Cellular Control Flashcards
What is gene mutation
This is a relatively small change to a nucleotide sequence in the DNA as a result of mistakes in copying. When a gene is modified or altered as a result of a change in the sequence of bases, a new form of gene is formed.
Why may the gene mutation not cause a change in the protein made
The alteration of the bases may not cause any change at all in the protein that it produces because the modified triplet may still code for the same amino acid in the protein. This only happens because genetic code is degenerate code. Also the mutation could occur in non-coding section of the DNA and so has no effect on the amino acid sequence at all.
What is a degenerate code
Several triplets coding for the same amino acid
Why may the change in amino acid sequence
The allele may be recessive and be masked by a dominant allele and so may not be expressed.
The gene may be one of several that are all involved in the expression of a characteristic, thus reducing the effect of a single mutation. Or one gene may specifically prevent the expression of others, called the epistasis effect.
Chromosome mutation
This can either change the number of chromosomes in a nucleus or the structure of individual chromosomes.
Gene mutation
This occurs when there is any change to a gene caused by changes in the DNA base sequence. This can be occured by substitution and insertion/deletion
Substitution gene mutation
Substituting one base pair for a different base pair will result in a change in one triplet only.
Insertion/deletion gene mutation
This involves inserting or deleting one or more base pairs results in a complete shift in the code, often called a frameshift.
What is a stutter mutation
This is where triplets are repeated many times. Huntington’s disease is caused by a stutter mutation.
Difference between substitution and insertion/deletion
Substitution is where a different base is used in the base sequence in place of the original base, giving a different triplet.
What is a silent mutation
The change in the genetic mutation may result in no change at all if the triplet codes for the same amino acid. Or it may result in a change in the amino acid coded for and used in polypeptide chain. This may produce an altered protein but not cause a drastic change.
What does a frameshift affect when a base is inserted
The result is not a change to a single triplet but a completely different sequence following the changed bases, since the code will now be read as a different sequence of triplets.
What does a frameshift cause when a base is deleted
They will affect the polypeptide chain because there has been a shift by the deletion. A similar situation occurs with an insertion when an extra base is added to the sequence. A stop codon will prevent the addition of further amino acids to the chain, causing it to be cut short.
What are examples of a beneficial mutation
In sunny countries such as Africa, the local people have dark skins because they have high concentration of melanin. This protects from the harmful UV radiation from the Sun but still allows vitamin D to be synthesised because of the high intensity of the sunlight. Paler skinned humans were less likely to survive in these circumstances due to skin cancer and burned skin.
In cooler temperature climates, dark-skinned humans would have been less able to synthesise vitamin D and so paler skinned humans would have an advantage.
What problems can lack of vitamin D have
Rickets which can cause a deformed pelvic girdle. This is dangerous in females during child birth.
Vitamin D deficiency also leads to reduced protection against heart disease and cancers.
Example of harmful mutation
Sickle cell anaemia and haemophilia
In 70% of cystic fibrosis sufferers the mutation responsible for the disease is the deletion of three base pairs in the gene coding for the protein CFTR at amino acid position 508, resulting in the loss of the amino acid phenylalanine.
Symptoms of cystic fibrosis
Lung and pancreatic problems as a result of particularly thickened mucus.
Another example of harmful mutation- Huntington’s disorder and PKU
Caused by a stutter mutation where the triplet code cytosine-adenine-guanine is repeated many times. The triplet codes for the amino acid glutamine and the repeat creates a poly glutamine sequence.
Once the repeats reach the critical level, usually more than 36, then there is a high probability that the disorder will occur, usually later in life.
It causes an increase in the decay of certain neurones in the brain. Some diseases occur because the mutation results in no protein production at all. Phenylketonuria (PKU) is an example of a disease resulting from this type of mutation in the gene coding for the enzyme needed to metabolise the amino acid phenyalanine. The mutation leads to a build up of phenylalanine, causing serious medical problems including mental impairment.
What are proto-oncogenes
These are growth-promoting genes. They code for growth factors or their receptors. They may be regulatory enzymes that can be switched off once the required cell division has been completed, or may restrict progress through the G1 stage of the cell cycle and so prevent progression if a previous step is incorrect or if the DNA is damaged.
What can a mutation in the proto-oncogenes cause
This may result in the gene becoming an oncogene, preventing the gene being switched off and leading to unregulated cell division and a tumour.
What is TP53 and what happens when it is mutated
This is a tumour-suppressor gene, one of another group of genes with a role in cancer development. It encodes the protein p53.
It can become mutated by some of the chemicals in cigarette smoke. Some mutations change p53 into an inactive form that prevents it from halting cell division at the G1 stage when damaged DNA or faulty copying has occured.
Is associated with an increased risk of lung cancer and is linked to cervical cancer when modified by the human papilloma virus, HPV.
What are neutral mutations
These are the result of a change in one DNA base that does not cause any difference in the amino acid coded for. In other cases the amino acid may be changed but the resulting polypeptide functions in the same way. Alternatively, if function is changed it makes no difference to survival, giving no advantage or disadvantage to the organism.
The ability to taste a chemical called PTC is due to the gene TAS2R38 being mutated. Brussel sprouts contain a bitter compound that is similar to PTC that some people cannot taste- those without the mutated gene cannot taste it.
