Chromosomes

When a cell divided it had to pass on a copy of all of its genes to both of its daughters. This was very difficult to do when the genes were all separate. So cells evolved which had all their genes joined together end to end. This long chain of genes is called a chromosome ("kro-mo-sowm"). As the cell divided it copied the chromosome and passed a copy on to each daughter. So they both got a complete copy of all the mother cell's genes.