Final answer:
Meselson and Stahl's experiment demonstrated that DNA replication is semiconservative, where each new DNA molecule consists of one old and one new strand. DNA replication is bidirectional and involves the synthesis of discontinuous Okazaki fragments on the lagging strand.
Step-by-step explanation:
Meselson and Stahl showed that DNA is replicated by a semiconservative system. Their landmark experiment provided significant evidence for this mode of replication, where each of the two parental DNA strands acts as a template for new DNA to be synthesized; after replication, each double-stranded DNA contains one parental or 'old' strand and one 'new' strand. This semiconservative model was one of three proposed models—the other two being conservative and dispersive. The conservative model suggested that the parental DNA is conserved while daughter DNA is newly synthesized as a separate entity. The dispersive model implied that the parental and daughter DNA would be interspersed in segments along the DNA molecule.
DNA replication is described as bidirectional because the replication forks proceed outward in opposite directions from the origin of replication, effectively duplicating DNA toward both ends of the parental strands. Additionally, replication is described as discontinuous on the lagging strand because it is synthesized in short segments known as Okazaki fragments, which are later joined to form a continuous strand.