticodon A G A A A U G G U U U G G C 5
Codons 3
mRNA 3

4. tRNA wobble 3 Amino acid attachment site Anticodon 5 3 Amino acidG 5 3
Amino acid
attachment
site A C C A 5 C G G C C G U G U A A U U A U * C C G A C A G U A * A G * C U C * G U G U C * C G A G G * * C A G U * G * G A G C
Hydrogen
bonds G C U A
A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long.
tRNA molecules can base-pair with themselves
Flattened into one plane, a tRNA molecule looks like a cloverleaf.
In three dimensions, tRNA is roughly L-shaped, where one end of the L contains the anticodon that base-pairs with an mRNA codon.
RNA)
Accurate translation requires two steps
First: a correct match between a tRNA and an amino acid.
Second: a correct match between the tRNA anticodon and an mRNA codon.
Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon. * G * A A C * U A G A
Anticodon
wobble 4

5. Ribosome P site (Peptidyl-tRNA binding site) Exit tunnel
A site (Aminoacyl-
tRNA binding site)
E site
(Exit site) E P A
Large
subunit
Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons during protein synthesis.
The large and small ribosomal are made of proteins and ribosomal RNAs (rRNAs).
In bacterial and eukaryotic ribosomes the large and small subunits join to form a ribosome only when attached to an mRNA molecule.
mRNA
binding site
Small
subunit 5

6. Ribosome Growing polypeptide Amino end Next amino acid to be added to
chain E
tRNA
mRNA 3
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
Codons 5 6

7. Building a Polypeptide
The three stages of translation
Initiation
Elongation
Termination
All three stages require protein “factors” that aid in the translation process 7 7

8. Initiation of translation
GTP P i
GDP 
P site P i 5 3
Large ribosomal subunit
completes the initiation complex.
Translation initiation complex
Large
ribosomal
subunit A E
Met
GDP  2 3 U A C 5
Met 5 A 3 U G
Initiator
tRNA
mRNA 5
Start codon 3
The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits.
A small ribosomal subunit binds with mRNA and a special initiator tRNA.
Then the small subunit moves along the mRNA until it reaches the start codon (AUG).
The start codon is important because it establishes the reading frame for the mRNA.
The addition of the large ribosomal subunit is last and completes the formation of the translation initiation complex.
Proteins called initiation factors bring all these components together.
Small
ribosomal
subunit
mRNA binding site 1
Small ribosomal subunit binds
to mRNA. 8

9. Elongation Amino end of polypeptide Codon recognition
mRNA P
site i 5 3
GTP  A
GDP
Codon recognition 1 E
mRNA
Ribosome ready for
next aminoacyl tRNA 5
Translocation P i
GTP 
GDP E A 3
Peptide bond
formation E P A 2 P A E i
GTP 
GDP
During elongation, amino acids are added one by one to the previous 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. P A 9

10. Termination of translation
Free
polypeptide 5 3
Release factor
promotes
hydrolysis. 2 2
GTP 
GDP P i 5 3
Ribosomal
subunits and other
components
dissociate. 3
Release
factor 3 5
Stop codon
(UAG, UAA, or UGA) 1
Ribosome reaches a
stop codon on mRNA.
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. 10

11. (a) Several ribosomes simultaneously translating one mRNA molecule
Figure 14.22a
Growing
polypeptides
Completed
polypeptide
Incoming
ribosomal
subunits
Polyribosome
Start of
mRNA
(5 end)
End of mRNA
(3 end)
In bacteria and eukaryotes multiple ribosomes translate an mRNA at the same time.
Once a ribosome is far enough past the start codon, another ribosome can attach to the mRNA.
Strings of ribosomes called polyribosomes (or polysomes) can be seen with an electron microscope.
(a) Several ribosomes simultaneously translating one
mRNA molecule 11

12. A change in the genetic material of a cell or virus.
Mutation
A change in the genetic material of a cell or virus.
Concept 14.5: Mutations of one or a few nucleotides can affect protein structure and function 12 12

13. Mutagens
Spontaneous mutations can occur during DNA replication, recombination, or repair.
Mutagens are physical or chemical agents that can cause mutations.
Most cancer-causing chemicals (carcinogens) are mutagenic, and the converse is also true.

14. Point mutations are chemical changes in just one or a few nucleotide pairs of a gene.
The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein. 14 14

16. The molecular basis of sickle-cell disease
Wild-type hemoglobin A G
mRNA 5 3 T C U
Sickle-cell hemoglobin
Val
Glu
Mutant hemoglobin DNA
Wild-type hemoglobin DNA 3 C T C 5 5 G A G 3
mRNA 5 G A G 3
Normal hemoglobin
Glu 16

17. Hemoglobin is made up of four polypeptides.
Normal hemoglobin
Sickle-cell hemoglobin
Val
Glu
Hemoglobin is made up of four polypeptides.
Hemoglobin alleles are codominant.
What would the hemoglobin of someone who was heterozygous for sickle cell anemia be like?
People who are heterozygous for sickle cell anemia don’t have the disease and they are resistant to malaria!
Normal polypeptide
Glu
Sickle-cell polypeptide
Val

18. 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. 18 18

19. Substitutions Nucleotide-pair substitution Silent mutations
replaces one nucleotide with another nucleotide.
Silent mutations
have no effect on the amino acid produced by a codon because of redundancy in the genetic code. 19 19

20. What would the amino acid sequence be in a silent mutation?
Figure 14.26a
Wild type
DNA template strand 3 T A C T T C A A A C C G A T T 5 5 A T G A A G T T T G G C T A A 3
mRNA 5 A U G A A G U U U G G C U A A 3
Protein
Met
Lys
Phe
Gly
Stop
Amino end
Carboxyl end
What would the amino acid sequence be in a silent mutation?
Phe
Gly
Met
Lys 3 5
Stop
A instead of G
Nucleotide-pair substitution: silent
U instead of C T A C G U 20

21. Substitution mutations are usually missense mutations.
Missense mutations still code for an amino acid, but not the correct amino acid.
Is sickle cell anemia a missense mutation?
Yes.
Substitution mutations are usually missense mutations.
Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein. 21 21

22. DNA template strand
mRNA
Stop
Carboxyl end
Protein
Amino end
Phe
Gly
Met
Lys 3 5
Wild type T A C G U
What would the amino acid sequence be if there were a nonsense mutation at the arrow?
Nucleotide-pair substitution: nonsense 3 5
Met
Stop
A instead of T
U instead of A T A C G U
Figure 14.26c Types of small-scale mutations that affect mRNA sequence (part 3: nonsense) 22

23. Frameshift mutations
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 of the genetic message, producing a frameshift mutation.

24. 3 nucleotide-pair deletion
Figure 14.26f
3 nucleotide-pair deletion
There is no frameshift, but one amino acid is missing 24