Mutation Types & Effects PDF

 Genetic Variation Mutation: A change in the base sequence of the DNA Mutations are changes in the genotype which may or may not affect the phenotype Mutations may be beneficial, neutral or harmful Many differences among microbes are the result of mutation or recombination, f...

 Genetic Variation Mutation: A change in the base sequence of the DNA Mutations are changes in the genotype which may or may not affect the phenotype Mutations may be beneficial, neutral or harmful Many differences among microbes are the result of mutation or recombination, followed by selection A base pair is mutated at a rate of 10-9-10-10 per generation, a gene of 1000 bp is mutated at ~10-6 per generation, and a bacterial genome is mutated at 3 x 10-3 per 3 generation.  Mutations - Causes Spontaneous mutations: Occur in the absence of mutation causing agents Induced mutations: Caused by mutagens, Base analogs (5-Bromouracil; 2-AP) Base modifiers (Hydroxylamine, Nitrous acid) Intercalators (acridine dyes) Agents that physically damage DNA (radiation) Mutagens increase the rate of mutation by inducing changes in DNA sequence, 4 directly or indirectly.  Chemical mutagens Example: Nitrousacid alters adenine such that it pairs with cytosine instead of thymine  Example: Ethidium bromide inserts between bases causing frameshift mutation Example: Nucleotide analogues-substitute for a base but have different pairing properties.  Radiation Ionizing radiation e.g., Xrays and gamma rays causes the formation of ions that can react with nucleotides (causing base changes) and the deoxyribose-phosphate backbone (causes chromosomes to break). UV radiation Induces formation of covalent bonds between adjacent thymines to form thymine dimers which can not be replicated Mutations May Cause Loss-of-Function or Gain-of- Function If the function is entirely lost, the mutation is called a null mutation. If is also possible that some function may remain, but not at the level of the wild type allele. These are called leaky mutations. 9 Have effect on the genes downstream- Operon Higher Organisms Recessive mutations are due to loss-of-function by the protein product. The wild type allele is functional, while the mutated allele does nothing. If brought together in a heterozygote, the functional allele would mask the nonfunctional allele. Thus, the mutation is recessive Dominant mutations result from a gain-of-function (Often, but not always). the wild type allele is expressed only at the appropriate level and time, the mutated allele is expressed at higher levels and/or at inappropriate times. Thus, the hyperactive mutated allele will mask the wild type allele in a heterozygote. Types of mutations Base substitutions (Point)  The most common type of mutation  A single base pair is replaced by another Frame shift mutations  One or more base pairs are inserted or deleted in the DNA  Results in a change in the reading of codons  Base substitutions (Point) A point mutation is a change in the sequence of DNA involving a single base pair. A transition is a mutation in which one pyrimidine is replaced by the other or in which one purine is replaced by the other. A transversion is a mutation in which a purine is replaced by a pyrimidine or vice versa. Base mispairing is a coupling between two bases that does not conform to the Watson-Crick rule, e.g., adenine with cytosine, thymine with guanine. Consequences of base substitutions Silent mutation: base change results in no change of the amino acid sequence of the translated protein Silent mutations have no effect on phenotype A result of the fact that multiple codons can code for the same amino acid  E.g., AGU and AGC both code for Serine  Consequences of base substitutions Missense mutation: base change results in the change of an amino acid in the translated protein The amino acid substitution induced by the missense mutation may have no effect on the function of the protein OR It may abolish the activity of the protein or alter its function having an effect on phenotype Example: sickle cell disease in humans is due to a missense mutation in the gene for globin. As a result the shape of red blood cells is altered affecting their movement through capillaries. Normal hemoglobin DNA Mutant hemoglobin DNA mRNA mRNA Normal hemoglobin Sickle-cell hemoglobin Glu Val Consequences of base substitutions Nonsense mutation Base change generates a stop codon in place of that coding for an amino acid Results in production of a truncated protein. Usually results in a non- functional protein  An insertion is the addition of a stretch of base pairs in DNA. Duplications are a special class of insertions. A deletion is the removal of a sequence of DNA, the regions on either side being joined together except in the case of a terminal deletion at the end of a chromosome. A transposon (transposable element) is a DNA sequence able to insert itself (or a copy of itself) at a new location in the genome, without having any sequence relationship with the target locus. Consequences of frameshift mutations Frameshift mutation: addition or deletion of one or more bases Results in misreading of the codons (changed reading frame) Almost always results in long stretches of altered amino acids and the production of inactive protein  Cystic Fibrosis Deadly inherited disorder Chloride ion pump protein is not properly secreted from the golgi or rough ER Result is an imbalance in the transport of fluid and ions across the plasma membrane  buildup of thick mucus outside of certain cells Respiratory, digestive and reproductive problems  People with the abnormal protein develop cystic fibrosis Three DNA bases are deleted in the mutated sequence, resulting in the deletion of an amino acid (phenylalanine) from the CF protein Microlesions- Mutations that affect single nucleotide Mutations that affect multiple nucleotide-macrolesions  Medical importance of mutations Bacteria can mutate to more virulent forms (more potent toxins) Bacteria can mutate to antimicrobial resistance or already resistant bacteria can achieve greater resistance. Important in hospitals and animal feedlots where there is constant exposure to antibiotics Once present, genes for virulence characteristics and antibiotic resistance can be transferred to other bacteria. BRCA2 mutations are usually insertions or deletions of a small number of DNA base pairs in the gene. As a result of these mutations, the protein product of the BRCA2 gene (tumor suppressor gene) is abnormal and does not function properly. the defective BRCA2 protein is unable to help fix mutations that occur in other genes. As a result, mutations build up and can cause cells to divide in an uncontrolled way and form a tumor.  Identifying bacterial mutants Mutants can be detected by  Positive (direct) selection where mutant cells grow or appear different. e.g., penicillin resistant mutants can be identified by exposure to penicillin. The mutant cells will grow the wildtype cells will not.  Negative (indirect) selection detects mutant cells because they do not grow. e.g., mutants in histidine biosynthesis cannot grow in the absence of histidine, but the wildtype cells can Negative selection-replica plating Esther Lederberg and Joshua Lederberg in 1952  The Ames Test for Chemical mutagens/Carcinogens Mutagens will cause reversion (or back mutation) of these mutants to allow them to grow in the absence of histidine About 90% of the substances identified as mutagenic by the Ames test have been confirmed as carcinogenic in animals In general, the more mutagenic the substance, the more carcinogenic it has been found to be.