Almost all senior secondary level biology courses cover the topic of genetics. This is often introduced through a study of the cell nucleus and specifically genetic material contained in the nucleus. Chromosomes contain the DNA, the genetic code by which cells are able to make the many proteins. DNA does not contain the information for the production of any molecules except proteins. The easiest way to examine chromosomes is to look at cells during mitosis because during this time the chromosomes contract from their usual very long thin form to a shorter fatter shape suitable for the mechanical processes of nuclear division. Two microscopic laboratory techniques often used to observe the structure of chromosomes are: (i) examination of a squash of an onion root tip, since the meristem just behind the tip of roots is an area of cell division where growth takes place (ii) examination of the salivary glands of some fruitfly larvae, where chromosomes may be easily seen during interphase. These, so called, polytene chromosome copies are arranged side by side in bundles. Why this occurs is not known, but they are a very useful preparation for examination.