Unit 2 - Molecular Genetics (3)

Question 1

Translation of mRNA

Answer 1

the conversion of a nucleic acid sequence to a polypeptide sequence
this occurs at a ribosome

Question 2

Ribosomes

Answer 2

consist of two subunits, one large and one small
each type of amino acid is carried to the ribosome by a specific type of tRNA
the anticodon loop of tRNA molecules base-pair with complementary mRNA codons in the codons in the ribosome and position their attached amino acid in the proper orientation for insertion into the growing polypeptide
the large ribosomal subunit has three noteworthy features
A-site, P-site, catalytic site

Question 3

Initiation

Answer 3

the small ribosomal subunit binds to the mRNA start codon (AUG)
the anticodon (UAC) of tRNA methionine base-pairs with the mRNA start codon
a large ribosomal subunit binds to the small subunit with the tRNA methionine occupying the P-site

Question 4

Elongation

Answer 4

the second mRNA codon specifies which tRNA amino acid complex base-pairs to the A site
the catalytic site of the large ribosomal subunit forms a peptide bond between the amino acids on the bound tRNA molecules
the first tRNA moves out of the P-site, leaving the tRNA dipeptide complex in the A-site
the ribosome moves one codon down the mRNA, moving the tRNA dipeptide complex to the P-site

Question 5

Termination

Answer 5

once an mRNA stop codon is reached (UAG, UAA, UGA), the finished peptide is released from the ribosome
the ribosomal subunits separate

Question 6

Regulating gene expression

Answer 6

metabolic pathways can be turned off by feedback inhibition
another strategy is to simply stop transcribing the genes for the enzymes of the pathway
this way, energy isn’t wasted making enzymes until they are needed

Question 7

Operons

Answer 7

enzymes of metabolic pathways in bacteria are sometimes grouped into operons and turned on or off simultaneously by a common control mechanisms
structural genes code for enzymes
the operator is a section of DNA between the promoter and the structural genes
a repressor is a protein that may bind to the operator and prevent RNA polymerase from transcribing the structural genes
a gene outside of the operon encodes for the repressor
operons allow cells to make one repressor protein, rather than several unnecessary enzymes

Question 8

The trp operon

Answer 8

E. coli synthesizes tryptophan using a metabolic pathway that includes 5 enzymes
these are encoded by the structural genes of the trp operon
when tryptophan is present in the environment, it need not be made, so the operon is turned off
tryptophan acts as a corepressor activating an inactive repressor
the repressor then binds to the operator, preventing RNA polymerase from transcribing the structural genes
as the bacterium runs out of tryptophan, the repressor becomes inactive and leaves the operator RNA polymerase is then free to transcribe the structural genes that will make the enzymes necessary for tryptophan synthesis

Question 9

The lac operon

Answer 9

the lac operon has structural genes that allow E. coli to metabolize lactose
this operon should be turned off unless the cell lacks energy and also has access to lactose

Question 10

Negative control

Answer 10

in the absence of lactose, an active repressor binds to the operator and prevents transcription
lactose acts as an inducer, binding to the repressor
this changes the repressor shape so that it is released from the operator, allowing RNA polymerase to transcribe the structural genes
when the cell has access to lactose, the lac operon is turned off

Question 11

Positive control

Answer 11

E.coli prefers glucose to lactose
when cells lack glucose, they tend to lack ATP, and have cyclic AMP
cAMP allows catabolite activator protein to bind to the promoter of the lac operon, which acts as a signal beacon to attract RNA polymerase
this turbo charges the lac operon so that lots of the lactose-metabolism enzymes are formed when the cell really needs the energy and also has access to lactose