Chromosomes contain genetic information. We often take this fact for granted, but just over a century ago, even the best biologists in the world were uncertain of the function of these rod-shaped structures. We now know that most eukaryotic chromosomes contain a single molecule of double-stranded DNA that forms a complex with proteins. This arrangement allows very long DNA molecules to be compacted into a small volume that can more easily be moved during mitosis and meiosis (Figure 2.1) and expressed during interphase. The compact structure also makes it easier for pairs of chromosomes to align with each other during meiosis. Finally, we shall see that chromosomal structure can affect whether genes are active or silent.