A2 Biology (Mutations and Gene Expression) Note on Cancer, created by Lucy Nove on 29/01/2017.
Lucy Nove
Note by Lucy Nove, updated more than 1 year ago
Lucy Nove
Created by Lucy Nove almost 7 years ago

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Mutations can cause uncontrolled cell growthMutations that occur in individual cells after fertilisation are called acquired mutations.If these mutations occur in the genes that control the rate of cell division, it can cause uncontrolled cell division.If a cell divides uncontrollably, the result is a tumour - a mass of abnormal cells. Tumours that invade and destroy surrounding tissue are called cancers.There are two types of gene that control cell division - tumour suppressor genes and proto-oncogenes. Mutations in these genes can cause cancer:Tumour suppressor genesTumour suppressor genes can be deactivated if a mutation occurs in the DNA sequence. When functioning normally, tumour suppressor genes slow cell division by producing proteins that stop cells dividing or cause them to self-destruct (apoptosis). If a mutation occurs in a tumour suppressor gene, the protein isn't produced. The cells divide uncontrollably, resulting in a tumour. Proto-oncogenesThe effect of a proto-oncogene can be increased if a mutation occurs in the DNA sequence. A mutated proto-oncogene is called an oncogene. When functioning normally, proto-oncogenes stimulate cell division by producing proteins that make cells divide. If a mutation occurs in a proto-oncogene, the gene can become overactive. This stimulates the cells to divide uncontrollably resulting in a tumour.

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Tumours can be benign or malignantTumours can develop over years without any obvious symptoms and can be quite large by the time they're discovered. Not all tumours are cancerous - there are two different types: Malignant tumours are cancers. They usually grow rapidly and invade and destroy surrounding tissues. Cells can break off the tumours and spread to other parts of the body in the blood stream or lymphatic system. Benign tumours are not cancerous. They usually grow slower than malignant tumours and are often covered in fibrous tissues that stop cells invading other tissues. Benign tumours are often harmless, but they can cause blockages and put pressure on organs. Some benign tumours can become malignant.

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Tumour cells can look and function differently to normal cells Tumour cells can differ from normal cells in many different ways: They have an irregular shape. The nucleus is larger and darker than in normal cells. Sometimes the cells have more than one nucleus. They don't produce all the proteins needed to function correctly. They have different antigens on their surface. They don't respond to growth regulating processes. They divide more frequently than normal cells.

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Abnormal methylation of cancer-related genes can cause tumour growth Methylation means adding a methyl group onto something. Methylation of DNA is an important method of regulating gene expression - it can control whether or not a gene is transcribed and translated. When methylation is happening normally, it plays a key role in many processes in the body. It's only when it happens too much (hypermethylation) or too little (hypomethylation) that it becomes a problem. The growth of tumours can be caused by abnormal methylation of certain cancer related genes: When tumour suppressor genes are hypermethylated, the genes are not transcribed - so the proteins they produce to slow cell division aren't made. This means that cells are able to divide uncontrollably and tumours can develop.Hypomethylation of proto-oncogenes causes them to act as oncogenes - increasing the production of the proteins that encourage cell division. This stimulates cells to divide uncontrollably, which causes the formation of tumours.

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Increased oestrogen may contribute to some breast cancersIncreased exposure to oestrogen over an extended period of time is thought to increase a woman's risk of developing breast cancer. The exact reasons behind this aren't fully understood, but there are a few theories as to how oestrogen can contribute to the development of some breast cancers: Oestrogen can stimulate certain breast cells to divide and replicate. The fact that more cell divisions are taking place naturally increases the chance of mutations occurring, and so increases the chances of cells becoming cancerous. This ability to stimulate division could also mean that if cells do become cancerous, their rapid replication could be further assisted by oestrogen, helping tumours to form quickly. Other research suggests that oestrogen is actually able to introduce mutations directly into the DNA of certain breast cells, again increasing the chance of these cells becoming cancerous.

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