There are multiple DNA polymerases, but we will not distinguish them since they all work about the same way. B. Direction of Chain Growth 1. DNA polymerases all add one way DNA polymerase adds nucleotides to the 3' end of a growing chain as in rxn 4 above, so the new chain is made from its 5' end to its 3' end. DNA polymerase will not add nucleotides on the the 5' end, and neither will any other enzyme, so all new chains must be made 5' to 3'. Once an enzyme is designed to hold the 3' end of a nucleotide so the 5' end of the nucleotide can hook up the 3' end of the growing chain, then the same enzyme cannot hold the 5' end of a nucleotide, which has a different shape and chemical make up, and so the enzyme cannot catalyze additions to the 5' end of the growing chain.
All chains grow antiparallel to template.
All nucleotide chains must be made 5' to 3' and each new single strand must be antiparallel to the its template strand, so the resulting double stranded molecule will have antiparallel strands. In both cases, the new strand is made from 5' to 3'. Then you must make the complement to the top strand in one direction and the complement to the bottom strand in the other direction. This is the only way both strands can be made 5' to 3'. This is diagrammed on handout 11-3 .