Lecture 17: Gene Expression: Translation

Question 1

What codons specify stop?

Answer 1

UAA UAG UGA

Question 2

what codon specifies start?

Answer 2

AUG

Question 3

tRNA

Answer 3

Transfer RNA (abbreviated tRNA) is a small RNA molecule that plays a key role in protein synthesis. Transfer RNA serves as a link (or adaptor) between the messenger RNA (mRNA) molecule and the growing chain of amino acids that make up a protein.

Question 4

What are the steps involved in charging a tRNA?

Answer 4

An enzyme (aminoacyltRNA synthetase),
recognises both a specific
amino acid and the
correct tRNA for this
amino acid and joins
them together

There are 20 different
(aminoacyl tRNA
synthetase) enzymes,
one for each amino acid

Question 5

What is translation?

Answer 5

Translation is the synthesis of proteins by ribosomes using mRNA as a set of instructions

Ribosomes contain both ribosomal RNA (rRNA) and proteins

Question 6

What is the location of the ribosomes?

Answer 6

Bound to the rER – synthesise proteins that are used within the plasma membrane or are exocytosed from the cell

Free in the cytosol – synthesise proteins that are released into the cytosol and used within the cell

Question 7

What are the three stages of translation?

Answer 7

Initiation
Elongation
Termination

Question 8

What happens during initiation?

Answer 8

A small ribosomal subunit
binds to an mRNA

The arrival of a large
ribosomal subunit completes
the initiation complex.

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 mRNA (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 9

What happens during elongation?

Answer 9

1. A ‘charged’ tRNA, with an anticodon complementary to the A site codon, lands in
   the A site.
2. Then 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
   ➠ A tRNA with the growing amino acid chain in the A site
   ➠ An empty/’uncharged’ tRNA in the P site
   ii. While the tRNAs are bound to the mRNA (in the P and A sites), the ribosome moves three nucleotides down the mRNA.
   ➠ The tRNA with the growing amino acid chain up in the P site (was in the A site) so that
   the chain of amino acids can exit through the tunnel located above the P site.
   ➠ ‘Uncharged’ tRNA in the E site (was in the P site).
3. In the E site, the ‘uncharged’ tRNA detaches from its anticodon and 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 10

What happens during temrination?

Answer 10

ribosomes reach a stop codon on mRNA a site accepts release factor

release factor promotes hydrolysis freeing peptide

ribosomal subunits and other components dissociate

Question 11

What is a genotype?

Answer 11

Genotype can also refer to a gene or set of genes that leads to a single trait or disease. For example, if your MC1R gene leads to you having red hair, then you have the genotype for red hair.

Question 12

What is a phenotype?

Answer 12

phenotype, all the observable characteristics of an organism that result from the interaction of its genotype (total genetic inheritance) with the environment. Examples of observable characteristics include behaviour, biochemical properties, colour, shape, and size.

Question 13

How many codons are there?

Does each codon encode for one amino acid only? Explain.

How many of the codons encode for an amino acid?

What are the codons called that do not code for amino acids?

Which amino acid is (almost always) the first one in a peptide sequence (protein)?

What binds to the mRNA codons and to the mRNA stop codons?

Identify the structure to the top right and label the two sites marked with arrows.

What are the features that make tRNA an ‘adaptor’ molecule?

What is a ‘charged’ tRNA and outline how a tRNA is ‘charged’?

Which macromolecules make up a ribosome?

What is being made during translation?

What are the three stages of translation?

Identify the structure on the bottom right and label the sites marked with arrows.

Describe the steps of translational initiation.

Describe the steps of translational elongation.

Describe the steps of translational termination.

What determines the amino acids sequence?

Where does translation take place in eukaryotes?

Below is an mRNA sequence with an underlined triplet codon.
For the underlined
triplet codon, what will the corresponding sequence of the tRNA anticodon be (write it
in the 5’ to 3’ direction) and what amino acid will it encode for? (Use the genetic code
table for the latter part of the question)