Gene Products And Protein Synthesis

Question 1

Why is the genetic code considered degenerate?

Answer 1

64 combinations of nucleotides in a codon possible yet only codes for 20 amino acid - some amino acids are coded for by more than one triplet

Question 2

What is the start codon

Answer 2

AUG, methionine

Question 3

What are the stop codons

Answer 3

UAA, UAG, UGA

Question 4

What are the types of point mutation?

Answer 4

Nonsense: amino acid to stop
Missense: amino acid to a different amino acid
Silent: amino acid doesn’t change (degenerate code)

Question 5

What is the mutation that causes sickle cell anaemia?

Answer 5

Mutation in B-globin gene; amino acid coded for at 6th position changed from glutamate to valine

Question 6

What is the mutation that causes cystic fibrosis?

Answer 6

A mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene

Question 7

What is tRNAs shape and why?

Answer 7

Single stranded RNA molecule with intramolecular double stranded regions which fold it up into a clover leaf shape.

Question 8

Where on a tRNA molecule will the amino acid attach?

Answer 8

To the 3’ end at the - CAA-OH group

Question 9

What is wobble pairing?

Answer 9

The last base on the codon loop can form non-Watson-Crick pairs; this does not change the amino acid sequence, just means that a single tRNA molecule can recognise more than one codon - good for degenerate code when last base in codon won’t effect the the acid called for

Question 10

What is the enzyme that produces aminoacyl-tRNA

Answer 10

Aminoacyl-tRNA-syntheases

Question 11

What is an aminoacyl-tRNA?

Answer 11

What forms when an amino acid bonds to a tRNA molecule

Question 12

Amino acid + tRNA + ATP (via aminoacyl-tRNA) =

Answer 12

= aminoacyl-tRNA + PPi + AMP

Question 13

What are the subunits (and their compositions) in prokaryotic ribosome?

Answer 13

70s complex, made up of a large 50s and a small 30s subunit.
The 50s subunit is made up of 23s RNA + 5s RNA + 34 proteins
The 30s subunit is made up of 16s RNA and 21 proteins

Question 14

What is a 50s subunit in a prokaryotic ribosome made up of?

Answer 14

23s RNA + 5s RNA + 34 proteins

Question 15

What is a 30s subunit in a prokaryotic ribosome made up of?

Answer 15

16s RNA + 21 proteins

Question 16

What are the three sites in the 30s subunit of a ribosome?

Answer 16

E, P, and A

Question 17

Describe the initiation step of translation in prokaryotes.

Answer 17

30s subunit binds initiation factors IF1 and IF3

Then the mRNA binds to the 30s subunit; with the AUG start codon binding near the P site (in the middle of E and A). The Shine-Dalgarno Sequence will bind upstream of the start codon to 16s RNA which ensures in-frame translation.

fmet-tRNA^fmet binds IF2-GTP which then enters P site and lines up with start codon.

Then the 50s subunit binds to 30s ribosome, this removes the initiation factors IF1,2,3 and hydrolyses GTP to GDP + Pi - releasing energy.

Question 18

Describe the elongation step of translation in prokaryotes

Answer 18

Aminoactyl-tRNA bound to EF-Tu-GTP (EF = elongation factor) binds to A site. Tu is a G-protein so factor complex can be called EF-G-GTP as below.

Pause for proof reading + GTP hydrolysis

Peptidyl transferase moves second amino acid on top of first, fMet will stay in same place as it was at P site. The 3’ end of tRNA hydrolysed to -OH group and moves over to E site at top while staying attached to P site (sitting diagonally) then tRNA to which fMet-AA is attached has top move over to P site while attached to A site (also sitting diagonally).

Then translocation occurs where bottom of tRNA lines up with top so top+bottom of tRNA at A and top+bottom of tRNA at P; leaving A site free. EF-G-GTP hydrolysed to EF-G-GDP + Pi

Then tRNA at E site released

repeats

Question 19

Describe the termination step of translation in prokaryotes

Answer 19

When a stop codon (UAA, UAG, UGA) presented to A site it causes the binding of the release factor and then the protein is released and everything dissociates

Question 20

Classes of antibiotics:

Answer 20

Chloramphenicol
Actinomycin
Puromycin
Tetracyclines

Alpha-amanitin
Rifomycin
Streptomycin
Erythromycin

Cyclohexamide
Diptheria toxin

Question 21

Chloramphenicol

Answer 21

Inhibits peptidyl transferase by binding on 50s subunit

Question 22

Actinomycin

Answer 22

Binding DNA at the transcription initiation complex - preventing elongation by RNA polymerase

Question 23

Puromycin

Answer 23

Causes premature chain termination during translocation (for both pro- and eukaryotes)

Question 24

Tetracyclines

Answer 24

Inhibits binding of aminoacyl-tRNAs to the ribosome

Question 25

Alpha-amanitin

Answer 25

Inhibitor of RNA polymerase II

Question 26

Rifomycin

Answer 26

Inhibits RNA synthesis by binding to prokaryotic RNA polymerase

Question 27

Streptomycin

Answer 27

Affects initiation (30s) causes misreading of codons

Question 28

Erythromycin

Answer 28

Binds to 50s subunit and prevents translocation

Question 29

Cyclohexamide

Answer 29

Inhibitor of protein biosynthesis in eukaryotes

Question 30

Diptheria toxin

Answer 30

RNA translocational inhibitor by inactivating elongation factor 2