Giant chromosomes, light micrograph. Polytene chromosomes are over-sized chromosomes that have developed from standard chromosomes. They are commonly found in the saliva of the fruit fly Drosophila melanogaster. They are formed from repeated sequences of DNA (deoxyribonucleic acid) replication without nuclear division. A process known as endoreduplication. Polytene chromosomes increase cell volume and metabolic rate, as there are more genes to be expressed. Magnification: x20 when printed at 10 centimetres wide.