Mutations

1. Mutations Year 13

2. Mutations • A mutation is a permanent genetic mistake in a gene or a chromosome. • Mutations can occur spontaneously or be induced. Spontaneous mutations arise from errors in replication. Induced mutations are caused by mutagens. • Mutations are the only way that new alleles can be created

3. Mutagens • Mutagens are chemicals or radiation that can induce mutations eg. Nuclear radiation, UV rays, x-rays, tobacco, agent orange, asbestos.

4. http://www.youtube.com/watch?v=abcsZZ9Duxw • http://www.youtube.com/watch?v=8RdvlNsQx-A

5. The effect of mutations • When mutations occur in the testes and ovaries they will be inherited by future generations (GAMETIC MUTATIONS). • Somatic mutations in body cells are not inherited but can effect the person during their life time.

7. Not all mutations are bad • Beneficial mutations: Examples: 1) bacteria becoming resistant to antibiotics 2) pesticide resistance 3) tolerance to high cholesterol levels (see page 143) 4) fast rates of mutations in the protein coat of viruses

8. Harmful mutations • Cystic fibrosis • Sickle cell disease • Albinism

9. Neutral mutations • Are neither harmful or beneficial but may have an effect in future generations. • An example of a neutral mutation is when the codon AUU is changed to AUC. They both code for the amino acid leucine, so the protein is unlikely to be affected.

10. Types of mutations • 1) Gene mutation – this is when a base change affects the DNA sequence of a single gene.

11. 2) Chromosomal rearrangements = block mutations • This is when blocks of genes within a chromosome are rearranged.

12. 3) Changes in chromosome number Aneuploidy is the loss or gain of whole chromosomes. Polyploidy is the loss or gain of complete sets of chromosomes.

13. 1. Gene mutations • Point mutations – changes in a single nucleotide. A nucleotide can be replaced by another (substitution), it can be removed (deletion) or an extra nucleotide can be added (insertion). • Tautomerism – abnormal base pairing.

14. Insertion mutations • When a single extra base is inserted into the DNA sequence a new sequence of codons can result due to a reading frame shift. • The protein that is then made is usually non-functional. The closer the insertion is to the start codon the more the protein will be affected.

16. Deletion mutations • A deletion of a base in a DNA sequence can have the same effect as an insertion mutation due to a reading frame shift.

18. Substitution mutations • Occurs when a base is substituted for another base. • Mis-sense substitution – change in codon leads to formation of protein but not the right protein. If the third base in a triplet is substituted, the amino acid may not actually be changed. • Non-sense substitution- the amino acid is changed to a stop codon resulting in a shorter, usually non-functional protein.

19. Mis-sense substitution

20. Non-sense substitution

21. Tautomerism Some point mutations may result from bases with an abnormal number of hydrogen-bonding sites. This results in abnormal base pairing. Pairs are called tautomers.

22. Inversion • When two nucleotides are inverted. If this happens within a codon, only the amino acid will be affected.

23. Do point mutation problems page 148Then 141-144

24. Inherited metabolic disorders – Page 146 biozone More than 6000 diseases attributed to diseases in single genes. • Sickle cell disease • B- Thalassaemia • Cystic Fibrosis • Huntington Disease

25. Sickle Cell Disease Single nucleotide substitution in HBB gene that codes for beta chain of haemoglobin. Autosomal recessive mutation.

26. Animation – sickle cell • http://www.hhmi.org/biointeractive/dna/DNAi_sicklecell.html

28. People that are heterozygous for sickle cell mutation are often resistant to malaria. Studies have shown that African Americans, who have lived in malaria-free areas for as long as ten generations, have lower sickle cell gene frequencies than Africans -- and the frequencies have dropped more than those of other, less harmful African genes. Similarly, the sickle cell gene is less common among blacks in Curacao, a malaria-free island in the Caribbean, than in Surinam, a neighboring country where malaria is rampant -- even though the ancestors of both populations came from the same region of Africa.

29. Cystic fibrosis Over 500 different recessive mutations of the CFTR gene have been identified. Most common mutation (70% of sufferers) is a triplet deletion (AAA), which means the 508th amino acid of the CFTR gene is missing (deletion mutation). Autosomal recessive mutation.

31. Pages 146-148 • http://www.youtube.com/watch?v=FMAOEOmLoUE

32. Chromosome mutations • Causes: • Errors in crossing over at meiosis • mutagens

33. Types of chromosomal (block) mutations • Inversion – pieces of chromosomes are flipped over so the genes appear in the reverse order. • There is no loss of genetic material:

34. 2) Translocation Pieces of chromosome are moved from one chromosome to another. Can cause major problems when the chromosomes are passed to gametes. Some will receive extra genes, some will be deficient.

37. 3) Duplication Pieces of chromosomes are repeated so there are duplicate segments. One chromosome “donates” a segment of chromsome to another. Some gametes will receive double the genes, others will have no genes for the affected segment.

39. 4) Deletion • Pieces of chromosome are lost when: • a middle piece of the chromosome falls out and the two ends rejoin, so some genes are lost. • The end of a chromosome may break off and is lost.

41. Do pages 150 and 151

42. Aneuploidy The diploid (2n) number of chromosomes in humans is 46. The haploid (n) number of chromosomes in humans is 23. Aneuploidy is the loss or gain of whole chromosomes. Extra or lost chromosomes can either be autosomes or sex chromsomes.

43. Some terminology • Disomy = 2n (normal) • Monosomy = 2n – 1 eg. Turner’s syndrome • Trisomy = 2n + 1 eg. Down’s syndrome

44. Trisomy in human autosomes • 1) Down’s syndrome: Trisomy 21

45. Causes of Down’s syndrome • 92% of cases due to non-disjunction of chromosome 21 during meiosis • 5% result from translocation of chromosome 21 (usually onto chromosome 14).

47. Non-disjunction in meiosis I Non-disjunction in meiosis II Normal meiosis

48. 2) Patau Syndrome: Trisomy 13 Usually die before 3 months A newborn male with full trisomy 13 (Patau syndrome).  this baby has a cleft palate, atrial septal defect, inguinal hernia, and postaxial polydactyly of the left hand.

49. 3) Edward syndrome: Trisomy 18

50. Many aneuploidies show a “maternal age effect” with incidence increasing with age of mother.