Chapter 10: Microbial Genetics part III
Chapter 10: Microbial Genetics part III 2420
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This 3 page Class Notes was uploaded by Siân L'Roy on Monday August 22, 2016. The Class Notes belongs to 2420 at Tarrant County College District taught by Mark Pulse in Summer 2016. Since its upload, it has received 6 views. For similar materials see Microbiology in Biology at Tarrant County College District.
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Date Created: 08/22/16
Microbial GeneticsPart III Bacteria must adapt to their environment in order to survive; type of adaptation include: o Genetic and phenotypic alterations by mutation o Acquisition of genetic material outside the cell o Altering genetic expression based on the environment Spontaneous or induced mutations can be inherited by vertical transference (parent to daughter cells); has to be in a stable environment External genetic material can be obtained by horizontal transference (cell to cellnot through division) Spontaneous Mutations are those that occur in nature; infrequent and can alter the phenotype or not (silent mutation) If a mutation is beneficial and stable, then it can be vertically inherited by progeny cells Types of Spontaneous Mutations include: o Base substitution: incorrect base inserted during DNA synthesis; usually occurs in a single basepair (point mutation) and can result in the following mutation types Missense mutation: Changes in the amino acid in the peptide sequence; protein retains partial functioning Nonsense mutation: Stop codon replaces an amino acid codon; results in nonfunctional or truncated protein( also known as null or knockout mutations) o Cleaving or adding nucleotides to the DNA sequence can affect gene expression by shifting the codon sequence of one or more genes (Frameshift mutation) Results in a change with the amino acid sequence, or the insertion of a stop codon and truncation of a protein o Transposable elements (jumping genes) can spontaneously move from one DNA sequence to another Often results in knockout mutations for one or more genes Genetic acquisition can occur by one of three methods: o Transformation: ‘naked’ DNA is transferred from the external environment into a competent cell o Transduction: DNA is transferred from one bacterial cell to another by a virus (bacteriophage) Microbial GeneticsPart III o Conjugation: direct contact between two bacterial cells that result in the transfer of DNA from one to the other (aka. Bacterial “sex”) Genetic material is transferred from a donor cell to a recipient cell If the acquired material is to be maintained in progeny cells, it must be either: o Incorporated into the DNA of the recipient (chromosomal or plasmid) o Contain a separate origin of replication (plasmid) Cells that retain the transferred genetic material are called recombinants(mutant cells); usually detected on selective media (antibiotics) Large chromosomal DNA fragments and plasmid DNA can be acquired by transformation Recipient cells receiving the DNA are naturally competent o Ability to takeup ‘naked’ DNA is associated with modifications to the cell wall and membrane composition (inner and outer) Natural transformation occurs by the following steps: o Doublestranded DNA molecule binds to the surface of a competent cell o Cellsurface nucleases degrade one strand of the DNA molecule and only one strand enters the cell DNA strand is incorporated into recipient cell’s DNA molecule and only one strand enters the cell DNA strand is incorporated into the recipient cell’s DNA (chromosomal or plasmid) by homologous recombination Complement copy of DNA strand is generated and it is not incorporated into the recipient’s DNA (plasmid) Transduction results in transference of genetic material through bacterial viruses known as bacteriophages Fragments of a donor cell’s DNA is contained within the protein coat of a bacteriophage; a fragment is inserted into the recipient cell during an infection Transduction occurs by the following steps: o Bacteriophage attaches (infects) to the donor cell o DNA material is injected and incorporated into the donor cell’s DNA o Bacteriophage protein coats are produced and phageencoded nucleases degrade the donor cell’s DNA Microbial GeneticsPart III o DNA fragments (phage or bacterial) are packaged into the protein coats o Donor cell lyses open, releasing bacteriophage containing both bacterial and phage DNA o Released bacteriophage infects the recipient cell and bacterial DNA is injected o Donor DNA fragment is incorporated into recipient cell’s DNA by homologous recombination o Genes associated with the DNA fragment are replicated and expressed within the recipient cell Conjugation involves the transfer of genetic material (plasmid or chromosomal) from a donor to a recipient cell by direct contact The best example for conjugation is associated with the fertility (F) plasmid in E.coli and the steps have been determined as follows: o The two cells involved includes a F+ (with plasmid) cell and a F(without plasmid) cell; F+: donor cell, fertility plasmid, sex pilus; F: opposite of F+ o Contact occurs between the two cells and the sex pilus of the F+ cell recognized surface associated receptors on the F cell o F plasmid is activated and one strand is cleaved for transfer into the F cell; pilus retracts pulling the cells closer together o Cleaved strand is transferred as a linear fragment into the F cell o Complement copies are generated for the singlestands in the donor and recipient cells o Both cells are F+ and produce the sex pilus
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