6. Protein synthesis

Question 1

Describe the central Dogma (3pts)

Answer 1

1. Begins with DNA which is transcribed into a primary RNA transcript.
2. The primary transcript is then processed through capping, polydenylation and splicing to give a precursor mRNA and then matured into a final MRNA transcript.
3. The mRNA is then exported out the nucleus and recognised by ribosomes leading to the process of translation to form proteins.

Question 2

Describe UTR’S and their effect on protein synthesis (3pt)

Answer 2

1. UTR= untranslated region
2. UTR is also coded by exons. UTRS’s perform important regulatory processes in the expression of a gene during protein synthesis.
3. As a result many genetic diseases occur due to mutations in UTR’s and other non coding regions of the gene.

Question 3

Describe the universal genetic code (4pts)

Answer 3

1. The universal genetic code supports evolution
2. There are 20 Amino acids with 9 essential Amino acids.
3. Triplet codon, 4 possible bases
4. 64 codons

Question 4

Describe Protein synthesis (5pts)

Answer 4

1. Is a multi stage process carried out by ribosomes.
2. Protein synthesis is carried out by 2 populations of ribosomes one bound to the endoplasmic reticulum whilst the other is free within the cytosol.
3. Protein synthesis is rapid
4. Protein synthesis occurs within a cleft between the large ribosomal subunit and the small ribosomal subunit.
5. Ribosomes add 20 Amino acids per second.

Question 5

Describe RNA in the cell composition (3pts)

Answer 5

Ribosomal RNA= 80- 85% abundance
Transfer RNA= 10-15% abundance
Messenger RNA= 2-5% abundance

Question 6

Describe a eukaryotic Ribosome (5pts)

Answer 6

1. Ribosome= 80S
2. Large subunit= 60S
   Small subunit= 40S
3. Large subunit component=
4. 28S, 5.8S, 5S
   RNA
   2. 49 proteins
5. Small subunit components= 1. 18S RNA
   2. 33 proteins
6. 3 active sites form within the cleft when the two subunits come together around the mRNA. Active protein synthesis only occurs when the two subunits are combined.

Question 7

Describe a prokaryotic Ribosome (4pts)

Answer 7

1. Ribosome= 70S
   2.Large subunit= 50S
   Small subunit= 30S
2. Large subunit components= 1. 23S and 5S
   RNA
   2. 31 Proteins
3. Small subunit components= 1. 16S RNA
   2. 21 proteins

Question 8

Describe the steps in protein synthesis (3pts)

Answer 8

1. Initiation
   - Assembly of the ribosome on mRNA together with the first tRNA.
   - the met-RNA binds to the p site instead of the a site
2. Elongation
   - charged tRNA’s delivered to the ribosome
   - Addition of Amino acids one at a time building the polypeptide chain
3. Termination
   - encounter with a stop codon
   - end of protein synthesis and release of the polypeptide chain
   - Dissociation of the ribosome

Question 9

Describe initiation and elongation factors (1pt)

Answer 9

Initiation and elongation factors are proteins that are directly involved in Translation

Question 10

Describe Initiation (8pts)

Answer 10

1. Multiple initiation factors bind to the cap and the poly A tail of the message
2. One initiation factor binds to met - tRNAi and GTP which is a source of energy forming a complex
3. The initiator TRNA and GTP attaches with the p site of the small ribosomal sub unit forming a 48s pre-initiation complex.
4. This initiates scanning in which the ribosome scans to find the start codon. This involved the consumption of energy via the hydrolysis of ATP forming ADP and Pi.
5. Once the anti-codon of the met-TNAi initiator recognises the correct start codon initiation pauses
6. MET- tRNA occupies P site
7. The 60s ribosomal subunit joins to form the 80s ribosome by the release of energy from GTP and the dissociation of some factors
8. 2nd charged trna enters the cleft and enters the A site with the elongation factor 1. This forms the first peptide bond between adjacent amino acids and the 80S translocates.

Question 11

Describe Elongation (5pts)

Answer 11

1. Amino acid+ ATP+ tRNA. GTP is hydrolysed.
2. This forms a peptide bond by peptidyltransferase attaching the elongating chain to the newly introduced Amino acid.
3. EF2 facilitated translocation of the ribosome along the message causing the TRNA to occupy the p and e site.
4. This means the A site is free and this further consumes energy in the form of GDP
5. Release of the spent Trna occurs from the E site allowing recharging of TRNA requiring more energy from ATP.

Question 12

Describe Termination (3pts)

Answer 12

1. Release factor binds to the stop codon
2. This triggers the hydrolysis of GDP and the terminal peptidy-tRNA which releases the polypeptide chain.
3. This results in the dissociation of the ribosome releasing the mrna allowing the recycling of factors and proteins involved in protein synthesis.

Question 13

Describe the antibiotic Streptomycin (2pts)

Answer 13

1. Targets the small ribosomal subunit

2. Effect on protein synthesis= inhibition of initiation misreading of genetic code

Question 14

Describe the antibiotic Tetracyline (2pts)

Answer 14

1. Targets the small ribosomal subunit

2. Effect on protein synthesis= inhibition of aminoacyl tRNA binding to ribosome.

Question 15

Describe the antibiotic Chloramphenicol (2pts)

Answer 15

1. Targets the large ribosomal subunit

2. Effect on protein synthesis= Inhibition of peptidyl transferase activity.

Question 16

Describe the antibiotic Erthromycin (2pts)

Answer 16

1. Targets the large ribosomal subunit

2. Effect on protein synthesis= inhibition of translocation