2.10—DNA replication and Meselson-Stahl evidence

Before cell division, DNA strands separate and each acts as a template. Free nucleotides pair by complementarity, DNA polymerase joins them into new strands, and each daughter DNA molecule contains one original and one new strand.

Helicase separates the strands; complementary nucleotides align; DNA polymerase forms phosphodiester bonds in the new strand. The two molecules then carry the same base sequence, apart from copying errors.

Meselson and Stahl grew bacteria with heavy nitrogen, moved them to light nitrogen and separated DNA by density. An intermediate-density band after one generation supported semi-conservative replication; later generations produced light and intermediate DNA.

The density pattern distinguishes semi-conservative replication from conservative and dispersive alternatives because it tracks old and newly synthesised DNA together.

DNA polymerase adds nucleotides to a template; it does not “copy a whole chromosome” in one step. Experimental evidence supports a model under stated conditions, not a claim that mutations never occur.