Gene Expression - Translation

Question 1

How many bases in polynucleotides?

Answer 1

4

Question 2

What specifies mRNA nucleotide sequence?

Answer 2

DNA

Question 3

What does the genetic code consist of?

Answer 3

non-overlapping triplets which are read from a fixed starting point

Question 4

Triplet codons:

Answer 4

64 codons specify 20 amino acids - 61 ‘sense’ and 3 ‘stop’ codons or nonsense codons

Question 5

Is genetic code identical in all organisms?

Answer 5

yes

Question 6

Is the genetic code universal?

Answer 6

yes

Question 7

What is the ‘open reading frame’?

Answer 7

the protein coding sequences in a gene

Question 8

For most amino acids…

Answer 8

• there is more than one codon
• the genetic code is ‘degenerate’

Question 9

The genetic code is not ambiguous meaning?

Answer 9

each codon specifies only on amino acid

Question 10

Types of RNA involved with translation:

Answer 10

1. mRNA
2. tRNA
3. rRNA

Question 11

Why is an adaptor or interpreter required?

Answer 11

as we have no direct pairing between the codons and amino acids

Question 12

How do tRNAs act as adapter molecules:

Answer 12

• bridges the codon to a specific amino acid
• each tRNA becomes covalently linked to one specific amino acid
• each tRNA has a triplet anticodon that recognises one or more codons in the mRNA by base pairing

Question 13

In response to a specific codon, tRNAs…

Answer 13

bring the correct amino acid to the ribosome

Question 14

What type of structure do tRNAs have?

Answer 14

cloverleaf structure

Question 15

3 functions of tRNA:

Answer 15

1. it binds to an amino acid, and is then “changed”; requires specific features in tRNA
2. it associates with mRNA molecules through the anticodon loop
3. it interacts with ribosomes

Question 16

tRNA charging:

Answer 16

specific enzymes couple each amino acid to its appropriate tRNA molecule

Question 17

What is tRNA catalysed by?

Answer 17

aminoacyl-tRNA synthetase

Question 18

The accuracy of tRNA charging is controlled by what?

Answer 18

aminoacyl -tRNA synthetase

Question 19

Amino acid selection and tRNA selection are subject to what?

Answer 19

proofreading

Question 20

What does proofreading do?

Answer 20

either disfavours the forward reaction or reverses the catalytic reaction if the wrong component has been selected

Question 21

What does proofreading avoid?

Answer 21

the need for absolute accuracy(which is slow)

Question 22

tRNA molecules match what to what in mRNA?

Answer 22

amino acids to codons

Question 23

Where does codon-anticodon recognition occur?

Answer 23

on the ribosome by the base pairing between the codon and anticodon

Question 24

What does ‘wobble’ and modified bases allow?

Answer 24

one tRNA to decode more than one codon

Question 25

What is 'wobble'?

Answer 25

flexibility in pairing

Question 26

Wobble hypothesis:

Answer 26

Watson-crick base pair rules between anticodon position 1 and codon position 3 are released due to flexibility and modifications of the anticodon loop

Question 27

Can single tRNAs recognise multiple codons?

Answer 27

yes

Question 28

Ribosomes:

Answer 28

holds mRNA and charged tRNAs in the correct positions to allow assembly of the polypeptide chain

Question 29

What type of proteins can ribosomes make?

Answer 29

any meaning they're not specific

Question 30

In eukaryotes, how many molecules of rRNA and proteins in a large subunit?

Answer 30

3 rRNA and 49 proteins

Question 31

In eukaryotes, how many molecules of rRNA and proteins in a small subunit?

Answer 31

1 rRNA and 33 proteins

Question 32

rRNA structure:

Answer 32

rRNA is highly structured and the major structural and functional component

Question 33

What is peptide bond formation catalysed by?

Answer 33

23s rRNA

Question 34

How many sites for tRNA binding on the largest ribosomal subunit?

Answer 34

3

Question 35

Codon-anticodon interactions between tRNA and mRNA occur at what sites?

Answer 35

P and A sites

Question 36

A site:

Answer 36

binds with the anticodon of charged tRNA

Question 37

P site:

Answer 37

where tRNA adds its amino acid to the growing peptide chain

Question 38

E site:

Answer 38

where tRNA sites before being released from the ribosome

Question 39

3 phases to polypeptide synthesis:

Answer 39

1. initiation 2. elongation 3. termination

Question 40

Initiation:

Answer 40

mRNA binding, start codon selection, binding the first aminoacyl-tRNA

Question 41

Elongation:

Answer 41

sequential addition of amino acids as specified by the codons in the mRNA

Question 42

Termination:

Answer 42

release of the completed polypeptide in response to a stop codon

Question 43

What is a fidelity function?

Answer 43

small subunit rRNA validates H-bonds and if they've not formed between all three base pairs, the tRNA must be an incorrect match, and it is rejected

Question 44

Do ribosomes have a fidelity function?

Answer 44

yes

Question 45

3 steps of elongation:

Answer 45

1. codon recognition - aminoacyl-tRNA entry into the A-site) 2. peptide bond formation(met is linked to Pro in A-site) 3. translocation(simultaneously, the free tRNA is moved to the E-site, the peptidyl-tRNA is moved to the P-site)

Question 46

Is elongation a repeated process?

Answer 46

yes

Question 47

What marks the end of translation?

Answer 47

stop codons

Question 48

What are stop codons recognised by?

Answer 48

protein release factors

Question 49

What do RRF(ribosome releasing factors) promote?

Answer 49

dissociation of the ribosomal subunits, tRNA and mRNA

Question 50

What are proteins made on?

Answer 50

polyribosomes

Question 51

What is a strand of mRNA with associated ribosomes called?

Answer 51

a polysome