Chromosomes are known as hereditary vehicles They are formed of strands of DNA molecules which contain information for the development of different characteristics and performance of various metabolic activities of the cells The coordination of various function is brought about through the formation of enzymes which are complex protein molecules

1. SPECIAL TYPE OF
CHROMOSOMES

2. WHAT IS A CHROMOSOME
DNA packaged tightly in thread like structures
Mainly seen in decondensed form but during cell division in
condensed form
DNA wrapped around proteins like histone results in
nucleosome , thus the complex of DNA plus histones and other
structural proteins is called chromatin
Chromosome have p or two short arms , q or two large arms
These arms are joined together by centromeres , they help to
keep chromosomes aligned during the complex process of cell
division

3. The two copies of a chromosome are called sister chromatids
The sister chromatids are identical to one another and are
attached to each other by proteins called cohesins
As long as the sister chromatids are connected they are still
considered to be one chromosome . However , as soon as they
are pulled apart during cell division , each is considered a
separate chromosome
Telomeres are repetitive stretches of DNA located at the ends
of linear chromosomes
They protect the ends of chromosomes in a manner similar to
the way the tips of shoelaces keep them from unravelling

4. In many types of cells , telomeres lose a bit of their DNA
every time a cells divides . Eventually , when all of the
telomere DNA is gone , the cell replicate and dies

5. CHROMOSOME STRUCTURE

7. CHROMOSOME
 Chromosomes are known as hereditary
vehicles
They are formed of strands of DNA molecules
which contain information for the development
of different characteristics and performance of
various metabolic activities of the cells
The coordination of various function is brought
about through the formation of enzymes which
are complex protein molecules

8. The information for the synthesis of these
protein molecules is contained in the DNA
molecules in the sequence of nitrogen bases
The sequences of three nitrogen bases that
codes for one molecule of a particular amino
acid constitutes a triplet codon
DNA of a particular cistron tanscribes its m
RNA

9. It carries genetic code similar to its DNA
It moves out of the nucleus and get attached to the
ribosome
The t RNA molecule pick specific activated amino
acids and recognize the codon on the m RNA by their
specific anticodons
The amino acids are thus linked together forming a
polypeptide chain , which is finally changed into
functional protein either singly or by the association of
several polypeptide chains

10. 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
B chromosomes/supernumerary chromosomes

11. GIANT CHROMOSOMES
Exceptionally enlarged and highly specialized
Found during some stages of life cycle of
certain cells
Two kinds of giant chromosomes

12. ENDOMITOSIS
The enormous size is the result of peculiar kind of chromosomal re-
duplication called endomitosis or endoduplication
It is repeated duplication of chromatids in geometrical progression ,
without accompanying cell division
In this process the DNA and chromatids keep on multiplying in
geometrical progression
Thus , endomitosis makes the chromosomes multistranded and
increases its DNA content nearly 1000 times or more
It is common in certain types of cells of plants or animals
Examples are human liver cells , the giant abdominal neuron of
Aplysia , and macronucleus of ciliates

13. POLYTENE CHROMOSOMES
Special types of interphase chromosomes that are large
These giant chromosomes are found in the cells of salivary
glands of Drosophila and Chironomus larvae and in the cells
of fat bodies of larval stages of certain dipterans
They were first observed in the salivary glands of
Chironomus and hence are called salivary gland
chromosomes
They are cable like multi stranded giant chromosomes

14. They are also seen in synergids and antipodal cells of
ovules of certain angiosperms , intestinal epithelium ,
malphigian tubules etc.
They were first observed by Balbani

16. STRUCTURE
The polytene chromosomes presents a distinct pattern of
transverse banding
It consist of dark coloured bands alternating with light
coloured interbands
Nearly 85% of the chromosomal DNA is in the dark bands
and the remaining 15 % is in the interbands (light)
Bands are mostly heterochromatic, and the interbands are
euchromatic

17. It is assumed that the band region is formed by the merging of
chromomeres of adjoining chromatids through condensation and
localized super coiling of chromatin filaments
The bands are specific for a particular chromosome and
banding pattern is almost similar in both the homologous
chromosomes of a pair
Therefore during meiosis the pairing between homologous
chromosomes is band to band
Each band consists of genes that can code for more than one
protein hence polygenic

20. Each band is believed to contain tissue specific genes
Each gene in the band may have multiple copies which are not
transcriptionally active all the time
On the other hand , interbands are believed to contain house
keeping genes , which are constantly transcribed and translated
into products necessary for the cell
The structure of giant chromosomes is modified in certain
regions such that one or more specific bands enormously
enlarge and bulge out , forming local swellings , called
chromosome puffs
They are formed by the uncoiling of chromatin filaments

21. Puffs are transcriptionally active expansions or extensions of
chromatin
So they are active centres of the synthesis and accumulation of
RNAs
In some cases , in addition to RNA , extra DNA is also
producing during puffing
Such puffs are the sites of specific gene amplification and are
responsible for differentiation and development
In some puffs , chromatin filaments are drawn out as a series of
lateral loops , known as BALBIANI rings

23. There are four categories of chromosome puffs
Stage specific puffs
Tissue specific puffs
Constitutive puffs
Environmentally induced puffs
Puffing is a cyclic and reversible process
At definite times , puffs appear , grow and finally disappear
to repeat the whole cycle over and over again

24. Puffing involves the regular succession of four major
events
Excessive accumulation of acidic proteins
Detachment of the DNA bound basic proteins to expose the DNA for
transcription
Despiralisation and decondensation of chromatin
Active synthesis and storage of RNAs
Puffing is mainly initiated during accumulation of acidic
proteins ,it is also shown that insect moulting hormone
called ecdysone induces puffing

25. Puffs and Balbiani rings are regarded as the active centres
for DNA duplication and RNA synthesis
Hence puffing is interpreted as a manifestation of gene
expression
Since active DNA duplication occurs in puffs , they may
contain heavy accumulation of redundant and repetitive
DNA sequences
These are the main characteristics of polytene
chromosome present in dipteran

26. In case of polytene chromosomes in ciliates they doesn't
have neither contain puffs , and Balbiani rings nor do they
show any sign of transcriptional activity
In them fully formed chromosomes first get transected by
‘membranous’ partitions , which appear in interband regions
These membranes consist of only a single diffuse layer of
proteins , which accompanies polytenization of
chromosomes
Then , each band and the adjacent interband get
completely enclosed within a membrane , forming a vesicle

27. Thus a series of vesicles are formed all along the chromosome
The vesicle stage would be followed by the shrinkage of
chromosome
The transection of chromosome , the formation of vesicles and
the shrinkage of the chromosome are associated with a
progressive reduction in DNA content
Nearly 85-90-% of the DNA is degraded to acid soluble
products , that are permanently lost from the chromosome
Soon after degradation and elimination of DNA , vesicles
disappear
The small amount of DNA that still remains and undergoes

29. LAMPBRUSH
CHROMOSOMES
Longest chromosomes of all
In oocytic nuclei of those animal which have large yolky eggs ,
the prophase of first meiotic division is extremely extended
During this phase the oocytes grow and synthesis nutrition for
the future embryo
In them , the chromosomes become greatly enlarged and
assume unusual configuration
A large number of loops projects out from the chromatid axis ,
giving it a lampbrush appearance . Hence are called lampbrush
chromosomes

30. The lampbrush chromosomes are bivalents , each consisting of
two chromatids
These persist during the prolonged diplotene phase of first
meiotic prophase
First observed by Flemming (1882) and described by Ruckert
(1892)
They are seen in diplotene phase of insects , sharks ,
amphibians , reptiles and birds
The lampbrush chromosomes in the oocytes of salamanders
are the largest of all known chromosomes

31. STRUCTURE
A lampbrush chromosome consists of a central axis and a
paired series of loop like lateral extensions
The central axis consists of two bivalent homologous
chromosomes , each with two chromatids , so altogether 4
chromatids are present
They are held together through contact points or
chiasmata
The chromatids are drawn out into paired lateral loops ,so
central axis extend to the lateral loops as loop axis
Loop axis is only a single chromatid of one of the

32. Lateral loops contain bundles of sub-microscopic fibrils
Both the central axis and the loop axis are formed of
deoxyribonucleoprotein
The loop axis may be coated with a matrix of RNA and
proteins
Lateral loop contain a fully extended DNA duplex ,they are
believed to be formed of uncoiling of sister chromatids ,
making their DNA available for transcription
Lateral loops are the active centres for RNA synthesis

33. Each loop may contain one or more transcription unit or transcriptons
Most of the newly synthesized RNA molecules associate with
proteins and form a ribonucleoprotein (RNP) matrix
 m RNA is also associated development of zygote
The chromosomal axis may contain paired series of granular
chromomeres , from were loop axes extend
Usually 1-9 loops may arise from each chromomere
Small swellings , without loops , may be present towards the end of
the central axis ,they represent the telomeres
Each bivalent may contain a loopless swelling also it represents the
centromere

36. There are to main hypothesis regarding lampbrush
chromosomes
According to Callen and Lyod - master and slave
hypothesis , each loop axis is polygenic , with repetitive
DNA sequences and multiple copies of genes
The centromere is believed to contain the master copy of
each gene (master gene) , and the loop axis contains its
duplicate copies (slave genes ). Only master gene take part
in transcriptional activity

38. According to spinning out and retraction hypothesis , a
chromomere is a fully transcribed gene from end to end by
spinning out a transient loop
The new loop material spins out on one side of a chromomere
at the thin end of loop and returns to a condensed stage on the
other side after completing the synthesis of RNA
These are associated with the rapid synthesis of yolk
(deutoplasmic) and RNA , protein in the maturing ovum
These disappear by the end of first prophase when
chromosomes become thick and more condensed

40. B CHROMOSOMES OR
SUPERNUMERARY
CHROMOSOMES
Accessory , aberrant or redundant chromosomes
Extra copies of normal chromosomes
Not homologous to normal chromosomes and are not
essential
Normally they are highly heterochromatic
They are present in many species of animals and flowering
plants like maize , rye ,some ants ,some grasshoppers etc.
Their increased presence in individual organisms produce
no marked phenotypic effects

41. They are often lost or gained in an individual organisms
Some features include;
Smaller in size , heterochromatic ,allocyclic and telocentric or
acrocentric
Exhibit structural polymorphism among cells and tissues of same
organism
Genetic effects is less compared to other chromosome
Present in large amount affect fertility and vigour
Relatively unstable , get eliminated due to lagging and non –
disjunction
Non – mendelian inheritance
No chiasmata pairing

42. Appreciable quantity of highly repetitive DNA with very low
cytosine and guanine content
More amount of histone and lesser protein content
Two main groups ;
Mitotically stable B chromosome whose number is same in the
cells of an individual organism
Mitotically unstable B chromosome whose number is different in
different cells of an organism

43. Effects of B chromosome
Delay the flowering time of plants
Adversely affect the plant character like height , weight etc.
Suppress or promote meiotic pairing
The current concept is that B chromosome might have
formed from small fragments through the clustering of highly
repetitive and heteropycnotic satellite DNA and other
structural re-arrangements
They eventually become as large as other chromosomes
by repeated duplication

44. Thank you