Operons

Question 1

prokaryotic regulation

Answer 1

takes place at the level of transcription initiation

Question 2

prokaryotic promoter

Answer 2

• RNA polymerase binds to a recognition sequence in the promoter to begin transcription
• different classes of promoters have different recognition sequences
• contain two sites that RNA polymerase recognizes

Question 3

sigma factor

Answer 3

• aids RNA polymerase in binding to the recognition sequence
• specific sigma factors have different affinities for binding to different -10 and -35 element sequences
• different promoters have different sigma factors, so transcription of genes depends on presence of sigma factors in the cell

Question 4

housekeeping genes

Answer 4

• genes that are normally expressed in actively growing cells
• have -10 and -35 elements (refers to how many bases upstream of the transcription start site they are located)

Question 5

What benefit is provided by having different promoter
recognition sequences for different genes?

Answer 5

allows transcription factors and RNA polymerase to bind selectively enabling the regulation of gene expression

Question 6

operon

Answer 6

set of structural genes controlled by a single promoter
starts with a promoter followed by an operator

Question 7

operator

Answer 7

sequence of DNA within an operon to which a molecule can bind to either induce or repress transcription

Question 8

polycistronic mRNA

Answer 8

genes in an operon are transcribed into a single mRNA
each gene produces different information for translation (recognition sequence, stop and start codon)

Question 9

lac operon

Answer 9

controls lactose metabolism in e. coli
structural genes LacZ, LacY, LacA encode three enzymes that help the cell use lactose for energy

Question 10

lacI gene

Answer 10

located close to the lac operon
encodes a protein called the lac repressor protein

Question 11

absence of lactose

Answer 11

• lac repressor protein binds to the operator and block RNA polymerase from binding to the promoter
• lacZ, lacY, lacA not transcribed

Question 12

presence of lactose

Answer 12

• allolactose binds to the repressor protein, changing its shape and making unable to bind to the operator
• RNA polymerase able to bind

Question 13

inducible operon

Answer 13

• transcription does not occur unless a messenger molecule is present in a large enough concentration
• allolactose is the co-inducer

Question 14

negative regulation

Answer 14

lac operon controlled by a repressor protein
prevents transcription

Question 15

positive regulation

Answer 15

• inducible operon
• activator protein: cAMP receptor protein (CRP)
• encourages efficient transcription of lac operon

Question 16

CRP

Answer 16

• inactive when glucose levels are high
• must be activate by cAMP messenger molecule, produced when glucose is low or absent
• cAMP binds to CRP, changes its shape, and binds near the promoter which helps the RNA polymerase bind

Question 17

repressible operon

Answer 17

transcribes by default
presence of trp must be high enough to bind to trp repressor protein and bind to operator

Question 18

trp operon

Answer 18

under negative regulation
when trp is present in a high concentration, it binds to the repressor protein which then binds to the operator

Question 19

catabolite repression

Answer 19

• the preferred energy source reduces other catabolic pathways
• presence of glucose represses the lac operon