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In a pair of ( chromosomes, sister chromatids, chromatids, homologous chromosomes, non-identical ), there are two ( chromatids, chromosomes, sister chromatids, homologous chromosomes, non-identical ) united in the center by a centromere.
( Sister chromatid, chromosomes, chromatids, non-identical, homologous chromosomes ) is only used during cell division when the structures are in an X-shape (when there are two copies connected by a centromere.
When a centromere joins two ( non-identical, chromosomes, chromatids, sister chromatid, homologous chromosomes ) copies of a chromosome (one from each parent); they are called ( homologous chromosomes, chromosomes, chromatid, sister chromatid, non-identical ).