12.3

Question 1

translation

Answer 1

-process of protein synthesis (converts RNA sequence into polypeptide sequence)
-basics same in prokaryotes and eukaryotes
-occurs in 3 stages: initiation, elongation, termination
-mRNA provides template
-tRNAs read template and deliver specified amino acids
rRNAs and proteins form translation machinery (ribosome)
-protein synthesis is N-terminus to C-terminus

Question 2

Prokaryotic mRNA

Answer 2

mRNA sequence is always read 5’ to 3’
prokaryotic mRNA often contains multiple coding sequences (due to operons)

Question 3

Eukaryotic mRNA

Answer 3

eukaryotic mRNA contains a single coding sequence

Question 4

genetic code

Answer 4

-each amino acid is specified by a sequence of 3 nucleotides, the codon
-64 possible codes but only 20 encode amino acids
-genetic code is practically universal

Question 5

tRNA

Answer 5

-single stranded RNA of 70-80 nucleotides, self folds into “cloverleaf”
-amino acids loaded onto 3’ acceptor end by aminoacyl tRNA synthetase
-anticodon loop recognized and binds to complementary codon in mRNA

Question 5

tRNA-mRNA base pairing

Answer 5

-cells often express fewer tRNAs than the number of possible codons (61)
-some tRNAs can recognize more than 1 codon
how is this possible: wobble pairing

Question 6

wobble pairing

Answer 6

involves less stringent base pairing between 3’ base of codon and 5’ base of anticodon

Question 7

ribosome

Answer 7

2 subunits: small and large
3 tRNA binding sites: E site, P site, A site

Question 8

small subunit

Answer 8

decode the mRNA

Question 9

large subunit

Answer 9

catalyzes peptide bond formation

Question 10

E site (exit)

Answer 10

binds tRNA of previous amino acid

Question 11

P site (peptidyl)

Answer 11

binds tRNA attached to growing peptide chain

Question 12

A site (aminoacyl)

Answer 12

holds tRNA attached to the next amino acid

Question 13

Initation: recognition of translation start site (prokaryotes)

Answer 13

1. small subunit binds ribosome binding site
2. Initator tRNA-N-formyl-Met binds ot start codon in the P site
3. large subunit binds
4. A site opens to accepts next tRNA-amino acid
5. Switch to elongation

Question 14

Initation: recognition of translation start site (eukaroytes)

Answer 14

1. small subunit bonds to 5’ cap to determine translation start site
2. initiator tRNA met binds to start codon in the P site
3. large subunit binds
4. A site is open to accept next tRNA amino acid
   switch to elongation

Question 15

termination of translation

Answer 15

-elongation continues until stop codon (UAA, UAG, UGA), is reached
-release factor recognizes stop codon -> triggers release of new polypeptide from P site tRNA
-ribosome shifts one “codon” and disassembles