<P> Chromosomes are normally visible under a light microscope only when the cell is undergoing the metaphase of cell division ( where all chromosomes are aligned in the center of the cell in their condensed form ) . Before this happens , every chromosome is copied once ( S phase ) , and the copy is joined to the original by a centromere , resulting either in an X-shaped structure ( pictured to the right ) if the centromere is located in the middle of the chromosome or a two - arm structure if the centromere is located near one of the ends . The original chromosome and the copy are now called sister chromatids . During metaphase the X-shape structure is called a metaphase chromosome . In this highly condensed form chromosomes are easiest to distinguish and study . In animal cells , chromosomes reach their highest compaction level in anaphase during segregation . </P>
when does a cell have condensed visible chromosomes also known as sister chromatids
metaphase