A mutation is an unpredictable change in the genetic material of an organism. Mutations create completely new alleles and are an important source of genetic variation. There are two types:- Gene mutation and Chromosome mutations. A mutation is a change in the quantity, arrangement or structure in the DNA of an organism.
- It may affect a single gene or a whole chromosome.
- Most mutations occur in somatic (body) cells
- Only those mutations which occur in the formation of gametes can be inherited
- Mutations are spontaneous random events which may provide a source of material for Natural Selection Pressure and therefore Evolution
- Mutation rates are normally very small, therefore mutations has less impact on evolution than other source of variation (avergae rate at which mutaion occur is 1 in 100,000)
- In general, organisms with short life cyces and more frequent meiosis show a greater rate of mutation
- The rate of mutation occuring can be increased by ionising radiation and mutagenic chemicals.
Mutagens and the Mutation Rate
Mutations happen naturally. However, sceintists have found that the mutation rate is increased if organisms are exposed to mutagens. These factors in the environment which include:
- X-Rays, Gamma Radiation and UltraViolet Light (UV)
- Chemicals such as polycyclic hydrocarbons in cigarette smoke
A mutagen which causes cancer is a carcinogen. Mutagens do not cause mutation. Increased exposure to mutagens increase the rate of muations occuring.
Mutataion happen in 2 ways:
- DNA is not copied properly before cell division. Sometimes mistakes are made in the copying process so that the new chromosomes are faulty. Usually they are small errors, involving only one gene, so they are called Gene Mutation or Point Mutation. This can be a serious problem for the individual if they are very important genes affected.
- Chromosomes are damaged and break. If chromosomes break they will normally repair themselves (the DNA will rejoin) but they may not repair themselves correctly. This can lead to large changes in the structure of DNA and may affect a large number of genes. These are called Chromosome Mutation.
A change in the structure of a DNA molecule, producing a different allele of a gene, is a Gene Mutation. Any gene can mutate but rates vary from one gene to another within an organism. Gene mutations are changes in the base pairs within the genes.
They can take forms of :
- substitution of bases
Whatever the change, the result is the formation of a modified polypeptide.
Modified Polypeptide means that it will not be able to perform its inherited function in the cells.
How can Mutations cause a change in the Phenotype?
- The Genetic Code, which ultimately determines an organisms characterisitics, is made up of a specific sequence of nucleotides on the DNA molecule
- Any change to one or more of these nucleotides, or any rearrangement of the sequence, will produce the incorrect sequence of amino acids in the protein it makes
- The protein made is often an Enzyme which may then be unable to catalyse a specific reaction. For example, a specific enzyme is necessary to convert a chemical precursor into the skin pigment, Melanin
- If a gene mutation results in the inability to produce this enzyme, the organism will lack a pigment
- The organism is referred to as an Albino
If the Gene Mutates it no longer codes for the enzyme, and the pathway will be blocked.
Sickle - Cell Anaemia
A gene mutation ( Substitution) in the gene producing haemoglobin reults in a defect called Sickle Cell Anaemia. The replacement of just one base in the DNA molecule results in the wrong amino acids being incorporated into two of the polypeptide chains which make up the haemoglobin molecule. The abnormal haemoglobin causes Red Blood Cells to become sickle shaped. These abnormal shaped red blood cells are less able to carry oxygen, resulting in Anaemia and possible death.
Haemoglobin S is produced instead of normal haemoglobin by a single base chain that causes Valine to be substituted for Glutamic Acid at the sixth position in the Beta Globulin chain. DNA codes for Glutamic Acid are CTT or CTC. Two of the codes for Valine are CAT and CAC. In either case the substitution of A for T as the second base would bring about the formation of haemoglobin S.
The mutant gene is Co-Dominant. In the Homozygous state the individula suffers the disease but in the Heterozygous state the individual has 30-40% sickel cells, the rest are normal. The Heterozygous condition is referred to as Sickle Cell Trait.
- Mutations causing changes in the structure or number of whole chromosomes in cells are known as Chromosome Mutations.
- They are most likely to occur during meiosis, when the process can go wrong as the paired chromosomes line up on the crowded equator at Metaphase and are pulled in Anaphase.
- Errors can result in the chromosome not being shared equally between Daughter Cells.
There are three types of changes:-
- In Structure
- In Numbers
- In Sets Of Chromosomes
Change in Structure
During Prophase I of Meiosis, Homologous chromosomes pair up and exchange of material takes place at Chiasmata.
Errors arise when chromosomes rejoin with corresponding pieces of chromosomes on its homologous partner.
Often the homologous chromosomes end up with a different gene sequence.
This makes it impossible for pairing up in meiosis to take place.
This type of mutation is important as a source of Variation.
Change in Numbers
Non-Disjunction is a process in which faulty cell division means that one of the daughter cells receives two copies of a chromosome while the other gets none. In Down's Syndrome chromosome number 21 is affected. If this happens in an ovary, it results in an Oocyte with either no chromosome 21 or with two copies instead of one.
Oocytes with no chromosome 21 will die but those with two copies survive and may be fertillised. The resulting zygote has three chromosome 21's with a total 47 chromosomes. This condition is known as Trisomy 21 and the zygote will develop into a child with Down's Syndrome.
Down's Syndrome occurs in approximately 1 in 700 births and the incidence of the mutation is related to the age of the mother,a result of the higher chance of mutation occuring during the formation of oocytes in older ovaries. At the age of 20 the risk is 1 in 2000, at 40 years 1 in 100 and after 45 years the risk has risen to 1 in 30.
Children with Down's Syndrome have open, slightly flattened faces. They are usually happy and friendly but there are varying degrees of mental retardation.
Change in sets of Chromosomes
Occasionally a Mutation can affect whole sets of Chromosomes. This is known as Polyploidy.
A defect in meiosis may result in a gamere receiving two sets of chromosomes. When this diploid gamete is fertilised by a normal gamete the zygote will be triploid, that is, having three sets of chromosomes.
If two diploid gametes fuse then a tetraploid will be produced. Tertraploid may also happen after fertilistaion if, during mitosis, the two sets of chromosomes double but fail to seperate.
Polyploidy is common in flowering plants and is associated with beneficial characteristics. Tomatoes and wheat are polyploidy. Triploidy are usually sterile as they cannot form homologous pairs.
Why are Mutations Important?
Mutations are important because they increase variation in a population. Most mutations are harmful to the organism concerned. Beneficial Mutations are very rare but they may give a selective advantage to an organism.
- If a mutation is in a body cell, it may cause cancer. For example increased exposure to UV light is linked to skin cancer
- If the mutation is in gametes, it will not affect the individual producing gametes, but will affect the zygote that develops from it, the offspring
- These mutations cause sudden and distinct differences between individuals.
- They are therefore the basis of Discontinuous Variation
- There are potential advantages from mutation that are beneficial and may increase variation.
- However, most mutations are recessive to the normal allele
- A recessive mutant allele must await replication in the gene pool over many generations before chance brings recessive allaeles together, resulting in their expression.
Substances that cause cancer are called Carcinogens. These affect the DNA in cells, resulting in mutations.
Mutations that occur in somatic cells often have no effect on an organism. Most mutated cells are recognised as foreign by the body's immune system and are destroyed.
Occasionally the mutation may affect the regulation of cell division. Cancers are thought to start when changes take place in these genes.
If a cell with such a escapes the attack of the immune system, it can produce a lump of cells called a tumour. Tumours are usually harmless or benign but sometimes the tumour cells are able to spread around the body and invade other tissues.
This type of tumour is described as malignant and the disease caused by such tumours are cancers. Any agent that causes cancer is called a carcinogen and is described as carcinogenic. Therefore some mutagens are carcinogens.
Smoking and Cancer
Tobacco smoke contains a number of harmful chemicals that affect human health. These include tar, nicotine and carbon monoxide.
Tar is a mixture of many toxic chemicals. It collects in the lungs as the tobacco smoke cools. Tar contains carcinogens which affect the DNA in the cells of the alveoli.
Normally, genes control cell division and division is halted when sufficient cells have been produced for growth and repair.
Tumour suppressor genes normally inhibit cell division. Carcinogens in tobacco smoke cause these genes to mutate, so that they do not carry out their normal function, leading to uncontrolled cell division.
Oncogenes and mutated supressor genes can both lead to lung cancer. About 25% of all cancer deaths in developed countries are due to carcinogens in tar of tobacco smoke.
Oncogenes = A mutated Gene that cause Cancer