M Ashcraft CHAPTER 8 BACTERIAL GENETICS Barbara McClinotck transposable elements Received the nobel prize in Medicine or Physiology fro 40 years of research on transposons and If the gene coding for a particular color was inactivated the kernel was not pigmented etc Studied corn and the interactions of the enzyme that caused cellular pigmentation By looking at the variations in color she observed variations in genetic function She concluded that something most likely pieces of DNA were moving in and out of genes to account for the differences in kernel color When a piece of DNA a transposable element moved into a gene the gene could no longer function and when it left the function returned When her results were published chromosomes were thought to be very stable and unchanging Genetic Change in Bacteria Natural selection Organisms must adapt in order to succeed o Adapt in two ways Regulating gene expression Genetic change Genotype the sequence of nucleotides in a its DNA Haploid have a single pair of genes making the affect of a change greater o This means that a change in genotype directly affects a change in phenotype o However the phenotype is also affected by environmental determinants Ex color depending on temperature when incubated Genetic change is brought about by mutation and gene transfer o Mutation is the change in the sequence of a cell s DNA The resulting cell is a mutant since a mutant will produce a mutant offspring or progeny this is called vertical gene transfer mutation changes the cell so it differs from the wild type organism the one that exists in nature this mutation could cause a cause in the phenotype and will not code for the correct protein even the substitute of 1 amino acid can lead to a dysfunction in protein synthesis changing the cell properties therefoe this could affect whether an organism needs a growth factor to survive Auxotroph a mutant that required a growth factor o Ex needs tryptophan Trp Prototroph a cell that can grow in the absence of any growth factor o Ex Trp o Gene transfer is the acquistion of genes from another microorganism Horizontal or lateral gene transfer emphasize that the cell gets the gene from another source Like mutants the changes are passed on to the progeny Bacteria have no equivlent of sexual reproduction Only part of the genome is transferred and can be transferred through several mechanisms If the gene is different than the recipient the recipient is permenantly changed M Ashcraft DNA is either A free plasmid Or integrated into DNA Spontaneous Mutations Spontaneous mutations are those that occur in the cell s natural environment o Each gene mutates spontaneously and infrequently at a certain rate o Occur without mutagens in the natural environment o The rate of a mutation will occur when a given gene divides and is expressed as a negative component basis for combination drug therapy Varies between E 4 and E 12 o Mutate independently of one another In order to calculate the probability the negative components are multiplied o Mutations are stable so that the progeny receives the mutation However on rare occasions the cell will revert make to normal via reversion o Helps the process of natural selection by making cells in population unique o Most common type that occurs during DNA synthesis when incorrect base is substituted Base substitution into DNA o Results in 1 of 3 outcomes Silent mutation in which the substituted codes for the same wild type amino acid Missence mutation in which the new bases code for a different amino acid Creates leaky or partially functional proteins Nonsense mutation when new codon is a stop codon resulting in a shorter or truncated protein Normally non functional and terms the gene null or knockout mutation o Oxygen can increase the amount of base substitutions because of unstable derivatives Reactive forms of oxygen can damage cellular DNA Normally causes DNA polymerase to code G A instead of G C Removal or Addition of nucleotides o The consequence depends on the number of nucleotides deleted If 3 are deleted that means that one amino acid is absent o Frame shift mutation when 1 or 2 nucleotides are added or subtracted and causes a significant mutation This affects translation when the DNA is transcribed into mRNA Normally this produces a stop codon which makes the gene a knockout mutation Transposable elements jumping genes o Distinct segments of DNA that can adjust their own movement in a process called transposition o Can jump to a different location within the chromosome or to plasmid vice versa o Insertional inactivation the gene that the transposons inserts no longer encodes a functional protein because the insertion disrupts the gene Most transposons contain transcriptional terminators affecting genes downstream Induced Mutations o Mutagens chemicals or radiation that increase the frequency of mutations Since mutations occur infrequently can increase the chance 1 000 fold Mutations are said to be induced by the mutagen M Ashcraft o Chemical Mutagens any chemical that alters the hydrogen bonding of purine and pyrimidine base in the DNA Chemical modification of purines and pyrimidines Alkylating agents highly reactive chemicals that add alkyl groups onto purines and pyrimidines short chains of carbon atoms Help kill cancer cells but also damage DNA in normal cells in body Nitroous acid converts amino to keto groups o Converting cytosine to uracil and removes amino groups from A Base analogs G Compounds that structurally resemble purine or pyrimidine closely enough to be mistakenly incorporated in place of the natural bases as nucleotides are synthesized 2 aminopurine o if incorporated instead of adenine pairs with cytosine 5 bromouracil o if incorporated in place of thymine pairs with guanine Intercalating agents increase the number of frameshift mutations Planar molecules of about the same size as a pair of nucleotides Do not alter the hydrogen bonding but intercalate or insert between adjacent base pairs If inserted into an old DNA strand will be replicated when a new strand is made If inserted into the strand being synthesized a deletion of a base pair will occur Ethidium bromide an intercalating agent used to stain bacteria in the lab o Carcinogn a cancer causing agent o Since transposons lack an origin of replication must insert themselves in order to be Transposition replicated o Usually cause a knockout mutation Radiation o Ultraviolet Causes thymine molecules on the same
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