My question pertains to Embryology and Development. When comparing the Sea Urchin with the Mammal, in terms of the developing embryo, when does DNA

My question pertains to Embryology and Development.

When comparing the Sea Urchin with the Mammal, in terms of the developing embryo, when does DNA replication begin?

I know that in sea urchins DNA replication begins after the 2 pronuclei have fused into diploid zygote nucleus… but what happens in mammals?

Also How is fusion of the male and female pronuclei different when one compares Sea urchin and mammal?

Thank you.