Meiosis I is the first of two meiotic nuclear divisions. Its defining operation is the separation of homologous chromosomes, not sister chromatids.

A diploid cell enters meiosis after one round of DNA replication. Each homolog therefore consists of two sister chromatids, and paired homologs can be connected by chiasmata created through meiotic crossover.

Homolog pairs orient on the meiosis-I spindle

At metaphase I, each paired homologous chromosome set forms a mechanically linked unit. The spindle geometry differs from ordinary mitosis:

• the two sister kinetochores of one homolog act together toward one spindle pole;
• the homologous chromosome's kinetochores act toward the opposite pole.

Thus the spindle biorients homologs as a pair rather than biorienting the two sisters of each individual chromosome.

A simplified representation is

pole A ← [homolog 1: two sisters] ╳ [homolog 2: two sisters] → pole B
                                      chiasma

Anaphase I separates homologs while sisters remain joined

At anaphase I, cohesion along chromosome arms is released sufficiently for chiasmata to resolve, allowing homologous chromosomes to move to opposite poles.

Crucially, cohesion near centromeres is protected during this division, so sister chromatids normally remain associated.

The chromosome unit moving to each pole is therefore still replicated.

Meiosis I reduces chromosome-set number

Consider a diploid nucleus with two homologs of each chromosome type. Before meiosis I it contains two chromosome sets, $2n$.

After homologs segregate, each daughter nucleus receives only one homolog from each pair:

before meiosis I:  homolog A + homolog a   → 2 sets

after meiosis I:   A in one nucleus, a in the other → 1 set each

Each resulting nucleus is therefore haploid, even though every chromosome still consists of two sister chromatids.

This is why meiosis I is called a reductional division: ploidy falls from two chromosome sets to one.

There is no second S phase before meiosis II

DNA is not replicated again between meiosis I and meiosis II. The haploid nuclei produced by meiosis I already contain replicated chromosomes, and meiosis II will separate those sister chromatids.

The central contrast is therefore:

mitosis:    sister chromatids segregate; ploidy is preserved
meiosis I:  homologous chromosomes segregate; ploidy is reduced