Lecture 18

Question 1

How was it determined that amino acids are coded for in triplets? What does this lead to ?

Answer 1

There are 20 amino acids and 4 bases, if DNA were coded in single bases this would lead to 4 amino acids, doublets would be 4^2 = 16 amino acids, triplets is the smallest amount which can code for all the amino acids leading to 4^3 = 64 possible amino acids, as such because there are 44 more combinations than amino acids there is a large amount of redundancy (some combinations lead to the same amino acid).

Question 2

What are the two common groups on all amino acids?

Answer 2

A carboxylic acid and amine group attached to the same carbon.

Question 3

How was it determined what codon sequences code for which amino acids?

Answer 3

Experimental analysis using 19 unlabelled amino acids and one radioactive one with a known mRNA sequence, repeated until all were known.

Question 4

What are the stop codons? What is the start codon and what amino acid does it code for?

Answer 4

The stop codons are UAA, UAG and UGA. The start codon is AUG and also codes for methionine.

Question 5

What was Crick’s adapter hypothesis? What did the adapter molecule end up being?

Answer 5

The adapter hypothesis was that the amino acid is added to the template by an adapter molecule and that the adapter is the part which actually fits on the RNA. It was found that this adapter is tRNA (transfer RNA),

Question 6

What is the structure of tRNA?

Answer 6

A single RNA strand of about 70-80 nucleotides in length with at least one tRNA for each amino acid and one amino acid and mRNA binding region on each tRNA. Semi joined by hydrogen bonds. Two dimensionally it has the amino acid attachment site at the 3’ end and the anticodon half way in between the 3’ and 5’ ends, looks kind of like a plus with no top. Three dimensionally it’s folded over towards the top and looks kind of like an asthma inhaler, anticodon at the bottom and amino acid attachment site at the top.

Question 7

How different codons are there? Compare this to how many tRNA anti codons there are, why is this the case?

Answer 7

64 different codons but only 32 tRNA types, this is because tRNA has some wobble pairing, the 5’ base of tRNA will bind with different 3’ bases in the mRNA to the normal base pairing rules:
G with U or C
C with G
A with U
U with A or G
I with A, U or C
Where I is a base unique to tRNA (inosine).
Note that for wobble pairing to work the first base of the 5’ end of the mRNA must be U, G or I, otherwise normal base pairing rules apply.

Question 8

How is tRNA charged?

Answer 8

aminoacyl-tRNA syntethase recognises a specific amino acid and the correct tRNA for that amino acid and joins them, there are 20 different versions of this enzyme, one for each amino acid.

Question 9

What is translation? What steps does it have?

Answer 9

Translation is the synthesis of proteins by ribosomes using mRNA as an instruction set. Steps are initiation, elongation and termination.

Question 10

What is the basic anatomy of a ribosome? What is the main similarity between prokaryotic and eukaryotic ribosomes?

Answer 10

Ribosomes have several parts, they are made up of both ribosomal RNA (rRNA) and proteins. The parts are the small subunit at the bottom of the RNA during translation, the large subunit (at the top during translation), an mRNA binding site on the small subunit, an exit tunnel at the top (from the p site), a P site (peptidyl-tRNA binding site), an E site (exit site) and an A site (aminoacyl-tRNA binding site).

Question 11

What steps are involved in translation initiation?

Answer 11

1. The small ribosomal subunits find the initiation (AUG) codon on the mRNA. The AUG codon is positioned in the P site of the small ribosomal subunit.
2. a charged tRNA carrying methionine binds to the P site
3. The large ribosomal subunit attached (forming the translation initiation complex)
4. This attachment requires energy, which is recieved from breaking down of guanosine triphosphate into guanosine diphosphate and inorganic phosphate.

Question 12

What steps are involved in translation elongation?

Answer 12

1. a charged tRNA carrying the complementary anticodon to the A site codon lands in the A site.
   2.these two processes occur at the same time: i. the ribosome breaks the bond binding the amino acid to the tRNA in the P site and transfers the amino acid chain to the newly arrived amino acid (A site).
   ii. The ribosome moves three nucleotides down the mRNA but leaves the tRNA in the same location, allowing the amino acid chain to feed through the exit tunnel and moves the uncharged amino acid to the exit site.
2. The tRNA in the E site is detached from the mRNA and is expelled.
3. repeat steps 1-3
   Note that energy is required for step 1 and step 3 (uses guanosine triphosphate).

Question 13

What steps are involved in translation termination?

Answer 13

1. Ribosome reaches a stop codon, causing a protein called release factor to bind to the A site.
2. The release factor uses hydrolysis to break the bond between the P site tRNA and the last attached amino acid, causing the polypeptide chain to detach and be released.
3. The small and large ribosomal sub units disociate from the mRNA and each other.
   Energy is also required here (using guanosine triphosphate.)

Question 14

What does a polyribosome refer to?

Answer 14

Polyribosome refers to when a single mRNA may be translated by several ribosomes at the same time.