Chapter 22: Translation

Question 1

What is Translation?

Answer 1

THe process of turning a transcript into a polypeptide chain using ribosomes

Question 2

Where does protein synthesis occur?

Answer 2

At the Endoplasmic reticulum on ribosomes embedded in the lumen

Question 3

How is the correct amino acid added to the polypeptide chain?

Answer 3

• tRNA has a complementary sequence to the mRNA

Question 4

How is it possible there is 64 combinations of three letters but only 20 amino acids?

Answer 4

Some amino acids can be coded for by more than one codon. It is called degeneracy

Question 5

What happens if a missense mutation occurs?

Answer 5

It is often a silence mutation because of the genetic code

Question 6

What is the wobble position and what happens if there is a mutation

Answer 6

The wobble position is the third position in a codon and only results in a change of amino acid 25% of the time

Question 7

What are the following mutations: Transition, silent, frame shift, and nonsense?

Answer 7

Transition: Pur replaced by Pur or Pyr by Pyr
Silent: When a mutation results in the same amino acid
Frame shift: when a nucleotide is added or deleted resulting in a shift of the codon reading
Non-sense: premature end

Question 8

What is an open reading frame?

Answer 8

A reading frame without a termination codon among 50+ codons

Question 9

What is a transversion?

Answer 9

• When a purine is replace by a pyrimidine
  Ex.
  Glu->Val = Sickle cell

Question 10

If there is 61 possible codons but only 32 tRNAs, How can they be accomidated?

Answer 10

• Third Base of a codon(wobble) can form non-canonical base pair with its complement in tRNA
• Some tRNA contain Inosinate (I) which can bond with U C and A

Question 11

What is an overlapping codon?

Answer 11

Read the codons in a non-overlapping format

Question 12

What happens if sequence has an open reading frame?

Answer 12

• A frame without a termination codon 50< codons
• List all three possibilities for the section
• Open reading frame must have stop and start

Question 13

What is involved in protein synthesis?

Answer 13

• Ribosome (rRNA and proteins)
• mRNA
• Charged tRNAs (aa activated)
• Initiation, Elongation, Release factors

Question 14

What are the five stages of protein translation?

Answer 14

• **Activation of AA **: tRNA is aminoacetylated
• Initiation of translation: mRNA and aminoacylated tRNA bind to ribosome
• Elongation: cycle continues till stop codon
• Termination and ribosome recycling: mRNA and protein dissociate, ribosome recycled
• Folding and processing: catalyzed by enzymes

Question 15

What does the tRNA accomplish?

Answer 15

1. Activates an amino acid for peptide bond formation
2. Ensures appropriate placement of amino acid in a growing chain
   (uses Mg 2+ cofactor)

Question 16

What is the general composition of the ribosome?

Answer 16

A smaller subunit and larger subunit

Question 17

Which rRNA subunit of the ribosome catalyzes the peptide bond formation?

Answer 17

30S ribosome subunit

Question 18

What does aminoacylation of tRNA do?

Answer 18

Attaches the correct amino acid to the corresponding tRNA

Question 19

How do aminoacyl tRNA make sure they have the right amino acid?

Answer 19

Proofreading by aminoacyl-tRNA synthetase

Question 20

Why is the interaction between amino-acyl tRNA synthetase and tRNA referred to as the second genetic code?

Answer 20

Because aminoacyl-tRNA synthetase are spcific for the amino acid and tRNA

Question 21

Which amino acid is the first to begin translation in bact, mito, chloro?

Answer 21

• tRNA^fMet
• Different from methionine found in the middle of the polypeptide chain

Question 22

What is the shiner dalgarno sequence? How does it help alignment of start codon and P site?

Answer 22

• 4-9 purine a few bp upstream of initiation codon
• Guide mRNA to the correct position on the 30s subunit

Question 23

What are three steps in initiation?

Answer 23

1. IF1, IF2-GTP, IF3 binds to 30S ribosomal subunit
2. mRNA + fMet bind to shine-Dal aligning sequence, IF3 released
3. GTP hydrolyzed helps bind 50S, release of IF1 + IF2 form 70S complex

Question 24

Which AA is first to start translation in eukaryotes?

Answer 24

Methionine

Question 25

What is the significance of circularization of euk mRNA in translation?

Question 26

What are three steps of elongation? (EF-Tu and EF-G)

Answer 26

1. Binding of incoming aminoacyl- tRNA: GTP hydrolyzed EF-Tu GDP released
   
   • (EF-Tu recycled)
2. Formation of peptide bond: cataylzed by 23S, amino group of tRNA in A attacks carbonyl tRNA in P site
3. Translocation: Met moves from P to E causing the rest to follow and continue
   
   • EF-G is GTP carrier that provides energy for codon shift

Question 27

How do release factors help termination of translation?

Answer 27

• Hydrolyze terminal peptide-tRNA bond
• Release polypeptide and tRNA from ribosome
• Cause subunits of ribosome to dissociate so that initiation can begin again

Question 28

What are polysomes?

Answer 28

When multiple ribosomes are translating an mRNA

Question 29

What modifications to proteins undergo after termination?

Answer 29

Folding and

Question 30

Know how antibiotics inhibit translation. Why don’t they inhibit eukaryotic transcription?

Answer 30

Antibiotics dont inhibit eukaryotic transcription because ribosomes are different shapes and sizes than that of bacteria.