Translation and Inhibitors of Protein Synthesis

Question 1

How is a peptide bond formed?

Answer 1

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.

Question 2

Define Translation.

Answer 2

Translation is the process in which the genetic information in mRNA molecule specifies the sequence of amino acids during protein synthesis.

Question 3

The first amino acid on the polypeptide forms the “__(a)__-terminal”. The last amino acid forms the “__(b)__-terminal”.

Answer 3

(a) N
(b) C

Question 4

How are ribosomal subunits identified?

Answer 4

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.

Question 5

How is each tRNA charged with the proper amino acid?

Answer 5

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.

Question 6

What is the wobble position in tRNA, and what is its significance?

Answer 6

Some tRNAs can pair with more than one codon. The third base of the anticodon is called the “wobble position,” and it can form base pairs with several different nucleotides.

Question 7

What is a codon, and how many possible codons are there?

Answer 7

A: 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.

Question 8

What does it mean that the genetic code is “degenerate”?

Answer 8

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.

Question 9

What is the start codon, and what amino acid does it code for?

Answer 9

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.

Question 10

What are stop codons, and what is their function?

Answer 10

There are 3 stop codons, also called “nonsense” codons, which signal the end of protein synthesis.
They are: UAA, UAG, UGA

Question 11

List the four broad stages of protein synthesis.

Answer 11

(1) activation
(2) initiation
(3) elongation
(4) termination

Question 12

Briefly explain the activation stage of protein synthesis.

Answer 12

This stage involves the preparation of amino acids for incorporation into a growing polypeptide chain.
Each amino acid is activated by bein attached to its corresponding tRNA molecule. This process it catalysed by a group of enzymes called aminoacyl-tRNA synthetases.
When attached to their amino acid (aminoacetylated), the tRNA are said to be “charged”.

Question 13

Discuss initiation of translation in Prokaryotes.

Answer 13

◾ Ribosomes bind to specific translation initiation sites.
◾ Bacteria have a large 50S and a small 30S ribosomal subunit.
◾ 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.

Question 14

Discuss initiation of translation in Eukaryotes.

Answer 14

🧬 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.
🧬 Eukaryotes have a large 60s and a small 40s ribosomal subunits.
🧬 Eukaryotic mRNAs code for only a single gene (although there are a few exceptions).
🧬 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).

Question 15

Elongation

What are the two sites for tRNAs on the ribosome during elongation?

Answer 15

The ribosome has two sites for tRNAs, called the P site (Peptidyl site) and the A site (Aminoacyl site).

Question 16

Elongation

What happens at the P site and A site during elongation?

Answer 16

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..

Question 17

Elongation

How is a new peptide bond formed during elongation?

Answer 17

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.

Question 18

What happens during ribosome translocation in elongation?

Answer 18

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.

Question 19

How is the elongation process repeated?

Answer 19

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.

Question 20

What happens when the ribosome reaches a stop codon?

Answer 20

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.

Question 21

What happens to the mRNA after the stop codon is reached?

Answer 21

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).