Mutations & Cancer

Mutations

A gene mutation is the alteration of a base in the sequence of bases for one gene. This is likely to occur during DNA replication, which is during interphase in the cell cycle. Gene mutations occur spontaneously, but the frequency of these occurring can be increased by certain factors which are called mutagenic agents.

Because mutations alter the gene, they can result in a different amino acid sequence in the encoded polypeptide. If the amino acid sequence changes then when protein is modified into the tertiary structure it will form hydrogen and ionic bonds in different places and fold differently. This will result in a different 3D shape, and therefore a non-functioning protein.

High Energy and Ionising Radiation

These mutagenic agents include radiation such as α and β particles and x-ray and gamma rays. Ultraviolet light is not ionising, but it is still high enough so can cause damage and disrupt the structure of DNA.

Mutations & Cancer, figure 1

Carcinogens

This is the term given to chemicals that can alter the structure of DNA and interfere with transcription. These include chemicals in tobacco smoke, mustard gas and peroxides.

Mutations & Cancer, figure 2

There are 5 different types of gene mutations:

  1. Addition
  2. Deletion
  3. Substitution
  4. Inversion
  5. Duplication
  6. Translocation

Additions

One extra base being added to the sequence.

Original: TAC TTC AGG TGG

Mutation: TAC __A__TT CAG GTG G

The impact of adding one base is that all subsequent codons are altered. This is known as a frame shift. This type of mutation can be very harmful because all the altered codons could potentially code for different amino acids and result in a very different sequence of amino acids resulting in a non-functioning protein.

Deletion

The deletion of a base in a sequence.

Original: TAC TTC AGG TGG

Mutation: TAC TCA GGT GG

This causes a frame shift to the left. This could result in a different polypeptide chain and a non-functioning protein.

Substitution

One bases has been changed for a different base, but the number of bases remains the same and there is no frame shift. This results in only one codon changing, and due to the genetic code being degenerate it may still code for the same amino acid and therefore have no impact.

Original: TAC TTC AGG TGG

Mutation: TAC __A__TC AGG TGG

Inversion

A section of bases detach from the DNA sequence, but when they re-join they are inverted, so this section of code is back to front. This results in different amino acids being coded for in this region.

Original: TAC TTC AGG TGG

Mutation: TAC GGA CTT TGG

Duplication

One particular base is duplicated at least once in the sequence. This causes a frame shift to the right and a different sequence of amino acids are coded for.

Original: TAC TTC AGG TGG

Mutation: TAC __TT__T TCA GGT GG

Translocation

A section of bases on one chromosome detaches and attach onto a different chromosome. This is a substantial alteration and can causes significant impacts on gene expression and therefore the resulting phenotype.

Mutations & Cancer, figure 1

Alterations to the genes can result in a mutation that causes cancer.

Mutations & Cancer, figure 2

Mutations & Cancer, figure 3

Cancer

Cancer is a result of mutations in genes that regulate mitosis. If these gene mutate and non-functioning proteins are made, then mitosis is not regulated, and it results in the uncontrollable division of cells and the creation of a tumour. Not all tumours are cancerous, they can be classified as either benign or malignant.

Benign Tumours

These can grow very large but at a slow rate. These are non-cancerous because they produce adhesion molecules sticking them together and to a particular tissue, they are often surrounded by a capsule, so they remain compact and therefore can be removed by surgery and they rarely return. For these reason, the impact is localised and often not life-threatening, depending on the tumour location.

Malignant Tumours

These are cancerous and grow large rapidly. The cell nucleus becomes large and the cell can become unspecialised again. They do not produce the adhesive, so instead metastasis occurs, meaning the tumour breaks off and spreads to other parts of the body. The tumour is not encapsulated and instead can grow projections into surrounding tissues and develop its own blood supply. For these reasons it can be life threatening and the removal of the tumour needs supplementary treatment (radiotherapy and chemotherapy) and recurrence is more likely.

Mutations & Cancer, figure 1

Mutations & Cancer, figure 2

Tumor Development

The development of a tumour is due to a gene mutation in either the tumour suppressor genes and/or oncogenes, the abnormal methylation of tumour suppressor genes and oncogenes or increased oestrogen concentrations.

Oncogenes

Oncogenes are mutated version of a proto-oncogene, which creates a protein involved in the initiation of DNA replication and mitosis cell division when the body needs new cells. Oncogene mutations can result in this process being permanently activated to make cells divide continually.

Tumour Suppressor Genes

These genes produce proteins to slow down cell division and to cause cell death if DNA copying errors are detected. If a mutation results in the tumour suppressor gene not producing the proteins to carry out this function, then cell division could continue, and mutated cells would not be identified and destroyed. BRCA1 and BRCA2 are two known mutated tumour suppressor genes that are linked to breast cancer.

Abnormal Methylation

This links to control of transcription- methylation can cause a gene to turn on or off.

Tumour suppressor genes could become hypermethylated, meaning an increased number of methyl groups attached to it. This results in the gene being inactivated and becomes turned off.

The opposite could occur in oncogenes, as they may be hypomethylated, reducing the number of methyl groups attached. This results in the gene being permanently switched on.

Increased Oestrogen Concentrations

Oestrogen is produced by the ovaries to regulate the menstrual cycle, but after the menopause this stops. Instead, fat cells in breast tissues can produce oestrogen and this has been linked with causing breast cancer in women post-menopause. This has a knock-on effect, as the tumour then results in even more oestrogen production which increase the tumour size and attracts white blood cells which can increase the tumour size further.

This could be because oestrogen can activate a gene by binding to a gene that initiates transcription, and if this a proto-oncogene the result is it is permanently turned on and activating cell division.

Correlation and Causation

When evaluating data there are many correlations between genetic and environmental factors and various forms of cancer, however this is not proof of causation. To determine that one factor directly causes an impact, cells must be exposes to this in a lab in controlled conditions.

What is a gene mutation?
Your answer should include: change / base
When would a gene mutation most likely occur?
Your answer should include: DNA / replication
Which type of tumour is cancerous?
malignant
Which type of tumour produces adhesives and encapsulates the tumour?
benign
Which type of tumour can metastasise?
malignant
What are the name of the two genes linked to cancer?
Your answer should include: suppressor / oncogene