Protein synthesis

Question 1

What are the key points of the genetic code?

Answer 1

1. There are 4 possible bases, each codon uses 3 bases so there are 64 different codons
2. Redundant - each amino acid is coded for by more than 1 codon
3. Universal - similar in all species
4. Some codon are stop codons
5. AUG codes for methionine = start codon

Question 2

How is the correct reading frame detected?

Answer 2

The AUG codon is identified

Question 3

How is tRNA involved in translation?

Answer 3

Each tRNA has an anticodon that binds to the complementary codon on the mRNA. Each tRNA will also be covalently bonded to the corresponding amino acid, peptide bonds will form between adjacent amino acids.
As a result redundancy there is more than 1 codon fro each tRNA.

Question 4

What is genetic ‘wobble’?

Answer 4

Some tRNA molecules only need complementary bonding at the first 2 bases, so some will bind to more than 1 codon, hence fewer tRNAs than codons

Question 5

Describe the structure of ribosomes.

Answer 5

Made of approximately 80 proteins and 4 rRNAs.
The large subunit is made of around 50 proteins and 3 rRNAs. It catalyses the formation of peptide bonds between adjacent amino acids and releases the tRNA.
The small subunit is made of around 30 proteins and 1 rRNA. It forms the complementary pairs with the anticodon and codons and holds the tRNA in place.
They are a type of ribozyme (an RNA molecule that is capable of acting as an enzyme)
Can be free in the cytoplasm but are often associated with the RER to allow for processing of the protein.

Question 6

How is protein synthesis (translation) initiated?

Answer 6

Translation imitation factors bind to the mRNA (5’ caps and 3’ poly-A-tails) - this allows the mature mRNA to leave the nucleus. The small ribosomal unit that codes for methionine will then search and bind for the start codon. The large subunit will then catalyse the formation of peptide bonds.

Question 7

How does translation stop?

Answer 7

Some codons code for stop codons, the complementary mRNA will not carry an amino acid so translation will stop.

Question 8

How is translation made more effective?

Answer 8

It uses polyribosomes. Ribosomes are bound at every codon, allowing for continuous synthesis

Question 9

How can proteins be modified?

Answer 9

Need to modified to become fully functional, by forming covalent bonds with modifications.

1. Phosphorylation - addition of phosphate group. activated, deactivates
2. Glycosylation - Addition of sugar group - cell signalling
3. Ubiquitination - addition of ubiquity - marks for degradation by proteases