Special Chromosomes

1. Special Chromosomes
B.Sc. (Botany)
THIRD YEAR
FIFTH SEM
Dr. Thirunahari Ugandhar
Head & Assistant Professor in Botany
Govt. Degree College Mahabubabad

2. • Special Chromosomes

3. • SPECIAL TYPES OF CHROMOSOMES
• In some organisms there are special
tissues in
• which chromosomes undergo structural
• Specializations
• Such specialized chromosomes includes
Giant chromosomes
• 1. Polytene chromosomes
• 2. Lampbrush chromosomes
• 3.B.Chromosomes/supernumerarychromo
somes

5. Polytene chromosomes:
•These are also giant chromosomes but relatively
smaller than lampbrush chromosomes, found in
the larvae of certain dipterans.
Such banded chromosomes occur in the larval
salivary glands, midgut epithelium, and rectum
and Malpighian tubules of various genera
(Drosophila, Sciara, Rhynchosciara, and
Chironomus).
In these larvae the salivary glands contain
salivary cells so large in size that they can easily
be seen with the lens power of a dissecting
microscope.

7. Ultrastructure of giant polytene (poly=many,
tene=strands) chromosomes:
It was first investigated by Beermann and Bahr (1954),
who observed numerous fine fibrils in the Balbiani
rings of Chironomus and estimated that each
chromosome contains 1000 to 2000 separate strands
(corresponding to the degree of ploidy).
Later Gay (1956) observed strands 200 to 500 A in
diameter in sectioned Drosophila salivary
chromosomes. The individual fibres in band and
interband regions are similar in appearance, but the
fibres in the bands exhibit a considerable degree of
metaphase-like folding and are much more tightly
packed.

14. Lampbrush chromosomes (amphibian oocyte)

15. A model for the structure of a
lampbrush chromosome

17. •These are the largest known chromosomes found in the yolk rich
oocytic nuclei of certain vertebrates such as fishes, amphibians,
reptiles and birds.
•They can be seen with naked eye and are characterized by fine
lateral loops, arising from the chromomeres, during first
prophase (diplotene) of meiosis.
These loops give it a brush-like appearance; that is why these are
called lampbrush chromosomes first discovered by Flemming in
1882 and were described in shark oocytes by Ruckert (1892).
Lampbrush chromosomes of certain urodele oocytes may reach
upto 5900µ in length.
It consists of longitudinal axis formed by a single DNA molecule
along which several hundred bead-like chromomeres are
distributed in a linear fashion. From each chromomere there
emerge two symmetrical lateral loops (one for each chromatid),
which are able to expand or contract in response to various
environmental conditions.

18. •చేపలు, ఉభయచరాలు, సరీసృపాలు మరియు పక్షులు వంటి కొన్ని సకశేరుకాల
యొకక పచచసొ న రిచ్ ఓసైటిక్ నయూకలియైస్‌లో కన్నపంచే అతిపద్ద కరోమోజోములు
ఇవి.
•వాటిన్న నగ్ి కననితో చయడవచనచ మరియు మెయోసస యొకక మొద్టి
పొొ ఫేస (డిపలి టిన్) సమయంలో కరోమోమీర్‌ల ననండి ఉత్పనిమయయూ చకకటి
పార్వ ఉచనచలు కలిగి ఉంటాయ.
•ఈ ఉచనచలు దీన్నకల బ్ొష్ ల ంటి రూపాన్ని ఇసాా య; అంద్నకే వీటిన్న ల ంప్ బ్ొష్
కరోమోజోములు అన్న పలుసాా రు,
•దీన్నన్న మొద్ట 1882 లో ఫ్ిమంగ్ కననగొన్నిరు మరియు వాటిన్న షారక
ఓసైట్స్‌లో రుకర్ (1892) వరిణంచనరు. కొన్ని యూరోడెలే ఓసైట్స యొకక ల ంప్
బ్ొష్ కరో మోజోములు 5900µ పొ డవు వరకు చేరవచనచ.
•ఇది ఒకే DNA అణువు దనారా ఏరపడిన రేఖ ంశ అక్షాన్ని కలిగి ఉంట ంది,
దనన్నతో పాట అన్ేక వంద్ల పూసల వంటి కరోమోమీర్‌లు సరళ పద్ధతిలో పంపణీ
చేయబ్డతనయ. పొతి కరో మోమీర ననండి రండు సనష్్ పార్వ ఉచనచలు (పొతి
కరోమ టిడ్‌కు ఒకటి) ఉద్భవించనయ, ఇవి వివిధ పరాూవరణ పరిసిత్ులకు
పొతిసపంద్నగా విసారించగ్లవు లేదన కుదించగ్లవు.

19. It consists of longitudinal axis formed by a single
DNA molecule along which several hundred bead-like
chromomeres are distributed in a linear fashion.
From each chromomere there emerge two
symmetrical lateral loops (one for each chromatid),
which are able to expand or contract in response to
various environmental conditions.
About 5 to 10% of the DNA is in the lateral loops.
Loop formation reduces the mass of the
corresponding chromomeres, implying a spinning out
of chromomere material into the lateral strands. The
centromeres also have the appearance of elongate
Feulgen-positive chromomeres but they
characteristically lack lateral loops.

20. •Lampbrush chromosomes can be dissected in
(toto) from oocyte nucleus. Individual
chromosomes are liable to stretching.
With extreme stretching, chromomeres begin
to separate transversely into two halves, so
that the paired loops form double stranded
bridges.
The axis between chromomeres is also double,
which can be seen in certain special regions
where two elements separate longitudinally
and bear single loops (Callan, 1955).

21. Functions of Lampbrush chromosomes,
(a) Synthesis of RNA:
Functions of lampbrush chromosomes involve
synthesis of RNA and protein by their loops.
RNA is synthesized only at the thin insertion and
then carried around the loops to the thick
insertion. There it may be either destroyed or
released into nucleus.
(b) Formation of yolk material:
There are some probabilities that lampbrush
chromosomes help in the formation of certain
amount of yolk material for the egg.

26. A polytene chromosome from
Drosophila salivary gland

29. Polytene chromosomes get their name from the
fact that they are formed by many parallel
chromatids, often more than a thousand strands,
which do not separate from one another
following duplication.
Along each chromatid strand some regions of
chromatin are tightly coiled and other regions
are less coiled, with the result that polytene
chromosomes appear to consist of light and dark
bands when observed under a microscope.
.

30. During larval development, specific areas on polytene
chromosomes become uncoiled, forming localized
regions called ‘pufs’. Puffs represent regions of active
RNA synthesis (transcription).
In the puff individual fibres remain continuous across
the puff and they become extended as short lateral
loops (Bahr, 1954).
DNA is concentrated almost entirely in the bands.
Protein and RNA is also found in puffs.
Puffing is due to the uncoiling of chromosome fibres
which are usually closely folded or coiled in the dense
band regions. These fibres then project in the form of
loops

31. Puffs and Balbiani rings:
•During their initial stages of development these
bands or interbands of the chromosomes exhibit
swellings or puffs.
•Their appearance depends on the stage of larval
development.
•It is probable that the metabolic activities,
required for the formation of puffs, are related
to the secretory function of the salivary glands.
The formation of this is controlled by certain
specific genes and the puffs are related with the
active synthesis of RNA and proteins.

32. Functions of giant polytene chromosomes:
(1) Main function of the polytene chromosome
is to carry genes which ultimately control
physiology of an organism. These genes are
formed of DNA molecules.
(2) Shifting of heterochromatin in respect to
euchromatin produces giant changes called
position effects. These effects cause mutations in
animals as well.

33. (3) Heterochromatic regions contain
fewer genes than euchromatic parts.
Production of nucleolar material is
entirely done by heterochromatin.
(4) Chromosomes also help in protein
synthesis indirectly. Nucleolus contains
RNA, and this RNA serves as a means of
transmission of genetic information to the
cytoplasm, leading to the formation of
specific protein.

34. Electron Microscope image of Drosophila polytene chromosome

35. B Chromosomes or Super Numerary Chromosome or
Accessory Chromosome:
In some organisms, chromosomes in addition to
normal autosomes are present as an extra
chromosomes that are not genetically necessary for
the individual and not homologous to any of the
normal chromosomes.
These chromosomes differ from the normal ones in
their variable number, smaller size and greater degree
of heterochromatinisation.
They are found more commonly in plant than in
animals (Locusta migratoria, Camunla pellucida,
Helix pomatia, Myrmeleo tettix maculatus).

36.  B-chromosome do not usually have any effect on the
phenotype and, hence, they are not genetically
desirable. In some plant their presence results some
deleterious effect, i.e., loss of vigour.
 Though they are found to be deleterious, yet the
occurrence of such chromosomes through
generations indicates that they must have some
positive adoptive role as well.
 B-chromosomes do not usually pair with normal
chromosome during meiosis though they may pair
with each other without the formation of chiasmata
when present in even number.

37.  B-chromosomes may be eliminated from certain
tissues or organs during embryogenesis.
 The origin of B-chromosome is rather obscure. They
may have been originated from the ordinary
chromosome.
 It has been suggested that the centric
heterochromatin part of an autosome is gradually
converted into B-chromosome by the elimination of
euchromatin part

38. • B క్రో మోజోమ్ యొక్క ప్ర భావాలు మొక్కల పుష్పించే
సమయాన్ని ఆలసయిం చేయిండి ఎత్తు , బరువు మొదలై న
మొక్కల పాత్ర ను ప్ర తికూలింగా ప్ర భావితత్ిం చేస్ు ింది.
• Me మెయోటిక్ జత్చేయడాన్నకి మదద త్త ఇవ్విండి లేదా
ప్రర త్సహించిండి B ప్ర స్ు త్ భావ్న ఏమిటింటే B
క్రో మోజోమ్ క్లిగి ఉిండవ్చ్చు చిని శక్లాలు నుిండి
క్ల సట రింగ్ దావరా ఏర్పడుత్తింది పునరావ్ృత్ మరయు
హెటెరోపై క్రిటిక్
• ఉప్గ్ో హ DNA మరయు ఇత్ర్ న్నరాాణాత్ాక్ పున re
ఏరాపట్లల చివ్రకి అవిత ఇత్ర్ క్రో మోజోమ్ల మాదిరగా
పదద విత అవుతాయి ప్దేప్దే నకిలీ దావరా

39. Thank you