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Higher Biology Mutations G Davidson Mutations Mutations are changes which alter the amount or structure of the genetic material in an organism. This can result in a change in the characteristics of an organism. Tuesday, May 23, 2017 G Davidson 2 Mutations Mutations are rare although most of the genes in existence today are probably the result of mutations. If mutations occur in gamete producing cells or gametes themselves, then the mutations can be passed on to the next generation. However, if a mutation was to occur in one of your skin cells, it might affect a small part of your skin but won’t be passed on to your children. Tuesday, May 23, 2017 G Davidson 3 Mutations Mutations can be very small changes, such as one base changing in the DNA, or can be large, such as increasing or decreasing the chromosome number. Tuesday, May 23, 2017 G Davidson 4 Mutations Considering the complexity of DNA, it is amazing that mutations are so infrequent. However, when a mutation does occur, resulting in a change in phenotype, the resulting individual is called a mutant. In a real sense therefore we are all mutants! Tuesday, May 23, 2017 G Davidson 5 Chromosome Numbers and Polyploidy Mutations can be brought about by a process called non-disjunction which sometimes happens during meiosis. To cause non-disjunction a spindle fibre fails and as a result, some gametes get an extra chromosome, while others lack one. Tuesday, May 23, 2017 G Davidson 6 Chromosome Numbers and Polyploidy One specific example of this is Down’s Syndrome in humans, caused by the egg having 24 chromosomes and a normal sperm with 23 chromosomes, producing a zygote with 47 chromosomes. Tuesday, May 23, 2017 G Davidson 7 Polyploidy Polyploidy is an extreme case of nondisjunction can result in the effect called Polyploidy. This results in a cell containing at least three times the normal chromosome compliment, (e.g. triploid = 3n and tetraploid = 4n). Tuesday, May 23, 2017 G Davidson 8 Polyploidy Polyploidy can be the result of a haploid gamete fertilising a diploid gamete or a diploid gamete fertilising another diploid gamete. This tends to be more common in plants than in animals and can often result in the formation of new species. Tuesday, May 23, 2017 G Davidson 9 Polyploidy Parental cells AA or 2n Parental Gametes AA or 2n Triploid Offspring diploid AA or 2n AAA or 3n Sterile A or n haploid AA or 2n AA or 2n Tetraploid Offspring diploid AA or 2n AAAA or 4n Fertile AA or 2n diploid Tuesday, May 23, 2017 G Davidson 10 Polyploidy Mutant or polyploid plants tend to show an increase in size, vigour, and disease resistance, and this is of great economic importance. Most cereals and many other crop plants are polyploid Tuesday, May 23, 2017 G Davidson 11 Polyploidy Scientists have developed a method of preventing spindle formation at mitosis using a chemical called colchinine. This leads to polyploid plants which, after the chemical is removed, divide normally, and a new species of crop plant can be produced. Tuesday, May 23, 2017 G Davidson 12 Chromosome mutations Chromosome mutations are normally large changes which are usually detectable under the microscope during cell division. They usually occur during crossing over when the number or sequence of genes may be altered. There are several ways this can happen: Tuesday, May 23, 2017 G Davidson 13 Chromosome mutations Type of Mutation Description Significance Deletion Genes chopped out Genes lost Inversion Genes rotate 180o Formation of nonviable gametes Duplication Genes copied twice Duplicated gene could mutate Translocation Genes stick to different chromosomes Problems of homologous pairing. Tuesday, May 23, 2017 G Davidson 14 Deletion 1 1 2 2 New chromosome 3 3 7 4 5 8 BREAK 6 4 7 5 Deleted genes 8 6 Original chromosome Tuesday, May 23, 2017 G Davidson 15 Inversion 1 1 2 2 3 3 4 4 5 6 BREAK 7 8 8 Original chromosome Tuesday, May 23, 2017 G Davidson 16 1 1 2 2 3 3 4 4 5 6 BREAK 7 8 8 Original chromosome Tuesday, May 23, 2017 G Davidson 17 1 1 2 2 3 3 4 4 5 6 BREAK 7 8 8 Original chromosome Tuesday, May 23, 2017 G Davidson 18 1 1 2 2 3 3 4 4 5 6 BREAK 7 8 8 Original chromosome Tuesday, May 23, 2017 G Davidson 19 1 1 2 2 3 3 4 4 5 6 BREAK 7 8 8 Original chromosome Tuesday, May 23, 2017 G Davidson 20 1 1 1 2 2 2 3 3 3 4 4 4 5 6 7 BREAK 6 7 8 5 8 Original chromosome Tuesday, May 23, 2017 8 New chromosome with G Davidson 21 Inverted genes Duplication 1 1 2 2 3 3 4 3 Duplicated 4 Genes from 4 BREAK 5 5 chromosome 6 5 7 6 7 8 8 Original chromosome Tuesday, May 23, 2017 Homologous G Davidson New chromosome 22 Translocation 1 1 2 2 3 Chromosome A 3 4 4 5 5 21 Translocated genes 21 22 22 Chromosome B Tuesday, May 23, 2017 BREAK 23 23 24 24 G Davidson 23 Gene mutations Gene mutations are relatively small, usually involving one or more nucleotides being changed. This leads to a different sequence of bases, which may result in one particular amino acid being altered and therefore cause a slightly different protein to be made. Tuesday, May 23, 2017 G Davidson 24 Gene mutations E.g. a single base change in DNA can cause the haemoglobin molecule to have one altered amino acid and this results in sickle cell anaemia. Some genes can cause death if they mutate and are called lethal genes. Tuesday, May 23, 2017 G Davidson 25 Gene mutations Type of Mutation Description Significance Substitution e.g. A swapped for T Changed amino acids lead to changed proteins Insertion Extra base in DNA All amino acids altered after inserted base Deletion Base chopped out All amino acids altered after deleted base Inversion Base order rotates through 180o One or two amino acids altered at inversion site, no more Tuesday, May 23, 2017 G Davidson 26 Substitution Normal DNA mRNA Amino acid Mutant C T T C A T G A A G U A Glutamic Tuesday, May 23, 2017 Valine G Davidson 27 Deletion Normal DNA A G U C A G U Mutant T C A C G T A A G U C A U G C C G T A mRNA Amino acid Serine Tuesday, May 23, 2017 Glutamine Serine G Davidson Arginine 28 Insertion Normal DNA A G U C A G U Mutant T C A C G A G U C A U G G C T C C A G mRNA Amino acid Serine Tuesday, May 23, 2017 Glutamine Serine G Davidson Proline 29 Inversion Normal DNA A G U C A G U Mutant T C A C G A G U C A U T A G C C G mRNA Amino acid Serine Tuesday, May 23, 2017 Glutamine Serine G Davidson Threonine 30 Mutation Frequencies Mutations, which cause defects, illnesses or death, are termed deleterious. Tuesday, May 23, 2017 G Davidson 31 Mutation Frequencies At this stage of evolutionary history, most genes are necessary and useful in their current forms, therefore a mutation is like to be deleterious and unlikely to give the organism any survival advantage. As a result, these harmful mutations tend to be lost quickly from a population, particularly if they are dominant alleles and lethal in their effect. Tuesday, May 23, 2017 G Davidson 32 Mutation Frequencies Recessive lethals can stay in a large population for longer as the chances of two recessive lethals of the same gene meeting in a zygote are very small, therefore the recessive will not be wiped out so quickly. The rate of mutations can be increased by mutagenic agents, e.g. Radiation such as X-rays, UV light, gamma rays Chemicals like colchinine, mustard gas etc. Tuesday, May 23, 2017 G Davidson 33 Mutation Frequencies These are termed induced mutations. Mutations are useful as they are the only source of new variation and without them there would be no source of variation. Without variation evolution would not be possible. Tuesday, May 23, 2017 G Davidson 34