According  to the model proposed by James Watson and  Francis Crick of DNA  molecule consists of two  long  strands coiled  around a common ,imaginary, central axis to form double helix .DNA is genetic material of living organism and is located in the chromosomes of each cell. If every organism most produce copies of itself in order to pass on genetic information to its young before it dies. DNA replication results is one DNA molecule becoming two daughter molecules each an exact copy of the original molecule. Each new DNA molecule consist of one old strand and one new strand .If the replication method were semi-conservative ,one round  of replication would yield  to DNA  molecule that each contain one strand of parental DNA  and one strand of new DNA . After replication each DNA molecule has one old and the other new strand. It shows that 50%part of the mother molecule is retained or conserver while remaining 50% part is newly constructed. Hence the process is referred to as semi-conservative replication.   DNA – polymerase catalyses polymerization only in one direction that is 3'-5'.  Consequently one stand replication is continuous, while on the other stand it is discontinuous.