Describe why DNA replication is semi-conservative

DNA replication involves the unwinding of double strand DNA by the enzyme helicase, resulting in two single DNA strands which can be used as templates for replication. DNA polymerase can then catalyse replication by binding free nucleotides to these template strands via complimentary base pairing. It is called 'semi-conservative' as both of the produced DNA molecules will contain 1 new strand and 1 old, 'template' strand. (i.e. 1 old DNA strand is 'conserved' in each)