Mutations

MutationsA mutation is a change in the structure of a gene, or a change in the structure of a chromosome, or a change in the number of chromosomes Mutations are rare events occur at random are usually harmful for cells and are the major cause of cancer are an important source of variation, especially when they occur in in the production of gametes as the offspring may carry or display the mutation may give the affected organism an advantage The rate of mutation can be artificially increased by chemicals or irradiation- these are called mutagenic agents e.g UV light, mustard gas and nuclear radiationGene Mutations These are alterations of a DNA base type or sequence of bases of a gene As a result the sequence of amino acids in a protein is altered. The altered protein is unable to carry out it's normal function.There are 4 types of gene mutation(a) SUBTITUTION one base pair is replaced by anotherAGA GTC TAC becomes AGT GTC TAC(b) INVERSIONtwo or more bases exchange positionAGA GTC TAC becomes AAG GTC TACIn both these mutations only one triplet codon is affected therefore only one amino acid in the protein is altered, the resulting protein is slightly changed and the protein function is slightly altered. Sickle Cell Anaemia A substitution in a gene which codes for an amino acid which produces haemoglobin S Haemoglobin S is a very inefficient carrier of oxygen- sufferers have a severe shortage of oxygen red blood cells tend to collapse into a sickle (c) shape Sickle cells stick together and cause blockages and organ damage (c) DELETIONone or more bases lostAGA GTC TAC becomes AGG TCT AC (d) INSERTIONone or more bases addedAGA GTC TAC becomes ATG AGT CTA C In both these mutations there is a frame shift in the triplet code and many triplet codes are affected therefore many amino acids are altered. The resulting protein is changed and the protein is non-functional. Cystic Fibrosis a deletion in a gene which codes for a membrane protein epithelial cells produce thick mucus secretions this causes blockage of airways and glands Deletion and insertion cause severe effects as the gene code suffers major change.These mutations are lethal as sufferers have reduced life expectancy.Chromosome MutationsThere are 2 types of chromosome mutation:Changes in structure of one chromosome(a) INVERSIONa section breaks free, rotates and rejoinsABCDEFGH becomes ABCFEDGH (b) DUPLICATIONa segment from the homologous partner becomes inserted and a set of genes is repeatedABCDEFGH becomes ABCDEFDEFGHInversion and Duplication lead to minor changes in the sequence of genes. They are usually harmful but may produce beneficial(c) DELETIONa section breaks free and is lostABCDEFGH becomes ABCGH(d) TRANSLOCATIONa section breaks free from one chromosome and becomes attached to another non-homologous chromosomeCHROMOSOME 1 ABCDEFGHCHROMOSOME 2- PQRSTUVbecomes FGHPQRSTUVDeletion and Translocation lead to major changes in the number of genes. The gametes are either non-viable or the offspring severely affected.Changes in numbers of chromosomesThis is caused by spindle failure at mitosis or meiosis. Chromosomes fail to separate and this is called non-disjunction. This may happen to only one pair of chromosomes or to a whole set.Down's Syndrome caused by non-disjunction of chromosome pair 21 individuals are mentally disabled and have reduced life expectancy the karyotype shows 47 chromosomes mostly due to higher number of mutations in older egg mother cells its frequency increases with the age of the mother egg mother cell --> non-disjunction at meiosis --> 2 ovums- one 22 pairs of chromosomes, one with 24 (containing extra chromosome 21)24 pair chromosome egg fuses with 23 chromosome pair sperm= abnormal zygotePolyploid increase in the number of sets of chromosomes by 3 times or more common in plants but very rare in animals occurs by complete non-disjunction at meiosis or mitosis allows sterile hybrids to be converted into fertile polyploids Polyploidy plants Important in crop production are larger with increased size of seeds and fruit show increased vigour and disease resistance e.g. tomato, coffee, apples can be seedless e.g. banana Now try the Mutations quiz by laldypop