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VCU BIOL 209 - Exam 3 Study Guide

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BIOL 209 1st EditionExam # 3 Study GuideDescribe the different ways that a microbe may adapt to changing growth conditions.1. Conjugation: genetic transmission through direct contact – a plasmid or fragment of DNA is transferred from a donor to a recipient cell through direct connection- Gram - and gram + bacteria can conjugate, but only gram -‘s can operate with a specialized plasmid (fertility/F factor)- Both the donor and recipient have to be alive- Bridge forms between cell surfaces to interact with pilus to transfer plasmid or just DNA in general- Rarely is the entire genome transferred – just a portion usually- High frequency transfer: the donor cell retains one strand of DNA and the other strand is transported to the recipient, in a higher frequency than other cells- Biomedical importance: R plasmids/factors bear genes resistant to antibiotics and other drugs, and are shared by bacteria through conjugation2. Transformation: capturing free DNA from solution – nonspecific, indirect acceptance of small fragments of soluble DNA from the surrounding environment- Griffith’s experiment with rats – live non-virulent cells picked up loose DNA from dead mice (who carried dead virulent cells) that had broken open, so they too became virulent- DNA, even from a dead cell, retains its genetic code- Chromosomes released by lysed cells break into fragments small enough to be picked up by recipient cells- Facilitated by special DNA-binding proteins on cell wall surface that capture DNA from surrounding environment and process it through the cell wall and membrane – requires no special appendages however- Cells that picks up free DNA is considered “competent” and is, of course, alive- Natural occurrences found in gram + and gram – species- Antibiotic resistance can be transferred this way- Requires 3. Transmission: viruses acting as vectors – indirect transfer by means of a bacteriophage pickingup random fragments of disintegrating host DNA from a donor cell during assembly and transferring it to a recipient cell- Donor/recipient must be of the same species because of virus-host specificity- Virtually any gene can be transferred this way- Transfer of drug resistance and gene regulators in gram – rods - Donor is lysed bacterial cell and carrier is defective bacteriophage- Donor cell must be alive- Transposons: jumping genes that shift from one part of the genome to anotherDefine "mutation" and provide examples of different sources.A mutation is a phenotypic change due to an alteration in the genotype, more specifically, in thenitrogen base sequence of DNA. Mutations can occur spontaneously or they can be induced. A spontaneous mutation is a random change in the DNA arising from errors in replication occurring without a known cause. An induced mutation results from exposure to known mutagens – physical or chemical agents that damage DNA and interfere with its functioning. Sources of mutations include: - (Physical) types of radiation – UV, ionizing (gamma rays, x rays) – carcinogenic to animals- (Chemical) acridine dyes, nitrogen base analogs, ethidium bromide, nitrous acid/bisulfite – most are carcinogensDescribe different ways that mutations can alter coding sequences in the genome and how these changes would affect gene products.Point mutations are small and affect only a single base on a gene. They involve addition, deletion, or substitution of single bases. Types of point mutations include: Substitution mutations1. Missense mutations: a change in the code that leads to placement of a different amino acid - Can create a non or less functional protein- Can produce a protein that functions differently- Can cause no significant alteration in function of the protein2. Nonsense mutations: changes a normal codon into a stop codon - Almost always results in a nonfunctional protein3. Silent mutation: substitutes a single base but still codes for the same amino acid4. Back mutation: a gene that has been mutated reverts back to its original base compositionFrameshift mutations – affect every amino acid after the change (unless it’s a multiple of 3, thenan amino acid is inserted or deleted but does not affect the reading of the remaining bases)1. Insertion- Almost always results in a nonfunctional protein2. Deletion- Almost always results in a nonfunctional proteinDescribe in qualitative terms how mutations may affect gene function, and the possible effects of mutation on microbial growth. Which effects are most prevalent? Which are most significant for adaptations?Mutations can sometimes create variant strains with alternate ways of expressing a trait – a natural response to their environment ends up giving them a new, advantageous trait. Most commonly, however, mutations result in cell death.Identify, and compare and contrast the different mechanisms of gene transfer discussed in class.Conjugation requires direct transfer of DNA. Transformation and transduction require indirect transfer of DNA. Conjugation and transduction involve live donor and recipient cells. Transformation requires only the recipient cell to be alive. How does the F. W. Griffith experiment investigating the virulence Streptococcus pneumoniae illustrate gene transfer? What trait was acquired by the strain that was initially non-virulent? What gene transfer mechanisms could explain the acquisition of virulence in his experiment?Griffith’s experiment shows gene transfer through transformation. The originally non-virulent strain acquired the S-strain, which was virulent, showing that the S cells injected into the dead mice actually broke open and released some of their DNA, which was then picked up by the live R cells.Describe the role of recombination in gene transfer: which mechanisms require recombination? What happens if recombination cannot occur? What, specifically, is required for recombination to occur?Recombination occurs when one bacterium donates DNA to another bacterium, resulting in a new strain different from both the donor and the original recipient strain. Recombinations are generally beneficial, providing additional genes for resistance to drugs and metabolic poisons, new nutritional and metabolic strategies, increased virulence, and adaptations to changing environmental conditions. If recombination cannot occur, research and technology would be at a great disadvantage. For recombination to occur, DNA is needed from a donor cell to pass to a recipient


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