Gene Expression - Translation

Question 1

What is a codon?

Answer 1

A triplet of three nucleotides that codes for a particular amino acid

Question 2

What are the key features of the genetic code?

Answer 2

• 61 of a possible 64 codons specify an amino acid.
• Most amino acids have more than one codon
• Three codons specify to stop (UAA, UAG, UGA)
• One codon specifies to start (AUG - this codon also specifies the amino acid methionine)

Question 3

What is an assumption you should never make?

Answer 3

That you should read it in the left to right direction. Even with codons and things you always need to be sure you are reading in the 5’ to 3’ direction

Question 4

What is tRNA’s role in translation?

Answer 4

To be the ‘adaptor’ molecule. Each tRNA has a region which can bind an amino acid AND a region which can interact with mRNA.
It is a single strand of RNA that is roughly 70-80 nucleotides in length. There is at least one tRNA for each amino acid.

Question 5

What are the important features of tRNA?

Answer 5

• The anticodon on a tRNA interacts with mRNA.
• The binding site for an amino acid which is found on the end of the structure.
• It’s 3D structure, which allows it to fit like a glove into the ribosome
  This use features are what allows it to function as an adaptor molecule.

Question 6

What is translation?

Answer 6

The synthesis of proteins by ribosomes using mRNA as a set of instructions.
- Ribosomes contain both ribosomal RNA (rRNA) and proteins.

Question 7

What are the steps involved with ‘charging’ a tRNA?

Answer 7

• An enzyme recognises both a tRNA and an amino acid that are correct for each other (and has sites for both to bind)
• This enzyme links the tRNA to the amino acid by using ATP to synthase covalent bonding between the two molecules
• The charged tRNA is now released from the enzyme

Question 8

Describe the anatomy of a ribosome

Answer 8

• It is made up of a large subunit and a small subunit (that start as separate components and then come together).
• There are three sites that the tRNAs move through, called A, P and E sites.
  The A site is where the tRNA is accepted into the ribosome.
  The P site is where the peptide bond with form between animo acids.
  The E site is where the empty tRNA molecule exits the ribosome.
• Above the P site there is a small tunnel for the amino acid chain to come out of as there is no space in the ribosome.
• The strand of mRNA goes inbetween the two subunits of the ribosome

Question 9

What is the difference between the ribosomes found on the rER and the ones found in the cytoplasm?

Answer 9

• The ribosomes bound to the rER synthesise proteins that are used within the plasma membrane or are exocytosed from the cell.
• The ribosomes that are free in the cytosol synthesise proteins that are released into the cytosol and are used within the cell.

Question 10

What are the three stages of translation?

Answer 10

Initiation, Elongation, Termination
(All of these stages require energy input)

Question 11

What happens in initiation?

Answer 11

1. A specific initiation tRNA (carrying methionine) binds to the small ribosomal subunit.
2. The small ribosomal subunit/initiator tRNA identifies the 5’ G cap and attaches to the mRNA.
3. The small ribosomal subunit/initiator tRNA complex moves along the RNA (5’ to 3’ direction) until it finds the initiation AUG codon.
4. The complex stops, with the initiator tRNA carrying the first methionine positioned in the P site.
5. The large ribosomal subunit attaches.

Question 12

What happens in elongation?

Answer 12

1. A ‘charged’ tRNA, with an anticodon complementary to the A site codon, lands in the A site.
2. Two things happen at the same time:
   i) The ribosome will break the bond that binds the amino acid to the tRNA in the P site, transfer the amino acid to the newly arrived amino acid (attached to the tRNA in the A site) and form a peptide bond between them
   ii) While the tRNAs are bound to the mRNA (in the A and P sites) , the ribosome moves three nucleotides down the mRNA.
3. In the E site, the anticodon of the tRNA detaches from the mRNA codon and the ‘uncharged’ tRNA is expelled
4. A new ‘charged’ tRNA with an anticodon complementary to the next A site codon enters the ribosome at the A site and the elongation process repeats itself.

Question 13

What happens in termination?

Answer 13

1. When the ribosome reaches a stop codon, a protein called release factor enters the A site.
2. The release factor breaks the bond between the P site tRNA (using water) and the final amino acid. This causes the polypeptide chain to detach from its tRNA and the newly made polypeptide is released.
3. The small and large ribosomal subunits dissociate from the mRNA and each other.