Topic 7.3: Translation

Question 1

Ribosomes

Answer 1

Site of polypeptide synthesis

Question 2

Composition of Ribosomes

Answer 2

1) Ribosomal RNA

2) Protein

Question 3

Subunits of Ribosomes

Answer 3

1) Small subunit (mRNA binding site)

2) Large subunit (tRNA binding sites (E, P, A))

Question 4

Polysome

Answer 4

Group of two or more ribosomes translating an mRNA sequence simultaneously

Question 5

Transfer RNA

Answer 5

Carries amino acids to the ribosome

Question 6

tRNA-activating enzymes

Answer 6

Catalyses the binding of a tRNA molecule with a specific amino acid in the cytoplasm

Question 7

Process of tRNA-activating enzymes

Answer 7

1) The enzyme joins ATP to an amino acid (‘charging’)

2) ‘Charged’ amino acid is linked to tRNA (AMP is released)

Question 8

Purpose of charging the amino acid (2)

Answer 8

1) Create a high energy bond that can be be used during translation
2) Ribosomes use this energy to synthesise peptide bonds

Question 9

Specificity of tRNA-activating enzyme

Answer 9

Specific to a particular amino acid, but may bind multiple tRNA (due to degeneracy)

Question 10

Translation

Answer 10

Process of polypeptide synthesis

Question 11

Steps of Translation (4)

Answer 11

1) Initiation
2) Elongation
3) Translocation
4) Termination

Question 12

Initiation (Component assembly) (3)

Answer 12

1) The small ribosomal subunit binds to mRNA and moves in a 5’ → 3’ direction to the START codon (AUG)
2) The complementary tRNA molecule binds to the START codon via its anticodoni
3) The large subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small subunit

Question 13

Elongation / Translocation (Polypeptide synthesis) (4).

Answer 13

1) A tRNA molecule pairs with the next codon (via A-site)
2) The ribosome covalently attaches the amino acid in the P-site to the amino acid in the A-site (via peptide bond)
3) The ribosome moves along one codon position and the deacylated tRNA molecule is released (from the E-site)
4) The elongation and translocation processes continue along the mRNA coding sequence in a 5’ → 3’ direction

Question 14

Termination (Component disassembly)

Answer 14

When a ribosome reaches a STOP codon, a polypeptide is released and the ribosome disassembles into subunits

Question 15

Translation in Prokaryotes

Answer 15

The absence of a nuclear membrane allows translation to occur immediately after transcription

Question 16

Transcription in Eukaryotes

Answer 16

After transcription, the mRNA must be transported from the nucleus (via nuclear pores) prior to translation by the ribosome

Question 17

Protein Destination in Eukaryotes

Answer 17

1) Free ribosomes (cytosolic) synthesise intracellular proteins
2) Bound ribosomes (rER) synthesise proteins destined for secretion from the cell or for use in lysosomes

Question 18

Determination of Protein Destination in Eukaryotes

Answer 18

1) Presence / absence of an initial signal sequence on a nascent polypeptide chain
2) Results in the recruitment of a signal recognition particle (SRP), which halts translation
3) The SRP-ribosome complex then docks at a receptor located on the ER membrane (forming rough ER)
4) Translation is re-initiated and the polypeptide chain continues to grow via a transport channel into the lumen of the ER
5) The synthesised protein is transported via a vesicle to the Golgi complex (for secretion) or the lysosome
6) Proteins targeted for membrane fixation (e.g. integral proteins) get embedded into the ER membrane
7) The signal sequence is cleaved and the SRP recycled once the polypeptide is completely synthesised within the ER