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Plasmids, the extra circles of DNA found in bacteria, have become vital tools in modern genetic technology. See especially Chapters 16, 18 and 22.

mosome is divided into 100 map units. The position of the thrABC genes was arbitrarily chosen as the zero/100 position (i.e., start and end). The origin of replication (oriC) is the point at which the chromosome starts to divide and the terminus (ter) is where replication terminates (see Ch 5).

Note that bacteria only have a single chromosome and therefore only contain a single copy of most of their genes. Therefore, dominant and recessive alleles of the same gene in the same bacterial cell are not normally found. However, bacterial cells map unit A subdivision that is one hundredth of the length of the bacterial chromosome origin of chromosome (oriC) Origin of replication of a chromosome terminus of replication (ter) The place on any DNA molecule where replication ends

Cell envelope

Chromosome

FIGURE 1.26 Plasmid within an E. coli Cell Shows Partial Diploidy

The bacterial chromosome is indicated in red; the large single-copy plasmid is indicated in green and the small multiple-copy plasmid in purple. Note that a segment of the bacterial chromosome, colored blue, has been duplicated and is carried by the larger plasmid, making the cell a partial diploid.

Large plasmid

Chromosome

Large plasmid

FIGURE 1.26 Plasmid within an E. coli Cell Shows Partial Diploidy

The bacterial chromosome is indicated in red; the large single-copy plasmid is indicated in green and the small multiple-copy plasmid in purple. Note that a segment of the bacterial chromosome, colored blue, has been duplicated and is carried by the larger plasmid, making the cell a partial diploid.

Small multi-copy plasmids

By convention, when bacterial geneticists describe the genotype of a bacterial strain they list only those genes with mutations. A gene that is not mentioned is assumed to be wild-type. Furthermore, the "-" sign that indicates a gene is mutated is also usually omitted. Thus merely listing a gene implies that it is mutated.

Consider the genotype: serA14 leu-6 thi

This genotype tells us that the bacterial strain in question has no defects in the genes for making the amino acid threonine. It does have a fully identified defect in one of the genes for making the amino acid serine (serA14). Its defect in leucine synthesis has been partly characterized and numbered, but which of the leucine genes is altered remains uncertain (leu-6).The mutation that prevents synthesis of the vitamin thiamine (thi) is still uncharacterized.

sometimes carry extra genetic elements known as plasmids. These are circular molecules of DNA that replicate in step with cell division, like the chromosome, but are generally much smaller (Fig. 1.26). They often carry genes that provide extra capabilities to the bacterial cell, but that are not essential for normal growth and division. Plasmids may be present in single or multiple copies. If a plasmid carries extra alleles corresponding to genes already on the bacterial chromosome, the bacteria are said to be partial diploids for those particular genes (Fig. 1.26).

Bacteria and the viruses that infect them (known as bacteriophages) were the most important organisms used when science made the transition from classical genetics to molecular genetics/biology. As will become apparent in subsequent chapters, the unveiling of the molecular basis of heredity allowed a much deeper understanding of genetic mechanisms. The next chapter will review the variety of living organisms and focus on those used most often in genetic analysis. Then, in Chapter 3, the structure of DNA will be examined and it will become apparent how DNA encodes the genetic information.

partial diploidy Situation in which a cell is diploid for only some of its genes plasmid Circular molecule of double stranded helical DNA which replicates independently of the chromosomes of the host cell. Rare linear plasmids have been discovered

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