19.2 Control of Gene Expression

Question 1

What do housekeeping genes code for? (1 mark)

Answer 1

Enzymes that are necessary for reactions present in metabolic pathways, like respiration, are constantly required.

Question 2

If every cell in an organism contains a complete set of the organism’s genes, why do they have different functions? (1 mark)

Answer 2

Only the genes needed for a cell’s specialised functions are switched on, the rest are switched off.

Question 3

State 4 ways genes can be regulated. (4 marks)

Answer 3

Transcriptional
Post-transcriptional
Translational
Post-translational

Question 4

What is a transcriptional factor? (1 mark)

Answer 4

A protein that binds to DNA and switches genes on or off by increasing or decreasing the rate of transcription.

Question 5

How does a transcriptional factor for a gene that is not to be expressed differ from one that is expressed? (2 marks)

Answer 5

The site on the transcription factor that binds to the DNA is blocked by an inhibitor molecule, so transcription is prevented.

Question 6

What is heterochromatin, and where is it found in the cell cycle? (2 marks)

Answer 6

Tightly wound DNA
Cell divison

Question 7

What is euchromatin, and where is it found in the cell cycle? (2 marks)

Answer 7

Loosely wound DNA
Interphase

Question 8

Describe how chromatin remodelling can affect the transcription of genes. (2 marks)

Answer 8

Transcription of genes is not possible in heterochromatin because RNA polymerase cannot access the gene, but it is possible in euchromatin.

Question 9

Are histones positively or negatively charged? (1 mark)

Answer 9

Positive

Question 10

Is DNA positively or negatively charged? (1 mark)

Answer 10

Negatively

Question 11

What is the relation between DNA and histones? (1 mark)

Answer 11

DNA coils around histones.

Question 12

How can histones be modified to increase the degree of packing? (2 marks)

Answer 12

Addition of methyl groups, makes them more hydrophobic, DNA coils more tightly.

Question 13

How can histones be modified to decrease the degree of packing? (2 marks)

Answer 13

Addition of acetyl or phosphate groups, decreases positive charge, DNA coils less tightly.

Question 14

What is the term for the control of gene expression by modification of DNA? (1 mark)

Answer 14

Epigenetics

Question 15

What does epigenetics mean? (1 mark)

Answer 15

The control of gene expression by modification of DNA.

Question 16

What is an operon? (2 marks)

Answer 16

A section of DNA that contains a cluster of genes which are controlled by a single promoter (expressed t the same time).

Question 17

What is a structural gene? (1 mark)

Answer 17

Codes for useful proteins, like enzymes.

Question 18

In E.Coli, what substrate is used for respiration when glucose is not available? (1 mark)

Answer 18

Lactose

Question 19

What is the lac operon? (2 marks)

Answer 19

A group of three structural genes that code for three enzymes and are transcribed onto a single long molecule of mRNA.

Question 20

What are the three structural genes in the lac operon? (3 marks)

Answer 20

Beta-galactosidase
Lactose permease
Transacetylase

Question 21

What is a regulatory gene? (1 mark)

Answer 21

Codes for a repressor protein.

Question 22

What is a repressor gene? (1 mark)

Answer 22

Prevents transcription of the structural genes.

Question 23

When and how does a repressor gene prevent transcription of the three structural genes in the lac operon? (2 marks)

Answer 23

When lactose is not present.
It binds to the operator, preventing RNA polymerase from binding and transcribing the genes.

Question 24

What does ‘promoter’ mean? (1 mark)

Answer 24

The section of DNA that the binding site for RNA polymerase.

Question 25

How is transcription of the lac operon up-regulated? (2 marks)

Answer 25

By cAMP receptor protein (CRP).
The transport of glucose decreases the levels of cAMP and therefore CRP.

Question 26

What is splicing in the context of changing pre-mRNA to mature mRNA? (1 mark)

Answer 26

Introns are removed and the exons are joined together.

Question 27

What is RNA processing? (2 marks)

Answer 27

The product of transcription is pre-mRNA, this is modified (by splicing) to form mature mRNA before it can bind to a ribosome and code for the synthesis of the required protein.

Question 28

What is RNA editing? (2 marks)

Answer 28

When an mRNA molecule is changed through base addition, deletion, or substitution.

Question 29

State two post-transcriptional controls. (2 marks)

Answer 29

RNA processing
RNA editing

Question 30

Describe three ways of translational control. (3 marks)

Answer 30

Degradation of mRNA- the more resistant the molecule is the longer it lasts in the cytoplasm.
Binding of inhibitory proteins to mRNA
Activation of initiation factors.

Question 31

What does an initiation factor do? (1 mark)

Answer 31

Aids the binding of mRNA to ribosomes.

Question 32

What are protein kinases? (1 mark)

Answer 32

Enzymes that catalyse the addition of phosphate groups to proteins.

Question 33

Why are protein kinases important? (1 mark)

Answer 33

Many enzymes are activated by phosphorylation, so protein kinases are important regulators of cell activity.

Question 34

State four ways proteins can be modified after they have been synthesised. (4 marks)

Answer 34

Addition of non-protein groups
Modifying amino acids
Folding or shortening of proteins
Modification by cAMP

Question 35

Describe how protein kinases can be activated by cAMP. (2 marks)

Answer 35

Binds to cell surface membrane of enzyme, and activates proteins by altering their 3D shape.
This changes the active site of an enzyme to become more or less active.