translation

Question 1

what is the process of translation?

Answer 1

• it is the process by which the base sequence in the mRNA is decoded, giving rise to the amino acid sequence of the polypeptide chain

Question 2

where does translation occur?

Answer 2

• in the ribosome

Question 3

what must first happen before the initiation of translation?

Answer 3

• amino acid activation, where amino acids are attached to their respective tRNA molecules using energy from ATP hydrolysis

Question 4

what are the 3 stages of translation?

Answer 4

1. initiation
2. elongation
3. termination

Question 5

how many steps are involved in translation initiation in eukaryotes?

Answer 5

• 3 steps

Question 6

what is the first step in translation initiation of eukaryotes?

Answer 6

• the small ribosomal subunit (binds first!!) binds to the 5’ cap of the mRNA through its mRNA binding site
• upon binding, it moves along the mRNA in the 5’ to 3’ direction until it reaches the start codon

Question 7

what is the second step in translation initiation of eukaryotes?

Answer 7

• initiator amino acyl-tRNA complex binds to the mRNA at the start codon, where complementary base pairing occurs between the anticodon of the 1st aminoacyl-tRNA complex & the start codon on the mRNA

Question 8

what amino acid does the 1st tRNA always carry?

Answer 8

• methionine, which is always the first amino acid in a polypeptide chain

Question 9

what is the 3rd step in translation initiation of eukaryotes?

Answer 9

• the large ribosomal subunit binds, & completes the formation of the translation initiation complex
• the 1st aminoacyl-tRNA complex is now positioned at the peptidyl-tRNA site of the ribosome

Question 10

what is the 1st step in transcription elongation of eukaryotes?

Answer 10

• the 2nd aminoacyl-tRNA complex with anticodon complementary to 2nd codon on mRNA binds to mRNA at the aminoacyl-tRNA (A) site of the ribosome
• the energy for this step is provided by GTP hydrolysis

Question 11

what is the 2nd step in translation elongation of eukaryotes?

Answer 11

• a peptide bond is formed between 2 adjacent amino acids by peptidyl-transferase on the large subunit of the ribosome

Question 12

what is the 3rd step of translation elongation of eukaryotes?

Answer 12

• the ribosome moves along the mRNA to the next codon in the 5’ to 3’ direction, using energy from the hydrolysis of GTP
• the 1st tRNA, which was previously at the P site, is now at the exit site, & released into the cytoplasm
• the 2nd aminoacyl-tRNA complex, which was previously at the A site, is now at the P site
• the A site of the ribosome is now available for the next aminoacyl-tRNA complex with anticodon complementary to the 3rd codon on the mRNA, where another peptide bond can be formed between 2 adjacent amino acids

Question 13

what happens to the tRNA at the E site?

Answer 13

• it is released into the cytoplasm without any amino acid
• it is recycled by attaching to a new amino acid & becoming ‘charged’

Question 14

which direction is the polypeptide chain synthesised from?

Answer 14

• the amino end to the carboxyl end

Question 15

what is the 1st step in translation termination of eukaryotes?

Answer 15

• a stop codon (UAG, UAA, UGA) occupies the A site on the ribosome, with a release factor recognising & binding to the stop codon at the A site

Question 16

what is the release factor in translation termination?

Answer 16

• it is a protein that binds to the stop codon at the A site (function), as there are no tRNAs with anticodons complementary to the stop codons

Question 17

what is the 2nd step in translation termination of eukaryotes?

Answer 17

• the polypeptide chain is released from the tRNA in a hydrolysis reaction involving the addition of a water molecule
• the polypeptide chain is also released from the ribosome, & it folds into its secondary & tertiary structure in the lumen of the endoplasmic reticulum or cytoplasm
• the folded polypeptide chain may also undergo chemical modification at the Golgi apparatus

Question 18

what is the 3rd step in translation termination of eukaryotes?

Answer 18

• the ribosome dissociates from mRNA, & separates into its subunits