What are Gaint chromosomes ?

The chromosomes which are extreamly large as compared to normal chromosomes.It is of 2 types-

  1.Lampbrush chromosomes-Occur in some insects and in some animals.These chromosomes occur  in the oocytes of ambhibians.Large synapsed(pair) homologolous chromosomes seen in the Diplotene stage (Prophase-1) of meiosis-I.It consists  of an axis from which paired loops extend in opposite directions like a lampbrush.The axis consists of nucleosomes in the chromomere region.The loops consist of transciptionally active DNA which synthesize m-rna.

Polytene chromosome of Drosophila

2.Polytene chromosomes-

Found in the salivary glands of fruitfly Drosophila larvae.Repeated rounds of dna replication without cell division(Endomitosis), to increase cell volume, forming a giant polytene chromosome.Polytene chromosomes have  light and dark banding patterns that can be used to identify chromosomal rearrangements . Dark banding - inactive chromatin, whereas light banding - higher transcriptional activity. The banding patterns identified in 1935 by Calvin B. Bridges. Chromosome puffs are diffused uncoiled regions of the polytene chromosome that are sites of RNA transcription. A Balbiani ring is a large chromosome puff. Polytene chromosomes are about 200 µm in length. The chromonema of these chromosomes divide but do not separate. Therefore, they remain together to become large in size.

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