Giant chromosome. Coloured high resolution scanning electron micrograph of part of a giant (polytene) chromosome in a fruit fly salivary gland cell. Chromosomes are the thread-like structures present in the nucleus of all plant and animal cells. They contain the sequences of genes which govern inherited characteristics. The giant chromosome of the fruit fly (Drosophila) is enlarged by repeated replications (polyteny). A clear pattern of transverse bands called chromomeres can be seen. Particular patterns aid in the identification of different parts of chromosomes and in the mapping of genes.