The Control of Gene Expression

Question 1

Explain why the effect of a substitution mutation may have less of an impact than a deletion mutation
(3 marks)

Answer 1

A deletion mutation cause a change in the base sequence and so all codons after the mutation will be affected. A change in the sequence of amino acids alters the tertiary structure of the protein formed.
A substitution mutation involves switching one base for another. It is a silent mutation and so the resulting protein will be unaffected.

Question 2

Explain how a carcinogen can lead to the formation of lung cancers
(3 marks)

Answer 2

They contain mutagenic agents which cause an increased methylation of tumour suppresser genes.
This results in a mutation which inactivates a tumour suppressor gene meaning it isn’t transcribed.
This causes uncontrolled cell division.

Question 3

Explain why the degeneracy of the genetic code might provide an advantage to organisms
(2 marks)

Answer 3

Degeneracy provides an advantage because:
- some amino acids are coded for by more than one triplet
- mutations may result in the same amino acid
- the resulting protein is unchanged

Question 4

Identify 2 mutagenic agents an explain how they might increase the occurrence of genetic mutations
(4 marks)

Answer 4

Ionising radiation: alters the structure of DNA
Nitrogen dioxide: alters the structure of DNA
Tobacco smoke: inactivates tumour suppressor genes

Question 5

What effect can a substitution mutation have on a protein structure?
(3 marks)

Answer 5

Coding for a stop codon so the amino acid sequence finishes early.
Acts as a silent mutation so no change in the amino acid sequence.
Codes for a different amino acid so results in a different primary structure.

Question 6

Distinguish between totipotent, multipotent and pluripotent cells
(3 marks)

Answer 6

Totipotent cells can differentiate into any type of cell in the body.
Multipotent cells can differentiate into a limited number of cell types.
Pluripotent cells can different into almost any cell type.

Question 7

Evaluate the use of pluripotent and induced pluripotent stem cells in the treatment of life changing diseases
(5 marks)

Answer 7

PLURIPOTENT AND INDUCED PLURIPOTENT STEM CELLS CAN:
- differentiate into nearly any other cell type
- can self-renew
- could be used to treat potentially life changing conditions
PLURIPOTENT STEM CELLS
- result in the destruction of embryos
- there may be ethical objections
- can be obtained from discarded IVF embryos
INDUCED PLURIPOTENT STEM CELLS:
- produced from a unipotent cell
- can originate from the patient so would be genetically identical
- no risk of rejection
- the patient can provide consent for taking stem cells

Question 8

Explain how transcription factors stimulate the expression of a gene
(3 marks)

Answer 8

Transcription factors move from the cytoplasm into the nucleus.
In the nucleus they bind to a specific base sequence on the DNA.
This initiates transcription of the gene.
mRNA is then translated.

Question 9

Explain how differential gene expression forms an organism with specialised cells
(4 marks)

Answer 9

Transcriptional factors are used to activate and deactivate genes.
mRNA is transcribed from these active genes.
This mRNA is translated to synthesise proteins.
The proteins that are produced give the cell a specific function.
mRNA is translated to synthesise proteins.

Question 10

Explain how changes to DNA methylation and acetylation can activate gene expression
(5 marks)

Answer 10

Acetylation of histones reduces the strength of the association between DNA and histones.
Removing methyl groups from cytosine bases makes the DNA accessible to transcription factors.
DNA is less condensed.
Binding of transcription factors allow transcription of the gene.

Question 11

Describe how gene expression can be inhibited at the point of translation
(4 marks)

Answer 11

Gene expression can be inhibited before translation by:
- small double stranded siRNA
- the double strand is separated to create 2 single siRNA strands
- this siRNA combines with an enzyme
- siRNA pairs with mRNA using complementary base pairing
- the associated enzyme cuts the mRNA so the mRNA cannot be translated

Question 12

Explain how an increase in oestrogen levels may lead to the development of breast cancer
(3 marks)

Answer 12

Oestrogen binds to the transcription factors of a gene controlling cell division.
This transcription factors then binds to the gene to initiate transcription.
Increased transcription of the gene leads to increased cell division and tumour formation.
OR
Oestrogen triggers a mutation of proto-oncogenes into oncogenes. The oncogene is permanently activated. This leads to uncontrolled cell division.

Question 13

Suggest how knowledge of epigenetics could lead to the development of more effective treatments for diseases such as cancer
(2 marks)

Answer 13

Treatment could be targeted at specific unhealthy cells.
Genes within these cells can then be activated by changes to acetylation and methylation.
Healthy cells are unaffected.
Therapy could also be used to detect diseases at the early stages to allow early treatment.

Question 14

Suggest how DNA sequencing can be used in the treatment of specific genetic diseases
(3 marks)

Answer 14

DNA sequencing allows identification of genes which result in genetic diseases.
Early diagnosis allows rapid treatment.
Patients could obtain access to genetic counselling.
Personalised medicines could be used to treat the patient

Question 15

Explain why the proteome of a human changes throughout their lifetime
(3 marks)

Answer 15

Proteins are produced depending on which genes are activated.
Gene activation is determined by the epigenome.
The epigenome is affected by the environmental influences which vary throughout a lifetime.

Question 16

Distinguish between the genome, epigenome and proteome
(3 marks)

Answer 16

The genome is all of the genetic material of an organism.
The proteome is all of the proteins that are expressed by the genome.
The epigenome is all of the chemical markers on the DNA which determine gene expression

Question 17

Outline the mechanism by which interactions between the genome and epigenome determine the proteome of an organism
(3 marks)

Answer 17

The genome provides the genetic code which codes for the synthesise of the proteins.
The epigenome markers on the DNA control which genes are expressed.
Activated of a gene leads to the transcription and translation of the protein.

Question 18

Explain why the genome of a bacterium is more simple to determine than the genome of a human
(2 marks)

Answer 18

Bacteria have a single loop of circular DNA whereas human DNA has large lengths of DNA associated with proteins.
Bacterial DNA contains no introns whereas human DNA contains introns.

Question 19

Compare the Sanger sequencing method compared to PCR
(4 marks)

Answer 19

SIMILARITIES
- both methods involve the use of an enzyme
- they both follow a similar sequence of events
DIFFERENCES
- the Sanger sequencing requires the extra addition of dideoxynucleosides
- Sanger sequencing uses DNA polymerase whereas PCR uses Taq polymerase
- the optimum temperature for the synthesise of new DNA in Sanger sequencing is 37c whereas the optimum temperature in PCR is 75c
- the Sanger sequencing method uses just one type of primer whereas PCR uses 2 different primers.

Question 20

Explain how electrophoresis separates DNA fragments
(2 marks)

Answer 20

DNA is negatively charges so it moves towards the positive electrode.
The fragments separate due to the size

Question 21

Describe and explain how the structure of DNA makes it possible to sequence the genome of an organisms
(3 marks)

Answer 21

DNA has 2 strands which means one can be used as a template.
The hydrogen bonds between the strands are weak and easily broken to expose the template strand.
A binds with T and C binds with G (complementary base pairing means it is easy to copy with minimal error)

Question 22

Outline how DNA sequencing facilitates the process of genetic screening for a disease
(5 marks)

Answer 22

A mutated version of a genes is sequenced to find the nucleotide base sequence.
Sequencing produces a fragment of DNA which complements the nucleotide base sequence.
These DNA fragments can be fluorescently labelled to make a DNA probed.
PCR is used to make multiple copies of the probe.
The probe is added to the DNA sample of an individual being tested.
If the individual has the mutant DNA the probe will bind to it and can identified due to the labelling.

Question 23

Discuss the ethical, legal and social consideration of DNA sequencing projects
(5 marks)

Answer 23

• use of sequencing for genetic screening may highlight undesirable genetic traits and lead to discrimination
• access and privacy of data must be considered
• phenotype also require knowledge of the individuals proteome
• sequencing date could be used to develop personalised medicines
• testing and treatments may be expensive so only accessible to the more wealthy individuals
• wider access to sequencing may lead to other genetic screening methods

Question 24

Describe 3 methods that could be used to obtain the DNA fragments that contain a specific gene
(3 marks)

Answer 24

Using reverse transcriptase to create cDNA from mRNA.
Using restriction endonucleases to cut the gene at a specific base sequence.
Using a gene machine to produce the desired fragment from a known DNA sequence

Question 25

Outline the role of a genetic counsellor
(3 marks)

Answer 25

Assist patients to make decisions after diagnosis.
Research patient family history.
Establish the probability of genetic diseases being passed onto offspring.
Give advice on future decisions
Explain the potential consequences of the disease

Question 26

Explain how genetic fingerprinting could be used to make comparisons between the VNTR sequences in different organisms
(6 marks)

Question 27

Suggest why bacterial cells are used in the production of human insulin for diabetic patients
(2 marks)

Answer 27

Bacteria contain plasmids which are easy to alter.
Plasmids are a suitable vector to transfer the genes.
Bacteria reproduce rapidly to produce the product.

Question 28

Compare the use of in vivo cloning compared to in vitro gene cloning for the production of gene fragments
(3 marks)

Question 29

Suggest how plantibodies could be used to combat disease in humans
(3 marks)

Answer 29

Plantibodies isolated from the parent plant could be administered by injection to provide passive immunity to the disease

Question 30

Evaluate the use of recombinant DNA technology to treat disease