Normal prion protein (PrPC)

Abnormal prion protein (PrPSc )

Transcription and translation

Normal prion protein (PrPC)

Abnormal prion protein (PrPSc )

In some cases the PrP gene is mutated.

.and PrPC may spontaneously misfold into PrPSc.


In some cases the PrP gene is mutated.

.and PrPC may spontaneously misfold into PrPSc.


Bacterial reproduction was discussed in Chapter 2, and a number of the principles and techniques covered in this chapter are linked to topics in future chapters. Bacterial chromosomes will be considered in more detail in Chapter 11, and bacterial replication, transcription, translation, and gene regulation will be the topics of Chapters 12 through 16. Bacteria are central to recombinant DNA technology, the topic of Chapter 18, where they are often used in mass producing specific DNA fragments. Many of the tools of recombinant DNA technology, including plasmids, restriction enzymes, DNA poly-merases, and many other enzymes, have been isolated and engineered from natural components of bacterial cells. Engineered viruses are common vehicles for delivering genes to host cells.

Some transposable genetic elements (discussed in Chapter 11) are closely related to viruses, and considerable evidence suggests that viruses evolved from such elements. Because their mutations are easily isolated, bacteria also play an important role in the study of gene mutations, a topic examined in Chapter 17. Chapter 20 deals with mitochondrial and chloroplast DNA, which in many respects are more similar to bacterial DNA than to the nuclear DNA of the cells in which these organelles are found. Finally, viruses cause some cancers, and the role of viral genes in cancer development is studied in Chapter 21.


• Bacteria and viruses are well suited to genetic studies: they are small, have a small haploid genome, undergo rapid reproduction, and produce large numbers of progeny through asexual reproduction. When spread on a petri plate, individual bacteria grow into colonies of identical cells that can be easily seen.

• The bacterial genome normally consists of a single, circular molecule of double-stranded DNA.

• Plasmids are small pieces of bacterial DNA that can replicate independently of the large chromosome. Episomes are plasmids that can exist either in a freely replicating state or can integrate into the bacterial chromosome.

• DNA may be transferred between bacteria by means of conjugation, transformation, and transduction.

• Conjugation is the union and the transfer of genetic material between two bacterial cells and is controlled by a fertility factor called F, which is an episome. F+cells are donors, and F- cells are recipients during conjugation. An Hfr cell has F incorporated into the bacterial chromosome. An F' cell has an F factor that has excised from the bacterial genome and carries some bacterial genes.

• The rate at which individual genes are transferred from Hfr to F- cells during conjugation provides information about the order and distance between the genes on the bacterial chromosome.

• In transformation, bacteria take up DNA from their environment. Frequencies of cotransformation provide information about the physical distances between chromosomal genes.

• Viruses are replicating structures with DNA or RNA genomes that may be double stranded or single stranded, linear or circular. Bacteriophages are viruses that infect bacteria. An individual phage can be identified when it enters a bacterial cell, multiplies, and eventually produces a patch of lysed bacterial cells (a plaque) on an agar plate.

• Phage genes can be mapped by infecting bacterial cells with two different strains of phage. The numbers of recombinant plaques produced by the progeny phages are used to estimate recombination rates between phage genes.

• In generalized transduction, bacterial genes become incorporated into phage coats and are transferred to other bacteria during phage infection. Rates of cotransduction can be used to determine the order and distance between genes on the bacterial chromosome.

• In specialized transduction, DNA near the site of phage integration on the bacterial chromosome is transferred from one bacterium to another.

• Benzer mapped a large number of mutations that occurred within the rII region of phage T4 and showed that intragenic recombination takes place. The results of his complementation studies demonstrated that the rII region consists of two functional units (cistrons).

• A number of viruses have RNA genomes. In positive-strand viruses, the RNA genome codes directly for viral proteins; in negative-strand viruses, a complementary copy of the genome is translated to form viral proteins. Retroviruses encode a reverse transcriptase enzyme used to make a DNA copy of the viral genome, which then integrates into the host genome as a provirus.

• Prions are infectious agents consisting only of protein; they are thought to cause disease by altering the shape of proteins encoded by the host genome.

[important terms minimal medium (p. 199) complete medium (p. 200) colony (p. 200) plasmid (p. 202) episome (p. 203) F factor (p. 203) conjugation (p. 203) transformation (p. 203) transduction (p. 203) pili (p. 205)

competent cell (p. 211) transformant (p. 212) cotransformation (p. 212) virus (p. 213) virulent phage (p. 214) temperate phage (p. 214) prophage (p. 214) plaque (p. 214) generalized transduction (p. 216)

specialized transduction (p. 216) transducing phage (p. 217) transductants (p. 217) cotransduction (p. 217) attachment site (p. 218) intragenic mapping (p. 220) positive-strand RNA virus (p. 223)

negative-strand RNA virus

(p. 223) retrovirus (p. 224) reverse transcriptase (p. 224) provirus (p. 224) integrase (p. 224) oncogene (p. 224) prion (p. 225)

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