16.3.1 Gene control - Lac operon (part of inheritance)

Question 1

The nucleus of every cell in the human body contains the same ..?

Answer 1

genes

Question 2

What is not expressed in every cell?

Answer 2

every gene
- Not all of these genes are expressed all the time

Question 3

There are several mechanisms that exist within cells to make sure the correct genes are expressed in the correct cell at the right time.
What do they involve?

Answer 3

regulatory genes

Question 4

What is a structural gene?

Answer 4

codes for a protein that has a function within a cell
For example, the F8 gene codes for the protein Factor VIII involved in blood clotting

Question 5

What is a regulatory gene?

Answer 5

codes for a protein that helps to control the expression of another gene

Question 6

Structural and regulatory genes that work together are usually found..?

Answer 6

close together

Question 7

When are inducible enzymes only synthesized?

Answer 7

when their substrate is present

The presence of the substrate induces the synthesis of of the enzyme by causing the transcription of the gene for the enzyme to start

Question 8

repressible enzymes synthesized as normal. Until?

Answer 8

repressor protein binds to an operator

The presence of the repressor protein represses the synthesis of the enzyme by causing the transcription of the gene for the enzyme to stop

Question 9

Why can controlling synthesis of enzymes be beneficial for cells?

Answer 9

as it stops materials and energy being wasted

For example, using materials and energy to synthesize an enzyme when its substrate is not present and it can’t carry out its function would be highly wasteful

Question 10

What do regulatory genes control?

Answer 10

• control structural genes and their levels of protein production
• sometimes have control over several structural genes at once

Question 11

What do regulatory genes control?

Answer 11

• control structural genes and their levels of protein production
• sometimes have control over several structural genes at once

Question 12

What can structural genes in prokaryotes form?

Answer 12

an operon

Question 13

What is an operon?

Answer 13

a group or a cluster of genes that are controlled by the same promoter

Question 14

Which operon is found in some bacteria which is the most well known?

Answer 14

lac operon

Question 15

what does the lac operon control the production of?

Answer 15

the enzyme lactase (also called β-galactosidase) and two other structural proteins

Question 16

What does lactase break down?

Answer 16

the substrate lactose so that it can be used as an energy source in the bacterial cell

Question 17

What type of enzyme is lactase?

Answer 17

inducible enzyme that is only synthesized when lactose is present

This helps prevent the bacteria from wasting energy and materials

Question 18

Structure of the lac operon
The components of the lac operon are found in the following order:

Answer 18

• Promoter (for structural genes)
• Operator
• lac Z
• lac Y
• lac A

Question 19

What is a promoter?

Answer 19

A region of DNA required to allow transcription of a gene to take place

Question 20

what does structural gene lacZ code for?

Answer 20

lactase

Question 21

What does structural gene lacY code for?

Answer 21

permease (allows lactose into cell)

Question 22

What does structural gene lacA code for?

Answer 22

transacetylase

trans-acet-y-lase

Question 23

Located to the left (upstream) of the lac operon on the bacterium’s DNA there is also the:

Answer 23

Promoter for regulatory gene
Regulatory gene lacI that codes for the lac repressor protein

Question 24

How many binding sites does the lac repressor protein have?

Answer 24

• 2 binding sites that allow it to bind to the operator in the lac operon and also to lactose (the effector molecule)

Question 25

When the lac repressor protein binds to the operator: describe events

Answer 25

it **prevents the transcription of the structural genes** as **RNA polymerase cannot attach to the promoter**

Question 26

When the lac repressor protein bind to lactose: describe event

Answer 26

the **shape** of the repressor protein **distorts** and it can **no longer bind to the operator**

Question 27

When lactose is absent The following processes take place when lactose is absent in the medium that the bacterium is growing in:

Answer 27

- The regulatory gene is transcribed and translated to produce lac repressor protein - The **lac repressor protein binds to the operator** region upstream of lacZ - Due to the **presence** of the repressor protein **RNA polymerase is unable to bind to the promoter region** - **Transcription** of the structural genes **does not** take place - **No lactase enzyme is synthesized**

Question 28

When lactose is present The following processes take place when lactose is present in the medium that the bacterium is growing in:

Answer 28

- There is an uptake of lactose by the bacterium - The **lactose binds to the second binding site** on the repressor protein, **distorting its shape** so that it **cannot** bind to the operator site - **RNA polymerase** is then **able to bind to the promoter region** and **transcription takes place** - The mRNA from all three structural genes is translated -**Enzyme lactase is produced** and lactose can be broken down and used for energy by the bacterium

Question 29

What do prokaryotes use to control the expression of genes in cells?

Answer 29

operons

Question 30

What do Eukaryotes use to control gene expression?

Answer 30

transcription factors

Question 31

What is a transcription factor?

Answer 31

is a **protein** that **controls the transcription of genes** by **binding to a specific region of DNA**

Question 32

What do transcription factors insure?

Answer 32

They ensure that **genes are being expressed in the correct cells**, at the **correct time and to the right level**

Question 33

What do transcription factors allow organisms allow organisms to respond to?

Answer 33

their environment

Question 34

How transcription factors work

Answer 34

**Some** transcription factors **bind to the promoter region of a gene** This binding can either **allow** or **prevent** the transcription of the gene **from taking place**

Question 35

the presence of a transcription factor will either..?

Answer 35

**increase or decrease the rate of transcription of a gene** For example, **PIF** is a transcription factor found in **plants** that activates the transcription of the amylase gene

Question 36

What do plant cells use in a similar way to animal cells?

Answer 36

transcription factors

Question 37

What is Gibberellin?

Answer 37

- a **hormone found in plants** (eg wheat and barley) that **controls seed germination** by stimulation the **synthesis** of the **enzyme amylase**

Question 38

How does giberellin control seed germination by stimulating the synthesis of the enzyme amylase?

Answer 38

It does this by **influencing transcription of the amylase gene** When gibberellin is applied to a germinating seed there is an increased amount of the mRNA for amylase present

Question 39

What is necessary for the synthesis of amylase?

Answer 39

**the breakdown of DELLA protein by gibberellin**

Question 40

The process of the breakdown of DELLA protein by gibberellin

Answer 40

- DELLA protein is **bound** to PIF, **preventing** it from **binding to the promoter** of the amylase gene so **no transcription can occur** - Gibberellin binds to a gibberellin receptor and enzyme which **starts the breakdown of DELLA** - **PIF** is **no longer bound to DELLA** protein and so it **binds to the promoter** of the amylase gene - Transcription of amylase gene begins -Amylase is produced