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Lecture 9

01:119:131 Lecture 9: Chapter 8: Gene Transfer (Lecture 9,10)

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Biological Sciences

CHAPTER 8: GENE TRANSFER Lecture 9 (2.20.17) I. Types and significance of gene transfer A. Gene transfer refers to the movement of genetic information between organisms B. Recombination is the combining of genes (DNA) from 2 different cells C. Vertical gene transfer is when genes pass from parents to offspring (like binary fission) D. Horizontal (lateral) gene transfer) is when genes are passed to other microbes of their same generation D.1. Transformation- naked DNA D.2. Transduction- Bacteriophage D.3. Conjugation- cell to cell contact by conjugation pilus/bridge II. Transformation A. A change in an organism’s characteristics because of transfer of genetic information A.1. Naked DNA: DNA that has been released from an organism after the cell is lysed and DNA is no longer incorporated into chromosomes A.2. Competence factor is released into the medium and apparently facilitates the entry of DNA into the cell A.2.a) Not all organisms can be transformed, only one that can are the ones that make these factors (A.2.a.1) Not all organisms can be transformed, only one that can are the ones that make these factors A.3. DNA transport proteins and DNA exonuclease (an enzyme that cuts up DNA) is also needed A.3.a) Can’t bring in an entire chromosome, too big B. The discovery of transformation: Griffith’s experiment with pneumococcal infections in mice (Figure 8.1) B.1. B.1.a) Heat killed S type (Smooth) with capsule → mouse lives B.1.b) Live, virulent S type with capsule → mouse dies B.1.c) Live, non virulent R type (rough) with capsule → mouse lives B.1.d) Mix both heat killed S type with live, nonvirulent R type → mouse dies (B.1.d.1) Autopsy of mouse finds live S type with capsule and live R type without capsule (B.1.d.2) R type transformed into S type because R type picked up naked DNA from dead S type and those naked DNA had genes with ability to make capsule- hence R transformed into S with capsule and killed mouse C. Mechanism of Bacterial Transformation (Figure 8-2) C.1. C.1.a) Naked DNA come into competent cell-- naked DNA comes in as single strand C.1.b) Usually only one DNA makes it to recombination phase, extraneous DNA in cytosol is broken down by enzymes III. Transduction A. Method of transferring genetic material using a bacteriophage A.1. Bacteriophage (phage) is a virus that can infect bacteria A.1.a) Phages: composed of a core of nucleic acid covered by a protein coat A.1.b) Capable of infecting a bacterium attached to a specific receptor site on the cell wall A.2. Virulent phage is capable of causing infection and the destruction and death of a bacterial cell A.3. Temperate phage ordinarily does not cause a disruptive infection A.4. Prophage is a phage DNA that is incorporated into a host bacterium’s DNA A.4.a) Carry things that make a bacterial strain virulent. Prophages are the ones that make the bacteria very toxic A.5. Lysogeny: persistence of a prophage without phage replication and destruction of bacterial cell B. Bacteriophage life cycles (figure 8.3) B.1. B.1.a) Temperate goes through lysogenic B.1.b) Virulent goes through lytic B.2. Lytic cycle B.2.a) Phage attaches to receptor site on bacteria cell wall, penetrates it and inserts its DNA B.2.b) Phage DNA directs the cell’s metabolism to produce viral components (B.2.b.1) Proteins and copies of phage DNA B.2.c) Empty phage heads are synthesized, heads are packed with DNA B.2.d) Collars, sheaths, and base plates have been attached to the heads-- tails are added last B.2.e) Bacterial cell lyses, releasing completed infective phages B.3. Lysogenic Cycle- can revert to lytic cycle at any time B.3.a) Phage attaches to receptor site on bacteria cell wall, penetrates it, inserts its DNA B.3.b) Phage DNA inserts itself (as a prophage) into bacterial chromosome, recombination B.3.c) Phage is replicated along with bacterial chromosome in binary fission B.3.d) Binary fission is completed, each cell has the phage DNA incorporated C. Transduction is either generalized or specialized C.1. Specialization (figure 8-4) C.1.a) (C.1.a.1) Can induce and every once in a while the phage will pop out and take with it a bit of the host DNA (C.1.a.2) Goes through lytic cycle and infects other bacteria (C.1.a.3) Now host cell chromosome acquires both phage DNA and genes from previous host (C.1.a.3.a)Phage DNA always inserts itself into host cell DNA as it replicates it take a bit of the host cell’s DNA with it. Cell lyses and infects new bacteria (C.1.a.4) Always inserts at a particular site, take genes from adjacent sides (C.1.a.5) Temperate phage C.2. Generalized Transduction (Fig 8-5) C.2.a) C.2.b) C.2.c) Caused by virulent phage, random area of chromosome C.2.d) Bacteriophage infection of a host bacterium initiates lytic cycle C.2.e) Chromosome is broken into fragments, can be picked up and packaged with phage DNA C.2.f) Particles are released and infect another bacterial cell C.2.g) Host acquires genes that were brought along (transduced) from another cell C.3. Difference C.3.a) Specialization- DNA that is going to be incorporated into host DNA can only be adjacent to where prophage is C.3.b) Generalization- each bacterial DNA fragment ha equal chance of becoming part of phage DNA as it is replication Lecture 10 (2.22.17) I. The discovery of conjugation (figure 8.6) A.
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