L6- Protein Synthesis

Question 1

What subunits make up a eukaryotic ribosome?

Answer 1

60S and 40S.

Question 2

What is the sedimentation coefficient of eukaryotic ribosomes?

Answer 2

80S.

Question 3

Where are ribosomes usually found in the cell?

Answer 3

In the cytoplasm or attached to the endoplasmic reticulum.

Question 4

What is the fate of proteins synthesized by ribosomes on the endoplasmic reticulum?

Answer 4

They are either exported from the cell or become part of the cellular membrane.

Question 5

What is a codon?

Answer 5

A trinucleotide sequence of mRNA that determines which amino acid is added to a nascent protein during translation.

Question 6

What is the sequence of the start codon?

Answer 6

AUG.

Question 7

The start codon codes for which amino acid?

Answer 7

Methionine.

Question 8

What are the sequences of the stop codons?

Answer 8

UAA, UAG, and UGA.

Question 9

In which direction does a ribosome read RNA?

Answer 9

From 5’ to 3’ (the same direction as nucleic acid synthesis by DNA and RNA polymerases).

Question 10

What is a peptide bond?

Answer 10

A covalent bond between an amino group of one molecule and the carboxyl group of another. Peptide bonds are formed between amino acids during protein synthesis.

Question 11

What is the Wobble Hypothesis?

Answer 11

There are fewer tRNAs than codons. It was proposed that the 3’ end of the codon allows for a more relaxed bonding with the 5’ base of the anticodon. The potential for non-Watson-Crick base pairing at this position allows a single tRNA to pair with several different codons.

Question 12

What class of enzymes activate and attach amino acids to their respective tRNAs?

Answer 12

Aminoacyl tRNA synthetases.

Question 13

What are the three sites on ribosomes that interact with tRNAs during elongation of a peptide?

Answer 13

A, P and E sites.

Question 14

What is the purpose of elongation factors?

Answer 14

Elongation factors (EF) position amino acyl-tRNAs on the ribosome and promote RNA translocation.

Question 15

What is monocistronic mRNA?

Answer 15

mRNA that will code for only one polypeptide sequence. With very few exceptions, eukaryotic mRNA is monocistronic. Bacteria, on the other hand, frequently have polycistronic messages.

Question 16

In bacteria, the region of the 3’ terminus of the 16S ribosomal RNA is complementary to the _______ (which sequence).

Answer 16

Shine-Dalgarno Sequence. This is the ribosome binding site on the mRNA. In eukaryotes the equivalent sequence is known as the Kozak sequence.

Question 17

What is a missense mutation?

Answer 17

A single base substitution in which one amino acid replaces another.

Question 18

What is a nonsense mutation?

Answer 18

A change in a codon that converts it to a stop codon, leading to premature termination of translation.

Question 19

What is a frameshift mutation?

Answer 19

A mutation that causes the codons to be read out of frame, thereby leading to a misinterpretation of mRNA.

Question 20

What is the sedimentation coefficient of bacterial ribosomes?

Answer 20

70S.

Question 21

What is translation?

Answer 21

The process by which an mRNA template is used to synthesize a polypeptide chain.

Question 22

Every tRNA has a ____ sequence at its 3’ end.

Answer 22

CCA.

Question 23

What kind of bond links the carboxyl group of an amino acid to the 3’ CCA sequence of a tRNA molecule?

Answer 23

An ester bond.

Question 24

What is an aminoacyl-tRNA?

Answer 24

A tRNA molecule bound to its corresponding amino acid.

Question 25

What is an anticodon?

Answer 25

A tRNA trinucleotide sequence that is complementary, and therefore binds to, an mRNA codon.

Question 26

What is the first tRNA binds to the small ribosomal subunit?

Answer 26

Met-tRNA (note that is binds regardless of whether or not there is a codon for methionine).

Question 27

Which initiation factor is required for the initiator tRNA (Met-tRNA) to bind to the small ribosomal subunit in eukaryotes?

Answer 27

eIF2 (along with GTP).

Question 28

To which binding site does the initiator tRNA (Met-tRNA) bind to on the ribosome?

Answer 28

It binds directly to the P site.

Question 29

True or False. Interaction of the eIF2-GTP-Met-tRNA complex with the 40S ribosomal subunit must occur in the presence of mRNA.

Answer 29

False. It typically occurs in the absence of mRNA.

Question 30

What is the function of eIF2 during translation?

Answer 30

It binds the 40S subunit to tRNA.

Question 31

What happens to eIF2 when the anticodon of the initiator tRNA matches the AUG start codon?

Answer 31

GTP is hydrolyzed, allowing eIF2 to dissociate from the ribosome and the 60S subunit to bind to the 40S subunit.

Question 32

Which elongation factor is required for tRNA to bind to the A site of the ribosome?

Answer 32

EF1.

Question 33

Which elongation factor is required for translocation of tRNA from the A site to the P site of the ribosome?

Answer 33

EF2.

Question 34

Which enzyme is thought to catalyze the nucleophilic attack on the carbonyl group of the peptidyl-tRNA by the amino group of the aminoacyl-tRNA? Where does this reaction occur on the ribosome?

Answer 34

The enzyme is peptidyl transferase and it occurs in the A site of the ribosome.

Question 35

What is the function of the soluble protein called termination factor?

Answer 35

It promotes the cleavage of the polypeptide chain from the tRNA in the P site, releasing the mRNA from the ribosome.

Question 36

What is the shape of a typical tRNA?

Answer 36

Cloverleaf.