Mutation and DNA repair mechanism

Mutation and DNA repair mechanisms are briefly described here

1. Addis Ababa University School of Medicine Department of Biochemistry Advanced M olecular B iology Seminar Mutation and DNA repair Mechanism by Ayetenew Abita Wednesday December 20, 2017GC 7/30/2019 1

2. OUTLINE Introduction Causes of mutation Point mutation Frame shift mutation Abnormal intron removal and exon splicing Summary Reference 7/30/2019 2

3. INTRODUCTION The blue print of genetic information The master plan of genetic information The genetic database Is stored in the nucleus as DNA Central Dogma of Molecular Biology 7/30/2019 3

4. INTRODUCTION DNA damage Mutation  is a permanent change DNA sequence or changed in the genetic message carried by gene. Mutagen   is an agent that can bring about a permanent alteration to the physical composition of a DNA gene. Damaged mRNA lead to altered polypeptide sequence. 7/30/2019 4

5. CAUSE OF DNA MUTATION Mutations occur continuously through endogenous and exogenous DNA damage . Examples of endogenous DNA damage: Reactive oxygen species (ROS), DNA replication errors. Examples of exogenous DNA damage : tobacco carcinogens, toxins in food, pollution, ultraviolet light from the Sun, gamma and X-radiation, medications, viruses. 7/30/2019 5

6. TAUTOMERISATION The bases of DNA are subject to spontaneous structural alterations called tautomerisation . Some of the hydrogen atoms change their location producing a tautomer An amino group (-NH 2 ) can tautomerise to an imino form (=NH) A keto group (-C=O) can tautomerise to an enol form (=C-OH) Ex: Thymine ( keto form) shifts to enol form , which pairs with guanine. 7/30/2019 6

7. CAUSE OF DNA MUTATION 7/30/2019 7

8. UV (Ultraviolet) UV is normally classified in terms of its wavelength: UV-C (180-290 nm)--"germicidal"--most energetic and lethal, it is not found in sunlight because it is absorbed by the ozone layer. UV-B (290-320 nm)-- major lethal/mutagenic fraction of sunlight. UV-A (320 nm visible)" near UV“ also has deleterious effects primarily because it creates oxygen radicals, but it produces very few pyrimidine dimers . 7/30/2019 8

9. CAUSE OF DNA MUTATION 7/30/2019 9

10. Base Analog Bromouracil (structural analog of Thymine) Enzyme of nucleotide synthesis and DNA synthesis treat Bromouraci l as thymine and incorporate it into DNA , where it pairs with adenine. 7/30/2019 10

11. Alkylation Agent Alkylating agents are chemicals that add bulky chemical groups like methyl (CH 3 ) and ethyl (CH 2 -CH 3 ) chains to bases Ethyl methansulfonate (EMS) is such an alkylating agent The addition of ethyl group distorts the helix and leads to incorrect base pairing.. 7/30/2019 11

12. Deaminating agents Deaminating agents are chemicals that remove amino groups Adenine deamination with nitrous acid produces hypoxanthine, which can mispair with cytosine, inducing a T C mutation 7/30/2019 12

13. CAUSE OF DNA MUTATION 7/30/2019 13

14. THE EFFECTS OF MUTATIONS Some mutations cannot be passed on to offspring and do not matter for evolution.  Somatic mutations  occur in non-reproductive cells and won't be passed onto offspring. The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called  germ line mutations . 7/30/2019 14

15. EFFECTS OF GERM LINE MUTATIONS A single germ line mutation can have a range of effects No change occurs in phenotype. Some mutations don't have any noticeable effect on the phenotype of an organism. The mutation occurs in a protein-coding region, but ends up not affecting the  amino acid sequence of the  protein . Small change occurs in phenotype. Big change occurs in phenotype. Mutations that cause the death of an organism are called lethals . Little mutations with big effects: Mutations to control genes 7/30/2019 15

16. EFFECTS OF GERM LINE MUTATIONS 7/30/2019 16

17. TYPES OF DNA MUTATION 7/30/2019 17

18. Point mutation A  point mutation  is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a sequence of  DNAor  DNA. Point mutation is a random SNP (single nucleotide polymorphism ) mutation in the deoxyribonucleic acid ( DNA) that occurs at one point . 7/30/2019 18

19. Transition/ transversion categorization In 1959 ,  Ernst Freese Transition Mutations (Alpha) are due to replacement of a  purine  base with another  purine or a  pyrimidine  with another pyrimidine . Transversions Mutations (Beta) are replacement of a purine with a pyrimidine or vice versa. Transition mutations are about ten times more common than transversions . 7/30/2019 19

20. Transition/ transversion categorization 7/30/2019 20

21. Functional Categorization 1)  Nonsense Mutations a) Stop-gain is a mutation that results in a premature  termination codon  ( a stop was gained ), which signals the end of translation. This interruption causes the protein to be abnormally shortened.   b) Stop-loss is a mutation in the original termination codon ( a stop was lost ), resulting in abnormal extension of a protein's carboxyl terminus. c)Start-gain creates a TAC start codon upstream of the original start site. d) Start-loss is a point mutation in a transcript's TAC start codon , resulting in the reduction or elimination of protein production. 7/30/2019 21

22. 1) Nonsense Mutation Examples of truncated protein formed in nonsense mutation converts an amino acid codon into a termination codon . 7/30/2019 22

23. 2) Missense Mutations Code for a different amino acid. A missense mutation changes a codon so that a different protein is created, a non-synonymous change . a) Acceptable mutation Eg : Normal Hemoglobin A molecule , 67th amino acid in beta chain 7/30/2019 23

24. 2) Missense Mutations b)  Partially acceptable: Partially functional protein Example Sickle cell anemia ( HbS ) 7/30/2019 24

25. 2) Missense Mutations . c)  Unacceptable Mutation Incompatible with normal life Eg : HbM Distal histidine of alpha chain 7/30/2019 25

26. 3) Silent mutations • Since the genetic code are redundant single nucleotide can change, but the new codon specifies the same amino acid, resulting in an unmutated protein. A silent mutation has no effect on the functioning of the protein. • This type of change is called synonymous change , since the old and new codon code for the same amino acid. This is possible because 64 codons specify only 20 amino acids. 7/30/2019 26

27. Functional Categorization 7/30/2019 27

28. FRAMESHIFT MUTATION • Inserting or deleting one or more nucleotides Changes the “reading frame” like changing a sentence Proteins built incorrectly • A frameshift mutation is not the same as a single-nucleotide polymorphism in which a nucleotide is replaced, rather than inserted or deleted. 7/30/2019 28

29. Cont’d 7/30/2019 29

30. Splicing Errors Mutation of a splice site resulting in loss of function of that site. Results in exposure of a premature stop codon , loss of an exon , or inclusion of an intron . Mutation of a splice site reducing specificity. May result in variation in the splice location , causing insertion or deletion of amino acids, or most likely , a disruption of the reading frame. Displacement of a splice site, leading to inclusion or exclusion of more RNA than expected , resulting in longer or shorter exons . Although many splicing errors are safeguarded by a cellular quality control mechanism termed nonsense-mediated mRNA decay (NMD ), a number of splicing-related diseases also exist . 7/30/2019 30

31. SUMMARY Mutations occur continuously through endogenous and exogenous DNA damage Mutation might be point, frame shift, splicing error. lethal mutation : causes the developing organism to die prematurely. loss-of-function mutation: either reduces or abolishes the activity of the gene. gain-of-function mutation : increases the activity of the gene or makes it active in inappropriate circumstances . 7/30/2019 31

32. REFERENCE Harper’s Review of Biochemistry Lehniger’s principle of Biochemistry Lippincott’s Illustrated Review of Biochemistry Text Book of Biochemistry with clinical correlations- Devlin TM Text Book of Biochemistry by Vasudevan Text book of biochemistry, satyanarayana Principle of biochemistry, William H. simmons . 7/30/2019 32

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