A fellow student asks, "How is it possible that six feet of DNA
can fit inside a single, microscopic nucleus of a cell?" If you
were to unravel a ball of yarn,
it would probably be able to circle
around your house (that's a
long distance!). Use this
comparison to answer
your fellow student's question.

DNA is wrapped around proteins known as histones to form chromatin. Packing the DNA into nucleosomes condenses the DNA approximately sevenfold. It can be further organized by winding the chromatin into more compact structures. Coiling the chromatin around itself decreases the space it occupies by another sixfold or so. Through a series of similar compacting strategies, the entire genome can fit inside the nucleus of a single cell.

Explanation:

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).