A general account of special types of chromosomes - Giant chromosomes (Polytene chromosomes or salivary gland chromosomes and Lampbrush chromosomes ) & B chromosomes

1. Dr. Saji Mariam George
Associate Professor (Retired)
Assumption College Autonomous
Changanacherry
SPECIAL TYPES OF CHROMOSOMES

2. SPECIAL TYPES OF CHROMOSOMES
1. GIANT CHROMOSOMES
i).POLYTENE ( SALIVARY GLAND)
CHROMOSOMES ii). LAMPBRUSH CHROMOSOMES
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3. i) POLYTENE (SALIVARY GLAND)
CHROMOSOMES
• First observed by Balbiani
(1881) in the salivary glands
of Chironomous and hence
the name Salivary gland
chromosome.
Chironomous

4. Chironomous (Midges) – Salivary
gland Chromosome.

5. POLYTENE (SALIVARY GLAND)
CHROMOSOMES…..
• Laterally magnified,
transcriptionally active,
interphase chromosome.
• Remain in a despiralised state
which is characteristic of
interphase chromosomes.
B - Bands
IB - Interbands
P - Puffs
POLYTENE NATURE

6. • Occur in salivary glands, malpighian tubules,
fat body cells, gut, trachea etc. among the
Dipteran larvae.
• Cell division is arrested in certain tissues like
salivary glands, malpighian tubules etc.
• Cell continue to grow in size.

7. • The chromosome undergo repeated duplication –
Endomitosis – the products do not separate – result in a
bundle of several hundreds or thousands of chromatids
– provide additional genetic material needed to support
to the high level of secretory activity of these enormous
cells.
• In this process of polytenization, the chromosomes gain
enormous increase in length and diameter – giant
chromosomes.
• In fruit fly ( Drosophila melanogaster ) ,
total length of somatoidal chromosome - 7.5 µ
salivary gland chromosome - 2000 µ.

8. Polytene chromosomes of Drosophila
Credit
POWER AND SYRED / SCIENCE PHOTO LIBRARY
https://www.sciencephoto.com

9. POLYTENE (SALIVARY GLAND)
CHROMOSOMES…..
Image:https://www.quora.com

10. CHARACTERISTICS OF POLYTENE(SALIVARY
GLAND ) CHROMOSOMES
1. Polytene nature : Depending on the number of
replication cycles, the polyteny of the chromosome
may be 8192, 16384 , ……..
2. Enormous length : Extremly longer than somatic
chromosome.
3. Somatic pairing : The homologous pairs are closely
associated and are considered to be in a permanent
prophase – Paired units of homologous
chromosomes – the number of elements in such
polytene cells correspond to the haploid number of
chromosomes.
4. Transverse bands : Along the length of the
chromosome is a series of dark bands alternating
with clear zones called interbands.

11. Bands :
• Rich in DNA
• Stain intensively
• Represent the location of the genes in the
chromosomes
• Formed as a result of enlargement,
duplication and aggregation of homologous
chromomeres.

12. • At certain times, bands become enlarged to
form swellings called Chromosome puffs or
Balbiani rings
• Puffs represent regions where the tightly
coiled chromosomal fibres open out to form
many loops.
• The puffing is due to unfolding or uncoiling of
individual chromomeres in a band.
• Puffs are active genes and represent sites of
RNA synthesis.

13. Balbiani rings
Image:https://www.toppr.com

14. Interbands:
• Contain some amount of DNA, which
probably connects DNA segments located in
the adjoining chromomeres.
• They do not stain with basic stains .

15. BANDS
• Stain intensely with basic
stains
• Fuelgen( A DNA specific stain)
positive
• Regions of high DNA
concentration
• Chromonemata tightly
packed by folding
• Absorb Ultraviolet light at
2600 A °
INTERBANDS
• Do not stain intensely
• Fuelgen negative
• Regions of low DNA
concentration
• Chromonemata relatively
extended
• Absorb little Ultraviolet
light.

16. ii) LAMPBRUSH CHROMOSOMES
• Larger than polytene chromosomes.
• In some Salamander Oocytes, they may reach
a length of 5900 µ.
• First observed by Flemming (1882) in
amphibian oocytes .

17. • Ruckert (1892) studied the chromosomes of
the oocytes of Shark and coined the name
Lampbrush chromosomes because of their
resemblance to brushes used in those times
to clean the chimneys of oil lamps – have
many fine lateral projections giving them the
characteristic hairy appearance.
• Occur at the diplotene stage of the meiotic
prophase in oocytes of animal species like
amphibians, fishes, reptiles, birds etc.

18. • Composed of a main axis and lateral loops.
• The main axis is composed of four
chromatids or two bivalent chromosomes.
• The chromonema of the chromatids gives out
fine loops at the lateral sides.

19. • Loops are rich in RNA (mRNA) and proteins.
• Synthesis of proteins and the yolk takes place
near the loops and when the synthesized
materials are given off, the loops are
collapsed or reduced.

20. Lampbrush Chromosomes
Image:http://www.expertsmind.com

21. Lampbrush Chromosomes ….
Image:https://fineartamerica.com

22. Lampbrush Chromosomes ….

23. 2. B CHROMOSOMES
(Accessory chromosomes or Supernumerary
chromosomes)
• Discovered by Wilson
(1905) in Metapodius
terminalis ( a
Hemipteran insect )
Metapodius terminalis

24. • Reported in about 600
species of Flowering plants
(e.g. Maize) and in more
than 100 animal species
(Jones, 1975) .
• Chromosomes over and
above the normal zygotic
complement.

25. B chromosomes in Maize
Image:http://www.funpecrp.com.br

26. • Have a normal structure, smaller than the
autosomes .
• Mostly heterochromatic (e.g. in many insects,
Maize ).
• Some are euchromatic (e.g. in Rye,
Tradescantia (Rheo sps. ).

27. • Number may vary – In maize,
0 – 30.
• Non – homologous group of chromosomes.
• Do not show any pairing affinity with the A
chromosome.

28. • Morphology is different from the normal
chromosomes (A chromosomes ) – globular
in appearance.
• The genetic constitution do not strongly
influence the individual.

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