translation

Question 1

translation

Answer 1

• Process in which a protein chain is assembled
• Occurs at the ribosome in the cytoplasm, which consist of a large and small subunit
  o Each subunit is made of proteins and ribosomal RNA
• Ribosome moves along the mRNA one triplet codon at a time, in the gap between large and small subunit
• The sequence of bases in the DNA molecule is transcribed to a complementary sequence of bases in mRNA
• Free amino acids in the cytoplasm attached to tRNA
• In ribosomes, tRNA anticodons align with mRNA codons by complementary base pairing  this brings amino acids side by side and peptide bonds are formed between them, forming polypeptide molecule
• By these steps, the DNA code is translated into amino acid sequence in a polypeptide

Question 2

Initiation

Answer 2

o On arrival at the ribosome, the mRNA binds to the small subunit at an attachment site
o In this position, there are 6 bases (2 codons) of the mRNA exposed to the large subunit at any time
o The first 3 exposed bases (start codon) of mRNA are always AUG
o A molecule of tRNA with the complementary anticodon UAC forms hydrogen bonds with this codon, the amino acid methionine is attached to this tRNA molecule

Question 3

Elongation

Answer 3

o A second tRNA bonds with next 3 bases of the mRNA, bringing another amino acid alongside the methionine molecule
o Whilst the 2 amino acids are held close together within the ribosome, a peptide bond is formed between them by condensation reaction  catalysed by an enzyme found in the large subunit, forming a dipeptide
o The ribosome moves along the mRNA in the 5’ -> 3’ direction and the next codon is read
o At the same time, the 1st tRNA without its amino acid leaves the ribosome
o Then, a third tRNA bonds with next codon of the mRNA, bringing another amino acid alongside the 2nd amino acid residue of the dipeptide  a peptide bond is formed immediately, forming a tripeptide that starts to emerge from a hole within the large subunit
o Again, the ribosome moves along the mRNA and the next codon is read
o At the same time, the 2nd tRNA leaves the ribosome
o By these steps, constantly repeated, a polypeptide is formed and emerges from the large subunit

Question 4

Termination

Answer 4

o Eventually, a “stop” codon is reached  UAA, UAG, or UGA
o At this point, the completed polypeptide is released from the ribosome into the cytoplasm
o Many ribosome occur freely in the cytoplasm – sites of synthesis of proteins
o Common for several ribosomes to move along mRNA at one time  polysome (mRNA, ribosomes and their growing protein chains)
o Other ribosomes are bound to the membranes of the ER (endoplasmic reticulum) – sites of synthesis of proteins that are subsequently secreted from cells