Translation and Inhibitors of Protein Synthesis Flashcards
How is a peptide bond formed?
Each amino acid has an amino group (-NH2) and a carboxyl group (-COOH). A peptide bond is formed when the carboxyl group of one amino acid reacts with the amino group of the other. During this reaction, a molecule of water is released.
Define Translation.
Translation is the process in which the genetic information in mRNA molecule specifies the sequence of amino acids during protein synthesis.
The first amino acid on the polypeptide forms the “__(a)__-terminal”. The last amino acid forms the “__(b)__-terminal”.
(a) N
(b) C
How are ribosomal subunits identified?
Ribosomal subunits are identified by their S (Svedberg unit) values, sedimentation coefficients that refer to their rate of sedimentation in a centrifuge.
Further notes:
The S values are not necessarily additive when subunits are combined, because rates of sedimentation are affected by shape as well as mass.
How is each tRNA charged with the proper amino acid?
Each tRNA has its corresponding amino acid attached to the 3’ end. A set of enzymes, called aminoacyl-tRNA synthetases, are used to “charge” the tRNA with the proper amino acid.
List nucleosides that are unique to tRNA.
(a) Pseudouridine
(b) Dihydrouridine
(c) Inosine
(d) Thymidine
(e) Queuosine
(f) Methylguanosine
What is the wobble position in tRNA, and what is its significance?
The third base of the anticodon is called the “wobble position,” and it can form base pairs with several different nucleotides.
What is a codon, and how many possible codons are there?
Each group of 3 nucleotides on the mRNA is a codon. There are 43 = 64 possible codons, which must code for 20 different amino acids.
What does it mean that the genetic code is “degenerate”?
More than one codon is used for most amino acids, making the genetic code “degenerate.” This means that it is not possible to take a protein sequence and deduce exactly the base sequence of the gene it came from.
What is the start codon, and what amino acid does it code for?
AUG is the start codon, and it codes for methionine. All proteins are initially translated with methionine in the first position, although it is often removed after translation.
What are stop codons, and what is their function?
There are 3 stop codons, also called “nonsense” codons, which signal the end of protein synthesis.
They are: UAA, UAG, UGA
List the four broad stages of protein synthesis.
(1) activation
(2) initiation
(3) elongation
(4) termination
What happens in the activation stage of protein synthesis?
tRNA molecules are charged with their respective amino acids, forming aminoacyl-tRNA complexes.
Compare the ribosome subunits of prokaryotes and eukaryotes based on Svedberg units.
◾ The ribosome of prokaryotes comprises a large 50S subunit and a small 30S subunit.
◾ The ribosome of eukaryotes comprises a large 60S subunit and a small 40S subunit.
Briefly discuss initiation of translation in Prokaryotes.
🧬 Ribosomes bind to specific translation initiation sites.
🧬 There can be several different initiation sites on a messenger RNA: a prokaryotic mRNA can code for several different proteins.
🧬 Translation begins at an AUG codon, or sometimes a GUG.
🧬 In prokaryotes, the modified amino acid N-formyl methionine is always the first amino acid of the new polypeptide.
Briefly discuss initiation of translation in Eukaryotes.
🧬 Ribosomes bind to the 5’ cap, then move down the mRNA until they reach the first AUG, the codon for methionine. Translation starts from this point.
🧬 Translation does not start at the first base of the mRNA. There is an untranslated region at the beginning of the mRNA, the 5’ untranslated region (5’ UTR). Also there is untranslated region at the end, the 3’ untranslated region (3’ UTR).
🧬 Eukaryotic mRNAs code for only a single gene (although there are a few exceptions).
Elongation
What are the two sites for tRNAs on the ribosome during elongation?
The ribosome has two sites for tRNAs, called the P site (Peptidyl site) and the A site (Aminoacyl site).
Elongation
What happens at the P site and A site during elongation?
The initial tRNA with the attached amino acid is in the P site. A new tRNA, corresponding to the next codon on the mRNA, binds to the A site.
Elongation
How is a new peptide bond formed during elongation?
The ribosome catalyzes the transfer of the amino acid from the tRNA in the P site to the amino acid on the tRNA in the A site, forming a new peptide bond. This reaction is catalyzed by peptidyl transferase.
What happens during ribosome translocation in elongation?
The ribosome moves down one codon on the mRNA. The now-empty tRNA in the P site is displaced off the ribosome, and the tRNA with the growing peptide chain moves from the A site to the P site. This process requires the hydrolysis of GTP.
How is the elongation process repeated?
The tRNA at the P site holds the peptide chain, and a new tRNA binds to the A site. The peptide chain is transferred onto the amino acid attached to the tRNA in the A site. The ribosome moves down one codon, displacing the empty tRNA from the P site and moving the tRNA with the peptide chain from the A site to the P site.
What happens when the ribosome reaches a stop codon?
When the ribosome reaches a stop codon, there is no tRNA that binds to it. Instead, proteins called “Release Factors” (RFs) bind, causing the ribosome, the mRNA, and the new polypeptide to separate.
What happens to the mRNA after the stop codon is reached?
The mRNA continues on past the stop codon. The remaining portion is not translated and is referred to as the 3’ untranslated region (3’ UTR).
The third base of the anticodon is called the
(a) degenerate base
(b) nonsense base
(c) 3’ end base
(d) 5’ cap
(e) wobble base
(e) wobble base
Which one of the following nucleosides is unique to tRNA?
(a) cytidine
(b) guanosine
(c) inosine
(d) adenosine
(e) uridine
(c) inosine
The following nucleosides are unique to tRNA except ________.
(a) dihydrouridine
(b) pseudouridine
(c) cytidine
(d) inosine
(e) methyladenosine
(c) cytidine
The following nucleosides are unique to tRNA except ________.
(a) guanosine
(b) methyladenosine
(c) inosine
(d) pseudouridine
(e) dihydrouridine
(a) guanosine
Which of the following nucleosides is unique to DNA and tRNA?
(a) methyladenosine
(b) pseudouridine
(c) inosine
(d) dihydrouridine
(e) thymidine
(e) thymidine
Further notes:
In tRNA, thymidine is present in a modified form known as ribothymidine.
tRNA must have the following except
(a) Must have an attachment site for the amino acid.
(b) Must be recognized by ribosomes.
(c) Must be recognized by a specific aminoacyl-tRNA synthetase that adds the correct amino acid.
(d) Must have an anticodon.
(e) Must be polyadenylated.
(e) Must be polyadenylated.
The following statements about puromycin are correct, except
(a) It inhibits protein synthesis.
(b) It is capable of peptide bond formation to form peptidyl-puromycin.
(c) It is an antibiotic.
(d) Its structure is very similar to the 3’ end of an aminoacyl-tRNA.
(e) It activates the process of polypeptide synthesis.
(e) It activates the process of polypeptide synthesis.
Which of the following drugs is not an inhibitor of protein synthesis in pathogens?
(a) Streptomycin
(b) Chloramphenicol
(c) Diphtheria toxin
(d) Fusidic acid
(e) Puromycin
(c) Diphtheria toxin
The initiation complex during translation in prokaryotes comprises the following except
(a) AUG codon
(b) A site
(c) charged tRNA
(d) 30S ribosome subunit
(e) mRNA
(b) A site
In prokaryotes, the start codon AUG specifies the amino acid ________.
(a) methionine
(b) proline
(c) glycine
(d) threonine
(e) N-formylmethionine
(e) N-formylmethionine
In eukaryotes, the start codon AUG codes for the amino acid ________.
(a) threonine
(b) methionine
(c) serine
(d) cysteine
(e) proline
(b) methionine
The initiation complex during translation in eukaryotes comprises the following except ________.
(a) P site
(b) charged tRNA
(c) ribosome
(d) mRNA
(e) release factor
(e) release factor
Which one of the following compounds inhibits protein synthesis in humans?
(a) diphtheria toxin
(b) erythromycin
(c) fusidic acid
(d) tetracycline
(e) chloramphenicol
(a) diphtheria toxin
