DNA replication
= DNA replication
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<DNA replication> copies a double-stranded <DNA> <molecule> into two daughter double-stranded <DNA> <molecules>. In <semiconservative replication>, each daughter retains one parental strand and receives one newly synthesized complementary strand. <DNA polymerases> build the new strands from incoming <nucleotides>; they do not make <RNA> or <proteins> in this copying step. See https://www.ncbi.nlm.nih.gov/books/NBK26850/[Alberts et al., Molecular Biology of the Cell, 4th edition (2002): DNA replication mechanisms].
For a five-<base pair> example, separate $5'\text{-}\mathrm{ATGCC}\text{-}3'$ from its partner $3'\text{-}\mathrm{TACGG}\text{-}5'$. <Base pairing> specifies a new TACGG partner for the old ATGCC strand and a new ATGCC partner for the old TACGG strand. With error-free copying, both daughter <DNA> <molecules> have the original <nucleotide sequence>, but different physical strand histories. One initial duplex contains ten <nucleotides>; one complete round produces two duplexes containing twenty <nucleotides>, ten retained and ten newly incorporated.
\Image[/dna-replication-semi-conservative.png]
{title=<Semiconservative replication>: follow two original <DNA> strands through two complete copying rounds}
{description=Blue O1 and purple O2 are the original strands; green strands were made in round 1 and orange strands in round 2. Each arrow connects a parent duplex to its daughters. After round 2, two of four duplexes retain an original strand, and the other two retain a round-1 strand. All eight strands have the appropriate complementary <nucleotide sequences>; colors identify strand age, not different <nucleobases>. The flattened duplexes show ancestry and orientation, not a replication fork or molecular shape.}
This is useful for inheritance: both daughter <biological cells> can receive a copy of the <DNA> record. The elegant feature is that <base pairing> lets each strand specify its missing partner. Direction matters too: a replicating <DNA polymerase> reads its template $3'\to5'$ and extends the new strand at its $3'$ end, so synthesis proceeds $5'\to3'$. These are opposite directions on <antiparallel nucleic acid strands>, not two alternative directions of synthesis. The enzyme needs a pre-existing paired strand end to extend; the diagram omits the machinery that starts and coordinates copying. See https://www.ncbi.nlm.nih.gov/books/NBK26850/[Alberts et al.: template-directed synthesis and the requirement for a primer].
The strand accounting predicts observable differences between copying mechanisms. In 1958, Meselson and Stahl grew <Escherichia coli> with the heavier <nitrogen> <isotope> $^{15}\mathrm{N}$, then supplied $^{14}\mathrm{N}$ and measured extracted <DNA> by density-gradient centrifugation. After one generation their samples contained intermediate-density <DNA>; after two, intermediate-density and light <DNA> occurred in equal amounts. This supported <semiconservative replication> in the tested <Escherichia coli> culture. It was an <isotope> assay of sampled <DNA>, not simultaneous testing of every kind of <DNA> or direct colored-strand photography. See <The replication of DNA in Escherichia coli by Matthew Meselson and Franklin W. Stahl>.