According to the model proposed by Watson and Crick, the DNA molecule consists of two polynucleotide chains arranged on a helix around an imaginary axis, rotating to the right (a double helix). That is, the DNA molecule is in the shape of a spiral staircase, in which the "steps" are made up of nucleotide nitrogen bases and the "handrails" are covalently linked phosphate and sugar - so it is said that DNA is shaped like a helical ribbon. Also according to Watson and Crick DNA replication is semi-conservative, because each strand of DNA will give rise to another strand that will be complementary to its nucleotides.
