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Mutations
Mutations are changes in genetic material that can be passed from parent cells to daughter cells. There are two main types of mutations: 1. Spontaneous mutations arise from errors in DNA replication and can include base pair substitutions like transitions/transversions and frameshift mutations from insertions/deletions of bases. 2. Induced mutations are caused by external mutagenic agents like chemicals or radiation that alter DNA structure. Mutations can have varying effects depending on if they result in nonsense/missense amino acid changes or are silent/neutral. Frameshift mutations alter all codons after the mutation. Mutations are an important source of genetic variation and play a role in evolution.
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Mutations
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- 2. Definition • A mutationis a sudden change in the genetic material of a cell that can be passed on from the cell to its daughter cells during the process of cell division.
- 3. • Mutations areimportant for several reasons. • First mutations are the source of new variation important for evolution. • Mutations are considered to be one of the ways that evolution happens.
- 4. • Mutations canlead to various human medical conditions. • Mutations can happen on a number of different scales. • Mutations involving only a single base pair in a DNA molecule are called point mutations.
- 5. Somatic vs germcell mutations • In multicellular organisms such as plants or animals mutations may occur in the somatic cells of the organism. • Somatic cells are the cells involved in growth and repair and maintenance of the organism.
- 6. • A mutationin these cells may lead to cancer and certain types of chromosomal mutations may be involved in aging.
- 7. • Other mutationshappen in the germ cells • These mutations may appear in the gametes and then in the offspring produced by sexual reproduction • These sorts of mutations are called germ cell/line mutations.
- 8. Causes of mutations •Mutations are often classified as: 1. Spontaneous and 2. Induced
- 9. • Spontaneous mutationsare mutations which arise because of errors in the DNA duplication process.
- 10. • As partof this process there is a proof reading mechanism involving one of the DNA polymerases. • But this process is not 100% perfect, leading to a low level of mutations per generation.
- 11. • For humansthis mutation rate is about 1 x 10-6 mutations per gene locus per generation. • This means that it is likely that most of us are carrying at least one new mutation not found in our parents.
- 12. • Induced mutationsare mutations brought about by exposure to chemicals or various forms of radiation. • These are called as Mutagens
- 13. 13 Two main typesof Mutations –Spontaneous mutations •Result from abnormalities in biological processes •Underlying cause lies within the cell –Induced mutations •Caused by environmental agents •Cause originates outside of the cell
- 14. 14 • Causes ofspontaneous mutations –Abnormalities in crossing over –Aberrant segregation of chromosomes during meiosis –Mistakes by DNA polymerase during replication –Alteration of DNA by chemical products of normal metabolic processes –Integration of transposable elements –Spontaneous changes in nucleotide structure
- 15. 15 • Induced mutationsare caused by mutagens –Chemical substances or physical agents originating outside of the cell –Enter the cell and then alter the DNA structure
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- 17. 17 CAUSES OF MUTATIONS •Spontaneous mutations are random events –Not purposeful –Mutations occur as a matter of chance • Some individuals possess beneficial mutations – Better adapted to their environment – Increased chance of surviving and reproducing • Natural selection results in differential reproductive success – The frequency of such alleles increases in the population
- 21. Types of Mutations •Transitions • Transversions Point Mutations • Frameshift mutations • These types are observed under spontaneous mutations Substitution Mutations
- 22. Mechanism of spontaneousmutation • Proposed by Watson & Crick • Structures of bases in DNA are not static • H atoms can move from one position to another (from an amino group to a ring Nitrogen) – tautomeric shifts • Can alter pairing potential of the bases
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- 25. • Thymine andGuanine – more stable keto forms • Adenine and Cytosine – more stable amino forms • Tautomeric shifts – less stable enol and imino forms respectively (infrequently)
- 26. • Bases areexpected to be in their less stable tautomeric forms for only very short periods of time • If the base existed in the rare form at the time of replication results in a mutation • adenine-cytosine and guanine-thymine base pairs • AT to GC or a GC to AT base pair substitution
- 30. • Mutations resultingfrom tautomeric shifts in the bases of DNA involve the replacement of a purine in one strand of the DNA with the other purine OR replacement of a pyrimidine in the complementary strand with the other pyrimidine • Such base pair substitutions are called transitions
- 31. • Base pairsubstitutions involving the substitution of a purine for a pyrimidine and pyrimidine for a purine are called transversions
- 33. Effects substitution mutations •Substitution mutations may vary in effect on the polypeptide resulting from translation. • Silent mutation the mutation does not alter the polypeptide at all. • While, nonsense mutations may lead to a radically altered polypeptide.
- 34. Normal TAC GTG ATACCA AAG TAG ACT AUG CAC UAU GGU UUC AUC UGA met his tyr gly phe ile -
- 35. Missense mutation TAC GTGATA GCA AAG TAG ACT AUG CAC UAU CGU UUC AUC UGA met his tyr arg phe ile -
- 36. Normal TAC GTG ATACCA AAG TAG ACT AUG CAC UAU GGU UUC AUC UGA met his tyr gly phe ile -
- 37. Nonsense Mutation TAC GTGATT CCA AAG TAG ACT AUG CAC UAA GGU UUC AUC UGA met his - - - - -
- 38. 41 EFFECTS OF MUTATIONS •Nonsense mutations –Normal codon is changed into a stop codon • e.g., AAA AAG (lys stop) –Translation is prematurely terminated • Truncated polypeptide is formed –Protein function is generally affected
- 40. 43 EFFECTS OF MUTATIONS •Missense mutations – Amino acid sequence is altered • e.g., GAA GTA (glu val) – Phenotype may be affected
- 41. Normal TAC GTG ATACCA AAG TAG ACT AUG CAC UAU GGU UUC AUC UGA met his tyr gly phe ile -
- 42. Neutral mutation TAC GTGATA CGA AAG TAG ACT AUG CAC UAU GCU UUC AUC UGA met his tyr ala phe ile -
- 43. 46 EFFECTS OF MUTATIONS •Neutral mutations –Type of missense mutation –Amino acid sequence is altered • e.g., CTT ATT (leu ile) • e.g., GAA GAC (glu asp) –No detectable effect on protein function • Missense mutations substituting an amino acid with a similar chemistry to the original is likely to be neutral
- 44. Normal TAC GTG ATACCA AAG TAG ACT AUG CAC UAU GGU UUC AUC UGA met his tyr gly phe ile -
- 45. Silent mutation TAC GTGATA CCG AAG TAG ACT AUG CAC UAU GGC UUC AUC UGA met his tyr gly phe ile -
- 46. • Addition ordeletion of one or a few base pairs - collectively referred to as frameshift mutations
- 47. The effects offrameshift mutation • In frameshift mutations, since a base is either added or removed from a codon, the effect is to shift the codons as read during translation by one position which radically alters the meaning of the mRNA in terms of how it is translated..
- 48. • Frameshift mutationchanges all the amino acids after the first one because codons are in groups of three non overlapping RNA bases.
- 50. • If messengerRNA resulting from the original DNA can be thought of as the following string of letters decoded by reading each group of three non overlapping letters: THEBIGCATATETHERAT = THE BIG CAT ATE THE RAT If the third letter(E) in the string is deleted we have: THBIGCATATETHERAT = THB IGC ATA TET HER AT which makes no sense when decoded.
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