B.sc. agri i pog unit 2 mutation
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Mutations are changes in DNA that can be caused spontaneously due to errors during DNA replication or induced by mutagenic agents like radiation and chemicals. There are two main categories of mutations: somatic mutations, which occur in body cells and are not passed to offspring, and germline mutations, which occur in sex cells and can be inherited. Common types of mutations include substitutions, insertions, deletions, and frameshift mutations. Bacteria have multiple DNA repair mechanisms like excision repair, photoreactivation, and post-replication repair to fix mutations through processes like removing and replacing damaged sections of DNA.
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B.sc. agri i pog unit 2 mutation
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- 2. Mutations Mutations in simple words are “change in DNA” Any sudden change occurring in hereditary material is called as mutation Term mutation was given by Devries in 1901 while studying evening primerose Oenothera lamarckiana Most of these were chromosomal variations Resultant effects – Positive Neutral Negative 2
- 3. In multicellular organism, two broad categories of mutations: Somatic mutations & germ line mutations Somatic mutations Arise in the somatic cells Passed on to other cells through the process of mitosis Effect of these mutations depends on the type of the cell in which they occur & the developmental stage of the organism If occurs early in development, larger the clone of the mutated cells 3
- 4. Germ line mutations They occur in the cells that produce gametes Passed on to future generations In multicellular organisms, the term mutation is generally used for germ line mutations 4
- 5. Base substitution is of two types: Transition: Purine is replaced with a purine Pyrimidine is replaced with a pyrimidine 5
- 6. Transversions: A purine is replaced by a pyrimidine or a pyrimidine is replaced by a purine 6
- 7. Missense mutation: a base is substituted that alters a codon in the mRNA resulting in a different amino acid in the protein product TCA AGT UCA TTA AAT UUA Ser Leu 7
- 8. Nonsense mutation: changes a sense codon into a nonsense codon. Nonsense mutation early in the mRNA sequence produces a greatly shortened & usually nonfunctional protein TCA AGT UCA TGA ACT UGA Ser Stop codon 8
- 9. Silent mutation: alters a codon but due to degeneracy of the codon, same amino acid is specified TCA AGT UCA TCG AGC UCG Ser Ser 9
- 10. Insertions & deletions: 2nd major class of gene mutation Addition or the removal, respectively, of one or more nucleotide pair Usually changes the reading frame, altering all amino acids encoded by codons following the mutation Also called as frame shift mutations Additions or deletions in the multiples of three nucleotides will lead to addition or deletion of one or more amino acids These mutations are called in-frame insertions and deletions, respectively. 10
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- 12. Run-on mutation Stop codon lost so protein is extra long (can also produce nonsense and run- ons) Summary of Mutation Types 12
- 13. Causes of Mutations A) Spontaneous errors (due to enzyme) B) Induced errors by mutagenic agents UV radiation X – rays Chemical Agents C) Transposable elements (Transposons)* Mutagen: Agent that causes mutations 13
- 14. *Transposable element A transposable element (TE, transposon or retrotransposon) is a DNA sequence that can change its position within the genome, sometimes creating or reversing mutations and altering the cell's genome size. Barbara McClintock's discovery of these jumping genes earned her a Nobel prize in 1983. 14
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- 16. On the basis of Causative agent of mutation: Spontaneous: Mutations that result from natural changes in DNA (1 in 109 bp) Occur in the absence of a mutagen Induced: Results from changes caused By environmental chemicals & radiations Any environmental agent that increases the rate of mutation above the spontaneous is called a mutagen such as chemicals & radiations 16
- 17. Ionizing Radiation: UV UV radiation causes thymine dimers, which block replication. Light-repair separates thymine dimers Sometimes the “repair job” introduces the wrong nucleotide, leading to a point mutation. 17Figure 8.20 3
- 18. Since replication errors and a variety of mutagens can alter the nucleotide sequence, a microorganism must be able to repair changes in the sequence that might be fatal. DNA is repaired by several different mechanisms besides proofreading by replication enzymes (DNA polymerases can remove an incorrect nucleotide immediately after its addition to the growing end of the chain). Repair in E. coli is best understood and is briefly described in this section. 18
- 19. Excision Repair Excision repair is a general repair system that corrects damage that causes distortions in the double helix. A repair endonuclease or uvrABC endonuclease removes the damaged bases along with some bases on either side of the lesion. The resulting single-stranded gap, about 12 nucleotides long, is filled by DNA polymerase I, and DNA ligase joins the fragments. This system can remove thymine dimers and repair almost any other injury that produces a detectable distortion in DNA. 19
- 20. Besides this general excision repair system, specialized versions of the system excise specific sites on the DNA where the sugar phosphate backbone is intact but the bases have been removed to form apurinic or apyrimidinic sites (AP sites). Special endonucleases called AP endonucleases recognize these locations and nick the backbone at the site. Excision repair then commences, beginning with the excision of a short stretch of nucleotides. 20
- 21. Another type of excision repair employs DNA glycosylases. These enzymes remove damaged or unnatural bases yielding AP sites that are then repaired as above. Not all types of damaged bases are repaired in this way, but new glycosylases are being discovered and the process may be of more general importance than first thought. 21
- 22. Removal of Lesions Thymine dimers and alkylated bases often are directly repaired. Photoreactivation is the repair of thymine dimers by splitting them apart into separate thymines with the help of visible light in a photochemical reaction catalyzed by the enzyme photolyase. Because this repair mechanism does not remove and replace nucleotides, it is error free. 22
- 23. Sometimes damage caused by alkylation is repaired directly as well. Methyls and some other alkyl groups that have been added to the O–6 position of guanine can be removed with the help of an enzyme known as alkyltransferase or methylguanine methyltransferase. Thus damage to guanine from mutagens such as methyl-nitrosoguanidine can be repaired directly. 23
- 24. Postreplication Repair Despite the accuracy of DNA polymerase action and continual proofreading, errors still are made during DNA replication. Remaining mismatched bases and other errors are usually detected and repaired by the mismatch repair system in E. coli. The mismatch correction enzyme scans the newly replicated DNA for mismatched pairs and removes a stretch of newly synthesized DNA around the mismatch. A DNA polymerase then replaces the excised nucleotides, and the resulting nick is sealed with a ligase. Postreplication repair is a type of excision repair. 24
- 25. Successful postreplication repair depends on the ability of enzymes to distinguish between old and newly replicated DNA strands. This distinction is possible because newly replicated DNA strands lack methyl groups on their bases, whereas older DNA has methyl groups on the bases of both strands. DNA methylation is catalyzed by DNA methyltransferases and results in three different products: N6-methyladenine, 5-methylcytosine, and N4- methylcytosine. After strand synthesis, the E. coli DNA adenine methyltransferase (DAM) methylates adenine bases in d(GATC) sequences to form N6-methyladenine. For a short time after the replication fork has passed, the new strand lacks methyl groups while the template strand is methylated. The repair system cuts out the mismatch from the unmethylated strand. 25
- 26. Recombination Repair In recombination repair, damaged DNA for which there is no remaining template is restored. This situation arises if both bases of a pair are missing or damaged, or if there is a gap opposite a lesion. In this type of repair the recA protein cuts a piece of template DNA from a sister molecule and puts it into the gap or uses it to replace a damaged strand. Although bacteria are haploid, another copy of the damaged segment often is available because either it has recently been replicated or the cell is growing rapidly and has more than one copy of its chromosome. Once the template is in place, the remaining damage can be corrected by another repair system. 26
- 27. The recA protein also participates in a type of inducible repair known as SOS repair. In this instance the DNA damage is so great that synthesis stops completely, leaving many large gaps. RecA will bind to the gaps and initiate strand exchange. Simultaneously it takes on a proteolytic function that destroys the lexA repressor protein, which regulates the function of many genes involved in DNA repair and synthesis. As a result many more copies of these enzymes are produced, accelerating the replication and repair processes. The system can quickly repair extensive damage caused by agents such as UV radiation, but it is error prone and does produce mutations. However, it is certainly better to have a few mutations than no DNA replication at all. 27
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- 29. References Fig 1 http://academic.pgcc.edu/~kroberts/Lecture/Chapter%20 7/mutation.html Fig 2 http://www.nature.com/nprot/journal/v6/n10/fig_tab/npr ot.2011.378_F2.html Fig 3 http://www- personal.ksu.edu/~bethmont/mutdes.html Fig 4 www.motifolio.com Genome 2 By T.A.Brown 29
- 30. Thank You 30