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BQ II > T.17 PROTEIN TRANSLATION > Flashcards

T.17 PROTEIN TRANSLATION Flashcards

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1
Q

What is translation in molecular biology?

A

Translation refers to the change from the nucleotide language of nucleic acids to the amino acid language of proteins; it occurs in the cytosol.

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2
Q

Does translation imply the protein is already functional?

A

No; the protein must be folded properly and undergo post-translational modifications to become active.

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3
Q

Why is translation of mRNA necessary?

A

Messenger RNA is recognized only by amino acids and not by nucleotides, so it must be translated and moved from the nucleus to the cytosol.

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4
Q

What is the nucleotide to amino acid ratio in translation?

A

For every three nucleotides of RNA, one amino acid is obtained.

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5
Q

Can translation happen via different routes?

A

Yes; it can be translated by the secretory route linked to the ER or by another route to go to different organelles.

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6
Q

When is translation associated with the rough ER?

A

When the protein is destined for the plasma membrane, lysosomes, etc., translation is associated with the rough ER.

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7
Q

When is translation free in the cytosol?

A

When the protein is destined for the nucleus, mitochondria, or peroxisomes, mRNA is translated freely in the cytosol.

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8
Q

Can proteins be synthesized and degraded at any time?

A

Yes; proteins are continuously synthesized and degraded.

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9
Q

How much energy does translation consume?

A

Translation uses 90% of the energy required for protein synthesis; the remaining 10% is used for transcription, RNA maturation, and transport.

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10
Q

How many macromolecules are involved in translation?

A

Translation involves 300 different macromolecules.

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11
Q

What is the error rate of translation?

A

Error rate is 10⁻⁴, which is higher than in transcription or replication.

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12
Q

What is the speed of translation?

A

Translation proceeds at 40 amino acids per second.

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13
Q

Is translation faster than transcription?

A

Yes; translation is a faster process than transcription.

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14
Q

Which RNAs cooperate during translation?

A

Ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA) all cooperate in translation.

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15
Q

What is the function of ribosomal RNA (rRNA)?

A

rRNA is part of the ribosome, has catalytic activity, and mediates protein synthesis.

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16
Q

What is the function of transfer RNA (tRNA)?

A

tRNA transports amino acids during synthesis and transfers them to specific codons in the polypeptide chain.

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17
Q

What is the function of messenger RNA (mRNA)?

A

mRNA carries the coding message and regulatory sequences for stability and translation control.

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18
Q

How many codons exist and how many are coding?

A

There are 64 codons total; 61 code for amino acids and 3 are stop codons.

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19
Q

What do stop codons do?

A

Stop codons (UAA, UAG, UGA) do not code for amino acids but signal the end of translation.

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20
Q

What determines the reading frame in translation?

A

The first/start codon (AUG) sets the reading frame and always codes for methionine.

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21
Q

What is an open reading frame (ORF)?

A

A coding region of at least 50 codons between the start and stop codons with no interruptions.

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22
Q

Do start and stop codons match the ends of mRNA?

A

No; the start codon does not match the cap end and the stop codon does not match the poly-A tail end.

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23
Q

What are UTRs in mRNA?

A

UTRs (untranslated regions) are parts of mRNA that are not translated; include 5’ UTR and 3’ UTR.

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24
Q

What is the function of UTRs?

A

UTRs control mRNA stability (protection from degradation) and regulate translation.

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25
Is the first nucleotide of mRNA translated?
No; it is within the 5’-UTR and is never translated.
26
What does "non-ambiguous" mean in the genetic code?
Each codon encodes a single, specific amino acid—no ambiguity.
27
What does "degenerate or redundant" mean in the genetic code?
Several codons may code for the same amino acid.
28
Is the genetic code universal?
Yes; the same code is used in mammals, plants, reptiles, etc.
29
What determines how many codons an amino acid has?
The frequency of the amino acid in proteins; more frequent amino acids have more codons.
30
Which amino acids have the most codons?
Serine (Ser), Leucine (Leu), and Arginine (Arg) have more codons due to their frequency.
31
Which amino acids have fewer codons?
Those that occur less frequently in proteins.
32
What is the structure of rRNA?
Single-stranded RNA with a 5’ and 3’ end and hairpin structures.
33
What is the function of rRNA?
It maintains the ribosome's structure and has catalytic (ribozyme) activity.
34
What is the size of prokaryotic ribosomes?
70S, composed of 50S and 30S subunits.
35
What is the size of eukaryotic ribosomes?
80S, composed of 60S and 40S subunits.
36
What is the structure of tRNA?
Single-stranded RNA with 5’ and 3’ ends; includes amino acid arm (3’-ACC) and anticodon arm.
37
What does the 3' end of tRNA contain?
The sequence ACC followed by a purine, which is the binding site for the amino acid (amino acid arm).
38
What is found at the anticodon arm of tRNA?
A sequence that base-pairs with the codon in mRNA, oriented 5’–3’ to match the mRNA codon 3’–5’.
39
How is amino acid specificity ensured in tRNA?
Each tRNA must be loaded with the correct amino acid based on its anticodon.
40
How many tRNAs exist for the 61 coding codons?
Only 32 different tRNAs due to the wobble base pairing system.
41
What is the wobble base pairing hypothesis?
Inosine (I) in the third position of the anticodon can pair with A, U, or C, allowing one tRNA to recognize multiple codons.
42
What is amino acid activation?
The process of loading specific amino acids onto their matching tRNAs.
43
What does "tRNAala" refer to?
A tRNA with an anticodon for alanine that is not yet loaded with alanine.
44
What does "ala–tRNAala" refer to?
A tRNA for alanine that is already loaded with alanine.
45
What enzyme catalyzes amino acid loading onto tRNA?
Aminoacyl-tRNA synthetase, specific to each amino acid.
46
What are the steps of amino acid activation?
1) Bind amino acid to AMP using ATP. 2) Load aminoacyl-AMP onto tRNA.
47
What are the two classes of aminoacyl-tRNA synthetases?
Class I and class II, based on whether AMP binds in one step or two.
48
How do aminoacyl-tRNA synthetases ensure specificity?
They recognize the anticodon and other tRNA arms, and they have proofreading ability.
49
What are the key characteristics of aminoacyl-tRNA synthetases?
Highly specific, contain proofreading mechanisms, and have a low error rate.
50
What is unique about protein synthesis in prokaryotes regarding transcription and translation?
It occurs simultaneously because there is no need to mature or transport mRNA to the cytoplasm as in eukaryotic cells.
51
In which direction is mRNA read during translation in prokaryotes?
5’ to 3’ direction.
52
What is the direction of protein synthesis in prokaryotes?
From the amino group to the carboxyl group (N-terminus to C-terminus)in the 5’ to 3’ direction.
53
How many times can the same mRNA be translated in prokaryotes?
Many times; many ribosomes can bind the same mRNA simultaneously.
54
What components are required for amino acid activation?
20 amino acids 20 aminoacyl-tRNA synthetases
55
What components are involved in initiation of translation in prokaryotes?
Mg2+ mRNA
56
What are the roles of the P and A sites on the 30S ribosomal subunit?
P site is where the peptide bond is formed; A site is where the aminoacyl-tRNA enters.
57
How does IF-1 function during initiation?
Blocks the A site to prevent entry of other amino acids.
58
What is the function of IF-3 during initiation?
Increases stability of the initiation complex.
59
What is the Shine-Dalgarno sequence?
A sequence before the initiation codon that is complementary to rRNA and guides ribosome binding.
60
What is the purpose of the Shine-Dalgarno sequence?
It allows the rRNA to identify the correct AUG initiation codon.
61
How does IF-2 function during initiation?
IF-2 bound to GTP brings N-formylmethionyl-tRNA to the P site; upon codon-anticodon pairing GTP is hydrolyzed and IF-2
62
What completes the assembly of the functional 70S initiation complex in prokaryotes?
The 50S subunit joins providing the A site
63
What components are required for elongation in prokaryotic translation?
Mg2+ 70S ribosome
64
What is the role of EF-Tu in elongation?
Transports aminoacyl-tRNA to the A site with GTP; upon pairing GTP is hydrolyzed and EF-Tu is released.
65
How is EF-Tu recycled?
GDP is replaced by EF-Ts which is then replaced by GTP to bind another tRNA.
66
How is the peptide bond formed during elongation?
Ribosomal RNA catalyzes nucleophilic attack by the amino group of the new amino acid on the carboxyl group of the previous one.
67
What happens after the first peptide bond is formed?
Both amino acids are attached to the tRNA in the A site; the P site tRNA becomes empty.
68
What is the role of EF-G in elongation? ’
Translocates the ribosome one codon 5’→3 moving the empty tRNA to the E site and the peptidyl-tRNA to the P site.
69
What signals the termination of translation in prokaryotes?
A stop codon (UAA, UAG)
70
Which release factors recognize the stop codons in prokaryotes?
RF-1: UAA UAG; RF-2: UGA
71
How does the release factor function?
Mimics tRNA binds to the stop codon
72
What happens after the polypeptide is released in prokaryotic termination?
The ribosome disassembles and translation ends.
73
Why is Mg2+ important in protein synthesis?
Provides enzymatic activity stability supporting DNA/RNA polymerases and other enzymes.
74
What occurs during folding and post-translational processing?
Removal of initiating residues and signal sequences proteolytic processing
75
Where does transcription occur in eukaryotes?
In the nucleus.
76
Where does translation occur in eukaryotes?
In the cytoplasm.
77
What is the main difference in translation between prokaryotes and eukaryotes?
Initiation—eukaryotes have more initiation factors.
78
List the initiation factors in prokaryotes. IF-1
IF-1 IF-2
79
List the initiation factors in eukaryotes.
eIF-1 eIF-1A
80
Match prokaryotic and eukaryotic elongation and termination factors.
EF-Tu = eEF1α; EF-Ts = eEF1βγ; EF-G = eEF-2; RFs = eRF-1 eRF-3.
81
What is the function of eIF-1?
Prevents premature tRNA binding to A site.
82
What does eIF-2 do?
Helps Met-tRNA bind to the 40S subunit.
83
What is the role of eIF-2B and eIF-3?
Bind early to the 40S subunit and facilitate further initiation steps.
84
What does eIF-4A do?
Acts as RNA helicase to remove mRNA secondary structure.
85
What is the function of eIF-4B?
Helps scan mRNA for the first AUG codon.
86
What does eIF-4E bind to?
Binds the 5’ cap of mRNA.
87
What is the function of eIF-4G?
Links eIF-4E and poly(A) binding protein (PAB) forming the eIF-4F complex.
88
What is the role of eIF-5?
Promotes release of initiation factors and allows 60S subunit binding.
89
What does eIF-6 do?
Facilitates dissociation of 80S ribosome into 40S and 60S subunits.
90
What does the mature mRNA do before translation in eukaryotes?
Exits the nucleus remains circularised
91
How is mRNA circularised in eukaryotes?
Through interactions between CAP-binding proteins and poly A tail binding proteins via the eIF-4F complex.
92
What sequence identifies the initiation codon in eukaryotes?
The Kodak sequence.
93
What distinguishes the eukaryotic AUG initiation codon?
It is embedded in the Kodak sequence unlike in prokaryotes.
94
How is elongation handled in eukaryotes?
eEF1α delivers aminoacyl-tRNAs; eEF2 facilitates ribosomal translocation.
95
What is the secretory pathway signal peptide structure?
n-region (1–5 positive aa) h-region (7–15 hydrophobic aa)
96
What is the function of the signal recognition particle (SRP)?
Recognizes the signal peptide and brings the ribosome to the ER membrane.
97
What is the role of the translocon?
A protein channel in the ER membrane where translation continues into the ER lumen.
98
What cleaves the signal peptide?
Signal peptidase.
99
What happens to proteins with a stop transfer sequence?
The signal peptide is cleaved but the stop transfer sequence anchors the protein in the membrane.
100
What direction do ER-translated proteins typically face?
Amino terminal inside the ER carboxyl terminal outside.
101
What does regulation by protein binding involve?
Proteins at the 3’ UTR regulate mRNA stability and prevent translation (e.g. ferritin regulation).
102
What happens to ferritin translation at high Fe2+ concentrations?
Iron binds IRE-binding protein preventing ferritin translation repression.
103
How does iron regulate the transferrin receptor mRNA?
High iron prevents IRE-binding protein binding RNase degradation of receptor mRNA.
104
What are other types of translational regulation?
Phosphorylation and microRNAs (miRs).
105
Why are 5’ and 3’ UTR mutations significant?
They affect translation and mRNA stability despite not coding for proteins.

BQ II (17 decks)

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