About chromosome,its characters

1. BRIEF STRUCTURE OF
EUKARYOTIC CHROMOSOME
By Jaydeep Bardhan
Dept of Botany, 3rd year
R.B.C. College,Naihati.

2. What is Chromosome ?
 A eukaryotic chromosome is a molecule of DNA together with associated
proteins .
 German biologist Walter Flemming in the early 1880s revealed that
during cell division the nuclear material organize themselves into visible
thread like structures which were named as chromosomes which stains
deep with basic dyes. The term “chromosome” was coined by W. Waldeyer
in 1888. “Chrome” is coloured and “Soma” is body, hence they mean
“colored bodies”. It contains many genes or the hereditary units, regulatory
elements and other nucleotide sequences.
 Chromosomes are the essential unit for cellular division and must be
replicated, divided, and passed successfully to their daughter cells so as to
ensure the genetic diversity and survival of their progeny.

3. Fig 1.1 :-A complete structure of
chromosome

4. Morphology Of Chromosome :-
During cell division chromosomes are much coiled therefore it seen very well. In
metaphase period chromosome have following parts –
 Chromatid :- A chromosome at mitotic metaphase consists of two
symmetrical structures called chromatids.
 Chromonemata :- In interphase chromosome consists of two very thin,
highly coiled filaments called chromonema or chromonemata. This
structure become more condensed and coiled during metaphase to form
chromatids.
Based on their own coiling it again divided into two types :-
1. Paranemic coil :- When those fibers are easy to separate from each other.
2. Plectonomic coil :- When those fibers are difficult to separate from each
other.
 Chromomere :- Both chromonemata remain intimately coiled in spiral
manner with each other and have a series of microscopically visible bead-
like swelling along its length called chromomeres.
 Primary constriction and centromere :- Each chromosome contains a
heterochromatic region therefore it doesn’t take stain and this region look like
a constriction. Scientist Darlington proposed it as centromere.

5.  There are four types of
chromosomes based upon the
position of the centromere (Figure
1.2)
1. Metacentric : In this type of
chromosome the centromere occurs in
the centre and all the four chromatids
are of equal length. During anaphase
showing ‘V’ or ‘U’ shape structure.
2. Submetacentric : In this type of
chromosome the centromere is a little
away from the centre and therefore
chromatids of one side are slightly
longer than the other side. During
anaphase showing ‘L’ shape structure.
3. Acrocentric : In this type of
chromosome the centromere is located
closer to one end of chromatid
therefore the chromatids on opposite
side are very close. During anaphase
showing ‘J’ shape structure.
4. Telocentric: In this type of
chromosome the centromere is placed
at one end of the chromatid and hence
only one arm. During anaphase showing
‘I’ shape structure. Fig 1.2 Showing different types
of chromosome depending on
their centomere.

6.  There are four types of chromosome based upon number of centomere :-
1. Acentric :- It does not consists any centromere.
2. Monocentric :- It consists only a single centromere E.g., Human chromosome.
3. Dicentric :- When chromosome contain two centomeres E.g., Maize, Drosophila
chromosome.
4. Polycentric :- More than two centomeres are present in a chromosome E.g., in plant
Luzula purpurea.
 Kinetochore :- Centromere in a chromosome contain specific DNA sequences with
special proteins bound to them, forming a disc shaped structure, called kinetochore.
In electron microscope the kinetochore appears as a plate or cup like disc, 0.20-
0.25 nm in diameter situated upon the primary constriction or centromere.
 Secondary Constriction :- Besides the primary constrictions or centromeres,
chromosomes also posses secondary constriction at any point of the chromosome
and are constant in their position and extent. These constrictions are helpful in
identifying particular chromosomes in a set.
NOR :- Chromosomes also contain Nucleolar Organizers Region which are
certain secondary constrictions that contain the genes coding
for 5.8S, 18S and 28S ribosomal RNA and induce the formation of nucleoli.
 Satellite :- Sometimes the chromosomes bear round, elongated or knob like
appendages known as satellites. This Types of chromosome are known as SAT
chromosome. SAT means Sine Acid Thymonuclenio.
 Telomere :- The end parts of chromosome is known as Telomere. It have a special
polarity which help to separate from other chromosome.

7. THE CONDENSATION LEVELS OF DNA IN
CHROMOSOME
 First level of packing :- Winding of DNA around a protein core to produce a
"bead-like" structure called a nucleosome. This gives a packing ratio of about 6.
This structure is invariant in both the euchromatin and heterochromatin of all
chromosomes. The protein core is composed of 8 histone proteins, two each of
H2A, H2B, H3 and H4. Histone H1 forms the linker between to nucleosomes. 146
bp of DNA i s wrapped around each nucleosome.
 Second level of packing :- Coiling of beads in a helical structure called the 30
nm fiber that is found in both interphase chromatin and mitotic chromosomes. This
structure increases the packing ratio to about 40. This appears to be a solenoid
structure with about 6 nucleosomes per turn. The stability of this structure
requires the presence of the last member of the histone gene family, histone H1.
 Third level of packing :- The 30 nm fibers is then organized into large looped
domains. Each containing 50 ×106 bp DNA. Six such loops attached to a scaffold to
form rossete, which is approximately 300 diameter.
 Final level of packing :- The final level of packaging is characterized by the 700
nm i.e., coil upon coils form eukaryotic structure seen in the metaphase
chromosome. The final packing ratio of about 1000 in interphase chromosomes
and about 10,000 in mitotic chromosomes.

8. • First level of
packing
Second level of
packing
Third level of
packing
Fig 1.4 -The condensation level of
chromosomal organization

9. Chromosome shape,size &
number :
The number and size of chromosome remain constant for a
perticular species. Each and every cell of an organism contains
the same number and types of chromosomes.
 Size: The size of chromosome is normally measured at mitotic
metaphase and may be as short as 0.25μm in fungi and birds
to as long as 30 μm in some plants such as Trillium. However,
most mitotic chromosome falls in the range of 3μm in Drosophila
to 5μm in man and 8-12μm in maize. The monocots contain large
sized chromosomes as compared to dicots. Organisms with less
number of chromosomes contain comparatively large sized
chromosomes.
 Shape: The shape of the chromosome changes from phase to
phase in the continuous process of cell growth and cell division.
During the resting/interphase stage of the cell, the chromosomes
occur in the form of thin, coiled, elastic and contractile, thread like
stainable structures, the chromatin threads. In the metaphase
and the anaphase, the chromosome becomes thick and
filamentous.

10. Organism No. of chromosome
Arabidopsis thaliana 10
Maize 20
Wheat(hexaploid) 42
Common fruit fly 8
Erthwarm 36
Tobaco(tetraploid) 48
Oat(hexaploid) 42
Human 46
Number of chromosome in different
organism

11. Polytene Chromosome
 Occurrence : In salivary gland cells, Intestinal tube, Malpigian tube
of the larve of dipterans like Drosophila.
 Discovery : This kind of chromosome first discovered by E. G.
Balbiani (1881) and named by Koller. The length of the
chromosome is 2000 micrometer i.e., 100 to 200 times larger than
normal chromosome.
 Structure : This is particularly surprising, since the salivary
gland cells do not divide after the glands are formed, yet their
chromosomes replicate several times (a process called
endomitosis) and become exceptionally giant-sized to be called
polytene or multistranded chromosomes (fig 1.6, A).
1. The polytene chromosome consists of band and interband. Band
cotains much amount of euchromatin and less amount of RNA and
based proteins and interband contain less amount of DNA and
acidic proteins but RNA is much amount.

12.  Lambrush Chromosome : In diplotene stage of meiosis, the yolk
rich oocytes of vertebrates contain the nuclei with many lamp
brush shaped chromosomes of exceptionally large sizes. The
lampbrush chromosomes (discovered by Ruckert in 1892) are
formed during the active synthesis of mRNA molecules lampbrush
chromosome contains a main axis whose chromonemal fibres (DNA
molecule) gives out lateral loops throughout its length. (fig- 1.6, B).
 B Chromosome : Many plant (maize, etc.) and animal (such as
insects and small mammals) species, besides having autosomes (A-
chromosomes) and sex-chromosomes possess a special category of
chromosomes called B-chroosomes without obvious genetic
function. These B chromosomess (also called supernumerary
chromosomes, accessory chromosomes, accessory fragments, etc.) B
chromosome occur more frequently in females and the basis is
non-disjunction (fig 1.6, C).

13. Fig 1.6 - SPECIAL TYPES OF
B
A
C

14.  Karyotype concept was developed by S. Navashin based on the
observations that the number of chromosomes and morphology.
The term “karyotype” from the Greek word meaning “kernel”, a
reference to the contents of the nucleus.
 Human somatic cells contain 46 chromosome(44 are autosome
and other 2 are sex chromosome.
 Depending on the differences between smallest and largest
chromosome of the set it may be symmetric (less difference) or
asymmetric (large difference).
 There some parameters are used to form karyotype –
1. The number of chromosome
2. The length of chromosome
3. The position of centromere
4. The position and size of the satellites etc.
Human Chromosome
Karyotype

15. Fig 1.3 :- Human karyotype

16. Staining methods to study
metaphase chromosome
 Staining is done to make the cell constituents visible under
the microscope helping in the study of their structure and
behavior.
 The stains are usually termed acidic, basic or amphoteric on
their chemical nature and behavior.
 Example of some common stains are –
I. Fuchsin (magenta red colored)
II. Orcein (deep purple colored)
III. Carmine (crimson colored)
IV. Crystal violet (bluish violet colored)
Cond.

17. b. With Hoechst 3325.
c. With chromomycine and DAPI
2. G – Banding or Giemsa banding
a. With Giemsa
b. With Felugen
3. C – Banding
a. CT – Banding
b. Cd - Banding
4. R – Banding or reverse banding
a. Giema Reverse Banding,
b. R – Banding by Fluorescence using acredine orange (AO)
5. T – Banding,
6. O – Banding or Orecin Banding,
7. N – Banding/ NOR Staining
8. Hy Banding
9. Restriction enzyme banding.

18. Fig 1.8 - Different Type Of Chromosome
Banding

19. Conclusion
 Chromosomes are the bearers of hereditary materials and vehicles
transmission from cell to cell and generation to generation. The number and
size of chromosomes are constant for a species to species but vary from
species to species.
 Chromosome is a vital adaptation in eukaryotic cell. The eukaryotic genome
is manifold larger than the prokaryotic genome. This needs to be
compressed and packaged for proper distribution and functioning during the
various stages of cell cycle. The chromosome structure enables this
compaction and tendons equal distribution of genetic material into the
daughter cells after each cell division.
 Chromosome structure plays a very important role in regulation of gene
expression in eukaryotic system. The euchromatine and heterochromatin
identify the chromosome into genetically active and inactive regions.
 Karyotype analysis the chromosomes are extensively studied in one of the
important feature of classification of organisms and each Karyotype form an
identifiable entity or fingerprints of a given species.

20. Reference
 Karp G (2010) in “Cell and Molecular Biology
Concepts and Experiments”, 6th edition. John Wiley
& Sons, Inc., USA.
 Lehninger A L, Nelson D L, Cox MM (2009) in
“Lehninger Principles of Biochemistry”. 5th edition,
Freeman and company, USA.
 Snustad P. D. , Simmons J. M. (2009) in “Principles
of Genetics” , 6th edition, Jhon Wiley & Sons, Inc.,
USA.
 Sen K, Midya T, Santra C D, (2015) in “Higher Biology”
, Reprint, Santra Publication Private Limited , India.
 Some Portable Document Format (PDF).
 And also reference from Prof. Dr. Shamyaeeta Sarkar,
from Dept of Botany ,R. B. C. College.