Translation 1- Genetic Code/Translation Machinery

Question 1

Why is the genetic code triplet?

Answer 1

There are 20 types of amino acid and 4 bases, meaning the code must be at least triplet.

Question 2

Why is the genetic code degenerate?

Answer 2

There is a redundancy and some amino acids are encoded by more than one codon.

Question 3

Is the redundancy in the genetic code random?

Answer 3

No- it is always in the 3rd base of a codon.

Question 4

Why is the redundancy in the genetic code always in the 3rd base of a codon?

Answer 4

Due to wobble base pairing- between the 3rd base of the codon and the 1st base of the anticodon of tRNA. This is because of modified bases in the tRNA.

Question 5

How did Crick and Brenner show that the genetic code was triplet?

Answer 5

By mutational analysis.

Question 6

What was Nirenberg’s experiment?

Answer 6

Adding synthetic polynucleotides to bacterial extracts and showed they could make polypeptides.

Question 7

How was the genetic code deciphered?

Answer 7

1. Synthesis of all triplet combinations, bound with ribosomes on an assay. 2. Identification of the amino acid produced by each combination.

Question 8

How are amino acids joined to tRNA molecules?

Answer 8

At CCA found at 3’ end of tRNA.

Question 9

Describe the structure of tRNA.

Answer 9

L-shaped with the anticodon at one end and the amino acid at the other. Folded on itself due to intramolecular base pairing. Base stacking due to stem loop stacking.

Question 10

Where are modified bases found in tRNA?

Answer 10

The D arm and the TWC arm.

Question 11

Where are the variable regions in tRNA?

Answer 11

Bases in the anticodon and the extra arm.

Question 12

What is tRNA charging?

Answer 12

The covalent linkage of amino acids to tRNAs.

Question 13

How do tRNAs bind to the correct amino acid?

Answer 13

Aminoacyl tRNA syntetases can recognise subtle differences in amino acid structure.

Question 14

What happens if a tRNA is mischarged?

Answer 14

tRNA synthetases has proof reading activity and will hydrolyse the incorrect amino acyl- tRNA bond.

Question 15

What is the size of a ribosome?

Answer 15

~18nm in diameter.

Question 16

What subunits are found in bacterial ribosomes?

Answer 16

50S and 30S.

Question 17

What subunits are found in eukaryotic ribosomes?

Answer 17

60S and 40S.

Question 18

How can RNA form the structures needed for catalysis and what does this suggest?

Answer 18

By folding on itself, can be considered a ribozyme, suggesting that life began in an RNA world.

Question 19

What is the rate of amino acid addition in ribosomes?

Answer 19

Eukaryotic- 2 amino acids per secondBacterial- 20 amino acids per second

Question 20

How many binding sites are there in a ribosome?

Answer 20

4 - one for the mRNA, three for the tRNA (A, P, E)

Question 21

Do tRNA genes have introns?

Answer 21

Some eukaryotic tRNAs have introns, however bacterial tRNAs do not.

Question 22

Why is wobble base pairing stable?

Answer 22

The binding of the first 2 bases provides enough binding strength to allow the wobble at the 3rd codon position.

Question 23

Describe the secondary structure of tRNA.

Answer 23

CCA at the 3’end- A involved in charging.
TWC loop- 5bp stem, contains pseudouridine
Variable arm- 3-21nts, contains modified bases
Anticodon arm- 5bp stem, contains anticodon
D-loop - 3/4 bp stem, contains dihydrouridine.
Acceptor stem- 7bp stem, containing G-U base pair.
Self complementary regions in each stem allow clover structure.

Question 24

How is translation fidelity increased in the anticodon arm?

Answer 24

Modified bases found either side of the anticodon strengthens anticodon-codon base pairing.

Question 25

Describe the tertiary structure of tRNA.

Answer 25

L- shaped, A-form RNA like helix. Maintained by hydrogen bonding and stacking interactions. Keeps most bases inaccessible to solvent (apart from anticodon and CCA).

Question 26

What is the stereochemical theory for genetic code evolution?

Answer 26

Thought that amino acids may have originally bound directly to mRNA codons by binding complementarity and similar hydrophobicity. Therefore the genetic code was based on this complementarity between codons and amino acids.

Question 27

What makes up the prokaryotic 50S ribosomal subunit?

Answer 27

Proteins L1-L31, 5S and 23S rRNA

Question 28

What makes up the prokaryotic 30S ribosomal subunit?

Answer 28

Proteins S1-S21 and 16S rRNA.

Question 29

What does the eukaryotic 60S ribosomal subunit consist of?

Answer 29

Proteins L1-L50, 28S, 5.8S and 5S rRNA

Question 30

What does the eukaryotic 40S ribosomal subunit consist of?

Answer 30

Proteins S1-S33 and 18S rRNA.

Question 31

How are rRNA genes expressed in prokaryotes?

Answer 31

As polycistronic transcripts that are cleaved by RNases and methylated to give mature rRNAs.

Question 32

How are rRNA genes processed in eukaryotes?

Answer 32

Can be self splicing

Question 33

Describe the ribosomal proteins.

Answer 33

Rich in basic amino acids (allows binding to nucleic acids). Contain the RNA recognition motif (RRM).

Question 34

Describe polyribosomes.

Answer 34

There is approximately 1 ribosome for every 80nts of mRNA. Once an active ribosome has passed its initiation site, another ribosome can also initiate translation- more efficient protein production.