16.3 Gene control

Question 1

Transcription Factors

Answer 1

proteins that bind to a specifc DNA sequence and control the flow of information from DNA to RNA by controlling the formation of mRNA.

Question 2

Structural Genes

Answer 2

Genes that code for proteins required by a cell

Question 3

Regulatory Genes

Answer 3

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

Question 4

Inducible Enzymes

Answer 4

Enzyme production only occurs when its substrate is present

Question 5

Repressible Enzymes

Answer 5

The presence of a particular substance allows the repressor to bind with the operator and stops the enzyme being produced

Question 6

Operon

Answer 6

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

Question 7

Most studied example of gene expression control

Answer 7

production of the enzyme lactase (β-galactosidase) in the bacterium Escheria Coli
- enzyme β-galactosidase hydrolyses the disaccharide
lactose to the monosaccharides glucose and galactose.

Question 8

Is lactase an inducible or repressible enzyme?

Answer 8

Inducible

- Bacterium only produces lactase when the sugar lactose is present

Question 9

Structure of Lac Operon

Answer 9

1) Promoter for regulatory gene
2) Regulatory Gene
3) Promoter for structural genes
4) Operator
5) Structural Genes

Question 10

Promoter

Answer 10

allows transcription of the structural genes to begin

Question 11

Operator

Answer 11

allows a molecule called a transcription factor to bind, which in this case stops the structural genes being transcribed

Question 12

The 3 Structural Genes

Answer 12

• lacZ, coding for β-galactosidase
• lacY, coding for permease (which allows lactose to enter the cell)
• lacA, coding for transacetylase.

Question 13

Regulatory Gene

Answer 13

codes for a protein called a repressor

Question 14

Lac Operon when lactose is present

Answer 14

• the lactase enzyme is inducible; its production occurs only when lactose is present
• When there is lactose in the medium, it binds to the repressor protein (allosteric site) and alters its shape.
• This prevents the repressor protein from binding to the operator region.
• Now RNA polymerase can bind to the promoter, and the structural genes can be transcribed.

Question 15

Lac Operon when lactose is not present

Answer 15

• When there is no lactose in the medium in which the bacterium is growing, the regulatory gene is expressed and therefore the repressor protein is produced.
• This repressor protein binds to the promoter for the structural genes, so they are not transcribed and no lactase is made.

Question 16

What happens when both glucose and lactose is present in the medium

Answer 16

• Glucose has a higher preference
• When a bacterium f nds 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
  dif erent transcription control factor.

Question 17

Gene control in Eukaryotes

Answer 17

• the bigger the size of the genome, the more transcription factors
• This means that Eukaryotes have many more
  ways of regulating gene expression than have Prokaryotes.
• The factors may bind to the promoter region of a gene.
• They may increase or decrease the transcription of the
  gene.
• Whatever the mechanism, their role is to make sure that genes are expressed in the correct cell at the correct time and to the correct extent.

Question 18

Effects of Transcription Factors

Answer 18

1) Necessary for transcription to occur. They form part of the protein complex that binds to the promoter region of the gene concerned.
2) factors activate appropriate genes in sequence,
allowing the correct pattern of development of body regions.
3) responsible for the determination of sex in mammals.
4) allow responses to environmental stimuli, such as switching on the correct genes to respond to high environmental temperatures.
5) Some transcription factors regulate the cell cycle, growth and apoptosis (programmed cell death)
6) Affected by hormones

Question 19

Example of Gene expression in Eukaryotes

Answer 19

• The plant hormone, gibberellin, controls seed germination by stimulating the synthesis of amylase
• a good example of how a hormone can influence transcription.
• It has been shown that, in barley seeds, application of
  gibberellin causes an increase in the transcription of
  mRNA coding for amylase.

Question 20

Gibberellin Hormone effect in plants

Answer 20

1) production of the enzyme amylase in germinating seeds depends on a transcription factor called PIF binding with the promoter region next to the gene that codes for amylase
2) When no gibberellin is present, PIF is bound to a protein called a DELLA protein. This prevents PIF from binding with the promoter for the amylase gene, so no amylase is made
3) When gibberellin is present, it binds to a receptor and an enzyme. This activates the enzyme, which breaks down the DELLA protein. PIF is now free to bind with the promoter for the amylase gene, and amylase is made