Mutations Flashcards
What is a mutation?
A change in the amount, arrangement or structure of DNA of an organism which may affect a single gene or a whole chromosome. This is a spontaneous random event which may provide a source of material for natural selection pressures and therefore evolution
What mutations can be inherited?
The mutations that occur in the formation of gametes
What organisms show greater rate of mutations?
Short life cycles and more frequent meiosis show a greater rate of meiosis.
How can mutations potentially be increased?
Ionising radiation and mutagenic chemicals
How can mutations occur?
1) DNA is not copied properly before cell division. Sometimes mistakes are made in the copying process so that new chromosomes are faulty. Usually they are small errors, involving only one gene so they are called gene mutation or point mutations.
2) Chromosomes are damaged and break. If chromosomes break they usually repair themselves (DNA re-joins) but they may not repair themselves correctly. This can lead to large changes in the structure of the DNA and may affect a large number of genes (chromosome mutation)
What are gene mutations?
A change in the structure of DNA molecule producing a different allele of a gene. They are changes in the base pairs within the gene and can take the form of duplication, insertion, deletion, inversion or substitution of bases. Whatever the change it results in the formation of a modified polypeptide.
How can mutations cause a change in the phenotype?
1) The genetic code (determine organism’s characteristics) is made up of a specific sequence of nucleotides on the DNA molecule
2) Any change to one or more of these nucleotides or any rearrangement of the sequence will produce a different sequence of amino acids in the protein it makes as translation machinery is still acting correctly
3) 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 us referred to as an albino.
What causes sickle cell anaemia?
A gene mutation (substitution) in the gene producing haemoglobin results in this defect. The replacement of just one base in the DNA molecule results in a different amino acid 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 which 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 cause valine to be substituted for glutamic acid at the sixth position 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 side the substitution of A for RT as the second base would bring about the formation of haemoglobin S.
What is the mutant allele in sickle cell anaemia?
Co-dominant. In the homozygous state the individual suffer the disease but in the heterozygous stat the individual has 30-40% sickle cells, the rest are normal.
What is the heterozygous condition in called?
Sickle cell trait
What is chromosome mutation?
Cause change in the structure or number of whole chromosomes in cells most likely to occur in meiosis as the paired chromosomes line up on the crowded equator at metaphase I and are pulled apart in anaphase I. Errors can result in the chromosomes not being shared equally between the daughter cells.
What changes can occur in the structure in chromosomes after mutation?
During prophase I of meiosis, homologous chromosomes pair up and exchange of material take place at chiasmata. Errors arise when chromosomes re-join with the corresponding pieces of chromosome on it 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.
What does non-disjunction mean?
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
What changes can occur in the numbers in chromosomes after mutation?
A faulty cell division where one of the cells receives two copies of a chromosome while the other gets none. In down’s syndrome chromosomes number 21 is affected. If this happens in an ovary it results in the oocyte with either no chromosome 21 or with two copies instead of one. Oocytes with no chromosome 21 die but those with 2 copies survive and may be fertilised. The resulting zygote has three chromosome 21s with a total 47 chromosomes. This conditions is called trisomy 21 and zygote will develop into a child with Down’s syndrome.
What changes can occur in sets of chromosomes after mutation?
A defect in meiosis may result in a gamete receiving two sets of chromosomes. When this diploid gamete is fertilised by a normal haploid gamete the zygote will be triploid, that is, having three sets of chromosomes. if two diploid gametes fuse then a tetraploid will be produced. tetraploidy may also happy after fertilisation if during mitosis the two sets of chromosomes double but fail to separate.
