Changes in chromosome structure, called rearrangements, encompass several major classes of events. A chromosome segment can be lost, constituting a dele-tion, or doubled, to form a duplication. The orientation of a segment within the chromosome can be reversed, constituting an inversion. Or a segment can be moved to a different chromosome, constituting a translocation. DNA breakage is a major cause of each of these events. Both DNA strands must break at two differ-ent locations, followed by a rejoining of the broken ends to produce a new chromo-somal arrangement ( Chromosomal rearrangements by breakage can be induced artificially by using ionizing radiation. This kind of radia-don, particularly X rays and gamma rays, is highly energetic and causes numerous double-stranded breaks in DNA.
To understand how chromosomal rearrangements are produced by breakage, several points should be kept in mind:
1. Each chromosome is a single double-stranded DNA molecule.
2. The first event in the production of a chromosomal rearrangement is the generation of two or more double-stranded breaks in the chromosomes of a cell (see Figure 16-19, top row at left).
3. Double-stranded breaks are potentially lethal, unless they are repaired.
4. Repair systems in the cell correct the double-stranded breaks by joining broken ends back together (see Chapter 15 for a detailed discussion of DNA repair).
5. If the two ends of the same break are rejoined, the original DNA order is restored. If the ends of two different breaks are joined together, however, one result is one or another type of chromosomal rearrangement.
6. The only chromosomal rearrangements that survive meiosis are those that produce DNA molecules that have one centromere and two telomeres. If a rearrangement produces a chromosome that lacks a centromere, such an acentric chromosome will not be dragged to either pole at anaphase of mitosis or meiosis and will not be incorporated into either progeny nucleus.
Therefore acentric chromosomes are not inherited. If a rearrangement produces a chromosome with two centromeres (a dicentric), it will often be pulled simultaneously to opposite poles at anaphase, forming an anaphase bridge. Anaphase-bridge chromosomes typically will not be incorporated into either progeny cell. If a chromosome break produces a chromosome lacking a telomere, that chromosome cannot replicate
Despite their simplicity of form, bacteria have in common with the larger and more complex members of other kingdoms the fundamental task of regulating the expression of their genes. One of the main reasons is that they are nutritional opportunists. Consider how bacteria obtain the many important compounds, such as sugars, amino acids, and nucleotides, needed for metabolism. Bacteria swim in a sea of potential nutrients. They can either acquire the compounds that they need from the environment or synthesize them by enzymatic pathways. Synthesizing the necessary enzymes for these pathways expends energy and cellular resources; so, given the choice, bacteria will take compounds from the environment instead. To be economical, they will synthesize the enzymes necessary to produce these compounds only when there is no other option—in other words, when these com-pounds are unavailable in their local environment. Bacteria have evolved regulatory systems that couple the expression of gene products to sensor systems that detect the relevant compound in a bacterium’s local environment. The regulation of enzymes taking part in sugar metabolism provides an example. Sugar molecules can be oxidized to provide energy or they can be used as building blocks for a great range of organic compounds. However, there are many different types of sugar that bacteria could use, including lactose, glucose, galactose, and xylose. A different import protein is required to allow each of these sugars to enter the cell. Further, a different set of enzymes is required to process each of the sugars. If a cell were to simultaneously synthesize all the enzymes that it might possibly need, the cell would expend much more energy and materials to produce the enzymes than it could ever derive from breaking down prospective carbon sources. The cell has devised mechanisms to shut down (re-press) the transcription of all genes encoding enzymes that are not needed at a given time and to turn on (activate) those genes encoding enzymes that are needed. For example, if only lactose is in the environment, the cell will shut down the transcription of the genes encoding enzymes needed for the import and metabolism of glucose, galactose, xylose, and other sugars. Conversely, the cell will initiate the transcription of the genes encoding enzymes needed for the import and metabolism of lactose. In sum, cells need mechanisms that fulfill two criteria:
1. They must be able to recognize environmental conditions in which they should activate or repress the transcription of the relevant genes.
2. They must be able to toggle on or off, like a switch, the transcription of each specific gene or group of genes. Let’s preview the current model for prokaryotic transcriptional regulation and then use a well-understood example—the regulation of the genes in the metabo-lism of the sugar lactose—to examine it in detail. In particular, we will focus on how this regulatory system was dissected with the use of the tools of classical genetics and molecular biology.
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