Griffiths Genetics Chapter 5
Griffiths Genetics Chapter 5 BIOLOGY 206
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This 3 page Class Notes was uploaded by Taylor Swifty on Sunday September 25, 2016. The Class Notes belongs to BIOLOGY 206 at University of Missouri - Kansas City taught by Michael O'Connor in Fall 2016. Since its upload, it has received 6 views. For similar materials see Genetics in Biology at University of Missouri - Kansas City.
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Date Created: 09/25/16
Chapter 5 The Genetics of Bacteria and Their Viruses Outline 5.1 – Working with microorganisms 5.2 – Bacterial conjugation 5.3 – Bacterial transformation 5.4 – Bacteriophage genetics 5.5 – Transduction 5.6 – Physical maps and linkage maps compared Learning Outcomes After completing this chapter, you will be able to Distinguish between the experimental procedures and analyses in the three main ways by which bacteria exchange genes. Map bacterial genomes using interrupted conjugation. Map bacterial genomes using recombinant frequency. Assess the outcome of double transformation experiments in terms of linkage. Predict the outcomes of transduction experiments using phages capable of generalized or restricted transduction. Map phage genomes by recombination in double infections of bacteria. Design experiments to map a mutation caused by transposon mutagenesis. Predict the inheritance of genes and functions borne on plasmids in bacterial crosses. DNA technology is responsible for rapid advances being made in genetics of all model organisms Examples: o full genome sequences of humans and chimpanzees in recent years o popularity of DNAbased forensic analysis in television shows and movies improvements have led to sequencing of genomes of many hundreds of species Such dramatic results are all based on use of methods that permit small pieces of DNA to be isolated carried from cell to cell amplified into large pure samples sophisticated systems that permit DNA manipulations of any organism are almost all derived from bacteria and their viruses The advance of modern genetics to present state of understanding was entirely dependent on the development of bacterial genetics However, the goal of bacterial genetics has never been to facilitate eukaryotic molecular genetics. Bacteria are biologically important are the most numerous organisms on our planet contribute to recycling of nutrients such as N, S, and C in ecosystems some are agents of human, animal, & plant disease some live symbiotically inside our mouths and intestines many types are useful for industrial synthesis of a wide range of organic products. belong to a class of organisms known as prokaryotes, which includes bluegreen algae (cyanobacteria). Defining feature of prokaryotes their DNA is not enclosed in a membranebounded nucleus. have genes composed of DNA arranged in a long series on a “chromosome.” organization of their genetic material is unique in several respects. o The genome of most bacteria is a single molecule of dsDNA in the form of a closed circle. o bacteria in nature often contain extra DNA elements called plasmids. most plasmids also are DNA circles but are much smaller than main bacterial genome. can be parasitized by specific viruses called bacteriophages (phages) Impetus for genetic dissection of bacteria has been the same as that for multicellular organisms to understand their biological function. Phages and other viruses are very different from the organisms that we have been studying so far. Viruses have some properties in common w/organisms their genetic material can be DNA or RNA, constituting a short “chromosome” most biologists regard viruses as nonliving because: o they are not cells o they have no metabolism of their own. Hence, for study of their genetics, viruses must be propagated in cells of their host organisms Bacteria, like unicellular eukaryotic organisms reproduce asexually by cell growth and division, one cell becoming two This asexual reproduction is quite easy to demonstrate experimentally. o is there ever a union of different types for the purpose of sexual reproduction? o how do the much smaller phages reproduce? o do they ever unite for a sexlike cycle? o These questions are pursued in this chapter. There is a variety of hereditary processes in bacteria and phages These processes act as models – as sources of insight into genetic processes at work in all organisms They can be cultured in very large numbers because they are so small consequently, it is possible to detect and study very rare genetic events that are difficult or impossible to study in eukaryotes. What hereditary processes are observed in prokaryotes? They can undergo both asexual and sexual reproduction mutation occurs in asexual cells in much the same way as it does in eukaryotes, and mutant alleles can be followed through both these processes in an approach analogous to that used in eukaryotes We shall follow alleles in this way in the chapter ahead. When bacterial cells reproduce asexually their genomic DNA replicates and is partitioned into daughter cells, but the partitioning method is quite different from mitosis. In sexual reproduction two DNA molecules from different sources are brought together. However, an important difference from eukaryotes is that, in bacteria, rarely are two complete chromosomes brought together; usually, the union is of one complete chromosome plus a fragment of another. The possibilities are outlined in Figure 52. Gene exchange first process to be examined will be conjugation, which is contact and fusion of two different bacterial cells. After fusion, one cell, called a donor, sometimes transfers genomic DNA to other cell This transferred DNA may be part or (rarely) all of bacterial genome. In some cases, one or more autonomous extragenomic DNA elements called plasmids, if present, are transferred Plasmids are capable of carrying genomic DNA into the recipient cell Any genomic fragment transferred by whatever route may recombine with the recipient’s chromosome after entry. A bacterial cell can also take up a piece of DNA from the external environment and incorporate this DNA into its own chromosome, a process called transformation. In addition, certain phages can pick up a piece of DNA from one bacterial cell and inject it into another, where it can be incorporated into the chromosome, in a process known as transduction. DNA transfer on a plasmid, by transformation or by transduction constitutes a process known as horizontal transmission, a type of gene transmission without the need for cell division This term distinguishes this type of DNA transfer from that during vertical transmission, the passage of DNA down thorough the bacterial generations. Horizontal transmission can spread DNA rapidly through a bacterial population by contact in much the same way that a disease spreads. For bacteria, horizontal transmission provides a powerful method by which they can adapt rapidly to changing environmental conditions. Phages can undergo recombination when two different genotypes both infect the same bacterial cell (phage recombination). Before we analyze these modes of genetic exchange, let’s consider the practical ways of handling bacteria, which are much different from those used in handling multicellular organisms.
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