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Gene and The Cell > Lecture 13 - Translation > Flashcards

Lecture 13 - Translation Flashcards

1
Q

Translation

A
  • Protein synthesis

- Process of converting the information present in mRNA to proteins (polypeptides)

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2
Q

Where is translation carried out?

A
  • By the ribosome
  • mRNA is read 5’ to 3’
  • Protein is synthesized from N to C terminus
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3
Q

What doe is mean for the genetic code to be redundant?

A

That for most amino acids there is more than one codon (third base degeneracy)

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4
Q

What does it mean for the genetic code to not be ambiguous?

A

That each codon specifies only one amino acid

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5
Q

Start codon

A

AUG

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6
Q

Stop codons

A

UAA
UAG
UGA

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7
Q

What three types of RNA are directly involved with translation?

A

mRNA - codons
tRNA - anticodons
rRNA - components of ribosomes

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8
Q

Function of tRNA in translation

A
  • tRNAs bring the correct amino acid to the ribosome in response to a specific codon
  • Each tRNA has a triplet anticodon that recognises one or more codon in the mRNA by base pairing
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9
Q

tRNAs universal structure

A
  • Amino acid attachment side (always CCA)

- Anticodon is at the midpoint of the tRNA sequence

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10
Q

Talk about the anticodon of tRNAs

A
  • It is the site of base pairing with mRNA

- It is unique for each species of tRNA

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11
Q

How does tRNA charging happen?

A
  1. Aminoacyl-tRNA synthetase for a specific amino acid activates the aa, catalyzing a reaction with ATP to form high energy AMP - aa and a pyrophosphate ion
  2. Pyrophosphatase drives the reaction forward
  3. The enzyme then catalyzes a reaction of the activated aa with the correct tRNA
  4. The specificity of the enzyme ensures that the correct aa and tRNA have been brought together
  5. The charged tRNA will deliver the appropriate aa to join the elongating polypeptide product of translation
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12
Q

If:
DNA = 3’-GCC-5’
mRNA = 5’-CGG-3’ (codon)
What is the anticodon on the tRNA?

A

3’-GCC-5’

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13
Q

Explain the wobble phenomenon

A

There are 61 different codons that encode for 20 aa. There are 2/3rds that number of tRNA species becasue the specificity for the base at the 3’ end of the codon (and the 5’ end of the anticodon) is not always strictly observed
-For example GCA, GCC, and GCU are all recognized by the same tRNA

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14
Q

Function of the ribosome in translation

A
  • Holds mRNA and charged tRNAs in the correct position to allow assembly of polypeptide chain
  • Not specific, can make any type of protein
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15
Q

By what are ribosomal subunits held together?

A
  • By ionic and hydrophobic forces

- When not active in translation the subunits exist separately

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16
Q

Explain the critical role of rRNA in the ribosome

A
  • rRNA is the major structural and functional component
  • Peptide bond formation is catalyzed by the 23S rRNA i.e. a ribozyme
  • Specific changes in rRNA conformation are associated with subunit association and tRNA binding
17
Q

Three phases in polypeptide synthesis

A
  1. Initiation
  2. Elongation
  3. Termination
18
Q

Explain the three sites to which tRNA can bind

A
  1. A (amino acid) site: the charged tRNA anticodon binds to the mRNA codon, lining up the correct aa to be added to the growing polypeptide chain
  2. P (polypeptide) site: Where the tRNA adds its amino acid to the polypeptide chain
  3. E (exit) site: where the tRNA, having given up its aa, resides before being released from the ribosome
19
Q

Explain the process of Initiation

A
  1. The small ribosomal subunits binds to its recognition sequence on mRNA
  2. Methionine-charged tRNA binds to the AUG “start” codon, completing the initiation complex
  3. The large ribosomal subunit joins the initiation complex, with methionine charged tRNA now occupying the P site
20
Q

What do initiation factors do?

A

They put together the mRNA, two ribosomal subunits, and methionine-charged tRNA

21
Q

Explain the process of Elongation

A
  1. Codon recognition: The anticodon of an incoming tRNA binds tot he codon at the A site
  2. Peptide bond formation: The new aa is linked to the previous by peptidyl transferase activity of the large subunit
  3. Elongation: Free tRNA is moved to the E site, and then released, as the ribosome shifts by one codon, so that the growing polypeptide chain moves to the P site
    * All the steps are assisted by ribosomal proteins called elongation factors
22
Q

Explain peptidyl transferase activity

A
  • Peptide bond formation is carried out by the 23S rRNA - a ribozyme
  • Occurs when A and P sites are appropriately occupied
  • Generates the new peptide bonds (polypeptide is now in the A site)
23
Q

How does translocation occur during the elongation process?

A
  • Translocation happens because the ribosome needs to be reset
  • The deacylated tRNA is ejected via the E-site
  • The peptidyl-tRNA is moved to the P-site together with its codon
  • The next codon is exposed in the A-site
24
Q

Explain the process of Termination

A
  1. When a stop codon enters the A site instead of binding any tRNAs it binds a protein release factor
  2. The release factor disconnect the polypeptide from the tRNA in the P site
  3. The remaining components (mRNA and ribosomal subunits) separate
25
Q

What is a polysome?

A

Multiple ribosomes and their growing polypeptide chains moving along an mRNA molecule

Gene and The Cell (16 decks)

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