Why must DNA replication be initiated with a primer rather than directly by DNA polymerase?

1. Leading strand runs 3′→5′ while the lagging strand runs 5′→3′
2. Leading strand is continuous; lagging strand is made in Okazaki fragments
3. The lagging strand is built from RNA while the leading strand is DNA
4. The lagging strand, unlike the leading strand, needs no ligase

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