Control of gene expression

Question 1

what is an operon

Answer 1

cluster of genes transcribed by the same promoter to make a polycistronic mRNA

genes are usually related, proteins formed might be all part of the same metabolic pathway for example

Question 2

what are the 3 proteins produced by the genes on the 3’ end of the lac operon and their functions

Answer 2

Beta galactosidase - hydrolyse galactose -> lactose and glucose

Permease - membrane protein, allows passage of beta galactosidase

Acetylase - function uknown dw bout it

Question 3

what produces the lac repressor monomer (and subsequent tetramer)

Answer 3

the LacI gene (thats an i not an L)

Question 4

what does the lac repressor tetramer do

Answer 4

binds to operator gene
acts as main supressor

see onenote for lac operon diagram

Question 5

what is the purpose of the lac operon

Answer 5

produces proteins that break down lactose

Question 6

how does the repressor work in the lac operon

Answer 6

binds to the operator,
RNA pol still binds but isn’t allowed to move along
when lactose is present = the repressor dissociates from the operator
RNA pol is allowed to move along and make the mRNA = hence the proteins are made

Question 7

how does the repressor in the lac operon demonstrate negative feedback

Answer 7

operator repressed
lactose becomes present
operator no longer repressed
glactosidase protein made
hydrolyses the lactose
so less lactose present
operator becomes repressed again

Question 8

what are trans-acting genes

Answer 8

genes that produce proteins that act as transcription factors

Question 9

what are trans acting factors

Answer 9

transcription factors generally called tranacting factors

it can diffuse across (trans)
and bind to specific DNA sequences
can wither activate or repress transcription

can regulate any gene anywhere when they can bind to the sequence

recessive

Question 10

what is a cis acting sequences

Answer 10

sequences to which transacting factors bind

e.g. promoters, operators

called cis acting cuz they have to be right next to the genes that they regulate

dominant

Question 11

mutation in lac operon - LacOc

Answer 11

c stands for constitutive = always activated

mutation in sequence of operator
means that repressor can’t bind
so gene always on and producing proteins

if we introduce a new LacI gene to make new repressors, would have no effect - cuz cis acting sequences are dominant
cannot be complemented

Question 12

mutations in lac operon - LacI

Answer 12

mutated lacI gene - won’t produce repressor or will produce non-functioning repressor

so operon is consitutive - always on

but

if introduce a wild type LacI that is functioning, it will solve the problem
repressor binds, functions as normal

hence trans-recessesive mutation
can be complemented - system can be restored

Question 13

what is catabolite repression

Answer 13

organisms prefer to use glucose before any other sugar
so several operons are repressed by glucose and only become active when glucose is used up

Question 14

2 conditions that need to be met for RNA to be transcribed in lac operon

Answer 14

CRP must bind to the promoter near RNA polymerase

lactose needs to be bound to repressor

Question 15

what is CRP

Answer 15

Catabolite Repressor Protein
(or catabolite activator protein)

ensure that glucose is the prefered carbon source

Question 16

what is cAMP

Answer 16

small nucleotide molecule that makes the CRP bind to the DNA (or not depending on glucose availability)

Question 17

how does catabolite repression work using CRP and cAMP

Answer 17

high glucose = low cAMP, so CRP cannot bind to DNA even if lactose available

low glucose = high cAMP, allows CRP to bind, allows the operon to be transcribed (if lactose is available ofc)

Question 18

TRP operon

Answer 18

responsible for synthesis of tryptophan, an important amino acid

see onenote for its structure in more detail

has an operator, same as lac operon

has a repressor, but acts in a slightly diff way

Question 19

how does the TRP repressor act

Answer 19

if tryptophan present, then operon needs to be off

is tryptophan not present, then operon needs to be on

hence, tryptophan is used as a co-repressor alongside trp repressor

when tryptophan binds to trp repressor, the repressor associates to operon, so tryptophan not produced

vice versa

Question 20

Key difference between the two operons

Answer 20

trp operon - REPRESSED by its PRODUCT
lac operon - INDUCED by SUBSTRATE