Increasing Variation through DNA Transfer Biotechnology Unit Increasing Variation through DNA Transfer

How does a bacterium fight off antibiotics? The three main ways are: Membrane proteins called efflux pumps literally pump the antibiotics out of the cell. The prokaryote can produce an enzyme that will degrade the antibiotic. The prokaryote can produce an enzyme that will change the structure of the antibiotic. Change in STRUCTURE = Change in FUNCTION You might have the students draw a typical bacterial cell and illustrate all three main ways of fighting off antibiotics. You should also mention that another way is to change the receptors on the outside of the cell so that the antibiotics cannot enter the cell.

How do these organisms become so resistant? Natural, or inherited, resistance The prokaryote might have been born without the proper cellular structures (transport system, membrane receptors, etc.) needed for the antibiotic to be effective. Acquired Resistance The organism obtains the needed resistance gene from another source, which it incorporates into its genome. Random mutations It has been estimated that one in every 108 – 109 bacteria will develop resistance when exposed to an antibiotic. This might seem like a rare event but please remember that bacterial growth is VERY rapid. This new gene is then given to all the new progeny. This process is known as vertical gene transfer. Have the students compare these with our own immune system as we also have acquired immunity, innate immunity, and also random mutations that occur when our cells undergo the cell cycle (S-phase).

Horizontal Gene Transfer There are ways, other than mutations, in which prokaryotes obtain antibiotic resistance through horizontal gene transfer. Transformation the update of naked DNA (usually a plasmid) by a prokaryote Transduction viral transmission of genetic information Conjugation One bacterium gives resistance to another bacterium. Transposition Movement of DNA segments within and between DNA molecules The students do NOT need to know specifics about horizontal gene transfer, but rather understand the process well enough to understand how it increases variation.

Transformation Transformation causes a change in the genotype, and even possibly the phenotype, and occurs when a cell takes in foreign genetic material and incorporates it into its own. This causes the cell to become a recombinant cell, i.e. one that contains genetic material from more than one source. This slide shows a picture of the experiment that Fredrick Griffith performed to show how heat killed virulent strains of Streptococcus pneumoniae conferred their pathogenicity to non-virulent strains. He gave mice living “S” cells which contain a outer capsule that protects the cells from the mice’s immune system. The mice died. Explain how this served as a control in the experiment. He then gave mice living “R” cells which do not have an outer capsule, thus the immune systems of the mice were able to fight off the infection. The mice lived. Explain how this served as the control in the experiment. Griffith then used heat to kill “S” cells and injected those into the mice. Since the bacteria were dead…the mice lived. He then mixed living “R” cells with heat-killed “S” cells (neither killed the mice when used individually in steps 2 and 3) and found that the mice died. He concluded that the non-virulent cells received a pathogenic substance from the “killed” strain. We know now that the DNA used to make the outer capsule was transferred to the non-virulent “R” cells which incorporated it into its own DNA thus giving it the ability to make the outer capsule needed to become a virulent strain.

Transduction Transduction is the process of viruses carrying prokaryotic genes from one bacterium to another. Do you remember how Hershey and Chase demonstrated that it was DNA, and not protein, that was the genetic material? You might want to remind the students about the famous Hershey and Chase experiment where they radioactively labeled sulfur (part of the protein nature of the virus) and phosphorus (part of the DNA structure) and saw that it was the sulfur that got injected inside the bacterium, not the protein.

Transduction The bacteriophage injects its genetic material into the cell. The bacterial DNA breaks apart. The students do NOT need to know specifics about this process, but rather understand the process well enough to understand how this process increases variation.

Transduction The viral DNA uses the cell’s machinery to make more viruses. Some of the viruses end up with the host cell’s genetic material. The students do NOT need to know specifics about this process, but rather understand the process well enough to understand how this process increases variation.

Transduction The virus particles with the host cell’s genetic material inject it into another cell. The new cell incorporates this new DNA into its own. The students do NOT need to know specifics about this process, but rather understand the process well enough to understand how this process increases variation.

Transduction Ask the students to explain how the processes above increase genetic variation in the population of prokaryotes.

Conjugation Some Prokaryotes can use a “sex pilus” and a “mating bridge” to transfer genetic information from one cell to another. Only one cell needs to have a pilus to perform conjugation. One of the bacteria sends out a sex pilus which attaches to the other one. Then the sex pilus pulls the two organisms together. A mating bridge is then formed which allows genetic information to be passed from the one with the sex pilus to the other one.

Conjugation The students do NOT need to know specifics about this process, but rather understand the process well enough to understand how this process increases variation. The students can be given this slide as a full page document for them to study. The slide shows two different ways bacteria use conjugation to transfer a gene to another bacterium. Shows a bacterial cell transferring an F plasmid. This plasmid confers the genetic information needed to make a sex pilus. Once this process is complete, the cell that just received the new F factor (gene) can produce a sex pilus and give the gene to another cell which is “negative” for the gene. Shows part of a cell’s genome (from a recombinant cell containing genetic information from two different sources) being copied and transferred to another cell which is not a recombinant. The new cell is now a recombinant F- bacterium. Ask the students to explain how both of the processes above increase genetic variation in the population of prokaryotes.

Horizontal Gene Transfer

Transposition Transposons have been characterized as “jumping genes.” Small DNA segments can be transferred from one place on a chromosome to another in the cell by: “Cut and Paste” “Copy and Paste” Retrotransposon movement An RNA intermediate is made and is used to make a new DNA segment using reverse transcriptase. That DNA is then inserted elsewhere in the genome. The students do NOT need to know specifics about this process, but rather understand the process well enough to understand how this process increases variation.

Transposons In 1983, at the age of 81, Barbara McClintock won the Nobel Prize for her discovery of “jumping genes” in corn. You can see how transposons have varied not only the genotype, but also the phenotype (kernel color) of this corn cob. The students do NOT need to know specifics about this process, but rather understand the process well enough to understand how this process increases variation. For an excellent student-friendly resource explaining how McClintock discovered “jumping genes” see http://www.weedtowonder.org/jumpingGenes.html