opt rdg 2.3

opt rdg 2.3 - DNA REPLICATION: Choosing a Place to Begin...

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DNA REPLICATION: Choosing a Place to Begin Joel A. Huberman* Before it can divide, a cell must first duplicate its own DNA, creating a copy of its genome for each daughter. In viruses, bacteria, and yeasts, DNA replication begins at sites in the genome called origins. Specific nucleotide sequences (or replicators) at each origin are required for initiation of DNA replication. Now in this issue of Science, Aladjem et al. (1) show that specific sequences are also necessary for initiation at origins near the human b-globin gene and are thus likely to be necessary for all replication origins in animal cells, as in viruses, bacteria, and yeasts. Aladjem et al. (1) used a genetic approach to characterize replication origins in animal chromosomes. Previous studies, which relied entirely on biochemical methods for identification of initiation sites, could not determine which nucleotide sequences are important. As a result, the field has been clouded by uncertainty. The advance by Aladjem et al. (1) dissipates the clouds by showing that replicators are essential and, in combination with improved biochemical methods (2), should now allow rapid progress in understanding the mechanism and regulation of the origin of replication in animal cells. To ensure that all sequences that they evaluated for origin function would be tested in the same environment, Aladjem et al. constructed a monkey cell line with a recombinase target site integrated at a single position in one of its chromosomes (see the figure). They also constructed plasmids containing the previously mapped (3, 4) human b-globin initiation region (green box in figure) or deletion derivatives of the initiation region plus a recombinase target site. The b-globin initiation region (about 8 kb) includes the complete b-globin gene plus noncoding flanking sequences. Simultaneous introduction into target cells of a plasmid and recombinase led to integration of the b-globin initiation region or one of its derivatives at the same chromosomal position in each recombinant cell line (1). Figure 1 In animals, too. Sequence-dependent DNA replication has been demonstrated in animal cells by Aladjem et al. (1), who inserted the intact human b-globin initiation region (green box) and various deletion derivatives of the initiation region (green boxes at bottom of figure) or the
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This note was uploaded on 10/07/2009 for the course GEN 409 taught by Professor Linda during the Fall '09 term at Iowa State.

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opt rdg 2.3 - DNA REPLICATION: Choosing a Place to Begin...

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