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Translation:�From RNA to Protein

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Overall Picture

Processed mRNA leaves the nucleus

mRNA

mRNA binds to ribosome

Ribosome

tRNA delivers amino acids

tRNA

Protein

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Universal Code

Codon

a sequence of 3 nucleotides that constitue a unit of genetic code. Each codon corresponds to a single amino acid or is a “stop codon”

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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 ribosome�with the help of �transfer RNA (tRNA)

​

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Structure of tRNA

glu

amino acid

A tRNA molecule bound to an amino acid is called an �amino - acyl tRNA�( aa-tRNA)

anticodon

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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

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tRNA

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tRNA

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Reusing tRNA

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Reusing tRNA

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How Many Codon Permutations?

A

U

C

G

43 = 64

?

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Universal AND Redundant

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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

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The Wobble Hypothesis

  • UCC and UCU both code for serine

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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

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Phases of Translation

  1. Initiation�
  2. Elongation�
  3. Termination

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Initiation

E

P

A

5´

3´

S

G

P

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G

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3’

5’

tRNA

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Initiation

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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

​

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Elongation

E

P

A

5´

3´

S

G

P

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Elongation

E

P

A

5´

3´

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G

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Elongation

E

P

A

5´

3´

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Elongation

E

P

A

5´

3´

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Elongation

E

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A

5´

3´

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Growing polypeptide chain

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Elongation

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Elongation

The 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

​

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Termination

E

P

A

3´

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5´

I’ll be back!

protein

mRNA

Release factor (protein)

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Termination

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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

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  • This cleaves the polypeptide from the tRNA and breaks apart the ribosomal sub-units

I love protein

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Polysome

  • One mRNA can be bound simultaneously to more than one ribosome. This is a polysome!�(sometimes called a polyribosome)

​

​

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Electron Micrograph

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3D Shape

  • During translation �polypeptides fold into a complex�3D 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

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Summary

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Summary

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Summary

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Detailed Video

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