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A seminar report on giant
chromosomes
By ABHISHEK GIRI
M.Sc. (Part-I)
Roll no. 1326056
Paper – III , Unit - I
Lamp brush & Polytene Chromosomes
A presentation on giant
chromosomes
By ABHISHEK GIRI
What is a giant chromosome?
Giant chromosome
 Chromosomes are decondensed during interphase.
 Some exception are lampbrush chromosomes of
vertebrate & polytene chromosome of insect.
 In both these chromosome The region that are actively
synthesizing RNA are least condensed.
 Giant chromosome are very long & thick (200 times)
during metaphase.
 Hence they are know as “Giant chromosomes”.
Lampbrush chromosome
INTRODUCTION
 First discovered by Ruckert in 1892.
 Occur in oocytes of vertebrates as well as in some invertebrates.
 Found in those cells which produce a lot of RNA and their cytoplasmic and
nuclear volume increases.
 Their detailed structure have been studied during the diplotene stage of
meiotic division.
 During diplotene stage, certain chr. Stretch out large loops of DNA, causing
the chr to resemble a lamp brush.
 They are visible under the light mocroscope.
A lampbrush chromosome
& the “original item”
a- telomeric loop,
b- side loops,
c- a chromatid without
loops.
a.
b.
c.
Morphology
 Each LBC’s consists of a main axis having two
chromatids.
 Main axis has a row of granules known as
chromomeres, which are held together by fine axial
fibre.
 Lateral loops in pairs project from the
chromomeres.
 About 1 to 9 loops may arise from a single
chromomere, Their size varies.
 They are held together at points of chiasma
formation.
 The loops of a paired chromosome form mirror-
image structures.
 This stage can last several months.
LBC transcription
 Transcription occurs either along the whole loop or at a parts of a loop.
 At the beginning of meiosis, when DNA replication is complete, the
homologous pairs lie immediately next to each other & form
characteristic structures composed of 4 chromatids.
 Lampbrush chromosomes are distinguished by an especially high rate
of RNA transcription.
 Lamp brush chromosomes are involved in the synthesis of RNA &
proteins.
 Each loop is believed to represent one long operon consisting of
repetitive cistrons.
 Each locus codes for RNA.
 The loop is supposed to synthesis at a high rate because of repetitive
gene sequence.
 There are also reports that the LBC help in the formation of yolk material
in the egg.
Function
Polytene chromosomes
INTRODUCTION
 First discovered by E.G Balbiani in 188, in squash of salivary cells of
Chironomous.
 They also occur in rectal epithelium & Malphigian tubules.
 They are many times larger than the normal chromosomes reaching a
length of 200µm and are visible even under a compound microscope.
 The enormous size is due to the duplication of chromonema which do
not separate.
 According to an estimate, the polytene chromosomes have 1000 times
more DNA than the normal somatic chromosomes.
 Because of these chromosomes actually consist of many strands, they
are called as Polytene chromosome.
Morphology
 Contain 5 long & 1 short arm radiating from a central point called
chromocentre, formed by the fusion of centromeres all the 8
chromosomes found in the cell.
 Of the 6 arms, the short arm represents the fused IV chromosome & the
longest represents the fused sex chr.
 About 80% of the DNA is located in bands, & about 15% in interbands.
 The chromatin in Darkly stained band is more condensed than chromatin
in interbands.
 Intensely stained chromosomal segments correspond to high degree of
packing & are genetically inactive (heterochromatin).
 Less tightly packed segments stain less distinctly & correspond to
segment with genetic activity (euchromatin).
 In Drosophila, 5000 bands have been found
in the 4 chromosomes of salivary gland cells.
 Chromomeres in bands are at right angle to
the long axis of chromosome.
 Bands have a high DNA content & absorb
U.V light.
 Painter (1933) & Bridges (1936) showed that
in Drosophila the bands are associated with
genes.
 Found in salivary glands & other tissues of flies.
 Seen in the nucleus during interphase.
 Show linear series of alternating bands & interbands,
distinctive banding for each chromosome in a given
species.
Functional stages in polytene
chromosomes
 Polytene chromosomes form structures that correlate with the functional
state
 During the larval development of drosophila, a series of expansions
(puffs) appear in temporal stages in the polytene chromosomes.
 Chromosome puffs are decondensed, expanded segments that
represent active chromosomal regions, i.e., regions that are being
transcribed.
 The location and duration of the puffs reflect different stages of larval
development
 The incorporation of radioactively labeled RNA has been used to
demonstrate that RNA synthesis, a sign of gene activity (transcription),
occurs in these regions
Chromosome puffs
 There are certain intresting structure associated with the
bands in the giant chromosomes called as chromosome puffs
or Balbiani rings.
 The swellings are called as chromosome puffs.
 These puffs are associated with the metabolic activities &
represents areas of active RNA synthesis.
Function
 Increasing the volume of the cells' nuclei and causing cell
expansion.
 Metabolic advantage as multiple copies of genes permits a
high level of gene expression.
 In Drosophila melanogaster, the chromosomes undergo
many rounds of endoreduplication, to produce large amounts
of glue before pupation.
 There is tandem duplication of various polytene bands
located near the centromere of the X chromosome which
results in the Bar phenotype of kidney-shaped eyes.
Reference
 CYTOLOGY & GENETICS BY SUMITRA SEN
 www.biotechnologia-journal.pl
 www.wikipedia.org/wiki/Lampbrush_chromosome
 www.wikipedia.org/wiki/Polytene_chromosome
 http://en.wikipedia.org/wiki/Chromosome_…
 www.sciencedirect.com
 www.tutorvista.com
Giant chromosomes-lampbrush & Polytene

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Giant chromosomes-lampbrush & Polytene

  • 1. A seminar report on giant chromosomes By ABHISHEK GIRI M.Sc. (Part-I) Roll no. 1326056 Paper – III , Unit - I Lamp brush & Polytene Chromosomes
  • 2. A presentation on giant chromosomes By ABHISHEK GIRI
  • 3. What is a giant chromosome?
  • 4. Giant chromosome  Chromosomes are decondensed during interphase.  Some exception are lampbrush chromosomes of vertebrate & polytene chromosome of insect.  In both these chromosome The region that are actively synthesizing RNA are least condensed.  Giant chromosome are very long & thick (200 times) during metaphase.  Hence they are know as “Giant chromosomes”.
  • 5. Lampbrush chromosome INTRODUCTION  First discovered by Ruckert in 1892.  Occur in oocytes of vertebrates as well as in some invertebrates.  Found in those cells which produce a lot of RNA and their cytoplasmic and nuclear volume increases.  Their detailed structure have been studied during the diplotene stage of meiotic division.  During diplotene stage, certain chr. Stretch out large loops of DNA, causing the chr to resemble a lamp brush.  They are visible under the light mocroscope.
  • 6. A lampbrush chromosome & the “original item” a- telomeric loop, b- side loops, c- a chromatid without loops. a. b. c.
  • 7. Morphology  Each LBC’s consists of a main axis having two chromatids.  Main axis has a row of granules known as chromomeres, which are held together by fine axial fibre.  Lateral loops in pairs project from the chromomeres.  About 1 to 9 loops may arise from a single chromomere, Their size varies.  They are held together at points of chiasma formation.  The loops of a paired chromosome form mirror- image structures.  This stage can last several months.
  • 8.
  • 9.
  • 10. LBC transcription  Transcription occurs either along the whole loop or at a parts of a loop.  At the beginning of meiosis, when DNA replication is complete, the homologous pairs lie immediately next to each other & form characteristic structures composed of 4 chromatids.  Lampbrush chromosomes are distinguished by an especially high rate of RNA transcription.
  • 11.  Lamp brush chromosomes are involved in the synthesis of RNA & proteins.  Each loop is believed to represent one long operon consisting of repetitive cistrons.  Each locus codes for RNA.  The loop is supposed to synthesis at a high rate because of repetitive gene sequence.  There are also reports that the LBC help in the formation of yolk material in the egg. Function
  • 12. Polytene chromosomes INTRODUCTION  First discovered by E.G Balbiani in 188, in squash of salivary cells of Chironomous.  They also occur in rectal epithelium & Malphigian tubules.  They are many times larger than the normal chromosomes reaching a length of 200µm and are visible even under a compound microscope.  The enormous size is due to the duplication of chromonema which do not separate.  According to an estimate, the polytene chromosomes have 1000 times more DNA than the normal somatic chromosomes.  Because of these chromosomes actually consist of many strands, they are called as Polytene chromosome.
  • 13. Morphology  Contain 5 long & 1 short arm radiating from a central point called chromocentre, formed by the fusion of centromeres all the 8 chromosomes found in the cell.  Of the 6 arms, the short arm represents the fused IV chromosome & the longest represents the fused sex chr.  About 80% of the DNA is located in bands, & about 15% in interbands.  The chromatin in Darkly stained band is more condensed than chromatin in interbands.  Intensely stained chromosomal segments correspond to high degree of packing & are genetically inactive (heterochromatin).  Less tightly packed segments stain less distinctly & correspond to segment with genetic activity (euchromatin).
  • 14.  In Drosophila, 5000 bands have been found in the 4 chromosomes of salivary gland cells.  Chromomeres in bands are at right angle to the long axis of chromosome.  Bands have a high DNA content & absorb U.V light.  Painter (1933) & Bridges (1936) showed that in Drosophila the bands are associated with genes.
  • 15.  Found in salivary glands & other tissues of flies.  Seen in the nucleus during interphase.  Show linear series of alternating bands & interbands, distinctive banding for each chromosome in a given species.
  • 16.
  • 17. Functional stages in polytene chromosomes  Polytene chromosomes form structures that correlate with the functional state  During the larval development of drosophila, a series of expansions (puffs) appear in temporal stages in the polytene chromosomes.  Chromosome puffs are decondensed, expanded segments that represent active chromosomal regions, i.e., regions that are being transcribed.  The location and duration of the puffs reflect different stages of larval development  The incorporation of radioactively labeled RNA has been used to demonstrate that RNA synthesis, a sign of gene activity (transcription), occurs in these regions
  • 18. Chromosome puffs  There are certain intresting structure associated with the bands in the giant chromosomes called as chromosome puffs or Balbiani rings.  The swellings are called as chromosome puffs.  These puffs are associated with the metabolic activities & represents areas of active RNA synthesis.
  • 19.
  • 20. Function  Increasing the volume of the cells' nuclei and causing cell expansion.  Metabolic advantage as multiple copies of genes permits a high level of gene expression.  In Drosophila melanogaster, the chromosomes undergo many rounds of endoreduplication, to produce large amounts of glue before pupation.  There is tandem duplication of various polytene bands located near the centromere of the X chromosome which results in the Bar phenotype of kidney-shaped eyes.
  • 21. Reference  CYTOLOGY & GENETICS BY SUMITRA SEN  www.biotechnologia-journal.pl  www.wikipedia.org/wiki/Lampbrush_chromosome  www.wikipedia.org/wiki/Polytene_chromosome  http://en.wikipedia.org/wiki/Chromosome_…  www.sciencedirect.com  www.tutorvista.com