Lecture 17

Question 1

If all cells have the same genes, how do they have different functions?

Answer 1

The genes within the cell are regulated so that different genes are expressed to produce different proteins with different functions.

Question 2

What part of the gene is the essential location for transcriptional regulation?

Answer 2

Promoter

Question 3

What is an operon? Do both prokaryotes and eukaryotes have operons?

Answer 3

An operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. They re predominantly found in prokaryotes.

Question 4

How many mRNAs are produced from one operon? How many polypeptides?

Answer 4

One mRNA is produced from one operon. More than one polypeptide may be produce depending on the operon.

Question 5

What is a promoter?

Answer 5

A sequence of DNA that is recognised by RNA polymerase to initiate the transcription of the gene.

Question 6

What is an operator?

Answer 6

An operator is a segment of DNA to which a transcription factor binds to regulate gene expression.

Question 7

What is the function (overall) of the gene in the trp operon?

Answer 7

The function of the gene in the trp operon is to produce more tryptophan.

Question 8

What binds to the promoter of the trp operon? What binds to the operator?

Answer 8

RNA polymerase may bind to the promoter to initiate transcription. Regulatory proteins (repressors/activators) can bind to the operator.

Question 9

What is produced by the trpR gene? What is the function of this protein?

Answer 9

trpR gene produces a Trp repressor (inactive) that is able to bind to the operator (when activated) on the trp operon to prevent to production of tryptophan.

Question 10

What molecule binds to the TrpR protein? Is the TrpR functional with or without this molecule?

Answer 10

Tryptophan binds to the TrpR protein for it to be activated.

Question 11

If tryptophan is present, are the trp genes expressed? Is an mRNA product? Are proteins produced?

Answer 11

If tryptophan is present, the trpR gene is transcribed and translated to produce a protein (activated by tryptophan) that will inhibit the expression of the trp gene.

Question 12

If a mutation in the promoter completely disrupts the RNA polymerase binding site, are the genes in the trp operon transcribed, translated, both or neither?

Answer 12

Neither

Question 13

If a mutation disrupts a critical codon in the trpE gene, does transcription occur, translation occur, and is tryptophan produced? Is trpE transcribed or translated?

Answer 13

All the genes on the trp operon will be transcribed and translated (except: trpE) Since, the genes make up enzymes for tryptophan synthesis, it would depend on what substrate is available for tryptophan to be produce.

Question 14

If a mutation disrupts a critical codon in the trpR gene, are the genes in the trp operon expressed?

Answer 14

Yes

Question 15

If a mutation critically disrupts the operator sequence of the trp promoter, but RNA polymerase can still bind, are the genes in the trp operon expressed in the presence and absence of tryptophan?

Answer 15

The trp operon will be expressed in both the absence and presence of tryptophan.

Question 16

If a base pair insertion occurs in the promoter, but this does not affect the binding of RNA polymerase or the repressor, is there a frameshift in the coding sequence?

Answer 16

NO.

Question 17

If a base pair insertion occurs in the trpE coding sequence, are the genes transcribed, or translated? Is there a frameshift only in trpE, or a frameshift mutation in all proteins?

Answer 17

The frameshift mutation would only affect trpE and not the other proteins.

Question 18

What is the function of the genes in the lac operon? (in general)

Answer 18

It produces proteins (enzymes) that assist in the hydrolysis and isomerisation of lactose to produce glucose and allolactose.

Question 19

What is the operator for?

Answer 19

The operator is a sequence of DNA that allows the bind of regulatory proteins to regulate the expression of genes.

Question 20

What protein is produced by the lacI gene, and what does it do?

Answer 20

It produces a Lac repressor (active) that is able to bind to the operator to prevent the synthesis of proteins.

Question 21

What molecule binds to the lac repressor protein? Where does this molecule come from (why is it around)? Does the binding of this molecule make the protein more active or less active?

Answer 21

Allolactose binds to the protein and make it less active. Allolactose is produced by the isomerisation of lactose.

Question 22

Is the lac operon expressed in the presence or absence of lactose?

Answer 22

Presence of lactose

Question 23

Which of the trp operon and lac operon is repressible and which is inducible? Why are they called this?

Answer 23

The trp operon is repressible while the lac operon is inducible. The trp operon is always switched “on” by default and needs to be repressed when needed. The lac operon is always switched “off” and needs to be induced when needed.

Question 24

What does it mean to say that a gene is regulated with combinatorial control?

Answer 24

There are usually several important regulators involved in the regulation of a single gene.

Question 25

Do each of the trp and lac operons show negative regulation or positive regulation?

Answer 25

Both show negative regulation. (where a repressor binds to prevent the synthesis of the gene)

Question 26

What is cAMP?

Answer 26

Cyclic AMP (cAMP) is a modification of ATP that accumulates in low glucose.

Question 27

What is CAP?

Answer 27

CAP is a activator that when active binds to its binding site in the promoter region of the gene and stimulate the transcription of the gene.

Question 28

Where does CAP bind?

Answer 28

CAP binds to the CAP-binding site on the promoter region.

Question 29

What does the bind of CAP achieve?

Answer 29

It stimulates increase levels of transcription.

Question 30

Is the lac operon expressed at high levels when there is low glucose and high lactose?

Answer 30

The lac operon is expressed at high levels.

Question 31

Is the lac operon

expressed when there is high glucose and high lactose?

Answer 31

The lac operon is expressed at a lower rate.

Question 32

When there is high glucose, is there high or low

cAMP?

Answer 32

There is low cAMP.

Question 33

What does binding of the lacI repressor do to the DNA of the lac promoter region?

Answer 33

It causes the promoter region to form a loop that prevents the RNA polymerase from binding.

Question 34

What are the characteristics of the DNA sequence of the lac operator that make it likely to be bound by a dimer? (note: for lacI these dimers then bind together in a tetramer)

Answer 34

The ends are reverse complementary of each other on the same strand.