= Cell-cycle DNA content
{title2=$(2n,2C)\to(2n,4C)\to2\,(2n,2C)$}
Let $n$ count <chromosomes> in one complete set, and let $C$ denote the nuclear <DNA> amount in one unreplicated <haploid> set of this <organism>. <Ploidy> counts sets, whereas $C$ units count material; $C$ is not a universal mass shared by all <species>. Here we count a paired replicated <chromosome> as one until its <sister chromatids> separate. A copied <diploid> <genome> is therefore still <diploid> through <S phase>: $2n$ persists even as $2C$ becomes $4C$.
An original bookkeeping example uses a hypothetical <diploid> <biological cell> with $n=3$. Name its three <homologous chromosome> pairs $A,a$, $B,b$ and $D,d$. Assume ordinary nuclear <DNA replication>, no <chromosome> loss, and one continuous double-stranded <DNA> <molecule> per unreplicated <chromosome> or replicated <chromatid>. Count nuclear material only, excluding <mitochondrion> <DNA>.
|| Stage and counting compartment
|| <Chromosomes>
|| Double-stranded <DNA> <molecules>
|| Nuclear <DNA> amount
| One <G1 phase> <biological cell>
| 6 unreplicated
| 6
| $2C$
| One <G2 phase> <biological cell>
| 6 replicated
| 12 in 6 pairs of <sister chromatids>
| $4C$
| Whole undivided <anaphase> <biological cell>
| 12 separated daughter <chromosomes>
| 12
| $4C$ total
| Each completed daughter <cell nucleus>
| 6 unreplicated
| 6
| $2C$ each
For example, $A$ becomes the paired copies $A_1,A_2$ during <S phase>. They remain one replicated <chromosome> until separation. Each completed daughter receives one $A$ copy and one $a$ copy, and similarly for the other two pairs. Across both daughters, $2\times6=12$ double-stranded <DNA> <molecules> and $2(2C)=4C$: division allocates the copied material without another round of <DNA replication>.
The whole undivided <anaphase> <biological cell> transiently contains $4n$ separately counted daughter <chromosomes>. This counting change does not establish a four-set <ploidy> in a completed daughter <genome>: each pole receives $2n$, and each completed daughter <cell nucleus> is <diploid>. Always state whether a number refers to one <cell nucleus>, one pole, or the entire undivided <biological cell>. Compare <meiotic DNA content>: after <meiosis I>, a <haploid> <cell nucleus> can contain $2C$, so amount alone cannot determine <ploidy>.
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