Extra base pairs may be added or deleted from the DNA of a gene. The number of bases can range from a few to thousands. Insertions and deletions of one or two bases or multiples of one or two cause frameshifts (shift the reading frame). These can have devastating effects because the mRNA is translated in new groups of three nucleotides and the protein being produced may be useless. The picture on the right shows an example of a frameshift mutation, in the second codon the deletion of 'c' leads to a shift in reading frame and multiple amino acid substitutions in the subsequent protein.Insertions and deletions of 3 or multiples of three bases may be less serious because they preserve the open reading frame. However, a number of trinucleotide repeat diseases exist including Huntingtons disease and fragile X syndrome. In Huntingtons disease, the repeated trinucleotide is 'cag'. This adds a string of glutamines to the Huntington protein. The abnormal protein produced interferes with synaptic transmission in parts of the brain leading to involuntary movements and loss of motor control. Fragile X syndrome is caused when a locus on the X chromosome contains a stretch of nucleotides in which the triplet 'cag' is repeated (as many as 400 times). This causes a constriction in the x chromosome making it quite fragile. Males who inherit this X chromosome are mentally retarded. Females are only mildly affected.
A chromosome mutation is any change in the structure or arrangement of the chromosomes. Mutations to chromosomes happen most frequently during the crossing over stage of meiosis. There are many different types of mutation that can change the chromosome structure resulting in detrimental changes to the genotype and phenotype of the organism. Chromosomal mutations effecting essential parts of the DNA can result in the abortion of the foetus before birth.
Translocations - Translocations are the transfer of a piece of one chromosome to a non-homologous chromosome. They are often reciprocal, with the two chromosomes swapping segments with each other. In most cases of chronic myelogenous leukaemia (CML), the leukaemic cells share a chromosomal abnormality known as Philadelphia chromosome. This abnormality is the result of a reciprocal translocation between chromosomes 9 and 22. An abnormal hybrid gene is created leading to the production of a novel protein that is not normally found in the cell. This protein prevents normal growth and development, leading to leukaemia.
Inversion - A region of DNA on the chromosome can flip its orientation with respect to the rest of the chromosome.
Deletions - A large section of a chromosome can be deleted resulting in the loss of a number of genes.
Duplications - In this mutation, some genes are duplicated and displayed twice on the same chromosome.
Chromosome non-disjunction - During cell division, the chromosomes fail to successfully separate to opposite poles, resulting in one of the daughter cells having an extra chromosome and the other daughter cell lacking one. If this non-disjunction occurs in chromosome 21 of a human egg cell, a condition called Downs syndrome (DS) occurs. A person suffering with DS has 47 chromosomes in every cell instead of the normal 46. They suffer from heart defects, mental retardation and stunted growth. However, it must be pointed out that the distributions of IQs of people with DS overlaps considerably with the IQ distribution of 'normal' (non-DS) population, mainly due to changes in education policy in the last 30 years.
Translocations - Translocations are the transfer of a piece of one chromosome to a non-homologous chromosome. They are often reciprocal, with the two chromosomes swapping segments with each other. In most cases of chronic myelogenous leukaemia (CML), the leukaemic cells share a chromosomal abnormality known as Philadelphia chromosome. This abnormality is the result of a reciprocal translocation between chromosomes 9 and 22. An abnormal hybrid gene is created leading to the production of a novel protein that is not normally found in the cell. This protein prevents normal growth and development, leading to leukaemia.
Inversion - A region of DNA on the chromosome can flip its orientation with respect to the rest of the chromosome.
Deletions - A large section of a chromosome can be deleted resulting in the loss of a number of genes.
Duplications - In this mutation, some genes are duplicated and displayed twice on the same chromosome.
Chromosome non-disjunction - During cell division, the chromosomes fail to successfully separate to opposite poles, resulting in one of the daughter cells having an extra chromosome and the other daughter cell lacking one. If this non-disjunction occurs in chromosome 21 of a human egg cell, a condition called Downs syndrome (DS) occurs. A person suffering with DS has 47 chromosomes in every cell instead of the normal 46. They suffer from heart defects, mental retardation and stunted growth. However, it must be pointed out that the distributions of IQs of people with DS overlaps considerably with the IQ distribution of 'normal' (non-DS) population, mainly due to changes in education policy in the last 30 years.
