Higher Biology

1. Higher Biology Mutations G Davidson

2. 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. G Davidson

3. 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. G Davidson

4. 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. G Davidson

5. 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! G Davidson

6. Chromosome Numbers andPolyploidy • 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. G Davidson

7. 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. G Davidson

8. Polyploidy • Polyploidy is an extreme case of non-disjunction 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). G Davidson

9. 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. G Davidson

10. Parental cells Parental Gametes AA or 2n diploid Triploid Offspring AA or 2n Sterile AAA or 3n A or n haploid AA or 2n AA or 2n diploid Tetraploid Offspring AA or 2n Fertile AAAA or 4n AA or 2n diploid AA or 2n Polyploidy G Davidson

11. 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 G Davidson

12. 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. G Davidson

13. 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: G Davidson

14. Chromosome mutations G Davidson

15. 1 2 3 7 8 1 2 3 4 5 6 7 8 New chromosome BREAK 4 5 6 Deleted genes Original chromosome Deletion G Davidson

16. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 Inversion BREAK Original chromosome G Davidson

17. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 BREAK Original chromosome G Davidson

18. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 BREAK Original chromosome G Davidson

19. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 BREAK Original chromosome G Davidson

20. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 BREAK Original chromosome G Davidson

21. 1 2 3 4 7 6 5 8 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 New chromosome with Inverted genes 8 BREAK Original chromosome G Davidson

22. 1 2 3 4 3 4 5 5 6 7 8 1 2 3 4 5 6 7 8 Duplicated Genes from Homologous chromosome BREAK Original chromosome New chromosome Duplication G Davidson

23. 1 2 3 4 5 21 22 1 2 3 4 5 Chromosome A Translocated genes 21 22 23 24 23 24 BREAK Chromosome B Translocation G Davidson

24. 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. G Davidson

25. 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. G Davidson

26. Gene mutations G Davidson

27. Normal Mutant DNA C T T C A T G A A G U A mRNA Amino acid Glutamic Valine Substitution G Davidson

28. Normal DNA A G A G T C T U C U C A G A U C U C G A mRNA Amino acid Serine Serine Glutamine Deletion Mutant A G A G C T Arginine G Davidson

29. Normal Mutant DNA A G A G G T C A G A G T C U C U C A G U C U C C A G mRNA Amino acid Serine Serine Glutamine Proline Insertion G Davidson

30. Normal Mutant DNA A G A T G C A G A G T C U C U C A G U C U A C G mRNA Amino acid Serine Serine Glutamine Threonine Inversion G Davidson

31. Mutation Frequencies • Mutations, which cause defects, illnesses or death, are termed deleterious. G Davidson

32. 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. G Davidson

33. 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. G Davidson

34. 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. G Davidson