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.

9. 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.

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

11. 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.

12. • 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.

13. Detailed stucture of lamp brush chromosomee

14. 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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