sl and HL translation

Question 1

Features of the genetic code
- Students should understand the reasons for a triplet code.
- Students should use and understand the terms “degeneracy” and “universality”.

Answer 1

• The genetic code carries the message for the sequence of amino acids in a polypeptide. Three bases on the transcribed strand of the DNA correspond to one triplet of bases on the mRNA. The triplet of bases is called a codon.
• The genetic code is said to be universal. All organism use the same 4 letter code
• Different codons can code for the same amino acid – this is referred to as degenerate.
• Degeneracy of the genetic code refers to the fact that different combinations of codons can result the expressing the same amino acid.

Question 2

Roles of mRNA

Answer 2

• mRNA has a site to which a ribosome can bind and a sequence of codons that specifies the amino acid sequence of the polypeptide (including start and stop codon). One mRNA molecule can be translated many times.

Question 3

Role of ribosomes

Answer 3

• The ribosomes act as the binding site for mRNA and tRNA. They catalyse the peptide bonds between amino acids of the polypeptide.

Question 4

role of transfer RNA

Answer 4

Transfer RNA (tRNA) molecules translate the base sequence of mRNA to an amino acid sequence. They have an anticodon of three bases that bind to a codon on mRNA via complementary base pairing. tRNA molecules carry amino acids corresponding to their codon

Question 5

role of transfer RNA

Answer 5

Transfer RNA (tRNA) molecules translate the base sequence of mRNA to an amino acid sequence. They have an anticodon of three bases that bind to a codon on mRNA via complementary base pairing. tRNA molecules carry amino acids corresponding to their codon

Question 6

Roles of mRNA, ribosomes and tRNA in translation (structure)
- Students should know that mRNA binds to the small subunit of the ribosome and that two tRNAs can bind simultaneously to the large subunit.

Answer 6

• Translation takes place in free, and membrane bound ribosomes using the mRNA strand produced during transcription.
• Ribosomes are composed of two subunits: a large and a small subunit. Both subunits are composed of long strands of rRNA.
• When synthesizing a new protein, the two subunits lock together with a messenger RNA trapped in the space between.
• The ribosome then walks down the messenger RNA three nucleotides at a time, building a new protein piece-by-piece.

Question 7

Translation

Answer 7

• Stepwise movement of the ribosome along mRNA and linkage of amino acids by peptide bonding to the growing polypeptide chain. Translation of an mRNA molecule is done by repeating cycle steps.
• Each cycle results in the addition of one amino acid to the growing polypeptide chain. During each cycle the ribosome moves three bases (one codon) along.

Question 8

3 stages of translation

Answer 8

Initiation: Assembly of components that carry out the process

Elongation: Synthesis of the polypeptide involves a repeated cycle of events

Termination: Disassembly of the components follows termination of translation

Question 9

Translation

Answer 9

1. The small ribosomal subunit attaches to the mRNA and slides along the molecule in a 5 prime to 3 prime direction until it recognizes the start codon with the bases AUG.
2. Now the first tRNA loaded with an amino acid specific to the corresponding anticodon (here methionine) attaches to a site on the ribosome called the P-site
3. The large subunit can now bind to close the complex.
4. The ribosome has three binding sites (E, P and A).
5. The second tRNA carrying an amino acid binds with its anticodon to the complementary codon at the A (amino acetyl) site of the ribosome.
6. Peptide bond formation between the amino acid on the tRNA and the amino acid in the P site. The polypeptide formed by this is transferred to the tRNA in the A-site.
7. The ribosome moves along the mRNA by one codon in 5’ -> 3’ direction, causing the second tRNA to move from the A to the P site, and the first from the P to the E site.
8. The empty tRNA in the E-site separates from codon on the ribosome.

Elongation is repeated multiple times until the stop codon is reached.

Question 10

Initiation of translation
- Include attachment of the small ribosome subunit to the 5’ terminal of mRNA, movement to the start codon, attachment of the large subunit, the initiator tRNA and another tRNA.
- Students should understand the roles of the three binding sites for tRNA on the ribosome (A, P and E) during elongation.

Answer 10

• Before translation starts, tRNA is activated by a tRNA activating enzyme
• A tRNA activating enzyme attaches the amino acid methionine to an initiator tRNA that has the anticodon UAC.
• Every amino acid and its corresponding tRNA must be activated initially. Each tRNA activation enzyme is specific to its tRNA and a specific amino acid. The activated tRNA then delivers amino acids to the growing polypeptide chain in translation.
• Each amino acid has its own specific tRNA activation enzyme.

Question 11

what is the a site/p site/e site?

Question 12

INITIATION OF TRANSLATION

Answer 12

• 5’ end of mRNA molecule is binded to small ribosomal unit at the mRNA binding site
• this is joined by the initiator tRNA (w/ amino acid methionine) at the start codon (AUG) at the P-site
• large ribosomal unit is attached
• the next codon of mRNA at the A-site is available for a tRNA with the appropriate anticodon. Its arrival places the two activated amino acids into position and forms a peptide bond btw. them (as a result of condensation rxns)

Question 13

ELONGATION OF TRANSLATION

Question 14

TERMINATION OF TRANSLATION

Question 15

Modification of polypeptides into their functional state
- Students should appreciate that many polypeptides must be modified before they can function.

Answer 15

• Changes to the side chain of amino acids (phosphorylation/glycosylation)
• Folding of polypeptides to stabilize the tertiary structure
• Removing part of the polypeptide chain
• Combining polypeptides into their quaternary structure
  Forming conjugated proteins by adding a non-polypeptide component.

Question 16

• The examples chosen should include the two-stage modification of pre-proinsulin to insulin.

Answer 16

The insulin gene is transcribed in pancreatic beta cells to produce mRNA copies, which subsequently are translated in ribosomes of the ER.

After translation, two post-translational steps of modification are required to turn the first product (pre-proinsulin) into mature insulin.

Question 17

Stage 1

Answer 17

First, the 110 amino acid long pre-proinsulin is cleaved by 24 amino acids by a protease enzyme and proinsulin is formed. Proinsulin now becomes folded with three disulfide bonds at 3 different sites.

Question 18

Stage 2

Answer 18

After this, peptide bonds in the proinsulin are broken at two different positions to form two separate chains held together by disulfide bonds. This is the mature protein insulin.

Question 19

Recycling of amino acids by proteasomes
- Limit to the understanding that sustaining a functional proteome requires constant protein breakdown and synthesis.

Answer 19

• Most proteins have a short life span in the cell due to loss of function caused by free radicals or chemicals, misfolding, denaturation or simply because it is no longer used.
• Proteins which are no longer needed are broken down by proteasomes. These proteins are tagged with a chain of small proteins called ubiquitin. This acts as a signal for proteasomes that these proteins should be digested.

Question 20

Recycling of amino acids by proteasomes
- STEPS

Answer 20

1. A proteosome is composed of regulatory subunits at either end.
   These subunits recognize ubiquinated proteins.
2. Proteins which are not needed anymore become “marked” with ubiquitin.
3. Once recognized by the proteoasome, the proteins are unfolded and fed into the core chamber of the proteosome.
   The protein is broken down into short chains of amino acids and released.

Question 21

Mutations that change protein structure
- Include an example of a point mutation affecting protein structure.

DNA to mRNA

Answer 21

• A gene mutation is a change to the base sequence of a gene.
• Sickle cell anemia is a disease which is caused by a mutation in the DNA which changes the oxygen transporting polypeptide structure of the protein hemoglobin which is contained in red blood cells.
• In the sickle cell disease, the human chromosome number 11 is the one which experiences a base substitution mutation of a gene (Hb) resulting in the polypeptide beta-globin to change shape and structure.
• The polypeptide chain of the mutated beta-globin changes its structure due to the altered interactions between amino acids. The hydrophobic interactions between amino acids in the protein transform the hemoglobin into rigid fibers, resulting in a change in shape. This new structure is referred to as a sickle shape.