The basis of this theory is that the double-stranded DNA “unzips” into two single strands, one “sense” and one “antisense”. Once the mRNA has created a new strand complementary to the antisense strand, a new double-stranded DNA molecule is produced. Each of the two daughter molecules produced in each round of this process contains one strand from the parent DNA, ergo “semiconservative”: each strand from a double-stranded molecule of DNA being replicated is conserved.
“Each chain then acts as a template for the formation onto itself a new companion chain, so that eventually we shall have two pairs of chains where we only had one before. Moreover, the sequence of pairs of bases will have been duplicated exactly.”