Primases. DNA

The answer is b. (Murray, pp 412—434. Scriver, pp 3-45. Sack, pp 3—29. Wilson, pp 99-121.) A special DNA polymerase called telomerase is responsible for replication of the telomeric DNA. Telomerase contains an RNA molecule that guides the synthesis of complementary DNA. Telomerase is therefore an RNA-dependent DNA polymerase in a category with reverse transcriptase. Telomerase does not require an RNA primer, initiating synthesis of the leading strands at 3 ends within the telomeric DNA. Synthesis of the lagging strands uses primase, DNA polymerase III, and DNA polymerase I, as with the replication of other chromosomal regions. 

Helicase, Primase, DNA Polymerases, and Other Proteins Participate in DNA Replication... 

Fludarabine phosphate, a fluorinated deamination-resistant nucleotide analog of the antiviral agent vidarabine (9-P-D-arabinofuranosyl-adenine), is active in CLL and low-grade lymphomas. After rapid extracellular dephosphorylation to the nucleoside fludarabine, it is rephosphorylated intracellularly by deoxycytidine kinase to the active triphosphate derivative. This antimetabolite inhibits DNA polymerase, DNA primase DNA ligase, and ribonucleotide reductase, and is incorporated into DNA and RNA. The triphosphate nucleotide is an effective chain terminator when incorporated into DNA, and the incorporation of fludarabine into RNA inhibits RNA function, RNA processing, and mRNA translation. 

It has been noticed that somatal cell lines cultured in vitro reproduce about 20 to 50 times and then the cells die. This has prompted biologists to specnlate that there is a natural end to multicellular BU in vivo. One reason for this may be events in the process of cellular mitosis, as chromosomes are split and replicated. Replication requires an involved series of steps, and includes RNA priming, DNA primase, DNA polymerase, and DNA ligase in an intricate set of maneuvers. These steps, however, are not able to replicate the end of the DNA strand. Errors in the replication process can... 

DNA Pol a-Primase, DNA Polymerase a, DNA Polymerase j3, and Terminal Deoxynucleotidyl Transferase... 

Yagura et al. [20] have reported that various eukaryotic cells contain at least two forms of DNA polymerase a. One possessed an ability to synthesize poly(dA) with poly(dT) as template and without primer in the presence of ATP and dATP [21]. They also showed that the unique property of this enzyme was due to the association of DNA primase activity with this enzyme [22]. As shown in Fig. 5, when bovine thymus DNA polymerase a fraction obtained from phosphocellulose column chromatography was further purified by DEAE-Sephadex A-50 column chromatography, the enzyme activity was separated into two major sharp peaks. Since one of the peaks was associated with the primase activity, we call this fraction DNA polymerase a with primase (DNA pol a-primase) and the other (fraction II in Fig. 5) DNA polymerase a in this study. When these two enzyme fractions were incubated in a reconstituted poly(ADP-ribos)ylating system, both DNA pol a-primase and DNA polymerase a were strongly inhibited (77 and 50% inhibition, respectively, Table 1). Thus, in order to clarify the mechanism of the inhibition, further study was carried out using mainly the DNA pol a-primase fraction. All the components of poly(ADP-ribos)ylation... 

RNA primase DNA polymerase a (also known as RNA primase) associates with DNA polymerase 5 forming a complex. Primase makes the RNA primer to which DNA is added by DNA polymerase 8... 

Mitsis P.G., Chiang C.-S., and Lehman I.R. 1995. Purification of DNA polymerase-primase (DNA polymerase a) and DNA polymerase 5 from embryos of Drosophila melanogaster. Methods Enzymol. 262 62-77. 

F%. 2 Herpesvirus DNA-rephcation fork showing the site of action of helicase/primase inhibitors... 

Beard PM, Duffy C, Baines JD (2004) Quantification of the DNA cleavage and packaging proteins UL15 and UL28 in A and B capsids of herpes simplex vims type 1. J Virol 78 1367-1374 Betz UA, Fischer R, Kleymann G, Hendrix M, Rubsamen-Waigmann H (2002) Potent in vivo antiviral activity of the herpes simplex vims primase-helicase inhibitor BAY 57-1293. Antimicrob Agents Chemother 46 1766-1772... 

Primosome The mobile complex of helicase and primase that is involved in DNA replication. 

DNA replication differs between the leading strand and the lagging strand of the DNA double helix. In cells, replication Af the lagging strand involves the formation of short RNA primers by action of an enzyme called RNA primase (or primase for short). Such RNA primers are made at intervals on the lagging strand and are then removed and replaced with DNA by DNA polymerase. 

In 4>X174, however, replication begins with a single stranded closed circle, a rather atypical situation. First, primase brings about the synthesis of a short RNA primer, beginning at one or more specific initiation sites on the DNA. 

The RNA oligonucleotides are complementary to a sequence on one of the strands of the DNA template and base pair with a portion of the DNA molecule. Subsequently, deoxyribonucleotides are covalently attached to the RNA primer. The synthesis of the primer itself is catalyzed by a special RNA polymerase called primase. Similar RNA polymerase-like enzymes are used to prime the synthesis of certain viral DNAs and eukaryotic DNA. 

Conserved catalytic (adenylation) domain in NAD-dependent (bacterial) and ATP-dependent (archaeal-eukaryotic) DNA ligases (Aravind and Koonin, 1999) Conserved nucleotide joining-cleaving domain in type I and II topoisomerases, DnaG-type primases, OLD nucleases, and RecR (Toprim domain) (Aravind et al, 1998b)... 

The effect of sphingosine on other enzymes may also contribute to its apoptotic effect. These include the inhibition of calcium/calmodulin-requiring enzymes and DNA primase and the stimulation of casein kinase II and several unidentified kinases (Alessenko, 2000). In addition, sphingosine can increase the cellular concentration of cyclic AMP, which is inhibitory for proliferation in many cell types (Pyne and Pyne, 1996). 

Primase synthesizes a short (about 10 nucleotides) RNA primer in the 5 —>3 direction, beginning at the origin on each parental strand. The parental strand is used as a template for this process. RNA primers are required because DNA polymerases are unable to initiate synthesis of DNA, but can only extend a strand from the 3 end of a preformed primer. ... 

The lagging strand is synthesized discontinuously as a series of small fragments (about 1,000 nucleotides long) known as Okazaki fragments. Each Okazaki fragment is initiated by the synthesis of an RNA primer by primase, and then Completed by the synthesis of DNA using DNA polymerase III. Each fr ment is made in the 5 - 3 direction. 

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