Transcript No Slide Title

Bio 1010
Dr. Bonnie A. Bain
92
CHAPTER 10
DNA Structure and Function
Part 3
Translation
(translate the mRNA code into a polypeptide)
The Players
*mRNA
*tRNA
*Ribosomes (rRNA + protein)
The Process
Initiation
Elongation
Termination
Initiation of Translation
1. An mRNA molecule binds to a small ribosomal
subunit
A special “initiator” tRNA then binds to the Start
codon on the mRNA
The “initiator” tRNA carries the amino acid,
methionine, on its amino acid attachment site
Anticodon on tRNA is UAC
Start codon on mRNA is AUG
Initiation of Translation
2. A large ribosomal subunit fits on to the small
subunit
The “initiator” tRNA fits in to the P site on the
ribosome
Figure 10.18
Translation: Elongation
Once initiation is complete, then amino acids are
added one by one
Each addition to the amino acid chain requires 3
steps (Fig. 10.18, p. 185):
1. Codon Recognition
2. Peptide Bond Formation
3. Translocation
1. Codon Recognition
The anticodon of the tRNA pairs with the
codon of the mRNA
Occurs at the A site of the ribosome
Figure 10.19
2. Peptide bond formation
The polypeptide (on the tRNA at the P site)
attaches to the new amino acid on the tRNA at
the A site
The polypeptide is attached to the “new” amino
acid by a peptide bond
Figure 10.19
3. Translocation
The P site tRNA leaves the ribosome
The ribosome translocates (moves) the other
tRNA from the A site over to the P site
This movement then exposes the next mRNA
codon to be translated (at the A site) and the
process then repeats itself
Figure 10.19
Termination of Translation
Elongation continues until a stop codon reaches
the A site
Stop codons: UAA, UAG, UGA
When the stop codon appears, translation stops
The completed polypeptide breaks off and
leaves the ribosome
Figure 10.19
Translation Rate: very fast
1 ribosome can make 1 polypeptide in less than
1 minute
As it is being made, the polypeptide bends and
folds into its secondary and then tertiary shape
After it is made, several may come together to
form a protein with a quaternary structure
Figure 10.20
Mutations
Any change in the nucleotide sequence of the
DNA molecule
Types of mutations:
1. Base Substitutions
2. Base Insertions or Base Deletions
1. Base Substitutions
Replacement of one base or nucleotide with
another
Can result in: no change to a protein
or
insignificant change (Missense mutation)
NOTE: SOME MISSENSE MUTATIONS, LIKE
SICKLE CELL ANEMIA CAN ALSO HAVE
MAJOR EFFECTS—textbook is misleading here
or major, life-threatening change (Nonsense
mutations)
1. Base
Substitutions
Can result in:
no change to a protein
Example
GAA changes to GAG
No change in the protein because both codons
code for the same amino acid (Glu)
1. Base Substitutions
Or they can result in insignificant change (but
see sickle cell anemia)
Called Missense Mutations
Example
GGC changes to AGC
GGC codes for Glycine
AGC codes for Serine
Missense Mutations
May or may not have a major effect on the
protein
Example of a major effect:
One base change causes sickle-cell anemia
see next slide (p. 186 in textbook)
Figure 10.21
1. Base Substitutions
Nonsense mutations
These turn an amino acid codon into a Stop
codon
Causes the polypeptide to be terminated
prematurely
Example: AGA changes to UGA
AGA codes for arginine
UGA is a Stop codon
2. Base Insertions or Base Deletions
Cause Reading Frame errors
(frameshift mutations)
Since mRNA is read as a series of nucleotide
triplets, any shift in where to start reading will
cause errors
2. Base Insertions or Base Deletions
Example of a base deletion:
AAG-UUU-GGC-GCA
Remove the first U
AAG-UUG-GCG-CA
Figure 10.22b
Stopped here on Monday Nov. 7, 2011
AAG-UUU-GGC-GCA
Lys – Phe – Gly- Ala
Remove the first U
AAG-UUG-GCG-CA
Lys – Leu – Ala- ?
Insertions of extra nucleotides can also cause
these problems (frameshift mutations)
But, if 3 nucleotides are added (or if 3 are taken
away), then it is no longer a frame-shift mutation
Point Mutations:
A change in a single nucleotide
Frameshift Mutations:
An insertion or deletion of 1 or 2 nucleotides
Remember:
Mutations are the ultimate source of diversity of
life on our planet
Figure 10.23