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Molecular Genetics - Midterm 1 > Translation Initiation > Flashcards

Translation Initiation Flashcards

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1
Q

What is translation?

A

The conversion of mRNA into a growing peptide chain of amino acids

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2
Q

What are anti-codons?

A

o Each codon is associated with an anti-codon
o Anti-codon comes from tRNA that carries an amino acid
o Also read 5’ to 3’
o A codon and its anti-codon are complementary

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3
Q

How do peptides grow? (Include the direction)

A

• Peptides grow from N-terminal to C-terminal
o The incoming amino acid will join its N-terminus on the last amino acid’s C-terminus
o The first amino acid has a free N-terminus

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4
Q

In which direction does the ribosome move down the mRNA?

A

5’ to 3’

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5
Q

What are ribosomes? (What are they made of?)

A

All ribosomes are large ribonucleoprotein complexes (protein that contains RNA)

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6
Q

Compare bacterial vs. mammalian ribosomes

A

o 70s vs. 80s
o Size: mammalian is larger
o Amount of RNA: bacteria have more
o Number of proteins: mammalian have more
o Large and small subunits (both larger in mammals)

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7
Q

What are the roles of rRNA?

A

1) structural role

2) catalytic activity

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8
Q

What are the roles of ribosomal proteins

A

1) structural role
o Provide ribosomal site for translation
o Form 3d active sites to facilitate translation
o Provide proximity
o Provide scaffolding by binding to rRNA or other proteins
2) catalytic roles

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9
Q

What are the major ribosomal binding sites?

A

• A site: aminoacyl site
o Binds incoming tRNA (codon/anti-codon recognition)
• P site: peptidyl site
o Site of addition of amino acid to growing chain
• E site: exit site (only in bacterial ribosomes)
o Deacylated (empty) tRNA exits the ribosome after the ribosome has moved forward 1 codon

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10
Q

What are the stages of translation?

A

1) Initiation: all reactions prior to the first peptide bond
2) Elongation: all reactions between the 1st and the last peptide bond
3) Termination: all reactions following the last peptide bond
I. Release of completed protein
II. Disassembly of the intact ribosome

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11
Q

What are the steps of initiation?

A
  1. Small subunit binds to mRNA
  2. Binding of first tRNA
  3. Completion of ribosome: bringing of large subunit
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12
Q

What are the factors involved in bacterial translation initiation?

A

o IF-1
 Binds close to partial A site (the A site is completed when the ribosome is completed)
 Only as part of the initiation complex
o IF-2
1. Binds the initial AA-tRNA and controls entry into partial P site
2. GTPas activity: GTP hydrolysis
• Provides energy for the LSU to SSU binding
• Results in conformational change, allowing release of Ifs
o IF-3
 Binds free cytosolic SSU, maintaining the dissociated state of ribosome in the cytosol
 Facilitates SSU binding to mRNA RBS (ribosome binding site)
 Confirms accuracy of initator AA-tRNA

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13
Q

What is the RBS?

A

Ribosomal binding site.
Based on 2 highly conserved sequences
o Shine-Dalgarno sequence: AGGAGG
o Translation initiation codon: AUG

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14
Q

Describe the 1st step of bacterial translation initiation.

A
  • Base pairing ensures that the SSU docks at the right place (i.e. on the RBS)
  • IF-1 blocks potential entry of any tRNA
  • IF-3 also binds the SSU
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15
Q

What is the initial bacterial tRNA?

A

tRNAMetF

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16
Q

What are the types of methionine that exist in bacterial cells and what distinguishes them?

A 
1.	tRNAMetF
	Only in initiation
	Binds IF-2
	It has an unusual tertiary structure
	f-met contains a formyl group (it is formylated)
	fMet-tRNAF is the initation AA-tRNA
2.	tRNAMetM
	Only in elongation
	tRNA with a conserved structure
	It binds the intact A site only
17
Q

Describes the 3rd step of bacterial initiation.

A
  • Once the joining is completed, translation shifts from initiation to elongation
  • GTP hydrolysis (GTPGDP) releases energy that induces conformational change that releases IF-2
  • IF-1 and IF-3 are released
  • The incoming AA-tRNA can bind
18
Q

What are the types of methionine that exist in eukaryotic cells and what distinguishes them?

A

• Initiator AA-tRNA is Met-tRNAI
o There are 2 different eukaryotic tRNAMet
 tRNAMetI : used for initiation
 tRNAMet¬M : used for elongation
• SSU binds to the 5’ G-cap of mRNA (which is away from the AUG initiation codon)
• More initiation factors involved than bacterial initiation
o eIF (analgous to bacterial IF)
o Binding order of factors is different

19
Q

Where does the initiation complex form?

A

At the 5’ G-cap (away from the AUG initiation codon)

20
Q

What is the coorrect consesus sequence for eukaryotic initiation?

A

1 purine, 3 nucleotide upstream of AUG

5’G ……..NNN(A⁄G)NNAUGG

21
Q

What is the 43S complex? What elements does it comprise? Where does it form?

A

43S pre-initiation complex
o Factors: eIF2, eIF3, eIF1, eIF1A
o Met-tRNAI
Forms on the eukaryotic SSU (40S).

22
Q

What are the roles of eIF2?

A

o Binds and docks the initiator Met-tRNAI

o Has GTPase activity

23
Q

What are the roles of eIF3?

A

o Binds and maintains the dissociated state of SSU

24
Q

What are the roles of eIF1 and eIF1A?

A

o Enhance the dissociation function of eIF3

25
Q

What is the cap binding complex? What does it comprise? Where does it form?

A 
It forms on the mRNA
•	eIF4E
•	eIF4A
•	eIF4B
•	eIF4G
26
Q

What are the roles of eIF4E?

A

o Cap binding subunit

27
Q

What are the roles of eIF4A?

A

o Helicase activity

o Unwinds the secondary structure at the 5’end of mRNA

28
Q

What are the roles of eIF4B?

A

o Stimulates the helicase activity

29
Q

What are the roles of eIF4G?

A

o Scaffolding subunit
o mRNA needs to be circularized prior to translation
o Connects the 5’G cap and 3’PABP/Poly A tail (circularizing mRNA)

30
Q

What is the 48S scanning complex? What does it comprise? Where is it formed?

A 
•	43S complex binds to the cap-binding complex on the 5’ end of mRNA
Factors
•	eIF1 and eIF1A
•	eIF5
•	eIF5B
31
Q

What are the roles of eIF1 and eIF1A?

A

o Facilitate movement from 5’ to 3’

32
Q

What are the roles of eIF5?

A

o Facilitates the eIF2 GTPase activity, which then releases eIF2 and eIF3

33
Q

What are the roles of eIF5B?

A

o chaperones LSU into correct position

o Formation of intact ribosome

Molecular Genetics - Midterm 1 (10 decks)

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