Special chromosomes

a. Lamp brush chromosome:

• They are found in oocytes of many invertebrates and all vertebrates except mammals.
• They have also been reported in human and rodent oocytes but most extensively studies in amphibian oocytes.
• From each of a majority of the chromosomes, generally a pair of lateral loops extends in the opposite directions perpendicular to the main axis of chromosome.
• These lateral loops give these chromosome, the appearance of lamp brush so called as ‘lamp brush chromosome’.
• As the oocytes progresses from diplotene to metaphase I, the loops are slowly withdrawn and reassembled into chromomeres.
• Each loop represents one chromatid of a chromosome and is composed of one DNA double helix.
• One end of each loop is markedly thinner and other is thicker.
• Loop represent the sites of gene action 9transcription) and the function of lamp brush chromosome is to produce large number and quantities of protein and RNAs stored in eggs.

b. Giant chromosomes:

• Giant chromosome are found in certain tissues like salivary glands of larvae, gut epithelium, malphigian tubules and some at bodies of some diptera .
• These chromosomes are very long in size during mitosis metaphase in case of drosophila and very thick hence known as giant chromosomes.
• Homologous chromosomes pair to form giant chromosomes so that the total number of giant chromosomes in a 2n nucleus is only n.
• Each giant chromosomes is composed of numerous strands, each strand representing one chromatid. so, these are known as polytene chromosomes and condition is referred as polyteny.