Lecture 6- Regulation of Gene expression Flashcards
structure of a gene
-contains a promoter, coding sequence, and a terminator
Within the promoter
- is the controlling site
- where induction (increased expression of a gene) occurs
Operon
-regulates genes
-one promoter serving several genes
ex- typtophan and lactose
Operator
- the operon’s switch for controlling transcription
- where the repressor binds
- determines whether RNA polymerase binds to the promoter and produces proteins or not
Repressor
- switches the operon off
- blocks attachment of RNA polymerase, therefore blocks transcription
- is specific
- is allosteric: 2 different shapes, active and inactive
co-repressor
molecule that binds to repressor to switch an operon off
-tryptophan
Tryptophan absent
- RNA polymerase binds to promoter
- transcribes mRNA to make more tryptophan
- operon on, repressor off
Tryptophan present
a lot of tryptophan-> tryptophan acts as a co-repressor
- tryptophan binds to repressor and activates it
- repressor attaches to operator
- RNA polymerase can no longer transcribe
Tryptophan operon
- example of repressible operon
- turned off by the end product of metabolic pathway
Lactose operon
- example of an inducible operon
- usually inactive
- can be turned on by the precursor/first chemical of the metabolic pathway
Lactose not present
- repressor binds to operator and blocks RNA polymerase
- mRNA cannot be made and proteins/enzymes cannot be made
Lactose present
- lactose binds to repressor, changing repressor’s shape and making it unable to bind to operator
- RNA polymerase binds to promoter and transcribes to make mRNA
- mRNA is used to make the enzymes that break down lactose
Regulatory genes
- codes for production of allosteric repressor protein that can switch off operon
- ex: lacI
Structural genes
ex- the enzymes for lactose, break down lactose
Mutations
- a permanent change in DNA
- source of new genes
Mutagens
-physical and chemical agents that promote mutations ex: -ionizing radiation -ultraviolet radiation -chemical mutagens -pyrimidine dimer -spontaneous mutations
Point mutation
- change in single nucleotide pair of a gene
- change of a single nucleotide in the DNA’s TEMPLATE STRAND leads to altered mRNA and abnormal protein
- some have no effect on the encoded protein due to the redundancy of the genetic code
Silent mutation
- no effect on amino acid sequence
- no observable effect on phenotype
Missense mutation
- changes one amino acid
- may have little effect, or big effect
Nonsense mutation
- causes translation to be ended early
- leads to a non functional protein
Insertion
additions of nucleotide pairs in a gene
Deletions
removal of nucleotide pairs in a gene
Insertions and Deletions
both alter the reading frame of mRNA
-more disastrous effect
Frame-shift mutation
- when insertion/deletion is not a multiple of three
- results in extreme missense mutations
- non functional protein
