Ch 17.2: Gene Expression from Gene to Protein

Question 1

What is an anticodon?

Answer 1

base-pairs with a complementary codon on mRNA

Question 1

What is the function of a tRNA?

Answer 1

tRNAs transfer amino acids to the growing polypeptide in a ribosome

Question 2

Where are the anticodons located?

Answer 2

on the bottom of a tRNA

Question 3

What is the function of aminoacyl-tRNA synthetase?

Answer 3

the enzyme corrects the match between a tRNA and an amino acid

Question 4

What are the two instances of molecular recognition in translation?

Answer 4

1. a correct match between tRNa and an amino acid (aminoacyl-tRNA synthetase)
2. a correct match between the tRNA anticodon and an mRNA codon

Question 5

What is a wobble?

Answer 5

flexible pairing at the third base of a codon and allows some tRNAs to bind to more than one codon

Question 6

What are ribosomal RNAs (rRNAs)?

Answer 6

proteins and rRNAs make up the two ribosomal subunits (large and small)

Question 7

What are the three binding sites for tRNAs?

Answer 7

P site, A site, and E site

Question 8

What is the function of the P site?

Answer 8

it holds the tRNA that carries the growing polypeptide chain

Question 9

What is the function of the A site?

Answer 9

it holds the tRNA that carries the next amino acid to be added to the chain

Question 10

What is the function of the E site?

Answer 10

it is the exit site, where discharged tRNAs leave the ribosome

Question 11

What are the three stages of translation?

Answer 11

initiation, elongation, termination

Question 12

What is initiation?

Answer 12

initiation starts when the small ribosomal subunit binds with mRNA and a special initiator tRNA (carries methionine), then the small subunit moves along the mRNA until it reaches the start codon, and proteins called initiation factors bring in the large subunit that completes the translation initiation complex

Question 13

What is elongation?

Answer 13

amino acids are added one by one to the C-terminus of the growing chain, each addition involves proteins called elongation factors, translation proceeds along the mRNA in a 5’–> 3’ direction, empty tRNAs released from the E site return to the cytoplasm, where they will be reloaded with the appropriate amino acid

Question 14

What is termination?

Answer 14

elongation continues until a stop codon in the mRNA reaches the A site, the A site accepts a protein called a release factor, the release factor promotes hydrolysis, freeing polypeptide, and the ribosomal subunits and other components dissociate

Question 15

What are the three steps in elongation?

Answer 15

codon recognition, peptide bond formation, and translocation (energy expenditure occurs in the first and third steps)

Question 16

What is the structure of a 3D polypeptide chain?

Answer 16

secondary and tertiary structure

Question 17

What might be required before a protein can begin doing its job?

Answer 17

post-transitional modification

Question 18

What are the two populations of ribosomes?

Answer 18

free ribosomes and bound ribosomes

Question 18

What is the function of free ribosomes?

Answer 18

they synthesize proteins that function in the cytosol

Question 19

What is the function of bound ribosomes?

Answer 19

they make proteins of the endomembrane system and proteins that are secreted from the cell

Question 20

What occurs during polypeptide synthesis?

Answer 20

begins in the cytosol and ends in the cytosol unless the polypeptide signals the ribosome to attach to the ER

Question 21

What signals the ribosome to attach to the ER?

Answer 21

a signal peptide?

Question 22

What is the function of the signal-recognition particle (SRP)?

Answer 22

SRP binds to the signal peptide, the SRP escorts the ribosome to a receptor protein built into the ER membrane, and the signal peptide is then removed by an enzyme

Question 23

What is a polyribosome (or polysome)?

Answer 23

multiple ribosomes can translate a single mRNA simultaneously forming a polysome