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The steps of the paleoelongation differ from those of eukaryotes

Posted by star on 2018-09-19 01:58:40
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    The machinery required for the elongation stage of DNA synthesis in the arcnaea is very similar to that used by the bacteria and eukaryotes. Lagging and leading strand synthesis are coordinated. Processive DNA synthesis requires tethering DNA polymerase to a sliding clamp. The archaeal sliding clamp is similar in sequence and structure to PCNA in eukaryotes. The clamp loader appears to also be similar to its eukaryotic counterpart. Two different kinds of archaeal DNA polymerases, PolB and FolD, participate in the elongation process; however, not all archaeal species have PolB.

    It is not yet clearwhat specific contributions each of these polymerases make to DNA replication. Perhaps one participates in leading strand synthesis and the other in lagging strand synthesis. The archaeal counterparts of eukaryotic Fen-1 and RNase H remove RNA from the 5'-ends of the Okazaki fragments and DNA ligase joins the fragments. Archaeal ligase, like its eukaryotic counterpart, requires ATP for ligation. It therefore differs from the NAD+-dependent bacterial ligase.

    The archaeal replication system has one major surprise. Thermophiles require an ATP-dependent reverse gyrase to introduce positive supercoils. The reverse gyrase works by introducing transient nicks into a single strand and so functions by a different mechanism from the bacterial gyrase. Thermophiles may require reverse gyrase because they grow at high temperatures that tend to unwind DNA. Undoubtedly the archaeal replication machinery will have other surprises in store as we learn more about its components and how they work together. Perhaps the most remarkable surprise of all is how similar the replication process is in all three domains of life.



Cas9 Targeted DNA Cutting

Posted by star on 2018-09-19 01:56:52
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    The working principle of this system is that crRNA(CRISPR-pried RNA) binds to tracrRNA/crRNA by base pairing to form a tracrRNA/crRNA complex, which directs the nuclease Cas9 protein to be paired with crRNA. The sequence target point cuts double-stranded DNA. Through the artificial design of these two RNAs, sgRNA(sigle-Guide RNA) can be modified to form a guiding effect, which is sufficient to guide Cas9's fixed-point DNA cutting.

    As an RNA-guided dsDNA binding protein, Cas9 effect nuclease is the first known unifying factor that can collectively locate RNA, DNA, and proteins, thus having great potential for transformation. The protein is fused with Cas9 nuclease-free Cas9 and expresses an appropriate sgRNA that can target any dsDNA sequence, and the end of the sgRNA can be attached to the target DNA without affecting the binding of Cas9. Therefore, Cas9 can bring any fusion protein and RNA to any dsDNA sequence, which has great potential for research and transformation of organisms.



The role of Orc1/Cdc6 in MCM

Posted by star on 2018-09-18 01:53:23
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    Some archaeal chromosomes appear to have a single origin of replication whereas others have two or more origins. Homologs of some of the proteins involved in the initiation of eukaryotic DNA synthesis have been identified in archaeal genomes. However, different archaeal species appear to have ditftrent variants of the replication machinery components. These differences may reflect the great variation in environmental conditions under which the archaea live.

    Tne archaea produce a single protein, designated Ore1/Cdc6 which binds to the archaeal origin of replication. This protein share some sequences in common with the eukaryotic ORC. Once bound to the origin, Ore1/Cdc6 recruits MCM to the origin. The MCM helicase appears to work in the same way as the eukaryotic helicase. The helicase opens and unwinds the double-stranded DNA.

    RPA (single-stranded DNA binding protein) binds to the exposed single-stranded DNA.

    Primase associates with the RPA· DNA complex and synthesizes the short RNA primers required to initiate DNA synthesis. Then DNA polymerase associates with the replication bubble, initiating rapid and processive bidirectional DNA synthesis.



DNA Replication in the Archaea

Posted by star on 2018-09-17 02:00:30
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    DNA replication in the archaea has not been as extensively investigated as it has in bacteria or eukaryotes. Furthermore, investigators working on archaeal DNA replication in different laboratories have not devoted their efforts to studying a single organism or even a few closely related organisms but have instead examined a variety of different organisms. Therefore, much less is known about DNA replication in the archaea than in the other two domains. Nevertheless, a picture of archaeal DNA replication has started to emerge. Investigators initially expected that the archaeal replication machinery would be more similar to the bacterial replication machinery than the eukaryotic replication machinery because the archaea, like the bacteria, are prokaryotes that for the most part have circular chromosomes.

    This initial expectation has proven to be wrong. DNA sequences have been determined for a few of the archaea. Based on these sequences, it is clear that the components of the archaeal replication machinery are homologous to the components of the eukaryotic and not the bacterial replication machinery.

    DNA replication in archaea follows the same general pattern as it does in the other two domains Archaeal DNA replication is semiconsetvative, bidirectional, and semidiscontinuous. It also can be divided into three stages: initiation, elongation, and termination. Very litter is known about replication termination in the archaea and so the discussion that follows is limited to the initiation and elongation stages.



A telomerase polypeptide is a reverse transcriptase

Posted by star on 2018-09-13 23:30:16
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    Joachim Lingner and Thomas Cech purified Euplotes aediculatus telomerase by taking advantage of the fact that an oligonucleotide with a telomere sequence can bind to the RNA subunit in fully functional telomerase. Their approach was to pass a partially purified nuclear extract through a column containing beads linked to an oligonucleotide with the telomere sequence so that telomerase would bind to the beads. Then they displaced the bound telomerase by adding a solution that contained oligonucleotides with an even greater affinity for the oligonucleotides on the beads.

    Purified telomerase had a molecular mass of about 230-kDa and contained a 123-kDa polypeptide, a 43-kDa polypeptide, and a 66-kDa RNA subunit. When the amino acid sequence of the large polypeptide subunit was compared to sequences available in the genetic data bank, they were found to resemble a yeast polypeptide. Although the yeast polypeptide had not been isolated, genetic studies showed that the gene that specifies it, EST2, is essential for telomere maintenance. This sequence similarity suggested the 123-kDa subunit is probably essential for telomerase activity and therefore likely to be the reverse transcriptase.

    This conclusion was supported when sequence comparisons with known reverse transcriptases revealed that the 123-kDa polypeptide subunit has a reverse transcriptase domain at its carboxyl terminus. Furthermore, conserved amino acid residues in the catalytic site of reverse transcriptase are also present in both the 123-kDa polypeptide subunit and the ESTZ gene product. Yeast mutants with alterations in these conserved residues cannot maintain their telomeres. The catalytic subunit of telomerase has now been identified in many other organisms including humans. While the function of the 43-kDa polypeptide in E. aediculatus is not yet known, it seems likely to have an ancillary function in telomere maintenance. S. cer......

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