Chapter 19 Translation

Question 1

What is meant by translation

Answer 1

It is the conversion of the mRNA into polypeptide via rRNA & tRNA

Question 2

What are the stop codons?

Answer 2

1) UAA
2) UGA
3) UAG

Question 3

What is the start codon?

Answer 3

AUG

Question 4

How many different codon sequences are there?

Answer 4

64 (4^3)

Question 5

Which amino acid has the most number of codon that codes for its synthesis?

Answer 5

Leucine (6 codons)

Question 6

Which amino acids has the least number of codons that codes for its synthesis?

Answer 6

1) Tryptophan (1 codon)
2) Methionine (1 codon “the start codon AUG)

Question 7

What are the properties of the genetic code?

Answer 7

1) specific: Each codon is a signal for a specific amino acid

2) degenerate: most amino aids are coded by multiple codons

3) nonoverlapping: mRNA is read by the ribosome in a continuous sequence of 3 bases at a time

4) almost universal, except for some deviations from the code found in the mitochondria

Question 8

What is the codon-Anticodon interactions?

Answer 8

• tRNA carries the amino acid with the anticodon that must be bound to the codon in a antiparallel way
• The attachment of the amino acid to a specific tRNA is catalyzed by the aminoacyl-tRNA synthetase enzyme being attached to the CCA 3’ end of the tRNA

Question 9

How is the binding of amino acid to the tRNA regulated?

Answer 9

• It has a high accuracy such that an error occurs in 1 of 10^4 amino acid synthesized due to:

1) codon-anticodon base pairing and the mechanism of amino acid attachment to their conjugate tRNA
2) Because it is enzyme specific (There is at least one aminoacyl-tRNA synthetase for each of the 20 amino acids)

Question 10

What are the function of the amino acyl-tRNA synthetases?

Answer 10

• Each enzymes links its specific amino acid to the appropriate tRNA
• They are a diverse group of enzymes by their molecular weight, primary sequence, and the number of their subunits
• They have a high specificity being able to discriminate between amino acids with similar structure
• They have a proofreading site which helps in correcting the mistakes

Question 11

Why are the stages of translation?

Answer 11

1) Initiation
2) Elongation
3) Termination

• It requires a variety of protein factors and GTP other than the enzymes

Question 12

What is the process of Initiation?

Answer 12

• Begins with the binding of small ribosomal subunits to the mRNA and the subsequent binding of the initiator tRNA which binds to the AUG codon
• It ends with the binding of the large ribosomal subunit
• Two sites on the ribosome are used for codon/anticodon interactions (A-site “aminoacyl site”, P-site “peptidyl site”)

Question 13

What is the process of Elongation?

Answer 13

It involves the synthesis of polypeptides according to their specification of the genetic message

• mRNA is read from 5’ to 3’, assembling the protein from the N-Terminus to the C-Terminus
• Elongation cycle: 1. Codon/anticodon pairing at the A site, 2. Peptide bond formation (formed between the peptidyl bond in the P site and the amino acid in the A site via peptidyl transferase enzyme), 3. Transfer of the peptidyl-tRNA to the P-Site
• Due to the formation of the peptide bond both amino acids are attached to the A-site tRNA releasing the tRNA in the P-Site, translocation then moves the ribosome along the mRNA to the next codon until a stop codon is encountered

Question 14

What is the process of Termination?

Answer 14

• Due to a stop codon binding to an aminoacyl-tRNA, which will bind to a protein releasing factor causing the peptidyl transferase to cleave the bond between the completed polypeptide and the last tRNA
• Ribosome will then release the mRNA dissociating into the large and small subunits
• Some proteins will fold directly after translation while others require posttranslational modifications

Question 15

What is meant by polysomes?

Answer 15

The reading of an mRNA by multiple ribosomes producing multiple proteins

Question 16

What is meant by postransational modifications?

Answer 16

Some proteins can fold immediately after their synthesis while others cannot those that cannot requires a posttransational modification, which prepares the protein for function and direct it to a specific location, the modifications are:

• Removal of amino acids
• Side chain modification
• Combining other polypeptides

Question 17

How does translation occur in prokaryotes?

Answer 17

• Occurs in the ribosome
• Large subunit (50s composed of 23s (where the enzyme peptidyl transferase core is located) & 5s rRNA & 34 proteins), which contains the catalytic site for peptide bond formation
• Small subunit (composed of 16S rRNA & 21 proteins), it serves as a guide for the translation factor
• The translation starts after the Shine-Dalgarno sequence in mRNA

Question 18

What are the posttransational modifications done in prokaryotes?

Answer 18

• Starts as the newly synthesized polypeptide emerges from the exit tunnel where it first encounters the molecular chaperone “Trigger factor (TF)”
• They then undergo covalent alteration & chemical modification
• Covalent modification includes (proteolytic processing “trimming”), the removal of single peptides and formylmethionine residue
• Chemical modification includes methylation, phosphorylation, covalent linkage to lipid molecules

Question 19

How is the protein synthesis process in eukaryotes different than those in prokaryotes?

Answer 19

1) the initiation stage is more complex, because:

• The mRNA secondary sequence, which requires extra processing
• The mRNA scanning, where the ribosomes binds a cap scanning for the start site not making use of the shine-dalgarno sequence

FYI: Eukaryotes have more than 12 initiating factors (eIFs)

Question 20

What are the posttransational modifications in eukaryotes?

Answer 20

• Prepares the protein
• There are more than 200 types of modifications, grouped into:

1) Proteolytic cleavage: A regulatory mechanism (removing the N-terminal methionine & signal peptides), it also converts proproteins into their active form like converting preproinsulin into preinsulin into insulin

2) Glyosylation (adding a carbohydrate to hydroxyl or a functional group): serves a structural/informational purpose, N-linked protein glycosylation protects the endoplasmic reticulum from misfiled glycoproteins, which will be translocated into the cytoplasm and get degraded via ubiquitin proteasome system

3) Hydroxylation (introducing a -OH group): Required by the proteins (proline & lysine) for their structure to form collagen and elastin, scurvy (weak collagen fiber structure) is due to the inadequate intake of Vitamin-C which is required in the hydroxylation of proline

4) Phosphorylation (adding a phosphate group): Plays a role in metabolic, control, signal transduction & in protein-protein interactions

5) Lipophilic modifications (covalent attachment of lipids to a protein): the most common ways are acylation & prenylation (adding a prenyl “isoprene”), which improves the membrane binding capacity & protein-protein interactions

6) Methylation (adding a methyl group): which flags/marks the proteins for repair, degradation or changing its function

7) Carboxylation (adding a carboxyl group “C (=O)-OH”): Common in protein clotting factors, which increases the proteins sensitivity to Ca2+ modulations

8) Disulfide bond formation: found only in secretory proteins and some membrane proteins, which is formed as the polypeptide goes into the lumen of the ER, forming spontaneously due to non-reducing environment via disulfide exchange