Higher Biology

1. Higher Biology Mutations Mr G R Davidson

2. Mutations • Any change in the quantity or structure of an organism’s DNA is called a mutation. • Mutations can be very small changes from a single base alteration, to a major change involving altering the number of chromosomes. • Any change which alters the characteristics of and therefore, what the individual looks like, results in a mutant. • These can even be lethal. G R Davidson

3. Mutations • Gene mutations are very rare and occur randomly. • We express the rate of mutation as the number that occur per million gametes. • This frequency varies from species to species and gene to gene, e.g. in humans the mutation rate for haemophilia is 5, which means 1 in every 200,000. G R Davidson

4. Point Mutations • Point mutations are a source of variation. • They involve one of the base pairs in a single gene being altered. • This can result in the production of a new protein. • Three types of point mutation are: • Insertion • Deletion • Substitution G R Davidson

5. Insertion • An extra base is inserted into the sequence which results in all the bases after this point being moved along. • This is also referred to as a frame-shift mutation, as every amino acid after this mutation is altered. G R Davidson

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

7. A base is removed from the sequence and this also results in all the bases moving along. This is also referred to as a frame-shift mutation, as every amino acid after this mutation is altered. Deletion G R Davidson

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

9. In a substitution mutation one base is replaced by another different base. This only affects one amino acid in the protein which may not even have an effect, but if it is at a critical position in the protein it could result in something more serious such as sickle cell anaemia in humans. Substitution G R Davidson

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

11. Since there are 64 possible triplet combinations and only 20 different amino acids, some amino acids can actually be coded for by a number of different triplets. Point mutations can result in five different possibilities. Point Mutations G R Davidson

12. Neutral mutations This happens when a base is substituted and a different amino acid is translated. However, the replacement amino acid has similar properties to the correct one and so there is little or no effect on the protein. Point Mutations G R Davidson

13. Silent mutations This happens when a base is substituted but the same amino acid is translated. This results in the protein being the same as the original one was supposed to be and therefore, nothing changes. Point Mutations G R Davidson

14. Nonsense mutations This occurs when a substitution causes the new base sequence to have a stop codon. This means the polypeptide chain stops being put together at this point and is therefore shorter than it should be. Point Mutations G R Davidson

15. Missense Mutations This is also caused by a substitution and the protein gets a new amino acid at this point. This results in a protein being formed but it has a different function to the original. Point Mutations G R Davidson

16. Frame-shift mutations These are brought about by insertion or deletion mutations. They result in the entire sequence of the DNA being altered after the point of the mutation. Point Mutations G R Davidson

17. This is a mutation involving the introns and exons. If an intron is not cut out, it may remain in the mRNA, and this could result in it being translated and so the protein will have extra amino acids in its structure. Splice Site Mutations G R Davidson

18. Mutations are the only source of new variation in organisms. Occasionally a mutation results in an allele which provides the organism with a better survival advantage. If this happens, the mutant then reproduces and passes the new allele on to the next generation and so, the mutation frequency is increased – evolution. Importance of Mutations G R Davidson

19. Chromosome mutations happen on a much larger scale and involve chromosomes breaking. The broken chromosomes can join together and create new chromosomes. There are four different types of chromosome mutation. Chromosome Mutations G R Davidson

20. This happens when a number of genes from one of the homologous chromosomes is transferred to the other. This results in these genes being repeated. Duplication G R Davidson

21. 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 R Davidson

22. In this type of mutation the chromosome breaks in two places. The middle segment comes out and the two end pieces rejoin to form a shorter chromosome. Deletion G R Davidson

23. 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

24. In this type of mutation a piece of one chromosome breaks off and becomes attached to a different chromosome. This can lead to problems at meiosis when the chromosomes are pairing up. The result is usually gametes which are non-viable. Translocation G R Davidson

25. 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

26. This type of mutation occurs when a chromosome breaks in two places and the centre section revolves and joins up again. This means some of the genes are in reverse order. This can also result in non-viable gametes. Inversion G R Davidson

27. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 Inversion BREAK Original chromosome G Davidson

28. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 BREAK Original chromosome G Davidson

29. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 BREAK Original chromosome G Davidson

30. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 BREAK Original chromosome G Davidson

31. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 BREAK Original chromosome G Davidson

32. 1 2 3 4 1 2 3 4 5 6 7 8 5 6 7 8 1 2 3 4 7 6 5 8 BREAK Original chromosome New chromosome with inverted genes G Davidson

33. Polyploidy • Polyploidy is an extreme case of non-disjunction. • This results in a cell containing at least three times the normal chromosome compliment, (e.g. triploid = 3n and tetraploid = 4n). G Davidson

34. 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

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

36. 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

37. 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