Ploidy describes how many complete sets of homologous chromosomes a cell or nucleus contains. It does not directly describe how much DNA is present.

A cell with one complete chromosome set is haploid and is often written $n$. A cell with two homologous sets is diploid and is written $2n$.

In a diploid cell, each chromosome type is represented by a pair of homologous chromosomes. The two homologs carry the same kinds of genes in corresponding chromosomal regions, although their DNA sequences need not be identical.

One homolog may have been inherited from one genetic parent and the other from the other parent.

Homologs are not sister chromatids

These relationships must be distinguished.

• Sister chromatids are the two DNA copies produced by replication of one chromosome.
• Homologous chromosomes are corresponding chromosomes occupying the same position in two chromosome sets but inherited as separate chromosome copies.

Consider a diploid cell with three chromosome types. Label the two homologs of each type $A_1/A_2$, $B_1/B_2$ and $C_1/C_2$.

Before S phase:

A1 A2    B1 B2    C1 C2

After S phase, each homolog has replicated:

A1×2 A2×2    B1×2 B2×2    C1×2 C2×2

There are now twice as many DNA molecules, but there are still two homologous chromosome sets. The cell remains diploid.

DNA replication changes DNA content, not ploidy

This gives a crucial comparison:

G1 diploid cell: 2 chromosome sets, unreplicated chromosomes
G2 diploid cell: 2 chromosome sets, replicated chromosomes

S phase doubles DNA content and chromatid number, but it does not create an additional homologous chromosome set.

Ploidy changes when homologous sets are separated

If a diploid cell segregates one homolog from each pair into different daughter nuclei, each daughter receives one complete chromosome set and becomes haploid.

That is the defining reduction accomplished by meiosis I.

By contrast, ordinary mitosis separates sister chromatids while preserving the number of homologous chromosome sets.

Ploidy is therefore a statement about sets of homologous chromosomes. Chromosome number, chromatid number and DNA mass can all change during the cell cycle without necessarily changing ploidy.