Gene function 2 - Translation

Question 1

What is transfer RNA (tRNA)?

Answer 1

When a single stranded RNA is able to fold into a 3D structure due to lots of intramolecular hydrogen bonding. Amino acid attaches to the 3’ end.

Question 2

What is the anitcodon?

Answer 2

This is located on the opposite side of the molecule and is the area that will base paid with the codon on the mRNA.

Question 3

What is the enzyme that adds a specific amino acid to a specific sequence of tRNA?

Answer 3

Aminoacyl-tRNA-synthetase

Question 4

What is the role of this enzyme?

Answer 4

The amino acid binds to the enzyme and a high energy phosphate bond is added.
This bond is then broken to join the amino acid to the correct tRNA.
The enzyme is specific for both amino acid and tRNA, although some enzymes accept multiple tRNA’s for an amino acid.

Question 5

What is a ribosome?

Answer 5

A functional ribosome is a structure made of proteins and RNA.

Question 6

What are the 2 subunits of ribosomes?

Answer 6

Large subunit and small subunit.

Question 7

What are the binding sites for tRNA in ribosomes?

Answer 7

P site (peptidyl-tRNA binding site) - the tRNA here carries growing amino acid chain
A site (aminoacyl-tRNA binding site) - the tRNA carries the next amino acid

Question 8

What is the flow of tRNA at these sites?

Answer 8

tRNA enters at A, moves to P and then to E. At E they can no longer bind readily and so exit from the ribosome.

Question 9

What does initiation being with during translation?

Answer 9

The small ribosomal subunit binds to the RNA and the initiator tRNA (AUG - methionine).

Question 10

What is the leader sequence?

Answer 10

This is the RNA upstream of the start codon and ensures the correct position of the start codon.

Question 11

What is the role of the 5’ cap during processing?

Answer 11

It is added to the mRNA and helps direct it to the ribosome and ensure it is in the correct orientation - 5’ to 3’.

Question 12

What does guanosine triphosphate (GTP) do?

Answer 12

Provides energy to the large ribosomal subunit then binds with the initiator RNA in the P site.

Question 13

What are the 3 steps in translation elongation?

Answer 13

1) Codon recognition
2) Peptide bond formation
3) Translocation

Question 14

What does codon recognition entail?

Answer 14

The next aminoacyl-tRNA that has the correct anticodon complimentary to the next RNA codon is brought in to the A site. This process requires 2xGTP.

Question 15

What does peptide bond formation involve?

Answer 15

A peptide bond is formed between the growing polypeptide and the new amino acid (enzymatically catalysed by the large ribosomal subunit). No GTP is required here.

Question 16

What does translocation involve?

Answer 16

This is the shifting of the ribosome along the mRNA. Requires a GTP molecule for energy.
The tRNA in the A site is moved to the P site; the tRNA in the P site is moved to the E site where it can no longer bind to the mRNA and is ejected.

Question 17

What is the release factor?

Answer 17

This is a protein that recognises the occurrence of a stop codon in the mRNA during termination.

Question 18

What is the effect of the release factor?

Answer 18

Causes the last tRNA to leave the ribosome and the newly synthesised polypeptide is released.

Question 19

What happens to mRNA during termination?

Answer 19

Ribosomal subunits separate (to be reused) and mRNA is released. The m-RNA is also re-used, how long the mRNA lasts determines how much protein is made.

Question 20

What is the process of eukaryotic gene expression?

Answer 20

DNA is transcribed to pre-mRNA –> pre-mRNA is processed –> addition of a cap and poly(A) tail –> parts of the pre-mRNA (introns and some exons) are spliced out –> the processed mRNA then leaves the nucleus and undergoes translation into proteins.

This occurs on ribosomes (in cytoplasm or in rough ER).

Amino acids are joined to tRNA by aminoacyl-tRNA synthetase –> amino acids are then joined in sequence to form polypeptide –> this polypeptide is then released

Question 21

What do antibiotic drugs target?

Answer 21

1) Action of bacterial RNA polymerase - rifampicin
2) Binding of mRNA to small ribosomal subunit (30S inhibitors) - tetracycline, streptomycin
3) Peptide bond formation in the large ribosomal subunits (50S inhibitors) - erhythromycin, chloramphenicol

Question 22

How does control of gene expression occur in prokaryotes?

Answer 22

Occurs at the level of transcription - whether or not a gene is actually transcribed. When mRNA is formed it is translated immediately.

Question 23

What does control of gene expression involve in eukaryotes?

Answer 23

1) Level of packing - packing the genetic material with histones limits gene expression
2) Blocking of transcription - the most important level
3) RNA processing - how it is spliced
4) Transport to cytoplasm - how quickly/slowly
5) Degradation of mRNA - mRNA’s ability to resist breakdown means more/less protein will be made
6) Blocking of translation - by regulatory proteins
7) Cleavage/modification/transport to cellular destination of a protein - some proteins must be processed to become functional: cleavage into two or more polypeptides; modification e.g addition of other molecules; transport to cellular destinations
8) Degradation of the protein