Microbio Exam 2 (Lecture 9)

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Genotype

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-the precise genetic makeup of an organism. It is designated by three lowercase letters
followed by a capital letter (all in italics).
-Example: hisC , which indicates the gene encoding for an enzyme in the biosynthetic pathway of histidine.

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Phenotype

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-the observable characteristics of an organism. It is designated by a capital letter followed by two lower case letters (not italicized), with either a + or - superscript to indicate the presence or absence of that specific phenotype.
-Example: His+ means that the strain can synthesize its own histidine, whereas a His- strain can not.

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Mutation

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-an inherited change in the base sequence of nucleic acid of the genome of an organism. It usually brings about only a very small amount of genetic change. -Example: errors in DNA replication occur at a frequency of about 10-7 to 10-11 per base during a single round of replication (10-4 to10-8 in a 1 kb gene per generation).

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Recombination

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process by which genetic elements contained in two separate genomes are brought together in one unit, and physical exchange of genetic material between the genetic elements occurs. It usually involves much larger changes than mutation

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Mutant

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a strain carrying a mutation. A mutant differs from its parental strain (wild type) in genotype. Its phenotype may also be altered. Example: a mutation in the hisC gene may lead to a change from a His+
to a His- phenotype.

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Selection

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-placing an organisms under conditions where the growth of those with a particular phenotype will be favored
-example: drug resistance, a selectable mutation

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Mutations

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can lead to a change in phenotype

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Auxotroph

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An organism that has developed a nutritional requirement through mutation

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Selectable Mutations

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Replica plating method for detection of nutritional mutants

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Mutations

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can be either spontaneous or induced;arise because of changes in the base sequence of the nucleic acid of an organism's genome

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Point Mutation

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results from a change in a single base
pair,and can lead to a single amino acid change in a polypeptide or to no change at all, depending on the particular codon involved

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Nonsense Mutation

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he codon becomes a stop codon and an incomplete polypeptide is made

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Missense Mutation

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the sequence of amino acids in the ensuing polypeptide is changed, resulting in an inactive protein or one with reduced activity

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Frameshift Mutation

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-Deletions and insertions; cause more dramatic changes in the DNA, and often result in complete loss of gene function
-The normal reading frame is referred to as the 0 frame, that missing a base the -1 frame, and that with an extra base the +1 frame.

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Mutagens

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chemical, physical, or biological agents that increase the mutation rate; can alter DNA in many different ways, but such alterations are not mutations unless they can be inherited

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UV Radiation

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-is mutagenic, and induces the formation of pyrimidine dimers (cytosine or thymine become covalently joined, which leads to errors during replication).
-is absorbed by DNA and RNA at 260nm; and by proteins at 280 nm

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SOS Regulatory System

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a complex cellular mechanism activated as a result of some types of DNA damage and initiates a number of DNA repair processes, both error-prone and high-fidelity. Error-prone DNA repair leads to mutations

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Homologous Recombination

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arises when closely related DNA sequences from two distinct genetic elements are combined in a single element (“crossing over” in classical genetics)

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Genetic Recombination in Prokaryotes

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occurs because fragments of homologous DNA are transferred to a recipient cell by several processes. If recombination does not occur, the fragment will be lost (it can’t replicate independently)

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Selective Medium

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what can you use to detect genetic recombinants?

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Resolution

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occurs by cutting and ligating the cross-linked DNA molecules

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Homologous DNA Pairs

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pair and exchange DNA segments. The mechanism involves breakage and reunion of paired segments. Two of the proteins involved are a single-stranded binding (SSB) protein and the RecA protein

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RecA

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-protein involved in genetic recombination
-catalyzes the pairing of ssDNA with complementary regions of dsDNA.

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Recombination

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an important evolutionary process, and cells have specific mechanisms for ensuring that
recombination takes place.

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In Prokaryotes

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genetic recombination is observed because fragments of homologous DNA from a donor are transferred to a recipient cell by one of three processes: transformation ,transduction
or conjugation. After the transfer, when the DNA fragment from the host is in the recipient cell,
homologous
recombination
may occur.

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Three Ways for Genetic Recombination

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-transformation
-transduction
-conjugation

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Transformation

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transfer of bacteria genes involving free DNA

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Transduction

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transfer of host genes from one cell to another
mediated by a virus

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Competent Cell

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a cell that is able to take up a molecule DNA and be transformed

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Conjugation

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-transfer of genes from one prokaryotic cell (donor) to another (recipient) by a mechanisms involving
cell to cell contact and a conjugative plasmid.
-2 parts: plasmid transfer and then chromosome transfer

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Discovery of Transformation

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one of the seminal events in biology because it led to
experiments demonstrating that DNA is the genetic material

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Griffith's Transformation Experiments

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discovered that DNA is the genetic material

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Hershey-Chase Experiment ("Blender" Experiment)

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showed that the DNA, not the protein, of the phage virus contains the phage genes

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Competence (in Certain Prokaryotes)

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a state in which cells are able to take up free DNA released by other bacteria

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If Recombination Does Not Occur....

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the incoming DNA cannot replicate and will be lost

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Artificially Induced Competence

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cells are treated with calcium ions/cold exposure to make their membrane permeable

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Electroporation

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cells are exposed to pulsed electrical fields to open small pores in their membranes

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Transduction

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involves the transfer of host genes from one bacterium to another by bacterial viruses

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Generalized Transduction

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-any genetic marker can be transferred from donor to recipient; efficiency is low
-one possible mechanism by which virus (phage) particles containing host DNA can be formed

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Specialized Transduction (Phage Lambda)

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the DNA of a temperate virus excises incorrectly and takes adjacent host genes along with it; efficiency in this case may be very high.

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Lysogeny

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Profage is integrated into host DNA at specific site

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Lambda Specialized Transducing Units

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can be constructed by genetic engineering to contain genes from any organism

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Plasmids

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genetic elements that replicate independently of the host chromosome

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Conjugative Plasmid

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plasmids that govern their own transfer by cell-to-cell contact

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Genetic Information that Plasmids Carry

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-may confer a selective growth advantage under certain conditions.
-examples include antibiotic resistance, enzymes for degradation of unusual organic compounds, special metabolic pathways and virulence factors.

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Conjugation

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-controlled by genes carrying certain plasmids
-process involves transfer of a plasmid from a donor to a recipient cell

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Plasmid DNA Transfer

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involves replication via the rolling circle mechanism

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Rolling Circle Replication

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conjugation process of a plasmid transfer
-both cells end up with a copy of the plasmid (replicative process)

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High Frequency of Recombination (Hfr)

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strains that transmit chromosomal genes at high frequency

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TRUE

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true or false:
Because of breakage of the DNA during transfer, only a part of the donor chromosome is transferred. Since this partial chromosome cannot replicate in the recipient cell, donor genes will become part of the recipient genotype only if recombination between the incoming fragment and the recipient chromosome occurs.

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Lederberg and Tatum (Yale University)

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detection of genetic conjugation (like we did in lab)

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Interrupted Mating

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Rate of formation of recombinants containing different genes after mixing Hfr and bacteria by the process known as....

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Complementation Test

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If a cell is treated so that it contains two copies of a region of its genome, ______ _____ can determine if two mutations are in the same or different genes
-complementation= in different genes
-no complementation= in the same gene

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Complementation Test

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if two bacteria are crossed to each other, and each is mutated in the same gene, function will not
be restored. On the other hand, if the mutations are in different genes, then the good copy of each gene in
the other bacterium will fulfill the function of the mutated gene, or complement it