A chromosome changes physical state when its DNA is replicated, but its identity is not determined simply by how many DNA molecules are visible.

Before DNA replication, an unreplicated chromosome contains one continuous double-stranded DNA molecule packaged with proteins.

After replication, that chromosome consists of two sister chromatids. Each sister chromatid contains one newly replicated double-stranded DNA molecule, and the two sisters carry nearly identical sequence information because they were produced from the same parental chromosome.

Thus

before replication: one chromosome = one chromatid

after replication:  one replicated chromosome = two sister chromatids

until those sisters separate.

Sister chromatids are held together

Protein complexes including cohesins hold sister chromatids together after replication. This physical linkage is especially important near a chromosome region called the centromere.

The centromere is not simply the geometric midpoint of a chromosome. It is a specialized chromosomal region that helps organize the attachment machinery used during segregation.

The kinetochore is the attachment structure

A kinetochore is a multiprotein structure assembled on centromeric chromatin. During cell division, spindle microtubules attach to kinetochores and transmit forces to chromosomes.

Each sister chromatid has its own kinetochore. Before separation, the two kinetochores belong to the two sisters of one replicated chromosome.

Chromosome counting follows centromere identity, not DNA mass

A replicated chromosome still counts as one chromosome while its sister chromatids remain joined. When the sisters separate during chromosome segregation, each former chromatid becomes an independent chromosome because it now has its own segregating centromere and DNA molecule.

For example, suppose a cell contains one particular chromosome before replication:

before replication: |          one chromosome, one DNA molecule

after replication:  X          one replicated chromosome, two sister chromatids

after separation:   |    |     two separate chromosomes

The total amount of DNA doubles during replication, but chromosome number does not double until sister chromatids physically separate.

This distinction is essential for reasoning about cell division: DNA content, chromatid number and chromosome number are related but are not interchangeable quantities.