In this diagram of the process of DNA replication at a replication fork, the black boxes labeled D and E are:

A RNA primers

The initiation of replication always starts with a short RNA piece. The DNA replicating enzymes will only add nucleotides to the 3' end of DNA or RNA, and thus an RNA primase must start replication. The DNA polymerases must correct errors or "proofread", a function that is not compatible with initiation.

B DNA template strands

C Okazaki fragments

D DNA polymerase

E Newly synthesized DNA strand