lecture 17 - translation

Question 1

How are amino acids coded for?

Answer 1

Triplets of base pairs called codons.

Question 2

How many codons are there?

Answer 2

4^3 = 64 codons

Question 3

How many codons specify an amino acid?

Answer 3

61

Question 4

How many amino acids code for STOP?

Answer 4

3

Question 5

What codon specifies start?

Answer 5

AUG

Question 6

Why are there only 20 amino acids when there are many more codons?

Answer 6

Several codons code for the same amino acid. e.g. CAT and CAC both code for His (H)

Question 7

What is tRNA?

Answer 7

transport RNA - an adapter that carries specific amino acids to the template strand (mRNA) and allows them to fit together.

Question 8

What is the structure of tRNA

Answer 8

single strand, 70-80 nucleotides long that folds up, giving it an Amino acid attachment site at one end and an anticodon at the other end.

Question 9

What does the anticodon of tRNA interact with?

Answer 9

Codon sequence of mRNA

Question 10

What is the process of charging tRNA?

Answer 10

An enzyme (aminoacyl-tRNA synthetase) recognises both a specific specific amino acid and the appropriate tRNA molecule, joining them together at the tRNA attachment site. ATP is used to bind them covalently at the active site of the enzyme.

Question 11

How many different aminoacyl tRNA synthesise enzymes are there?

Answer 11

20 - one for each different amino acid.

Question 12

What is translation?

Answer 12

The synthesis of proteins by ribosomes using mRNA as a ‘set of instructions’.

Question 13

What do ribosomes contain?

Answer 13

Ribosomal RNA (rRNA) and proteins.

Question 14

Where are ribosomes found?

Answer 14

rER and free in cytosol of cell.

Question 15

What are the components of a ribosome? (4)

Answer 15

Large subunit, small subunit, E, P, A sites, exit tunnel.

Question 16

What are the 3 stages of translation?

Answer 16

Initiation, elongation, termination

Question 17

What stages of translation require an energy input?

Answer 17

All stages (initiation, elongation, termination)

Question 18

What is the small ribosomal unit?

Answer 18

A component of a ribosome to which mRNA and tRNA binds. the small subunit moves along the mRNA during translation.

Question 19

How does initiation begin in the process of translation?

Answer 19

Specific initiation tRNA, carrying the methionine ‘start’ amino acid bind to the small ribosomal subunit.

Question 20

What is the ‘start’ amino acid, coded for by the codon AUG/ATG?

Answer 20

methionine / Met (M)

Question 21

What is the process of initiation in translation?

Answer 21

initiation tRNA binds to small ribosomal subunit, which identifies the 5’ G-cap and attaches to mRNA at the binding site of subunit. Complex moves along mRNA (5’ to 3’ direction) until it finds initiation AUG codon. Large ribosomal subunit attaches, and the tRNA sits in the P site.

Question 22

Where does the initiator tRNA sit within the large ribosomal subunit when the complex stops and the large subunit binds?

Answer 22

The P site.

Question 23

Where does methionine sit while the initiator tRNA sits within the Psite of the large ribosomal subunit?

Answer 23

In the exit tunnel.

Question 24

What is the structure of the large ribosomal subunit?

Answer 24

Contains 3 sites for tRNA - E (exit) site, P (peptidyl-tRNA binding) site, A (aminacyl-tRNA binding) site. Has an exit tunnel for the growing polypeptide chain.

Question 25

How does elongation begin in translation?

Answer 25

A charged tRNA with an anticodon complementary to the A site codon lands at the A site, upstream of the initiator tRNA, which sits in the P site.

Question 26

In what direction is mRNA translated?

Answer 26

Read in 5’ to 3’ direction, and produces an anti parallel 3’ to 5’ strand in the 5’ to 3’ direction.

Question 27

What two processes occur simultaneously during the elongation step of translation?

Answer 27

Ribosome breaks bond between amino acid and tRNA in P site, transferring it to the amino acid in the A site. Simultaneously, the ribosome moves 3 nucleotides down the mRNA to reveal a new codon. tRNA moves to next site in large ribosomal subunit (E released, P to E, A to P).

Question 28

When, during elongation of translation, the amino acid in the P site is transferred to the amino acid in the A site, how are they joined?

Answer 28

Peptide bonds.

Question 29

How would tRNA in the E site be described?

Answer 29

Uncharged (contains no amino acid)

Question 30

Where are uncharged tRNA expelled from during elongation of translation?

Answer 30

the E site.

Question 31

What is the final step in elongation of translation?

Answer 31

A new charged tRNA with a complementary anticodon to the newly exposed mRNA section binds, entering the A site.

Question 32

When does termination occur in translation?

Answer 32

When the ribosome reaches a stop codon.

Question 33

When a ribosome reaches a STOP codon during translation, what occurs?

Answer 33

A protein called release factor enters the A site, promoting hydrolysis which releases the polypeptide from the ribosome via the exit tunnel.

Question 34

What is the function of release factor in the termination of translation?

Answer 34

Breaks bond between the P site tRNA and final amino acid, using water (hydrolysis), which causes the polypeptide chain to detach from the tRNA.

Question 35

How does release factor release the polypeptide chain from the P site tRNA?

Answer 35

Hydrolysis.

Question 36

Once the polypeptide chain is release from the ribosome in termination of translation, what occurs?

Answer 36

Small and large ribosomal subunits dissociate from mRNA and each other.

Question 37

What type of molecule is release factor?

Answer 37

Protein - it is NOT tRNA