Giant chromosomes

This ppt explains the different forms of giant chromosomes, polytene and lamp brush chromosomes, its structure and functions. It helps the Genetics, Human genetics and molecular biology, Genetic engineering, Entomology students to learn about the giant chromosomes.

1. Giant Chromosomes Dr.N.C.J.Packia Lekshmi Allied Health Sciences Noorul Islam Centre For Higher Education

2. Introduction There are chromosomes which are extremely large compared to normal chromosomes. Such, chromosomes, called as giant chromosomes, occur in some animal cells. Two types of giant chromosomes are known: Polytene Chromosomes Lampbrush chromosomes

3. Polytene Chromosomes Polytene chromosomes are large chromosomes which have thousands of DNA strands. Polytene chromosomes were first reported by E.G.Balbiani in 1881. These are giant chromosomes found in the salivary gland cells of the insects such as fruit fly, Drosophila . They provide a high level of function in certain tissues such as salivary glands of insects, they are also referred to as "salivary gland chromosomes".

4. They are many times larger than the normal chromosomes reaching a length of 2000 and are visible even under a compound microscope. The large size of the chromosome is due to the presence of many longitudinal strands called chromonemata ; hence the name polytene (many stranded). They are about 0.5 mm in length and 20 μm in diameter. These arms contain numerous chromonemata resulting from repeated replication of DNA, without separation into daughter chromosomes. This type of division is called endomitosis .

5. The polytene chromosome contains two types of bands, dark bands and interbands . The dark bands are darkly stained and the inter bands are lightly stained with nuclear stains. The dark bands contain more DNA and less RNA. The interbands contain more RNA and less DNA. The amount of DNA in interbands ranges from 0.8 - 25%. The dark bands are euchromatic regions. Some of the dark bands temporarily swell up and form enlargements called chromosomal puffs or Balbiani rings. These regions contain actively transcribing DNA involved in the synthesis of RNA types.

6. The bands of polytene chromosomes become enlarged at certain times to form swellings called puffs. The formation of puffs is called puffing. In the regions of puffs, the chromonemata uncoil and open out to form many loops. The puffing is caused by the uncoiling of individual chromomeres in a band. The puffs indicate the site of active genes where mRNA synthesis takes place. The chromonemata of puffs give out a series of many loops laterally. As these loops appear as rings, they are called Balbiani rings after the name of the researcher who discovered them. They are formed of DNA, RNA and a few proteins. As they are the site of transcription, transcription mechanisms such as RNA polymerase and ribonucleoproteins are present. In protozoans , there is no transcription, since the puff consists only of DNA.

7. Another peculiar characteristics of the polytene chromosome is that the maternal and paternal homologous chromosomes remains associated side by side. This phenomenon is called somatic pairing. In female Drosophila, the polytene chromosomes are found in the form of five long and one short strands radiating from a single more or less amorphous chromosomes. One long strand corresponds to the X chromosome and remaining four long strands are the left and right arms of II and III chromosomes. The shortest strand represents the small dot like IV chromosome. Each of these chromosomes contains maternal and paternal homologous in somatic pairing which lacks in the sex chromosomes of male fruit flies.

8. Lamp Brush Chromosomes These chromosomes occur in the oocytes (germ cells in the ovary) of amphibians and in some insects. They are extremely large synapsed homologous chromosomes which can be seen in the diplotene stage of prophase I in meiosis. They measure about 1500 to 2000 in length. A lampbrush chromosome consists of an axis from which paired loops extend in opposite directions, giving the appearance of a lamp brush. The axis consists of chromomeres ( nucleosomes ) and interchromomere regions. The loops consist of transcriptionally active DNA which can synthesize large amount of mRNA, necessary for the synthesis of yolk.

9. A Lamp brush chromosomes and lamp brush a. Teromeric loop b. Side loop C. Chromatid without loop

10. Morphology The lamp brush chromosome occur at the diplotene stage of meiotic prophase in the primary oocytes of all animal species, both vertebrates and invertebrates. They are present in the form of bivalents in which the maternal and paternal chromosomes are held together by chiasmata . Each bivalent has four chromatids , two in each homologue. The axis of each homologue consists of a row of granules or chromomeres from which lateral loops extend. The loops can be catergorized by size, thickness and other morphological characteristics.

11. Lateral loops in pairs project from the chromosomes About 1 to 9 loops may arise from a singe chromosome . Their size varies . They are held together at points of chiasma formation . The loops of a paired chromosome form mirror image structure. This stage can last several months.

12. Detailed stucture of lamp brush chromosomee

13. Functions Lamp brush chromosomes are involved in the synthesis of RNA and proteins Each loop is believed to represent one long operon consisting of repetitive cistrons . Each loop is found to perform intense transcription of hnRNA . Each locus codes for RNA The loop is supposed to synthesis at a high rate because of repetitive gene sequence. There are reports that the LBC help in the formation of yolk material in the egg.

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