Chromosomes that are much larger than normal chromosomes are called giant chromosomes. These special types of chromosomes are:
1. Lamp Brush Chromosome: The chromosome which looks like lamp brush or lamp brush is called lamp brush chromosome. These chromosomes have several pairs of loops centered on their axis and look like chimney brushes or lamp brushes. The axis of the chromosome is composed of chromomeres and inter-chromomeres and loop transcribing genes. Its length is 1500-2000 micrometers. It is required for the formation of sperm and egg yolk. In 1882, the embryologist Rishabh Sarham was the first to discover this chromosome from the egg of an asymmetric amphibian. It is found in oocytes or immature eggs of fish, amphibians, reptiles, birds and insects. However, it is not found in the ovum of mammals. Embryologist Flemming first discovered this chromosome from the egg of the amphibian Amblystoma maxicanum. It is found in oocytes or immature eggs of fish, amphibians, reptiles, birds and insects. However, it is not found in the ovum of mammals.
2. Polytene Chromosomes: Those chromosomes which are large, multi-armed and consist of thousands of DNA are called polytene chromosomes. It is also called salivary gland chromosome. Its length is 2000 micrometers. It increases the nucleus and cell shape. Their multicopy genes result in high levels of gene expression in the organism.
Polytene chromosomes have 5 long and 1 short arm. Each arm has a black and white band. Black arms are euchromatin regions. Some parts of it swell and form large formations. It is called Balbiani ring. The chromosomal puff/ballbiani ring synthesizes mRNA. This chromosome is found in salivary gland of insects like Drosophila, Chironomus, Rhynchosciara etc. In 1881, French entomologist E.G. Balbiani discovered polytene chromosomes.
3. B Chromosome : One or more extra chromosomes other than the normal chromosome in the nucleus are called B chromosomes or supplementary chromosomes. B chromosomes are small, non-vital and have heterochromatin. This is sefish genetic material and can be easily distinguished. It contains few genes and does not follow Mendelian pattern. In 1905, Wilson first discovered the B chromosome from the beetle Metapodius. B chromosome is found in 475 species of plants belonging to 163 genera of 42 families. B chromosome is found in organisms such as Ulat Chandal, Bhuta, Brachycome iberidifolia, Lilium callosum, Secale cereale etc.
4. Sat Chromosome: The chromosomes that have satellites are called sat chromosomes. The satellite is separated from the parent chromosome by a thin chromatin. Sat chromosome plays a major role in the formation of cell nucleolus. In 1930, scientist Heitz first used the term SAT.