Translation: From RNA to Protein
Feb 24, 2016
Translation:From RNA to Protein
Processed mRNA leaves the nucleus
mRNA mRNA binds to ribosomeRibosome
tRNA delivers amino acids
tRNA
Overall Picture
Protein
Universal Code
tRNA• To translate the mRNA transcript into a
protein, the codons must be read one at a time to assemble amino acids in the right sequence
• This occurs in the ribosomewith the help of transfer RNA (tRNA)
Structure of tRNAgluamino acid A tRNA molecule
bound to an amino acid is called an amino - acyl tRNA( aa-tRNA)
anticodon
tRNA
tRNA
Structure of tRNA• Folded RNA molecule with an
anticodon loop– the anticodon contains a nucleotide
triplet which is complementary to the mRNA codon
• Each tRNA carries a particular amino acid corresponding to its anticodon
Reusing tRNA
How Many Codon Permutations?
AUCG
43 = 64
?
Universal AND Redundant
The Wobble Hypothesis• Though there are 61 codons coding for
amino acids, we do not need 61 types of tRNA
• The 3rd nucleotide in the anticodon is often less important than the first two(binds weakly / not specific)
• The third nucleotide is in the“wobble” position
The Wobble Hypothesis• UCC and UCU both code for serine
The Ribosome• A ribosome is a complex of protein
combined with ribosomal RNA (rRNA)• 2 subunits : small and large
3 binding sites for tRNA• P site
holds one aa - tRNA and the growing chain of amino acids
• A site receives the tRNA with the next amino acid to be added to the chain
• E site releases the used tRNA back into the cytoplasm
E P A
Small ribosomal subunit
Large ribosomal subunit
Phases of Translation
1. Initiation
2. Elongation
3. Termination
Initiation
1. In the cytoplasm a small portion of mRNA binds to rRNA on the small ribosomal subunit
2. A tRNA molecule with the start anticodon UAC (to complement the start codon AUG) binds to the mRNA - rRNA complex. The start tRNA carries the amino acid methionine
3. This complex then binds to the large ribosomal subunit
Initiation
E P A
5´ 3´S
G
P S
G
P S
U
P S
U
P S
A
P S
G
P S
A
P S
U
P S
C
PS
G
P S
G
P S
U
P S
A
P S
U
P S
G
P S
A
P S
U
P S
C
P
3’
5’
tRNA
Initiation
ElongationThe cycle of elongation has 3 steps:1. aa-tRNA binds to the A site.
2. The large ribosomal subunit catalyzes the formation
of a peptide bond with the previous amino acid. At the same time the polypeptide chain is passed from the tRNA in the P site to the tRNA in the A site. Translates 5’ 3’
3. Translocation: The ribosome moves 3 nucleotides (one codon) along the mRNA. This releases the used tRNA at the E site and frees the A site for a new aa-tRNA to bind
Elongation
E P A
5´ 3´S
G
P S
G
P S
U
P S
U
P S
A
P S
G
P S
A
P S
U
P S
C
PS
G
P S
G
P S
U
P S
A
P S
U
P S
G
P S
A
P S
U
P S
C
P
Elongation
E P A
5´ 3´S
G
P S
G
P S
U
P S
U
P S
A
P S
G
P S
A
P S
U
P S
C
PS
G
P S
G
P S
U
P S
A
P S
U
P S
G
P S
A
P S
U
P S
C
P
Elongation
E P A
5´ 3´S
G
P S
G
P S
U
P S
U
P S
A
P S
G
P S
A
P S
U
P S
C
PS
G
P S
G
P S
U
P S
A
P S
U
P S
G
P S
A
P S
U
P S
C
P
Elongation
E P A
5´ 3´S
G
P S
G
P S
U
P S
U
P S
A
P S
G
P S
A
P S
U
P S
C
PS
G
P S
G
P S
U
P S
A
P S
U
P S
G
P S
A
P S
U
P S
C
P
Elongation
E P A
5´ 3´S
G
P S
G
P S
U
P S
U
P S
A
P S
G
P S
A
P S
U
P S
C
PS
G
P S
G
P S
U
P S
A
P S
U
P S
G
P S
A
P S
U
P S
C
P
Growing polypeptide
chain
Elongation
Termination• There is no tRNA with the
complementary anticodon for the stop codon
• A protein release factor binds to the stop codon in the A site
• This cleaves the polypeptide from the tRNA and breaks apart the ribosomal sub-units
I love protein
Termination
E P A
3´S
G
P S
G
P S
U
P S
U
P S
A
P S
G
P S
A
P S
U
P S
C
PS
G
P S
G
P S
U
P S
A
P S
U
P S
C
P S
A
P S
U
P S
C
PS
U
P S
U
P S
C
P S
C
P S
A
P S
A
P S
C
P
I’ll be back!
protein
mRNA
Release factor (protein)
Termination
Polysome• One mRNA can be bound
simultaneously to more than one ribosome. This is a polysome!(called a polyribosome in your textbook)
Electron Micrograph
3D Shape• During translation
polypeptides fold into a complex3D shape
• NOT DONE YET! Most newly synthesized proteins need modification in the ER and/or Golgi:
Post-translational modifications: - some amino acids may be removed- polypeptide can be divided into pieces- sugar and phosphate may be added- several polypeptides can join to form quaternary
structure
Summary
Homework
Your homework this evening is to:
1. Write captions for the diagrams provided
2. Do the section review questions on p. 331 #1-8
Structure of tRNA
3’
5’
amino acid
anticodon
Ileu
A tRNA molecule bound to an amino acid is called an amino - acyl tRNA ( aa-tRNA)
Polysome