Humans have 23 pairs of chromosomes, with a diploid number of 46. Scientists number these chromosome pairs according to their size—the largest is chromosome 1 and the smallest is chromosome 23. Chromosomes consist chiefly of proteins and DNA (remember nucleotides form the structure of DNA).

Each cell contains enough DNA to form a thread extending about 1.5 m (about 5 ft). Proteins called histones play a key role in packaging DNA within chromosomes. Sections of the DNA molecule wind around clusters of histones to form units called nucleosomes, which resemble spools encircled with thread. Chromosomes become most condensed (as you see them above) when a cell is preparing to divide.