mutation-20.2.2019.ppsx
- 1. MOLECULAR BASIS OF MUTATION Prof. A. B. Das Dept. of Botany Utkal University, Vini Vihar Bhubaneswar -751004 E-mail: abdas.uubot@gmail.com
- 2. What is mutation? Mutation is the permanent hereditary changes of genetic make up of an individual which is caused by structural and compositional changes of genes. Mutation is of gene mutation or point mutations and chromosomal mutation
- 3. Hugo DeVries propose the ‘Mutation Theory’ & the term mutation during his rediscovery of Mendel's laws of Inheritance.
- 4. Mutations are of two type i) Somatic mutation ii) Germinal mutation. Somatic mutation – occur in the somatic cells that leads to phenotypic changes of the organs or structures. Somatic mutations are nonheritable and thus not carried forward to the next generation. In plants the mutation can be passed on to the progeny by vegetative reproduction like budding and grafting like in cacti.
- 5. Mutations are of two type i) Somatic mutation ii) Germinal mutation. Germinal mutations - occur in the germ cells or gametes. These are heritable and are expressed in the next generation.
- 6. Mutable genes - Each gene is a potential site for mutation, but some genes mutate more frequently than others and these genes are called mutable genes. These are mainly structural genes which on mutation produce a changed phenotype. For example, somatic mutations giving rise to leaf variegation are due to mutable genes. Example: R-gene in maize (that controls anthocyanin production) mutates more frequently at the rate of 01 out of 1,00,000 gamets (discovered by Emerson).
- 7. Mutator genes – the gene which influence the rate of mutation is called mutator gene. Ex: Dt gene influences gene a1 (of the locus a1 - a1 for green colour) to mutate to a1 so that the green leaves of maize become variegated (streaked with purple) and seeds are speckled with purple dots (Rhoades,1938). Haldane estimated that a human gene could remain without any mutation for 2,500,000 yrs. As a result Mc Kausick could trace out 1,364 heritable diseases in man. Till date 2,811 heritable diseases have been discovered.
- 8. Reverse mutation: Most mutant events consist of a change from normal or wild type to a new genotype (recessive or dominant). Such mutation events are called forward mutations, while if the same mutant genotype changes back to the wild type, it is back or reverse mutation.
- 9. Paramutations: In heterozygotes, alleles are unaffected by the presence of other alleles. The locus R in corn bears a number of alleles, concerned with the formation of anthocyanin pigment. Normally Rr gene in single dosage produces dark mottling and two or three produce full colour. But when Rr alleles extracted from heterozygous condition with Rsf or Rmb alleles from other localities produces considerable lighter pigment than before. This change in Rr is heritable and Brink has described this effect as paramutation and Rsf and Rmb are paramutagenic.
- 10. Detection of mutations 1.ClB method in Drosophila (Muller ClB method of induced mutation) C = represent a long inversion of chromosomes that prevents crossing over within the inverted segments. Gene l and B are located in this segment. l = recessive lethal mutant present on X chromosome. Female Drosophila homozygous for gene l/l and male hemizygous l/Y are non viable. B= Sex-linked dominant mutant producing bar-shaped eyes. Parents – ClB heterozygous female X irradiated wild type male. Mutagen - X-ray Result- Bar eyed female but no males. X2-progeny consists of only females and 50% surviving male if mutation is not lethal
- 11. 2. Muller – 5 method in Drosophila for detection for lethals in X-chromosomes. Gene for apricot eye and bar eye are used as genetic markers. Gene Wa – Apricot eye (a recessive mutant of the red colour) Gene B – Bar eye character (a dominant mutant of the normal eye) Locations of genes: These genes are located on the X-chromosome which has a long inversion in the area of these two genes. The inversion eliminates crossing over in this area. Parents: Homozygous apricot bar female (WaB) and wild male (irradiated to induced mutations with X ray). Result: The r2 generation 50% male will receive X-chromosomes with WaB genes and other 50% with irradiated X chromosomes. 50% R2 male will not survive if mutation is lethal. The 50% male with WaB will be apricot bar i.e. R2 offspring female and male are produced in 2:1 ratio. Of the females 50% are bar –eyed and 50% normal. Of the male 50% will survive and are apricot-bar while 50% do not survive because the irradiated X- chromosomes has lethal mutation.
- 12. Molecular basis of gene mutations The process of replication of DNA sometimes shows inaccuracy which may occur within the DNA due to some internal or external, natural or artificial factors. Such inaccuracy at any one of these levels introduces changes in the arrangement of nucleotides in a polynucleotide chain of a DNA molecule.
- 13. Molecular basis of gene mutations The smallest change may involve the addition, deletion or substitution of a single nucleotide pair in the DNA molecule, which may change the reading of genetic code and ultimately may be manifested into an altered phenotype. If mutation produced, codes for one of the terminator or nonsense codon (UAA, UAG or UGA), the reading of RNA would stop at that point. This would prevent a complete polypeptide chain from being formed and could result in a lethal mutation
- 14. Gene Mutation Substitution Mutations 1.Deletion Mutations 2. Insertion Mutations 1.Transition 2. Transversion Frame-Shift Mutations
- 15. 1. Transition Replacement of a purine or pyrimidine by another purine or pyrimidine in a polynucleotide chain, caused by: (a)Tautomerisation (a) Ionization (c) Base analoges (d) Deamination
- 16. 2. Transversion Replacement of purine by a pyrimidine or vice versa is caused by : (a) Alkylating agents (b) Depurination
- 17. Since these mutations include very limited segment of DNA, these are called point mutations. They are of following types : 1.Substitution mutations 2. Frame-shift mutations
- 18. I. Substitution mutations In a substitution mutation, a nitrogenous base of a triplet codon of DNA is replaced by another nitrogenous base or some derivative of the nitrogen base, changing the codon. The altered codon may code for a different amino acid and may result in the formation of a protein molecule with single amino acid substitution, whose effect may be seen in altered phenotype. The substitution mutations may be of the following two types : 1. Transitions Transitions are changes that involve replacement of one purine in a polynucleotide chain by another purine and correspondingly in the complementary chain the replacement of one pyrimidine by another pyrimidine. This is called copy error mutation. They are of following four types.
- 19. a.Tautomerisation: In a normal molecule of DNA, the purine- adenine (A) is linked to the pyrimidine-thymine (T) by two bonds, while the purine-guanine (G) is linked to pyrimidine-cytosine (C) by three bonds. However, all these four nitrogenous exist in alternate states. These states are called tautomers and are formed by the 'rearrangement in the distribution of hydrogen atoms (tautomeric shifts). Due to tautomerisation the amino (- NH2) group of cytosine and adenine is converted into imino (-NH) group and likewise keto (C=O) of thymine and guanine is converted to enol group (-OH). In its tautomeric state, a nitrogenous base can not pair to its normal partner. Rather a tautomeric adenine, pairs with the normal cytosine and tautomeric guanine with thymine. Similarly, tautomeric thymine pairs with normal guanine and cytosine with adenine. Such pairs of nitrogenous bases are known as 'forbidden base pairs' or 'unusual base pairs'.
- 20. a.Tautomerisation: The rare bases can introduce mutations during DNA replication. If for example, adenine in the chain of a parent DNA is in rare state, the complementary new chain formed from this chain will contain cytosine. At the time of next replication this cytosine would pair with guanine. This will produce a substitution of A = T base pair by G ≡ C pair. Similarly, a substitution of G ≡ C by A = T pair can be produced if cytosine is in tautomeric state. This situation is know as copy error and is not stable because at the next replication the tautomaric base returns to common state and pair with thymine. Common bases of DNA and their tautomers. Keto (C=O) form on nitrogenous base changes to enol (-OH) and amino (-NH2) form to imino (-NH) form.
- 23. MUTATION Standard base-pairing arrangements Anomalous base-pairing arrangements
- 24. b. Ionization: Transition may also be introduced by ionization of a base at the time of DNA replication. It involves the loss of the hydrogen from number 1 nitrogen of nitrogenous base. Example: Ionized T pair with normal G and ionized G pair with normal T . Forbidden base pairs of thymine and guanine resulting from the ionization of No. 1 nitrogen.
- 25. Two forms of 5-BU C. Base analogues Some of the chemical compounds have molecular structure similar to the nitrogenous bases present in DNA necleotides. These are called base analogues. These are usually deravatives of nitrogenous bases of DNA and occur as natural as well as artificial base analogues. Eg. Natural analogues - 5-methyl cytosine, 5-hydroxymethyl cytosine (in E. coli), 5- hydroxymethyl uracil. Artificial base analogues: 5-bromouracil (5-BU), 5-iodo uracil (5-IU)
- 26. Base analogues – bromouracil is base analogue of thymine Aminopuration Aminopurine a chemical artificial base analogue of adenine. It can substitute adenine as well as can pair with cytosine.
- 27. D. Deamination: Some chemical substances like nitrous acid (HNO2), hydroxylamine diethyl sulphate (DES), ethylmethane sulphonate (EMS) ethyl ethane sulphonate (EES) changes base sequences in DNA by a series of chemical steps. Nitrous acid and hydroxylamine causes deamination of nitrogenous bases by replacing amino group (-NH2) by hydroxyl group (-OH).
- 28. 2. Transversions: Certain chemicals like EMS (ethyl methane sulphate) , MMS (methyl methane sulphate) induce substitutions by two ways: A. Transition: by substituting a purine for purine or pyrimidine for a pyrimidine. B. Transversion: by substituting a purine for a pyrimidine or pyrimidine for a purine.
- 29. The cause of transversions is that these chemicals alkylate the purine nitrogenous base in the nitrogen at the 7th position in the guanine and adenine and finally lead to its separation from the DNA strand. This is also know as depurination
- 30. Mechanism of depurination caused by alkylating agent. Substitution of A=T by C≡G as a result of transversion. The removal of a purine from strand of DNA leaves a gap. At the time of replication any of the four base can possibly get inserted at this place in the complementary strand. If the nucleotide contains a pyrimidine it is a transition and if purine then it is transversion. In the next cycle of DNA synthesis a DNA molecule is formed which contains complete transversion which is nonreversible.
- 31. 2. Frame Shift Mutations The mutations caused by the addition or deletion of nitrogenous bases in the DNA or mRNA are known as frame-shift mutations, because these shift the reading frame of codons from the site of change onward. One deletion may be neutralised by one addition or vice-versa, provided they occur at the same place or very close to each other. Frame-shift mutation and the changes in the reading of genetic code
- 32. Types of Frame-shift Mutations The frame-shift mutations are of two types : 1. Deletion mutations : These mutations are caused due to the loss or deletion of one or more nucleotides. 2. Insertion mutations : These mutations are caused by the addition of one or more extra nucleotides in a DNA molecule at one or more places.
- 33. Mechanism of Origin of Frame-shift Mutation Acridine dyes have been found to cause deletion or insertion of a single base pair. Acridines like 5-aminoacridine and proflavin become intercalated between two adjacent purines and thus increase the distance between then from 3.4 Å to 6.8 Å. At the time of DNA replication, either a nitrogenous base pair is introduced in the gap or a nitrogenous base pair is lost. Frame-shift mutations caused by the deletions are also introduced by the removal of ethylated bases.
- 34. Biological Significance of Point Mutations All point mutations are not lethal. Transition and transversions are relatively benign, because these cause replacement of only one amino acid in the peptide chain coded, which might not produce any significant change. Such mutations are known as silent mutations. However, frame-shift (insertion and deletion) mutations cause all the DNA beyond the point of mutation to be misread. Such mutations are often lethal. If the deleted segment includes nucleotides in three or multiples of three, there is no frame-shift error beyond the site of mutation but protein produced may be defective.
- 35. Effect of some chemical mutagens