Cell Division, Diversity, and Organisation

• Each species has a unique number of chromosomes per cell: Humans (46, 23 pairs), chimpanzees (48, 24 pairs), fruit flies (8, 4 pairs), dogs (78, 39 pairs), and onions (12, 6 pairs)
• Chromosomes are found in pairs in the nucleus; humans have 23 homologous pairs (one of each from the mother and the other from the father), only one odd pair; sex chromosomes (XY → male and XX → female) Y is substantially smaller
• The majority of animal species and half the plant species are diploid (2n), a few are haploid (n) and some, including many plants, are polyploid
• Chromosome structure: One DNA molecule, short pieces of DNA are called genes, eukaryote DNA molecules are wrapped around proteins called histones; average human DNA spread out would stretch to the sun (from Earth) and back 400 times