Round and wrinkled peas

1. Round and wrinkled peas Fig. A, page 22

2. Mutations: Primary tools of genetic analysis • Mutations are heritable changes in base sequences that modify the information content of DNA • Forward mutation – changes wild-type to different allele • Reverse mutation – causes novel mutation to revert back to wild-type (reversion)

3. General observations of mutation rates • Mutations affecting phenotype occur very rarely • Different genes mutate at different rates • Rate of forward mutation is almost always higher than rate of reverse mutation

4. Mutations that cause a phenotypic change are very rare Fig. 7.3

5. Classification of mutations by effect on DNA molecule • Substitution – base is replaced by one of the other three bases • Deletion – block of one or more DNA pairs is lost • Insertion – block of one or more DNA pairs is added • Inversion – 180 degree rotation of piece of DNA • Reciprocal translocation – parts of nonhomologous chromosomes change places • Chromosomal rearrangements – affect many genes at one time

6. Fig. 7.2

7. Unequal crossing over creates one homologous chromosome with a duplication and the other with a deletion 7.10 a

8. Transposable elements move around the genome and are not susceptible to excision or mismatch repair Fig. 7.10 e

9. Trinucleotide instability causes mutations • FMR-1 genes in unaffected people have fewer than 50 CGG repeats. • Unstable premutation alleles have between 50 and 200 repeats. • Disease causing alleles have > 200 CGG repeats. Fig. B(1) Genetics and Society

10. Trinucleotide repeat in people with fragile X syndrome Fig. A, B(2) Genetics and Society

11. Are mutations spontaneous or induced? • Most mutations are spontaneous. • Luria and Delbruck fluctuation experiments and replica plating - simple ways to tell if mutations are spontaneous or if they are induced by a mutagenic agent

12. Fig. 7.4

13. Interpretation of Luria-Delbruck fluctuation experiments • Bacterial resistance arises from mutations that exist before exposure to bacteriocide • The bacteriocide is a selective agent killing the nonresistant cells, allowing only the preexisting mutant cells to survive. • Mutations do not arise as a direct response to environmental change • Mutations occur randomly at any time

14. Replica plating verifies preexisting mutations Fig. 7.5 a

15. Fig. 7.5b

16. Chemical and physical agents cause mutations • Deamination removes –NH2 group. Can change C to U, inducing a substitution to an A-T base pair after replication • Hydrolysis of a purine base, A or G occurs 1000 times an hour in every cell Fig. 7.6 a,b

17. X rays break the DNA backbone • UV light produces thymine dimers Fig. 7.6 c, d

18. Oxidation from free radicals formed by irradiation damages individual bases Fig. 7.6 e

19. Repair enzymes fix errors created by mutation Excision repair enzymes release damaged regions of DNA. Repair is then completed by DNA polymerase and DNA ligase Fig. 7.7a

20. DNA polymerase proofreading Fig. 7.8

21. Mutagens increase mutation rate using different mechanisms Fig. 7.12a

23. Fig. 7.12 b

24. Fig. 7.12 c

25. Consequences of mutations • Germ line mutations – affect the evolution of species • Somatic mutations – affect the survival of an individual • Cell cycle mutations may lead to cancer • Conditional mutations – produce changes in phenotype under one set of conditions but not under another • Conditional lethal mutations

26. The Ames test for carcinogens using his- mutants of Salmonella typhimurium Fig. 7.13

27. What mutations tell us about gene structure • Complementation testing - are two mutations in the same or different genes? • Benzer’s experiments - genes are linear sequences of nucleotide pairs • Some regions of chromosomes mutate at a higher rate than others – hot spots

28. Complementation testing Fig. 7.15 a

30. Fig. 7.15 b,c Five complementation groups (different genes) for eye color. Recombination mapping demonstrates distance between genes and alleles.

31. Recombination within a gene can generate wild-type Fig. 7.16

32. A gene is a linear sequence of nucleotide pairs • Seymour Benzer mid 1950s – 1960s • If a gene is a linear set of nucleotides, recombination between homologous chromosomes carrying different mutations within the same gene should generate wild-type • T4 phage as an experimental system • Can examine a large number of progeny to detect rare mutation events • Could allow only recombinant phage to proliferate while parental phages died

33. Benzer’s experimental procedure • Generated 1612 spontaneous point mutations and some deletions • Mapped location of deletions relative to one another using recombination • Found approximate location of individual point mutations by deletion mapping • Performed recombination tests between all point mutations known to lie in the same small region of the chromosome • Result – fine structure map of the rII gene locus

34. Working with T4 phage

35. Phenotypic properties of T4 phage Fig. 7.17 b

36. Complementation test for mutations in different genes

37. Detecting recombination between two mutations in the same gene Fig. 7.17 d

38. Deletions for rapid mapping of point mutations to a region of the chromosome Fig. 7.18 a

39. Recombination mapping to identify the location of each point mutation within a small region Fig. 7.18 b

40. Fine structure map of rII gene region Fig. 7.18 c

41. What mutations tell us about gene function • One gene, one enzyme hypothesis - a gene contains the information for producing a specific enzyme • Beadle and Tatum used auxotrophic and prototrophic strains of Neurospora to test hypothesis • Genes specify the identity and order of amino acids in a polypeptide chain • The sequence of amino acids in a protein determines its three-dimensional shape and function

42. Beadle and Tatum – One gene, one enzyme • 1940s – isolated mutants that disrupted synthesis of arginine • Auxotroph – needs supplement to grow on minimal media • Prototroph –needs no supplement • Recombination analysis - mutations in four distinct regions of genome • Complementation tests - each of four regions correlated with a different complementation group (4 different genes)

43. Fig. 7.20 a

44. Fig. 7.20 b

45. Interpretation of Beadle and Tatum experiments • Each gene controls the synthesis of an enzyme involved in catalyzing the conversion of an intermediate into arginine

46. Some proteins are multimeric, containing subunits composed of more than one polypeptide Fig. 7.24

47. How do genotypes and phenotypes correlate? • Alteration of amino acid composition of a protein • Alteration of the amount of normal protein produced • Changes in different amino acids at different positions have different effects

48. Summary of dominance relationships Fig. 3.2

49. Codominant blood group alleles Fig. 3.4b

50. Dominant or recessive alleles depend on the relationship between protein function and phenotype • Recessive – alleles that produce nonfunctional proteins (loss-of function) • Null mutations – no synthesis of protein or promote synthesis of protein incapable of carrying out any function • Hypomorphic mutations – produce much less protein or a protein with weak function • Incomplete dominance – phenotype varies in proportion to amount of protein