Replicative form synthesis

Hirose Y, Nagahori H, Yamada T et al (2009) Comparison of the effects of the synthetic pyrethroid metofluthrin and phenobarbital on CYP2B form induction and replicative DNA synthesis in cultured rat and human hepatocytes. Toxicology 258 64—69... 

Budding and release of progeny virus. B. Replicative cycle of an influenza virus, an example of an RNA virus. 1. Attachment. 2. Endocytosis. 3. Influx of H+ through M2 protein. 4. Fusion of the viral envelope with the endosomes membrane, dissociation of the RNP complex, and entry of viral RNA into the nucleus. 5. Synthesis of viral mRNA by viral RNA polymerase. 6. Translation of viral mRNA by host cell s ribosomes. 7. Replication of viral RNA, using viral RNA polymerase, via cRNA replicative form. 8. Assembly of virus particles, and 9. Budding and release of progeny virus. 

Canine parvovirus, first identified in 1978, is now endemic.455,456 Childhood fifth disease is also caused by a parvovirus.456,457 When these single-stranded DNA viruses infect cells a double-stranded replicative form of DNA arises by synthesis of the complementary negative strand alongside the original positive DNA strand. Many copies of the replicative form are then synthesized. The negative strands of the replicative forms serve as templates for synthesis of numerous new positive strands that are incorporated into the progeny viruses. The whole process may take only 20 minutes. Some parvoviruses are unable to reproduce unless the cell is also infected by a larger adenovirus. 

Current theories propose that the development of cell membranes accelerated early life. Enclosing chemicals responsible for replication, protein synthesis and energy generation within a confined space allows these essential life processes to occur far more frequently than they would do otherwise. Biological membranes comprise two phospholipid layers held together by interdigitation of their hydrophobic alkyl tails as shown in Fig. 5.1. They form a lamellar structure with their polar internal and external surfaces separated by a non-polar region. In their extended form each... 

Goulian, M. and Kornberg, A. (1%7) Enzymatic synthesis of DNA. 23. Synthesis of circular replicative form of... 

The rate of synthesis of each molecular species varies considerably during the infectious cycle viral s-s RNA synthesis is maximal during the linear phase. There is a constant production of d-s replicative form (RP) which, therefore, acoiMulates towards the end of the cycle. By contrast, the replicative intermediate Rl) is the major molecular species only during the early (exponential) phase of RRA synthesis (50). 

It is followed by the replication or synthesis of DNA (S-phase), and before cell division takes place a second preparation phase (G2 phase) is required. During the short interval of mitosis (M phase), the DNA is packed in an extremely condensed form, microscopically visible as chromosomes, which are then symmetrically distributed to the two daughter cells. The total time for a complete division cycle of typical mammalian cells under laboratory conditions is in the order of 12-24 h. 

DNA-dependent RNA synthesis in E.coli cells was inhibited under experimental conditions by actinomycin D. After 50-90 sec, almost 70% of the of the parent RNA was found in one fraction of RNA with a sedimentation coefficient of about 20S, which these workers called the replicative form of RNA, because it was resistant to ribonuclease (double-helical form). The quantity of P of ribonuclease-resistant RNA then fell sharply, and a new peak of this RNA appeared after 3.5 min (Fig. 12). In one case this wave of change of the parent RNA into the double-helical RNAase-resistant form was observed on four successive times. In addition, slight but definite incorporation of parent P material was observed into a special low-molecular weight 6S fraction (mol.wt. from 1 to 30 X10 ) was observed, which, however, contained much newly synthesized RNA (in this fraction the ratio of parent RNA/new RNA was much lower than in the 2OS fraction of RNA). 

After penetrating into bacterial cells the DNA of this phage undergoes the typical fate which we saw for RNA in cases of virus reproduction considered above it is replicated by complementary synthesis with the preliminary formation of double-stranded DNA. This process was discovered in later experiments (Sinsheimer, 1961 Sinsheimer et al., 1962) by fractionation of nitrogen- and phosphorus-labeled DNA of infected cells in a density gradient of cesium chloride at various times after infection. This double-stranded form of phage DNA was called the replicative form (RF). 

As the results of molecular hybridization and determination of base sequence have shown, RNA formed during phage reproduction in vivo on the basis of the circular double-helical replicative form of DNA (Hayashi et al., 1964) is complementary to only one of the two DNA chains, i.e., to that which is complementary, in turn, to the single-stranded DNA of the mature phage. In vitro when the circular structure of DNA is disturbed, both strands of replicative DNA can act as substrate for RNA-polymerase. The single-stranded DNA of this phage can also act as template for RNA synthesis in vitro, with the preliminary formation of DNA/ RNA hybrids (Chamberlin and Berg, 1964). 

The consequence of ADA deficiency is accumulation of adenosine and 2 -deoxyadenosine, substances toxic to lymphocytes, important cells in the immune response. 2 -Deoxyadenosine is particularly toxic because its presence leads to accumulation of its nucleotide form, dATP, an essential substrate in DNA synthesis. Elevated levels of dATP actually block DNA replication and cell division by inhibiting synthesis of the other deoxynncleoside 5 -triphosphates (see Chapter 27). Accumulation of dATP also leads to selective depletion of cellular ATP, robbing cells of energy. Children with ADA SCID fail to develop normal immune responses and are susceptible to fatal infections, unless kept in protective isolation. 

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