translation

Question 1

describe the structure of eukaryotic mRNA

Answer 1

• genes far apart in the chromosomes and are never co-transcribed
• there is an untranslated region at the ends of mRNA
• there is a single translational start site, and a single ORF
• message is capped and tailed
• ribosome binds at cap not start site

Question 2

how is bacterial mRNA different from eukaryotic mRNA?

Answer 2

genes are closer together, the message is polycistronic and there are multiple translational start sites from a single promoter

Question 3

how does protein synthesis occur on the ribosome, if the mRNA is longer than ribosomal diameter?

Answer 3

takes place on polyribosomes
the ribosome binds 5’ end of mRNA and continues to 3’ end. the ribosome moves aloong, taking the nascent peptide with it as it elongates

Question 4

why cant eukaryotes have coupled transcription-translation like bacteria?

Answer 4

in eukaryotes, transcription takes place in the nucleus and translation occurs in the cytoplasm

Question 5

in which direction does the ribosome move along the mRNA coding sequence?

Answer 5

5’-3’

Question 6

what is pseudouridine?

Answer 6

an isomer of the nucleoside uridine in which the uracil is attached via a carbon-carbon instead of a nitrogen-carbon glycosidic bond

Question 7

what is dihydrouracil?

Answer 7

modified uracil

Question 8

what is inosine?

Answer 8

a nucleoside formed when hypoxanthine is attached to a ribose ring via a B-N9 glycosidic bond

Question 9

what is the role of tRNA?

Answer 9

tRNA is the adaptor molecule that binds a specific codon and brings with it an amino acid for incorporation into the polypeptide chain

Question 10

describe the structure of tRNA

Answer 10

5’ terminus is a phosphorylated guanine, 3’ terminus has a free hydroxyl group.

Question 11

what are the common features amongst tRNAs?

Answer 11

1 - each is a single chain consisting of between 33-93 ribonucleotides
2 - the molecule is L-shaped
3 - they contain many methylated bases
4 - about half the nucleotides in tRNA are base-paired to form double helices
5- the 5’ end is phosphorylated
6 - an activated amino acid is attached to a hydroxyl group of the adenosine residue in the amino acid binding site
7 - the anticodon loop makes the 3 bases of the anticodon accessible

Question 12

how does the architecture of tRNA make it suitable for its function?

Answer 12

the anticodon is available to interact with an appropriate codon on mRNA, while the end that is linked to an activated amino acid is well positioned to participate in peptide bond formation

Question 13

what does the ‘wobble’ in wobble base pairing refer to?

Answer 13

the steric freedom in the pairing of the 3rd base to the codon

Question 14

what is the impact of wobble base pairing?

Answer 14

45 tRNAs can recognise 61 codons

Question 15

which two generalisations can be made about codon-anticodon interactions?

Answer 15

1 - the first two bases of a codon pair in the standard way - codons that differ in their first two bases must be recognised by different tRNAs
2 - the first base of an anticodon determines whether a particular tRNA can read one, two or 3 codons

Question 16

how is inosine formed and why is it frequently part of tRNA?

Answer 16

deamination of adenine. maximises the number of codons which can be read by a particular tRNA

Question 17

why is the wobble tolerated in the last base but not the first two?

Answer 17

on a 30S ribosome, there is 16S RNA containing conserved Adenine 1492, Adenine 1493, and Guanine 530. these form hydrogen bonds on the minor groove with correctly formed codon-anticodon base pairs. these interactions check for Watson-Crick base pairing in first two positions, but not 3rd

Question 18

what tRNA sparing strategy do bacteria use?

Answer 18

they omit A34, as G34 can recognise U3 and C3

Question 19

what is the amino acid binding site on tRNA?

Answer 19

3’ hydroxyl group of the adenosine residue at the 3’ terminus

Question 20

what is the sequence of the tRNA anticodon?

Answer 20

5’-ICG-3’

Question 21

which 5 groups of bases on tRNA are not base paired to form double helices?

Answer 21

-3’ CCA terminal
-TpseudoUC loop
-DHU loop
anticodon loop

Question 22

which bases on the 16S RNA on the 30S subunit are universally conserved? why?

Answer 22

Adenine 1492, Adenine 1493, Guanine 530

• these residues form hydrogen bonds on the minor groove but only with correctly formed base pairs of the codon-anticodon duplex
• these check if Watson-crick base pairing is present in the first two positions

Question 23

what bases can inosine pair with?

Answer 23

UCA