Although the location and distribution of cell wall synthetic activity varies

Although the location and distribution of cell wall

This preview shows page 10 - 12 out of 35 pages.

Although the location and distribution of cell wall synthetic activity varies with species, there seem to be two general patterns . Many gram-positive cocci (e.g., Enterococcus faecalis and Streptococcus pyogenes ) have only one to a few zones of growth. The principal growth zone is usually at the site of septum formation , and new cell halves are synthesized back-to-back. The second pattern of synthesis occurs in the rod-shaped bacteria Escherichia coli, Salmonella, and Bacillus. Active peptidoglycan synthesis occurs at the site of septum formation just as before, but growth sites also are scattered along the cylindrical portion of the rod . Thus growth is distributed more diffusely in rod-shaped bacteria than in the streptococci. Synthesis must lengthen rod-shaped cells as well as divide them. Presumably this accounts for the differences in wall growth pattern.
Image of page 10
Bacteria genetics Conjugation The initial evidence for bacterial conjugation, the transfer of genetic information by direct cell to cell contact, came from an elegant experiment performed by Joshua Lederberg and Edward L. Tatum in 1946. They mixed two auxotrophic strains, incubated the culture for several hours in nutrient medium, and then plated it on minimal medium. To reduce the chance that their results were due to simple reversion, they used double and triple auxotrophs on the assumption that two or three reversions would not often occur simultaneously. For example, one strain required biotin (Bio __ ), phenylalanine (Phe __ ), and cysteine (Cys _ ) for growth, and another needed threonine (Thr _ ), leucine (Leu _ ), and thiamine (Thi _ ). Recombinant prototrophic colonies appeared on the minimal medium after incubation. Thus the chromosomes of the two auxotrophs were able to associate and undergo recombination. Lederberg and Tatum did not directly prove that physical contact of the cells was necessary for gene transfer. This evidence was provided by Bernard Davis (1950), who constructed a U tube consisting of two pieces of curved glass tubing fused at the base to form a U shape with a fritted glass filter between the halves. The filter allows the passage of media but not bacteria. The U tube was filled with nutrient medium and each side inoculated with a different auxotrophic strain of E. coli . During incubation, the medium was pumped back and forth through the filter to ensure medium exchange between the halves. After a 4 hour incubation, the bacteria were plated on minimal medium. Davis discovered that when the two auxotrophic strains were separated from each other by the fine filter, gene transfer could not take place. Therefore direct contact was required for the recombination that Lederberg and Tatum had observed. F + X F - Mating In 1952 William Hayes demonstrated that the gene transfer observed by Lederberg and Tatum was polar. That is, there were definite donor (F + ) and recipient (F - ) strains, and gene transfer was nonreciprocal. He also found that in F + X F - mating the progeny were only rarely changed with regard to auxotrophy (that is, bacterial genes were not
Image of page 11
Image of page 12

You've reached the end of your free preview.

Want to read all 35 pages?

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask You can ask (will expire )
Answers in as fast as 15 minutes