section 1: cellular control - topic 1: regulating gene expression

Question 1

what are the 3 levels gene expression can be controlled at?

Answer 1

transcriptional.
post-transcriptional.
post-translational.

Question 2

how can gene expression be controlled at the transcriptional level?

Answer 2

by altering the rate of transcription of genes.

i.e. increased transcription = more mRNA = make more protein

Question 3

what is transcriptional level control controlled by?

Answer 3

transcription factors.

Question 4

what are transcription factors?

Answer 4

proteins that bind to DNA and switch genes on or off by increasing or decreasing the rate of transcription.

Question 5

what are factors that start transcription called?

Answer 5

activators.

Question 6

what are factors that stop transcription called?

Answer 6

repressors.

Question 7

what does the shape of a transcription factor determine?

Answer 7

whether it can bind to DNA or not.

Question 8

what can the affect the shape of a transcription factor?

Answer 8

can sometimes be altered by the binding of some molecules i.e. certain sugars & hormones.

Question 9

what do transcription factors do in prokaryotes?

Answer 9

control of gene expression often involves transcription factors binding to operons.

Question 9

what do transcription factors do in eukaryotes?

Answer 9

they bind to specific DNA sites near the start of their target genes - these are the genes they control the expression of.

Question 10

what is an operon?

Answer 10

a section of DNA that contains a cluster of structural genes that are all transcribed together, as well as control elements and sometimes a regulatory gene.

Question 11

what are the control elements?

Answer 11

a promoter.
an operator.

Question 12

what do the structural genes code for?

Answer 12

useful proteins, such as enzymes.

Question 13

what is a promoter?

Answer 13

a DNA sequence located before the structural genes that RNA polymerase binds to.

Question 14

what is an operator?

Answer 14

a DNA sequence that transcription factors bind to.

Question 15

what does the regulatory gene code for?

Answer 15

an activator or a repressor.

Question 16

describe the lac operon in e.coli.

Answer 16

e.coli respires glucose, but can use lactose if glucose isn’t available.
the genes that produce the enzymes needed to respire lactose are found on the lac operon.
lac operon has 3 structural genes - lacZ, lacY, lacA - produce proteins that help the bacteria digest lactose.
(inc. B-galactosidase & lactose permease)

Question 17

how does the lac operon work when lactose isn’t present?

Answer 17

the regulatory gene (lacI) produces the lac repressor - a transcription factor that binds to the operator site when there’s no lactose present.

this blocks transcription because RNA polymerase can’t bind to the promoter.

Question 18

how does the lac operon work when lactose is present?

Answer 18

when lactose is present, it binds to the repressor - changing the repressors shape so that it can no longer bind to the operator site.

RNA polymerase can now begin transcription of the structural genes.

Question 19

what are introns?

Answer 19

sections of genes in eukaryotic DNA that don’t code for amino acids.

Question 20

what are exons?

Answer 20

all the bits that do code for amino acids.

Question 21

what happens to the introns and exons of during transcription of mRNA?

Answer 21

they are both copied into the mRNA.
mRNA strands containing them both are called primary mRNA transcripts.
introns are removed from primary mRNA strands by splicing - introns are removed and the exons are joined together to form mature mRNA strands.
^ this takes place in the nucleus.
the mature mRNA then leaves the nucleus for translation.

Question 22

how do some molecules that control protein activation work?

Answer 22

by binding to cell membranes and triggering the production of cyclic AMP (cAMP) inside the cell.

Question 22

what is post-translational level control?

Answer 22

where some proteins are not functional straight after translation, so they need to be activated to become a functional protein.

like protein synthesis, protein activation is also controlled by molecules i.e. hormones & sugars.

Question 23

describe the activation of protein kinase A (PKA) by cAMP.

Answer 23

PKA is an enzyme made of 4 subunits.
when cAMP isn’t bound, the 4 units are bound together and are inactive.
when cAMP binds, it causes a change in the enzymes 3D structure, releasing the active subunits.
^ PKA is now active.

Question 24

what does cAMP do?

Answer 24

it then activates proteins inside the cell by altering their 3D structure.

i.e. altering the 3D structure can change the active site of an enzyme, making it become more or less active.