gene expression-17

Question 1

Key features of the genetic code:

Answer 1

61 of a possible 64 codons specify an amino acid
most amino acids have more than one codon
three codons specify stop (UAA, UAG and UGA)
one codon specifies start (AUG – this codon also specifies methionine)

Question 2

How many codons mean stop?

what are they?

Answer 2

3
UAA
UAG
UGA

Question 3

How many codons mean start?

what are they?

Answer 3

AUG

methionine

Question 4

Codons

Answer 4

61/64 specify amino acids

most amino acids have >1 codon

Question 5

tRNA:

Answer 5

acts as an adaptor
has a specific 3D shape
amino acids attach at one end
anticodon recognises the mRNA codon

Question 6

Charging’ tRNA:

Answer 6

Aminoacyl-tRNA synthetase recognises both a specific amino acid and the correct tRNA for this amino acid and actively (uses ATP) joins them together to form what is called “charged tRNA”.

Question 7

Translation:

Answer 7

the synthesis of proteins by ribosomes using mRNA as a set of instructions.

Question 8

Ribosomes able to interact with

Answer 8

mRNA and tRNA

Question 9

Binding Sites of a Ribosome:

Answer 9

A site (aminoacyl-tRNA): holds the tRNA carrying the next amino acid to be added to the chain

P site (peptidyl-tRNA): holds the tRNA carrying the growing polypeptide chain

E site (exit): site from which discharged tRNAs leave the ribosome

Question 10

Binding Sites of a Ribosome:
A site (aminoacyl-tRNA):

Answer 10

holds the tRNA carrying the next amino acid to be added to the chain

Question 11

Binding Sites of a Ribosome:
P site (peptidyl-tRNA):

Answer 11

holds the tRNA carrying the growing polypeptide chain

Question 12

Binding Sites of a Ribosome:
E site (exit):

Answer 12

site from which discharged tRNAs leave the ribosome

Question 13

Translation 3 Stages:

know what is happening at each stage and be able to draw a diagram

Answer 13

Initiation
Elongation
Termination

Question 14

Translation:
Initiation:

Answer 14

AUG codon in P site of small ribosomal subunit.
Small ribosomal subunit find initiation AUG codon on mRNA.
tRNA (charged) with amino acid Methionine (Met M) binds to P site.
Large ribosomal subunits attaches and completes initiation complex.

Question 15

Initiation:

Answer 15

Small ribosomal subunit binds to mRNA and a specific initiator tRNA,
large ribosomal subunit attaches to complete the translation initiation complex.
Proteins called initiation factors are required to bring all the translation components together.
GTP provides the energy for the assembly.
The initiatior tRNA is in the P site; and the vacant A site is ready for the next aminoacyl tRNA.

Question 16

Elongation

Answer 16

Codon recognition: anticodon of incoming aminoacyl tRNA base-pairs with complementary mRNA codon in A site.

Peptide bond formation: rRNA molecule of large ribosomal subunit catalyzes formation of peptide bond between new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. Removes the polypeptide from tRNA in P site attaching it to amino acid on the tRNA in A site.

Translocation: ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs, bringing the next codon to be translated into the A site.