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CORE Block 2 > Translation > Flashcards

Translation Flashcards

1
Q

How much energy is used by the cell for translation?

A

~50% of its energy reserves

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2
Q

What are the key stages of protein synthesis?

A
  1. amino acid activation
  2. initiation
  3. elongation
  4. termination
  5. recycling
  6. co/post-translational processing
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3
Q

What is the adaptor hypothesis?

A

base pairing of codon to anticodon in antiparallel fashion. Something needs to link the anticodon to the amino acid

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4
Q

Contrast degeneracy with lack of ambiguity

A

Degeneracy means that there are multiple codons for the same amino acid. Lack of ambiguity means that no one codon can code for more than one amino acid.

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5
Q

What phenomenon describes the fact that some cells have less tRNAs than codons

A

wobble hypothesis

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6
Q

Describe the wobble hypothesis.

A

The first base of the anticodon is sometimes converted to hypoxanthine (H) by deamination, and can pair with A, U, or C, meaning that the anticodon can interact with multiple codons and allow the cell to not need as many tRNAs as codons.

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7
Q

What does it mean for the genetic code to be quasi-universal?

A

It means almost all codons encode for the same amino acids, except for mitochondria usage of codons among different organisms.

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8
Q

Where do nonsense mutations normally occur?

A

In the last exon of the mRNA.

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9
Q

Which part of tRNA is phosphorylated?

A

The 5’ terminus.

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10
Q

What are the primary active sites of tRNA?

A

The anticodon loop and the CCA 5’ terminus where the amino acid is attached.

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11
Q

Which player in translation does the actual translation?

A

aminoacyl-tRNA synthetase

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12
Q

Describe the activation of amino acids/charging of tRNA.

A
  • carboxyl end of amino acid is activated by the addition of AMP, resulting in loss of pyrophosphate
  • pyrophosphate is hydrolyzed, driving forward the reaction
  • tRNA is charged by forming an ester linkage to the carbonyl oxygen, breaking the AMP ester bond.
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13
Q

how many aminoacyl-tRNA synthetases are there?

A

20! each one recognizes its amino acid based on D-loop and variable region and anticodon

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14
Q

How is the fidelity of tRNA charging maintained?

A

aminoacyl-tRNA synthetase has proofreading and editing function and can destroy an incorrect pair. This is important because the ribosome reads the anticodon, not the amino acid.

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15
Q

How are ribosome components measured and named?

A

Based on there sedimentation in a sucrose gradient. Dependent on size and shape.

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16
Q

What are the components of prokaryotic ribosomes?

A
  • 50S large subunit (more proteins than small)
  • 30S small subunit (less proteins than large) containing 16S RNA
  • 70S assembled ribosome
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17
Q

What are the components of eukaryotic ribosomes?

A
  • 60S large subunit
  • 40S small subunit
  • 80S assembled ribosome
18
Q

Describe the main function of the small ribosome subunit.

A

It decodes the anticodon

It controls the fidelity of pairing

19
Q

Describe the main function of the large ribosome subunit.

A

peptidyl transferase site

tunnel through which nascent protein is funnelled

20
Q

Which are the large translation events which dictate initiation?

A

mRNA binds small subunit and to initiating charged tRNA. The larger subunit then joins the small subunit and the A, P, E sites are formed.

21
Q

Describe the steps of translation initiation.

A
  • GTPase eIF2 binds initiating tRNA which associates with 40S subunit
  • eIF4 factors associated with mRNA direct it cap-first to the 40S subunit
  • 40S scans mRNA until AUG is found and interacts with initiating tRNA anticodon
  • 60S associates, eIF2 GTP is hydrolyzed (no more charge), and eIF2 and anti-association factor eIF3 bound to 40S is released
22
Q

Which site of the ribosome is initiating tRNA in upon initiatioon?

A

The P site.

23
Q

Describe the circularization of mRNA in eukaryotic translation and the role of eIF4 proteins.

A
  • eIF4E binds 5’ cap
  • PABP’s bind polyA tail
  • eIF4G bind eIF4E to PABP
24
Q

How does the ribosome find the initiation codon?

A

It uses energy and catalysis from eIF4A to scan along the mRNA.

25
Q

What are the general steps of translation elongation?

A

decoding, peptide bond formation, and translocation

26
Q

How are GTPases in translation regenerated?

A

Not by phosphorylation. EF1-alpha, for example, has GDP removed by guanine exchange factor EF-1beta-gamma. The levels of GTP in the cell allow EF1-alpha to immediately pick up another GTP.

27
Q

Describe the first two steps of translation elongation.

A
  • GTPas EF1-alpha carries in each new charged tRNA to the ribosome. If there is a match, it will enter the A site and the anticodon will interact with the codon
  • hydrolysis of EF1-alpha allows it to dissocite from its tRNA so it can go bring in a new tRNA

-

28
Q

Describe the 3rd and 4th steps of translation elongation.

A
  • 60S subunit catalyzes bond formation between initiating amino acid and the next by linking methionine C-term to 2nd amino acid N-term. Energy comes from hydrolysis of the high energy tRNA-amino acid ester bond
  • deacetylated initiation tRNA pushed to E site and first elongation tRNA pushed to P site with energy from EF-2 GTPase (translocase)
29
Q

Describe eukaryotic translation termination.

A
  • eRF1 recognizes termination sequence codon at A site
  • eRF1 forms complex with eRF3 GTPase
  • Hydrolysis of GTP from eRF1-eRF3 complex is used to release peptide from the P site.
30
Q

Describe eukaryotic translation recycling.

A
  • eRF1 still bound to ribosome after translation termination, binds recycling factor ABCE1
  • the ribosomal complex, release factors, and tRNA then dissociate
31
Q

What structure facilitates reinitiation of translation?

A

Circularized mRNA

32
Q

Describe the differences between eukaryotic and prokaryotic translation initiation.

A
  • No energy is used for scanning. Instead, 16S RNA binds complementary with shine dalgarno sequence upstream of AUG
  • the initiating tRNA carries a methionine which is formylated
  • overall, much less energy is needed
  • fewer initiation factors are required
33
Q

How does translation elongation differ between prok and euk?

A

Just the name of the elongation factors

34
Q

How does translation termination differ between prok and euk?

A
  • There are two different release factors that recognize the different termination codons
  • GTP energy from RF3-GTP is used to release termination sequence binding factors

-

35
Q

How does ribosome recycling differ between prok and euk?

A

Just different names.

  • In euk: ABCE1.
  • In prok: RRF (ribosomal recylcing factor)
36
Q

Where is ATP hydrolysis used in prok translation? Euk?

A

In prok there is no use of ATP. In euk, ATP is used for ribosomal sliding along mRNA.

37
Q

Which are the twenty first and twenty second amino acids?

A
  • selecnocysteine
  • pyrrolysine
38
Q

How is the UGA codon recoded in selenoproteins?

A
  • SECIS binding protein (SBP2) binds SECIS element at 3’ end of mRNA, downstream of stop codon
  • specialized tRNA bound to selenocystein binds the SBP2, causing the anticodon to interact with UGA stop codon and add selenocysteine to the growing polypeptide chain. All UGA will be recoded this way
  • special elongation factor also needed
  • actual termination codon must not be UGA
39
Q

How is selenocysteine formed?

A
  • Serine is added to tRNA with UGA anticodon
  • ATP energy is used to knock off -OH and add selenium
40
Q

What are human selenoproteins involved in?

A

oxidation reduction reactions (e.g. thioredoxin reductase, thyroid hormone deiodinases)

CORE Block 2 (9 decks)

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