DNA synthesis in bacteria

Okazaki fragment. A short segment of single-stranded DNA that is an intermediate in DNA synthesis. In bacteria, Okazaki fragments are 1,000-2,000 bases in length in eukaryotes, 100-200 bases in length. 

Jacob, F., S. Brenner, and F. Cuzin. 1963. On the regulation of DNA synthesis in bacteria. Cold Spring Harbor Symp. Quant. Biol., 28 329-348. 

The answer is b. (Hardman, p 1159.) Rifampin inhibits RNA synthesis in bacteria, mycobacteria, and chlamydiae by binding to the DNA-de pen dent RNA polymerase it also inhibits assembly of poxvirus particles. Rifampin is used as a single prophylactic agent for contacts of people with meningococcal or H. influenzae type b infections. Otherwise, it is not used alone because 1 in 10 organisms in a population exposed to rifampin will become resistant, possibly because of mutation or a barrier against rifampin s entry into cells. 

Thiram was genotoxic to insects, plants, fungi, and bacteria it induced sister chromatid exchange and unscheduled DNA synthesis in cultured human cells. Despite established genotoxicity in vitro, it showed no clastogenic and/or aneugenic activity in vivo after oral administration to mice at the maximum tolerated dose. ... 

DNA synthesis in irradiated sensitive bacterial cells is permanently inhibited by doses which produce about 5 thymine dimers per single strand of DNA (1000 p.). In the cells of resistant bacteria, DNA synthesis is inhibited only by doses of radiation which produce about 500 dimers per cell. The difference between the two types of bacteria is caused by the presence in the resistant cells of efficient enzymatic mechanisms for repairing the damage done by thymine dimerization. 

Quinolones (bacteriostatic inhibit DNA gyrase) e.g., ciprofloxacin, cinoxacin, enoxacin, norfloxacin Rifampin (bactericidal blocks mRNA synthesis in bacteria, inhibits RNA polymerase)... 

Epoxybutane induced morphological transformation, sister chromatid exchanges, chromosomal aberrations and mutation in cultured animal cells however, in a single study, it did not induce unscheduled DNA synthesis in rat primary hepatocytes. It induced sex-linked recessive lethal mutations and translocations in Drosophila melanogaster, mitotic recombination in yeast, and mutations in yeast and fungi. 1,2-Epoxybutane induced DNA damage and mutations in bacteria. 

Methyl chloride was mutagenic to bacteria and induced chromosomal aberrations in plants. It induced unscheduled DNA synthesis in cultured rat hepatocytes and, in rats exposed in vivo, there was a small increase in unscheduled DNA synthesis in hepatocytes but not in tracheal epithelial cells or spermatocytes. DNA strand breaks were induced by methyl chloride in the kidney cells of exposed mice. In cultured mammalian cells, it induced mutations and sister chromatid exchanges and enhanced viral cell transformation. It induced dominant lethal effects in rats. The last effect appears to be due to a failure of the males to fertilize the females, rather than to preimplantation embryonic death and can be partially inhibited by treatment with an anti-inflammatory agent (Chellman et al., 1986c). 

There is conflicting evidence concerning the mutagenicity of 1,2-dimethylhydrazine to bacteria. In a single study, it induced recombination in Saccharomyces cerevisiae. In vitro, 1,2-dimcthylhydrazine formed DNA adducts in human bronchial cells, provoked unscheduled DNA synthesis in rat hepatocytes and induced gene mutation in mammalian cells. It gave positive results in rodents in microbial host-mediated assays. 

Trichloroethane bound to DNA, RNA and protein and caused strong S-phase induction but not unscheduled DNA synthesis in rodents in vivo. It induced DNA damage and micronuclei in human lymphocytes and cell transformation in BALB/c-3T3 cells in vitro. 1,1,2-Trichloroethane caused chromosomal malsegregation in fungi and showed some evidence of mutagenicity in bacteria. 

Chloroethane was mutagenic to bacteria and at the hprt locus in a study with the Chinese hamster ovary cell line, but not did not induce transformation in BALB/c 3T3 cells. In B6C3F] mice exposed by inhalation, it did not induce either unscheduled DNA synthesis in hepatocytes or micronuclei in bone-marrow cells. 

As previously summarized, diethyl sulfate induced mutation and DNA damage in bacteria and induced reverse mutation and mitotic recombination in yeast. In plant cells, diethyl sulfate induced chromosomal aberrations. In a single study, diethyl sulfate did not induce heritable translocation in Drosophila melanogaster but did induce autosomal recessive lethal mutations, sex-linked recessive lethal mutations and genetic crossing-over. In cultured mammalian cells, diethyl sulfate induced chromosomal aberrations, mieronueleus formation, sister chromatid exchanges, forward mutation and DNA singlestrand breaks it also induced unscheduled DNA synthesis in primary cultures of rat... 

Morpholine did not induce mutations in bacteria, unscheduled DNA synthesis in primary cultures of rat hepatocytes or chromosomal aberrations in Chinese hamster lung fibroblasts. According to an abstract, it induced a small increase in tk locus mutations of mouse lymphoma cells and increased the frequency of morphologically transformed BALB/c 3T3 cells. In a transplacental exposure study with Syrian hamsters, it did not induce micronuclei, chromosomal aberrations or mutations in embryo cells (lARC, 1989). 

Plasmids are autonomously replicating pieces of extrachromosomal DNA present in bacteria. They are encoded with subtle, yet vital, changes for the synthesis of important cellular proteins. Because they are relatively large, they can contain information pertaining to several genes. One of these genes codes for beta-lactamase, an enzyme that can hydrolyze the four-member heterocyclic beta-lactam ring present in penicillins and cephalosporins. 

One classical and early example of selective inhibition of DNA biosynthesis is shown in Fig. 3 for the anitbiotic, mitomycin C29 A concentration of 0.1 pg/ml (3 x 10 7 M) completely inhibited DNA synthesis in E. coli B, while RNA synthesis, protein synthesis and growth meaning turbidity, i.e. cell mass increase, continued. However, after the experimental period of only 90 min, the number of viable bacteria had decreased by 85 per cent. By that time, bacterial filaments were visible under the microscope. 

---