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