Lecture 17 - Gene expression - translation

Question 1

How many codons are there?

Answer 1

64, 61 specify as an amino acid

Question 2

Does each codon encode for one amino acid only? Explain.

Answer 2

No, most amino acids have more than 1 codon

Question 3

What are the codons called that do not code for amino acids?

Answer 3

stop codons

Question 4

Which amino acid is (almost always) the first one in a peptide sequence (protein)?

Answer 4

AUG - methionine

Question 5

What binds to the mRNA codons and to the mRNA stop codons?

Answer 5

tRNA

Question 6

What are the features that make tRNA an ‘adaptor’ molecule?

Answer 6

it has a region which can bind an amino acid and a region which can interact with mRNA

Question 7

At which end is the amino acid attachment site located

Answer 7

3’

Question 8

What is a ‘charged’ tRNA and outline how a tRNA is ‘charged’?

Answer 8

an enzyme recognises a specific amino acid and the correct tRNA and joins them together which makes it “charged”

Question 9

Which macromolecules make up a ribosome?

Answer 9

ribosomal RNA and proteins

Question 10

What is being made during translation?

Answer 10

proteins

Question 11

What are the three stages of translation?

Answer 11

initiation, elongation, termination

Question 12

Describe the steps of translational initiation.

Answer 12

• a specific initiation tRNA binds to small ribosomal subunit
• this complex identifies the 5’Gcap and attaches to mRNA
• it moves along the mRNA until it finds the initiation AUG codon
• the complex stops with the initiator tRNA carrying the first methionine positioned in the P site
• the large ribosomal sub unit attaches

Question 13

Describe the steps of translational elongation.

Answer 13

• charged tRNA lands in A site with complementary anticodon
• ribosome breaks bond that bind amino acid and transfer to the newly arrived amino acid to form a peptide bond
• ribosomes moves 3 nucleotides down the mRNA
• the uncharged tRNA detaches and is expelled from the E site
  a new charged tRNA enters the ribosome at the A site

Question 14

Describe the steps of translational termination.

Answer 14

• when ribosome reaches stop codon, a protein (release factor) enters the A site
• release factors breaks bond between P site tRNA and the final amino acid to allow the polypeptide chain to detach from tRNA
• small and large ribosomal sub units dissociates from components

Question 15

What determines the amino acids sequence?

Answer 15

the genetic code in DNA

Question 16

Where does translation take place in eukaryotes?

Answer 16

cytoplasm