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The figure shows DNA in the process of replication (the production of two DNA molecules from one). The original (template) strands have green and white backbones. The new strands have yellow and white backbones. New nucleotides are added on the 3' ends which are shown with red arrowheads. The   leading strand (shown on the right) is the new strand which grows towards the replication fork. The lagging strand (left) grows away from the fork. Periodically, the lagging strand is joined (ligated) with the previous lagging strand to make one long piece. The joining occurs where the red arrowhead on the 3' end abuts the yellow connector on the 5' end. As the original DNA unwinds, a new lagging strand will grow and join with this one. Replication requires the action of many proteins which are not shown.