Why DNA polymerase can only work from 5' to the 3'?

DNA-polymerase can only work from the 5'-end to the 3'-end.

I think in order to understand, just think of the structure of a nucleotide.

1) A nucleotide has a free 5' phosphate end and a free 3' OH end.

2) A strand in 5' to 3' direction indicates a free 5' phosphate at one end and a free 3' OH at the other end.

3) DNA polymerase requires a free 3' OH end to add the incoming nucleotide. Nucleotides monomers  are added to the 3’ OH end of the growing strand one by one by DNA polymerase.  That is the bonding is between the 3' OH end of the first nucleotide and 5' P end of the incoming nucleotide (and is the phosphodiester bond) . There fore new polynucleotide  chain is always synthesised in 5’-3’ direction.

4) DNA pol III involved in replication cannot add the first nucleotide to start the synthesise of daughter DNA strand.

How DNA polymerisation is initiated?

DNA polymerase cannot initiate a synthesis but it requires a primer called primase, an RNA polymerase.

It synthesise a primer or a short sequence  of about 10 RNA nucleotides complementary to parental DNA strand. It is called primer RNA. DNA pol III recognises the primer that provide a free 3'OH end and adds DNA nucleotide to construct  new DNA strand. Later RNA primer is replaced by DNA nucleotides by DNA pol I.