Based on the image, the y and z line shows shorter ends of daughter molecules. When a molecule of a paired strands of DNA double helix undergoes one round of replication, both 3' ends are shorter in the daughter molecules than in the parental molecules. Both strands of parental DNA serve as templates for newly synthesized daughter strands that inherits a new double helix DNA containing one old and one new daughter strands arising from the parental duplex. This mechanism would require one daughter strand to polymerize in the 5′-to-3′ direction and the other in the 3′-to-5′ direction through the action of DNA ligase.