organisation of the prokaryotic genome

Question 1

what is the definition of an operon?

Answer 1

• an operon is a unit of prokaryotic gene expression & regulation

Question 2

what does an operon consist of?

Answer 2

• an operon consists of a cluster of structural genes that code for enzymes of a single biochemical pathway of related functions

Question 3

what are the structural genes in an operon controlled by?

Answer 3

• a common promoter & a common operator!

Question 4

what binds to the operator of the operon?

Answer 4

• the product of the regulatory gene of the operon

Question 5

what are the 4 regions in a typical bacterial operon?

Answer 5

• promoter
• operator
• structural genes
• terminator

Question 6

what are the control elements in an operon?

Answer 6

• promoter

- operator

Question 7

what is the definition of a promoter?

Answer 7

• it is a sequence of DNA where RNA polymerase recognises & attaches to initiate transcription

Question 8

what is the function of a promoter?

Answer 8

• it specifies the start site of transcription

Question 9

do different operons have the same promoter sequences?

Answer 9

• NO

Question 10

do all bacteria have the same promoter sequence?

Answer 10

• NO!!

Question 11

where is the promoter located in the operon?

Answer 11

• it is located upstream of the structural genes in the operon

Question 12

what are the 2 recognition sites in the promoter for RNA polymerase?

Answer 12

• RNA polymerase recognition site (-35 region)

- RNA polymerase binding site / Pribnow box (-10 region)

Question 13

what are the features of the RNA polymerase recognition site?

Answer 13

• it has a consensus sequence of 5’ TTGACA 3’ at the -35 region of the non-template strand

Question 14

what purpose does the consensus sequence of the promoter serve?

Answer 14

• the sigma factor subunit of the RNA polymerase will recognise the consensus sequence before transcription occurs

Question 15

what are the features of the Pribnow box (-10 region)?

Answer 15

• it has a consensus sequence of 5’ TATAAT 3’ at the -10 region of the non-template strand

Question 16

what purpose does the consensus sequence on Pribnow box serve?

Answer 16

• the region is AT-rich, and is where the RNA polymerase begins to unwind & separate DNA strands for transcription

Question 17

are there any basal transcription factors involved in prokaryotic transcription?

Answer 17

• NO !!!!

Question 18

if basal transcription factors are not involved in prokaryotic transcription, then what first recognises & binds to the promoter?

Answer 18

• the sigma factor!!

Question 19

where is the start site of prokaryotic transcription?

Answer 19

• +1

Question 20

what does the term ‘operator’ mean?

Answer 20

• the operator is a short DNA sequence that a repressor protein binds to, hence regulating the transcription of structural genes

Question 21

what is the function of the operator?

Answer 21

• it regulates the transcription of structural genes by controlling the access of RNA polymerase to the promoter

Question 22

where is the operator located in the operon?

Answer 22

• between the promoter & the 1st structural gene

Question 23

what happens when the repressor is not bound to the operator?

Answer 23

• RNA polymerase can bind to the promoter,
• & transcribe the structural genes of the operon
• thus, the operon is switched on

Question 24

what happens when the repressor is bound to the operator?

Answer 24

• the repressor blocks the attachment of RNA polymerase to the promoter,
• preventing the transcription of structural genes
• thus, the operon is switched off

Question 25

what is the difference between prokaryotes & eukaryotes in terms of the control elements regulating transcription?

Answer 25

- in eukaryotes, the silencer of a gene is located a few thousand nucleotides upstream from the gene to be transcribed, - while in prokaryotes, the operator is located next to the 1st structural gene in the operon

Question 26

what does the term 'structural genes' mean?

Answer 26

- a structural gene codes for a protein that forms part of a structure, - OR has an enzymatic function?

Question 27

where are structural genes of an operon located?

Answer 27

- the structural genes of an operon usually lie adjacent to one another

Question 28

how is a single polycistronic mRNA formed?

Answer 28

- RNA polymerase moves from 1 structural gene to the next, continuously transcribing all the genes into a single polycistronic mRNA

Question 29

what happens to the polycistronic mRNA after it is translated?

Answer 29

- it is translated into several distinct polypeptides that become various proteins / enzymes of a metabolic pathway

Question 30

what is the definition of a regulatory gene?

Answer 30

- a regulatory gene codes for a specific protein product that regulates the expression of the structural genes?

Question 31

where is the regulatory gene found?

Answer 31

- upstream of an operon

Question 32

does each regulatory gene have its own promoter?

Answer 32

- YES!!

Question 33

are there introns in prokaryotic mRNA?

Answer 33

- THEY ARE ABSENT

Question 34

what causes termination of transcription?

Answer 34

- a terminator at the end of each operon

Question 35

what are present upstream of each operon?

Answer 35

- regulatory genes, which code for regulatory proteins that control the switching on & off of the operon

Question 36

why are genes clustered into operons in bacterial DNA?