Gene Control in Prokaryotes and Eukaryotes (Chapter 16)

Question 1

What is transcription of a gene controlled by in prokaryotes and eukaryotes?

Answer 1

Transcription factors

Question 2

What are transcription factrors?

Answer 2

Proteins that bind to a specific DNA sequence and control the flow of information from DNA to RNA by controlling the formation of mRNA

Question 3

What are structural genes?

Answer 3

Genes that code for proteins required by a cell e.g. because they form part of a cellular structure of act as an enzyme

Question 4

What are regulatory genes?

Answer 4

Genes that code for proteins that regulate the expression of other genes

Question 5

What can repressible enzyme synthesis be prevented by?

Answer 5

Binding a repressor protein to the operator region on a bacterium’s DNA

Question 6

When does inducible enzyme synthesis occur and why?

Answer 6

When its substrate is present - transcription of the gene occurs as a result of the inducer (the enzyme’s substrate) interacting with the protein produced by the regulatory gene

Question 7

What is an operon?

Answer 7

• A length of DNA making up a unit of gene expression in a bacterium
• It consists of one or more structural genes and control regions of DNA that are recognised by the products of regulatory genes

Question 8

What enzyme hydrolyses lactose to glucose and galactose?

Answer 8

Beta-galactosidase

Question 9

How does the number of molecules of beta-galactosidase present in a bacterial cell vary?

Answer 9

• It varies according to the [lactose] in the medium in which the bacterium is growing
• The bacterium has one copy of the gene coding for the enzyme and ∴ to alter its concentration in the cell, it must regulate transcription of the gene

Question 10

What is the lac operon made up of?

Answer 10

A cluster of 3 structural genes and a length of DNA including operator and promoter regions

Question 11

What are the 3 structural genes in the lac operon and what do they code for?

Answer 11

1) lacZ - codes for beta-galactosidase
2) lacY - codes for permease which allows lactose to enter cells
3) lacA - codes for transacetylase

Question 12

What is close to the promoter but not part of the operon?

Answer 12

Its regulatory gene

Question 13

Explain what happens when there is no lactose present

Answer 13

1) the regulatory gene codes for a repressor protein
2) the repressor protein binds to the operator region, close to the gene for beta-galactosidase
3) in the presence of a bound repressor at the operator, RNA polymerase cannot bind to the promoter of the DNA
4) ∴ no transcription of the 3 structural genes can take place

Question 14

Why does the binding of lactose affect the repressor protein?

Answer 14

Because the repressor protein has two binding sites (it is allosteric) ∴ when it binds to a molecule at one site, this affects it ability to bind to a different molecules at the other binding site

Question 15

What are the two separate binding sites of the repressor protein?

Answer 15

One site binds to DNA (operator region) , the other one binds to lactose

Question 16

Why does lactose inhibit the repressor protein?

Answer 16

When lactose binds to its site, the shape of the protein changes so that the DNA-binding site is closed

Question 17

Explain what happens when lactose is present

Answer 17

1) lactose is taken up by the bacterium
2) lactose binds to the repressor protein, distorting its shape and preventing it from binding to DNA at the operator site
3) transcription is no longer inhibited ∴ mRNA is produced from the 3 structural genes - the genes have been switched on and are transcribed together

Question 18

What are the benefits of the mechanism of the lac operon?

Answer 18

1) it allows the bacterium to produce beta-galactosidase, permeate and transacetylase only when lactose is available
2) it allows the enzymes to produced in equal amounts
3) it avoids the waste of energy and materials in producing enzymes for taking up and hydrolysing a stager that the bacterium may never meet
4) sugar can be hydrolysed when it is available

Question 19

What kind of enzyme is beta-galactosidase?

Answer 19

Inducible - bc sugar can be hydrolysed when it is available

Question 20

What sugar does the bacterium use in preference to other sugars and what is the effect of this?

Answer 20

Glucose ∴ when a bacterium finds both glucose and lactose in the medium in which it is growing, it represses the use of lactose by suppressing the lac operon by means of a different transcription control factor

Question 21

Give a summary of how the lac operon works

Answer 21

• When the lac repressor protein is attached to the operator gene, RNA polymerase cannot attach to the DNA
• If lactose is present, it binds to the lac repressor protein, which is detached from the DNA, allowing RNA polymerase to bind and transcribe the operon’s structural genes

Question 22

What is the effect of transcription factors when they bind to the promoter region of a gene?

Answer 22

To increase or decrease transcription of the gene

Question 23

What is the role of transcription factors?

Answer 23

To make sure that genes are expressed in the correct cell at the correct time and to the correct extent

Question 24

What are the 6 effects of transcription factors in eukaryotes?

Answer 24

1) general transcription factors that are necessary for transcription to occur - they form part of a the complex that binds to the promoter region of the gene concerned
2) activation of appropriate genes in sequence, allowing correct pattern of development in body regions
3) determination of sex in mammals
4) allowing responses to environmental stimuli e.g. high temperatures
5) regulating the cell cycle, growth and apoptosis
6) hormones have their effect through transcription factors

Question 25

How does GA control seed germination?

Answer 25

By stimulating the synthesis of amylase

Question 26

How does GA stimulate the synthesis of amylase?

Answer 26

1) application of GA causes an increase in the transcription of mRNA coding for amylase
2) GA has this effect by binding to a receptor and an enzyme, initiating the breakdown of DELLA proteins which inhibit the binding of PIF (a transcription factor) to a gene promoter
3) by causing the breakdown of DELLA, GA allows PIF to bind to its target promoter
4) transcription of the gene can then take place, resulting in an increase in amylase production

Question 27

Summarise how GA controls gene transcription

Answer 27

1) PIF cannot bond to a gene promoter while it is bound to DELLA
2) GA bonds with a receptor and an enzyme, initiating the breakdown of DELLA
3) PIF can now bind with the promoter and transcription can be initiated