Mutation
Mutations are any changes in the DNA sequence of an organism. They can be caused spontaneously during DNA replication or repair, or can be induced by mutagens like chemicals, radiation, or viruses. Mutations are classified as point mutations, which change a single DNA base, or frameshift mutations, which insert or delete DNA bases. Cells have DNA repair mechanisms to correct mutations, such as base excision repair, nucleotide excision repair, and mismatch repair. Unrepaired mutations can be harmful, beneficial, or have no effect on the organism.
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More from Mona Othman Albureikan / King Abdulaziz University
Mutation
- 1. DNA Mutation Presented by Mona Othman AlBureikan
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- 3. What are mutations? -Any changes in the DNA sequence of an organism is a mutation. - DNA is made of a long sequence of smaller units strung together. There are four basic types of unit: A, T, G, and C. - Some parts of DNA are control centers for turning genes on and off. - some parts have no function. - And some parts have a function that we don't understand yet. - Organisms have mechanisms such as DNA repair to remove mutations.
- 4. What are mutations? -Other parts of DNA are genes that carry the instructions for making proteins . - Proteins are long chains of amino acids. - These proteins help build an organism. - Protein-coding DNA can be divided into codons — sets of three bases that specify an amino acid or signal the end of the protein.
- 5. Classifications of mutations OR Types of Mutation
- 6. A- Good orbad or neutral. - A harmful mutation Is a mutation that decreases the fitness of the organism. - A beneficial mutation Is a mutation that increases fitness of the organism, or which promotes traits that are desirable. - A neutral mutation Has no harmful or beneficial effect on the organism. Such mutations occur at a steady rate, forming the basis for the molecular clock.
- 7. B- Somatic orgermline
- 8. C- Spontaneous orInduced Mutations - Most mutations are spontaneous, rather than being induced by a mutagen. - Spontaneous mutation ; A mutation occurring in the absence of mutagens, usually due to errors in the normal functioning of cellular enzymes.
- 9. Spontaneous Mutations These mutationscan be caused by: • Tautomerism – A base is changed by the repositioning of a hydrogen atom, altering the hydrogen bonding pattern of that base resulting in incorrect base pairing during replication. • The ability of a molecule to exist in more than one chemical form is called tautomerism . • All the four common bases of DNA (A, G, C and T) have unusual tautomeric forms, which are rare.
- 10. Spontaneous Mutations - Depurination – In molecular genetics, depurination is an alteration of DNA in which the purine base (adenine or guanine) is removed from the deoxyribose sugar by hydrolysis of the beta-N-glycosidic link between them. - Loss of a purine base form an apurinic site (AP site). where the sugar phosphate backbone remains and the sugar ring has a hydroxyl (-OH) group in the place of the purine.
- 11. Spontaneous Mutations • Deamination– Hydrolysis changes a normal base to an atypical base containing a keto group in place of the original amine group.
- 12. Spontaneous Mutations - Slippedstrand mispairing - Denaturation of the new strand from the template during replication, followed by renaturation in a different spot lead to insertions or deletions. - (SSM) is a process that produces mispairing of short repeat sequences between the mother and daughter strand during DNA synthesis
- 13. Induced Mutations • Ingredientsthat cause mutations are called mutagens. Mutagen is divided into three, namely: 1- Mutagenic chemicals like; • Hydroxylamine NH2OH • Base analogs (e.g. BrdU) • Alkylating agents • Agents that form DNA adducts • DNA intercalating agents • DNA crosslinkers • Oxidative damage
- 14. Induced Mutations Nitrousacid converts amine groups on A and C to diazo groups, altering their hydrogen bonding patterns which leads to incorrect base pairing during replication.
- 15. Induced Mutations 2- Mutagenmaterials physics like; - Ultraviolet rays. (can cause skin cancer). - Radioactive rays. - Gamma rays.
- 16. Induced Mutations - Ultravioletlight is absorbed by the nucleic acid bases, and the resulting influx of energy can induce chemical changes.
- 17. Induced Mutations - The most frequent photoproducts are the consequences of bond formation between adjacent pyrimidines within one strand, and, of these, the most frequent are cyclobutane pyrimidine dimers (CPDs). - T CPDs are formed most readily, followed by T-C or C-T; C-C dimers are least abundant.
- 18. Induced Mutations 3- Mutagenbiological substances. suspected viruses and bakeries can cause mutations. Of the virus that can cause mutations is DNA.
- 19. D- Forward orReverse Mutations - In an organism when mutations created a change from wild type to abnormal phenotype, then that type of mutations are called forward mutations. - Most mutations are of forward type. - The forward mutations are often corrected by error, correcting mechanism, so that an abnormal phenotype changes into wild type phenotype. Such mutations are called back or reverse mutations.
- 20. E- Point orframeshift mutations
- 21. Point mutations - Pointmutations also called Single base substitutions are single nucleotide base changes in a gene's DNA sequence. - It exchange a single nucleotide for another. - These changes are classified as transitions or transversions.
- 22. Point mutations - Mostcommon is the transition that exchanges a purine for a purine (A ↔ G) or a pyrimidine for a pyrimidine, (C ↔ T). - Less common is a transversion, which exchanges a purine for a pyrimidine or a pyrimidine for a purine (C/T ↔ A/G).
- 23. Point mutations - Pointmutations that occur within the protein coding region of a gene may be classified into three kinds, depending upon what the erroneous codon codes for: • Silent mutations: which code for the same amino acid. • Missense mutations: which code for a different amino acid. • Nonsense mutations: which code for a stop and can truncate the protein.
- 24. Point mutations Illustration ofthree types of point mutations
- 25. frameshift mutations - Frameshiftmutation is a mutation caused by add or remove one or more DNA bases of nucleotides that is not evenly divisible by three from a DNA sequence. - Insertion or deletion mutations cause frameshift mutations.
- 26. frameshift mutations • Additionor deletion of one or two bases results in a new sequence of codons which may code for entirely different amino acids. This results in a drastic change in the protein synthesized.
- 27. DNA Repair • Cellshave developed a number of systems designed to repair DNA damage and correct mutations. 1- Photoreactivation - For Repair Thymine Dimers by A brief exposure to blue light following UV exposure can reverse the effects of the UV radiation.
- 28. DNA Repair • Anenzyme called photolyase or photoreactivation enzyme (PRE), which cleaves the covalent bonds linking the thymine dimers using the energy from a DNA Repair Pathway. This flow chart shows one way that damaged photon of blue light. DNA is repaired in E. coli bacteria.
- 29. DNA Repair • 2- Base excision repair The damaged or inappropriate base is removed from its sugar linkage and replaced. These are glycosylase enzymes which cut the base-sugar bond. • 3- Nucleotide excision repair - This system works on DNA damage which is "bulky" and creates a block to DNA replication and transcription. - (UV-induced dimers and some kinds of chemical adducts). - It probably recognizes not a specific structure but a distortion in the double helix.
- 30. DNA Repair 4- Recombinational(daughter-strand gap) repair - This is a repair mechanism which promotes recombination to fix the daughter-strand gap--not the dimer--and is a way to cope with the problems of a non-coding lesion persisting in DNA. - Double Strand Break (DSB) Repair. Shown is an overview of the main steps and factor requirements for DNA DSB repair by homologous recombination (left) and non-homologous end-joining (right).
- 31. DNA Repair • Mismatch repair - This process occurs after DNA replication as a last "spellcheck" on its accuracy. - In E. coli, it adds another 100-1000-fold accuracy to replication. - It is carried out by a group of proteins which can scan DNA and look for incorrectly paired bases (or unpaired bases) which will have aberrant dimensions in the double helix. - The incorrect nucleotide is removed as part of a short stretch and then the DNA polymerase gets a second try to get the right sequence.
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Editor's Notes
- #30 example: uracilglycosylase--enzyme which removes uracil from DNA.Several proteins are involvedin this process (in prokaryotes these are the products of the 'uvr' genes, for 'UV repair').