Protein Synthesis

Question 1

What is the central dogma of molecular biology?

Answer 1

It describes the two-step process, involving transcription and translation, by which the information in genes flows into proteins.

Question 2

What is synonymous alteration (in genes)?

Answer 2

It is when a nucleotide changes in the chain, but the coding remains the same.

Question 3

What does the Svedberg unit (S/Sv) measure?

Answer 3

It is a unit for sedimentation rate ie. how quickly a particle ‘settles’.

Question 4

What are the two subunits of a ribosome called, and what rRNA (and proteins) are they themselves made up of?

Answer 4

The ribosome is split into two subunits, a smaller 40S subunit, and a larger 60S subunit. The 40S is made up of 18S rRNA (and 33 proteins), while the 60S is made up of 5S, 5.8S and 28S rRNA (and 49 proteins).

Question 5

What are the three sites in the ribosomal subunits called?

Answer 5

Aminoacyl tRNA binding site, Peptide binding site and Exit (APE).

Question 6

Name and describe (in a few sentences) the three stages of protein synthesis.

Answer 6

INITIATION: assembly of the ribosome on mRNA together with the first initiator tRNA - aminoacyl tRNA (met-tRNAi)
ELONGATION: charged tRNAs are delivered to the ribosome, and the amino acids are added one at a time, building the polypeptide
TERMINATION: a stop codon is encountered, which signals the end of protein synthesis and the release of the polypeptide

Question 7

What is the name of the eukaryotic initiation factor that binds to the 40S subunit at the beginning of initiation, and what is its purpose?

Answer 7

It’s called EIF3, and it’s there to prevent the association of the 40S subunit with the 60S subunit.

Question 8

What two substances attach to the met-tRNAi before it settles into one of the sites?

Answer 8

EIF2 and GTP

Question 9

In which site does the met-tRNAi settle?

Answer 9

The P site

Question 10

What term do we use to refer to the complex consisting of the 40S ribosomal subunit, EIF3, met-tRNAi, EIF2 and GTP?

Answer 10

The 48S preinitiation complex

Question 11

What happens when a start codon is found?

Answer 11

The EIF3 dissociates, and the GTP gets hydrolyzed along with it, releasing energy that allows the 60S subunit to join the complex.

Question 12

Which site does a second tRNA occupy, and with the help of which factor?

Answer 12

It occupies the A site, with the help of EIF1.

Question 13

What enzyme mediates the charging of tRNA?

Answer 13

Aminoacyl tRNA synthase

Question 14

What does peptidyl transferase do?

Answer 14

It forms peptide bonds between adjacent amino acids.

Question 15

What does translocase do?

Answer 15

It aids in the movement of met-tRNAis between sites in the ribosomal subunit.

Question 16

What is a polysome?

Answer 16

An mRNA with multiple different ribosomes attached to it.

Question 17

What causes the release of the polypeptide chain?

Answer 17

The recognition of a stop codon, and the consequent hydrolysis of the terminal peptidyl-tRNA.

Question 18

What is the energy cost for charging of tRNAs with amino acids?

Answer 18

It is 1 ATP per amino acid.

Question 19

What is the energy cost for the initiation of polypeptide synthesis?

Answer 19

It is 1 GTP and nATP (the n being the number of nucleotides scanned along the mRNA).

Question 20

What is the energy cost of the elongation of the polypeptide?

Answer 20

It is 2 GTP per amino acid.

Question 21

What is the energy cost of the termination of polypeptide synthesis?

Answer 21

It is 1 GTP.

Question 22

List the properties of a prokaryotic ribosome (ribosome name, subunit names, large and small subunit components).

Answer 22

• 70S ribosome
• 50S and 30S subunits
• 50S: 23S and 5S rRNA (31 proteins)
• 30S: 16S (21 proteins)

Question 23

List the properties of a eukaryotic ribosome (ribosome name, subunit names, large and small subunit components).

Answer 23

• 80S ribosome
• 60S and 40S subunits
• 60S: 5S, 5.8S and 28S (49 proteins)
• 40S: 18S (33 proteins)

Question 24

For the antibiotic Streptomycin, list its target in bacteria and its effect on protein synthesis.

Answer 24

It affects the small ribosomal subunit, and inhibits the initiation misreading of the genetic code.

Question 25

For the antibiotics Tetracyclines, list their target in bacteria and their effect on protein synthesis.

Answer 25

They affect the small ribosomal subunit, and inhibit the met-tRNAis binding to the ribosome.

Question 26

For the antibiotic Chloramphenicol, list its target in bacteria and its effect on protein synthesis.

Answer 26

It affects the large ribosomal subunit, and inhibits peptidyl transferase activity.

Question 27

For the antibiotic Erythromycin, list its target in bacteria and its effect on protein synthesis.

Answer 27

It affects the large ribosomal subunit, and inhibits translocation.

Question 28

For the antibiotics Neomycins, list their target in bacteria and their effect on protein synthesis.

Answer 28

They affect multiple sites, and inhibit the several effects.