Bacterial Endosymbionts of Killer Paramecium

Paramecium is a protozoan that cruises around in freshwater and feeds by swallowing bacteria whole and digesting them. This lifestyle has led to the development of killer strains of Paramecium that use bacteria as a biological weapon to kill other, sensitive strains of Paramecium. Such killer Paramecium produce a toxin that kills all sensitive Paramecium that get too close. Killers are immune to their own toxin, but different brands of killer Paramecium exist that kill each other. Killer...

Human Viral Diseases Are Common

Many common childhood diseases such as measles, mumps and chickenpox are caused by viruses, as are the common cold and flu. More dangerous viral diseases include polio, smallpox, herpes, Lassa fever, Ebola and AIDS. Do viruses ever do anything useful Yes infection by a mild virus can provide resistance against a related but more dangerous virus (see Ch. 17). Viruses may carry genes from one host organism to another, in a process known as transduction (see Ch. 18), and have thereby played a...

Bacterial Viruses are Known as Bacteriophage

Viruses that infect bacteria are often called bacteriophage or phage for short. Phage is derived from the Greek for eat and refers to the way in which bacterial viruses eat holes or plaques in a lawn of bacteria growing on the surface of agar (Fig. 17.06). Bacterial viruses were heavily used in the early days of molecular biology to investigate the nature of the gene. Because viruses only contain DNA or RNA surrounded by a protein coat and because bacteriophage infect the simplest of all cells,...

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Looped Protein Scaffold

FIGURE 10.05 Methylation of Insulator Sequences and Binding A) The Igf2 (insulin-like growth factor-II) gene is distant from an enhancer element. B) When the insulator is not methylated, the CTCF protein binds to the insulator and the enhancer can only affect the H19 gene. C) When the insulator and the H19 gene are methylated, the CTCF protein does not bind, allowing the enhancer to activate the Igf2 gene. Insulators can be inactivated by methylating their CG sequences. isms, such as the...

Supercoiling Causes Problems for Replication

Binding Dna Strand

Several major problems must be solved to accomplish bacterial DNA replication. First, there are the topological problems due to DNA being not only a double helix but also supercoiled. Because the two strands forming a DNA molecule are held together by hydrogen bonding and are twisted around each other to form a double helix, they cannot simply be pulled apart. The higher level supercoiling further complicates the problem of separating the strands. Consequently, before new DNA can be made, first...

Detecting Insertions in Vectors

Inserts in a vector can be checked by isolating DNA, cutting with a restriction enzyme, and seeing how many fragments are generated. Inserts are sometimes screened by the change in growth properties due to disrupting a gene on the vector. Inserts are often detected by blue and white screening with Xgal. Inserts abolish production of beta-galactosidase and result in white (rather than blue) colonies. Once a gene or other fragment of DNA has been cloned into a plasmid vector and transformed into...

Occasional Plasmids are Linear or Made of RNA

Single Stranded Plasmid Hairpin

Although most plasmids are circular molecules of DNA there are occasional exceptions. Linear plasmids of double-stranded DNA have been found in a variety of bacteria and in fungi and higher plants. The best-characterized linear plasmids are found in those few bacteria such as Borrelia and Streptomyces that also contain linear chromosomes (see Ch. 5). Linear DNA replicons in bacteria are not protected by incompatibility The inability of two plasmids of the same family to co-exist in the same...