Transcript Brooker Chapter 13

Translation
(CHAPTER 13- Brooker Text)
Sept 25, 2008
BIO 184
Dr. Tom Peavy
 CODON =
3 nucleotides
encode for
1 amino acid
 DEGENERACY=
more than one
codon encodes
for an amino acid
(wobble base)
 Code is UNIVERSAL
(nearly)
 Special codons=
start codon (AUG)
stop codons
(UAA, UGA, UAG)
• Figure 13.2 provides an overview of gene expression
Figure 13.2
Recognition Between tRNA and mRNA
• During mRNA-tRNA recognition, the anticodon in
tRNA binds to a complementary codon in mRNA
tRNAs are named
according to the
amino acid they bear
Proline
anticodon
Figure 13.8
 cloverleaf folding pattern
(stem-loop structures)
 contain modified bases
(>60 possible)
 amino acyl tRNA synthetase=
enzyme that conjugates
appropriate amino acid to
the 3’ end of tRNA
(20 of them, one for each a.a.)
 Wobble hypothesis=
codon-anticodon recognition
tolerates mismatches in the
third position (but not first and
second)
Figure 13.10 Structure of tRNA
RIBOSOME STRUCTURE AND
ASSEMBLY
• Macromolecular complex of rRNA and proteins
• Prokaryotes = one type of ribosome (70S)
– Found in their cytoplasm
• Eukaryotes = two types of ribosomes
– One type is found in the cytoplasm (80S)
– The other is found in organelles (70S, like prok.)
• Mitochondria ; Chloroplasts
Bacterial Ribosomes (and mitochondrial/chloroplast)
A ribosome is composed
of structures called the
large and small subunits
Each subunit is
formed from the
assembly of Proteins
+ rRNA
Note: S or Svedberg units
are not additive
Figure 13.13
Eukaryotic Ribosomes
Formed in the
cytoplasm during
translation
The 40S and 60S subunits are
assembled in the nucleolus
Then exported to the cytoplasm
Figure 13.13
Ribosomes contain three
discrete sites:
Peptidyl site (P site)
Aminoacyl site (A site)
Exit site (E site)
Three Stages: Initiation
Initiator tRNA
Release
factors
Elongation
Termination
Initiation Stage
• Initiation complex=
mRNA + initiation factors + ribosomal subunits
• The binding of mRNA to the 30S subunit is facilitated by a
ribosomal-binding site or Shine-Dalgarno sequence
– This is complementary to a sequence in the 16S rRNA
Hydrogen bonding
Component of the
30S subunit
Figure 13.17
16S rRNA
Initiation completed
after assembly of
first tRNA
(=formyl methionine)
The only charged
tRNA that enters
through the P site
All others enter
through the A site
70S initiation
complex
This marks the
end of the first
stage
Figure 13.16
Elongation
• Charged tRNA binds to the A site
(use of GTP and Elongation factor)
• Bond between polypeptide chain
(P site) and incoming amino acid
(A site) catalyzed by peptidyl
transferase
• Polypeptide transferred to the A site
Figure 13.18
Elongation Cont’d
• Ribosome translocates one codon
to the right
(GTP and elongation factor)
• Uncharged tRNA released from
the E site
• Repeat Process until stop codon
Termination Stage
– In most species there are three stop or nonsense codons
• UAG
• UAA
• UGA
– These codons are not recognized by tRNAs, but by
proteins called release factors
• Indeed, the 3-D structure of release factors mimics that
of tRNAs
 Polypeptide synthesis has a directionality that parallels the
5’ to 3’ orientation of mRNA
Carboxyl group
Amino
terminus
Amino group
Carboxy
terminus
Condensation
reaction releasing a
water molecule
Levels of Structures in Proteins
• There are four levels of structures in proteins
– 1. Primary (its amino acid sequence)
– 2. Secondary (protein folds to form regular, repeating
known)
– 3. Tertiary (short regions of secondary structure in a
protein fold into a three-dimensional)
– 4. Quaternary (Proteins made up of two or more
polypeptides)
A protein
subunit
Figure 13.6