M. AshcraftCHAPTER 8: BACTERIAL GENETICSBarbara McClinotck- Received the nobel prize in Medicine or Physiology fro 40 years of research on transposons and transposable elements- Studied corn and the interactions of the enzyme that caused cellular pigmentation- If the gene coding for a particular color was inactivated, the kernel was not pigmented, etc.- 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 succeedo 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 phenotypeo 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 transfero 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 factoro Ex: needs tryptophan Trp-- Prototroph: a cell that can grow in the absence of any growth factoro 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 changedM. Ashcraft DNA is either:- A free plasmid- Or integrated into DNASpontaneous Mutations- Spontaneous mutations are those that occur in the cell’s natural environmento Each gene mutates spontaneously and infrequently at a certain rateo 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-12o 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 reversiono Helps the process of natural selection by making cells in population unique - Base substitutiono Most common type that occurs during DNA synthesis when incorrect base is substituted into DNAo 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 mutationo 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 nucleotideso 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 versao 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 Mutationso 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 mutagenM. Ashcrafto 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 groupso Converting cytosine to uracil and removes amino groups from A + G Base analogs- Compounds that structurally resemble purine or pyrimidine closely enough to be mistakenly incorporated in place of the natural bases as nucleotides are synthesized- 2-aminopurineo 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 labo Carcinogn: a cancer causing agent- Transpositiono Since