<P> Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus . This segregation process occurs during both mitosis and meiosis . </P> <P> During mitosis chromosome segregation occurs routinely as a step in cell division (see mitosis diagram). As indicated in the mitosis diagram, mitosis is preceded by a round of DNA replication, so that each chromosome forms two copies called chromatids . These chromatids separate to opposite poles, a process facilitated by a protein complex referred to as cohesin . Upon proper segregation, a complete set of chromatids ends up in each of two nuclei, and when cell division is completed, each DNA copy previously referred to as a chromatid is now called a chromosome . </P> <P> Chromosome segregation occurs at two separate stages during meiosis called prophase I and prophase II (see meiosis diagram). In a diploid cell there are two sets of homologous chromosomes of different parental origin (e.g. a paternal and a maternal set). During the phase of meiosis labeled "interphase s" in the meiosis diagram there is a round of DNA replication, so that each of the chromosomes initially present is now composed of two copies called chromatids . These chromosomes (paired chromatids) then pair with the homologous chromosome (also paired chromatids) present in the same nucleus (see prophase I in the meiosis diagram). The process of alignment of paired homologous chromosomes is called synapsis (see Synapsis). During synapsis, genetic recombination usually occurs . Some of the recombination events occur by crossing over (involving physical exchange between two chromatids), but most recombination events involve information exchange but not physical exchange between two chromatids (see Synthesis - dependent strand annealing (SDSA)). Following recombination, chromosome segregation occurs as indicated by the stages metaphase I and anaphase I in the meiosis diagram . </P> <P> Different pairs of chromosomes segregate independently of each other, a process termed "independent assortment of non-homologous chromosomes". This process results in each gamete usually containing a mixture of chromosomes from both original parents . </P>

When does segregation and independent assortment happen during meiosis