LIFESCI 4 Lecture Notes - Lecture 15: Repressor, Permease, Cistron
LIFESCI 4 - Lecture 15 - Ames Test and Lac Operon
Clicker 1: What is a mutation?
A) A change in DNA sequence that causes a change in phenotype
B) A change in DNA sequence that causes a change in protein or RNA sequence
C) A change in DNA sequence
Explanation:
- Mutation in DNA does not guarantee that it will be expressed in the phenotype or RNA
sequence
Clicker 2
Experiment: Expose bacteria to bactericide. Wild-type cells do not survive, but there are some
colonies on the plate. Thes colonies have mutations that allow them to grow on bactericide.
How did the mutations arise?
A) Exposure to bactericide causes mutations - create cells that are resistant
B) Mutations happen spontaneously - some mutations just happen to make cells
resistant to bactericide.
Explanation:
- Mutations randomly happen → help cell resistant to bactericide
How Does Bactericide Resistance Arise?
- First hypothesis: resistance is a response to bactericide
- Mutations caused by exposure to drug
- Second hypothesis: resistance arises from random mutations
- Already resistant before exposure to the drug → mutation not caused by drug
- This was supported by the experiment
Ames Test for Mutagenicity
Clicker: A mutagen is anything that increases the frequency of mutations. The Ames test is a
sensitive assay for mutagens
- Why add rat liver enzymes?
A) Everything's better with enzymes
B) To simulate mammalian metabolic processes
C) It provides amino acids
D) It preserves chemicals
Explanation:
- Function of liver: get rid of toxins
- Convert one chemical to another
- Liver enzymes placed to simulate mammalian metabolic processes
Clicker:
Are the colonies on the experimental plate likely to be revertants?
A) Yes
B) No
Explanation:
- For the colonies to grow on a plate without the amino acids, it must be aa+
- All needs to be revertants (be able to make aa+)
- With no mutagen/carcinogen, there are much more colony growth
- All should be revertants
- If steak are overcooked, it may become carcinogenic
Clicker 3:
- Answer: B
- Explanation:
- Negative control: expose to something unharmful
- # of colonies = 12
- Positive control: one that causes mutations
- # of colonies = 675
- Test substance with rat enzyme: high number of colonies similar to positive
control
- When consumed, it reverts to a mutagen
Lac Operon: The Lac Operon as a Model for Understanding Gene Regulation
The Lac Operon:
- Lac operon only turned on when needed
- Includes positive and negative control
- Promoter: RNA polymerase site
- Operator: Repressor site
- Structural gene: lacZ, Y, A sequence
- lacI: gene coded for repressor
- Not under control of promoter
- Has its own upstream promoter
- Lac operon RNA is polycistronic: can be used to make 3 proteins in 1 sequence
Active Repressor = no transcription
- lacI synthesize and code for repressor
- Repressor binds to operator
- RNA polymerase cannot move downstream → no transcription
- This is the natural state for E.coli
Active Lac Operon = there is transcription
- Lac operon is induced
- Inducer = lactose → simulates lac operon
- Lactose acts as substrate and binds to repressor → conformational change
-Repressor becomes inactive
→ RNA polymerase can bind and transcribe sequence and
make lac mRNA
- Lac mRNA contains coding sequence for 3 proteins
Clicker 4:
Polycistronic messages in prokaryotes are often:
A) Spliced to form the mature mRNAs
B) The result of frameshift mutations
Document Summary
Lifesci 4 - lecture 15 - ames test and lac operon. Clicker 1: what is a mutation: a change in dna sequence that causes a change in phenotype, a change in dna sequence that causes a change in protein or rna sequence, a change in dna sequence. Mutation in dna does not guarantee that it will be expressed in the phenotype or rna sequence. Wild-type cells do not survive, but there are some colonies on the plate. Thes colonies have mutations that allow them to grow on bactericide. How did the mutations arise: exposure to bactericide causes mutations - create cells that are resistant, mutations happen spontaneously - some mutations just happen to make cells resistant to bactericide. Mutations randomly happen help cell resistant to bactericide. First hypothesis : resistance is a response to bactericide. Already resistant before exposure to the drug mutation not caused by drug. Clicker: a mutagen is anything that increases the frequency of mutations.
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