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I. Introduction

     We have been learning about some of the most important processes in the inheritance and expression of genetic information: DNA replication, transcription, and translation. Although they are common to all living things, the events surrounding replication, transcription, and translation are most easily understood in the context of relatively simple biological systems - bacteria and their phages. When we that these systems are simple, it does not mean that they are not complex, or that they are completely understood. As it is used here, simplicity denotes ease of culture, haploid cells that reproduce asexually, the presence of mobile genetic elements, and the availability of genetic selections based on nutritional requirements and antibiotic resistance.

II. Virus Life Cycles

As you might expect for an organism that is able to execute a genetic takeover of another cell, viruses have life cycles tailored to the cells that they infect, the hosts. All viruses, though, share the characteristic of only being able to reproduce with the aid of the DNA expression and gene expression machinery of the host cell. Figure 18.4 shows some of the steps that occur during the reproduction of a virus with DNA genes. Note that RNA viruses like the HIV viruses budding from an infected T cell (left) must carry out reverse transcription to a DNA genome intermediate.

     Bacteriophages, viruses that infect bacterial cells, have two basic modes of reproduction - lytic and lysogenic. In the lytic cycle, phage adhere to cells and inject their DNA. Following this event (Figure 18.5) the phage genome directs the cell’s metabolism to construct and assemble phage. Finally, the phage digest the cell wall and lyse the cell, resulting in the release of hundreds of phage particles. In the lysogenic cycle, the phage DNA is inserted into the bacterial genome, and is replicated along with the host DNA. When the cell gets in trouble (or at other times) the phage can switch to a lytic mode.

     The complex life cycles of eukaryotic viruses reflect the complexity of the cells on which they subsist. Genomes can be DNA or RNA, and the viruses can be enveloped or have protein coats. HIV (Figure 18.7) is an example of a virus with a complex life cycle. The integration of HIV DNA makes this virus particularly difficult to clear from the host.

     The replication and packaging of viral DNA is often a chaotic process, one that often results in viruses with genetic differences from the parent. In fact, this is part of the viral survival strategy - mutability tends to defeat host defenses. It also results in viruses that can have hypervirulent strains (influenzae) jump host species (monkeypox) or increase rapidly due to human activity (hantavirus, HIV)

Check this page for more on viral life cycles.

III. Bacterial Genetics

E. coli cells undergoing binary fission

    As discussed above, bacteria make the perfect organisms in which to study genetics. Their 3000 genes, present in single copy, allow mutations to be detected, and the ability to culture at high density means that even rare genetic events can be detected with reasonable frequency. (E. coli has a mutation rate of about 10^-7 per gene per replication, although this can be increased by the addition of mutagens.) With a 20 minute doubling time, a single cell can give rise to a colony of 10^8 (mostly) identical cells overnight. An example of how a recombination event might be detected is shown on page 331. Genetic change in bacteria can take place by spontaneous or induced mutation, transformation, transduction, or conjugation.

     Transformation is the uptake and integration of foreign DNA, as in the Griffith experiments. Many bacteria have specialized mechanisms carrying out the uptake of foreign DNA, and the induced uptake in laboratory strains is important in molecular biological manipulations. Transduction results from the aberrant packaging of viral genomes, and in the generalized and specialized modes can lead to the replacement of short parts of the bacterial chromosome. Conjugation refers to the regulated transfer of genetic material between individuals of the same species (mating; Figure 18.14). When the F plasmid is integrated into the bacterial chromosome as an episome, an Hfr strain results, with the potential for the ordered transfer of the entire bacterial chromosome. Interrupted Hfr matings can be used to map the order of genes along the chromosome. The F plasmid is an example of a mobile genetic element. Plasmids have an origin of replication, and a few genes that specify functions that are not required under all circumstances but that are useful at some times (antibiotic resistance.)

     Transposons are another class of mobile genetic elements. Occurring in almost all living things, transposons contain the transposase gene (required for movement) flanked by inverted repeats. At low frequencies, these elements move, with potentially mutagenic consequences.

IV. Regulation of Gene Expression in Bacteria

     Figure 18.18 shows the biochemical steps in the synthesis of the amino acid tryptophan, and two overlapping levels of control of the enzymes of this pathway. First, enzyme activity can be regulated in response to the presence or absence of the product. Second, the levels of biosynthetic enzymes can be controlled by regulating gene expression. Since the genes for these enzymes are organized into an operon, coordinated control is a simple prospect: a repressor protein occludes the trp promoter when tryptophan is present (a corepressor; see Figure 18.19 and the figure below.)

     In the lac operon, a repressor / inducer pair regulates the expression of the genes coding for lactose utilization enzymes. This operon is also subject to positive control by the CAP/cAMP system, which senses the presence or absence of the sugar glucose, and allows a hierarchy of choices in the utilization of carbon source. See Figures 18.20 and 18.21.