| MadSci Network: Genetics |
A chromosome is a single molecule of double-stranded DNA. Most animals are diploid, which means that there are two homologous copies of each chromosome before DNA replication. During replication each chromosome is copied, resulting in 2 pairs of identical chromosomes - for a total of four DNA molecules. During mitosis, the identical sister chromosomes are segregated to each daughter cell, so that the final number is returned to two homologous copies per cell. The X structure of metaphase chromatin is two DNA molecules (the sister chromatids) held together at their centromeres prior to their separation during anaphase. In bacteria, there is only one chromosome composed of a single closed circle of double-stranded DNA.