Chromosomes are made of chromatin fibers containing DNA and proteins. Each chromosome contains two chromatids joined at the centromere. The centromere divides the chromosome into short and long arms. During cell division, the centromere and attached kinetochore allow chromosomes to align and separate. Telomeres are repetitive DNA sequences at chromosome ends that prevent fusion and shorten with each cell division due to the end replication problem, in which DNA polymerase cannot fully replicate chromosome ends.

1. Structural organization chromosome:
By
Dr. ASWARHHA HARINATH REDDY
Department of Biotechnology
Christ University

2. Structure of chromosome:
 Each chromosome is made up of two chromatids
(chromosomal arms) which are joined to each other at a small
constricted region called the centromere.

3.  The two chromatids are made up of very thin chromatin fibres
which contain 40% DNA and 60% histone proteins.
 Each chromatin fibre consists of one DNA strand coiled around
eight histone molecules form a complex; such a complex is
called nucleosome.

5. Short chromatid
Long chromatid

6.  Centromere which divides the chromosome into two sections or
“arms”.
 The short arm of the chromosome is labeled the “p arm” ,
 The long arm of the chromosome is labeled the “q arm”

7. • The chromatin condensation varies during the life cycle of the
cell and plays an important role in regulating gene expression.

8.  In interphase (nondividing) cells, most of the chromatin (called
euchromatin) is relatively decondensed and distributed throughout
the nucleus.
 In interphase of the cell cycle, genes are transcribed and the DNA
is replicated in preparation for cell division.

9.  Heterochromatin is a very highly condensed state at this stage
cells undergoing mitosis.
 Heterochromatin is transcriptionally inactive and contains
highly repeated DNA sequences.

10. Heterochromatin are of two types,
 Constitutive heterochromatin.
 Facultative heterochromatin.
 The regions that remain condensed throughout the cell cycle
are called constitutive heterochromatin.
 The regions where heterochromatin condensation state can
change are known as facultative.

11. 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, thread like structures, called
chromatin threads.
 In the metaphase and the anaphase, the chromosome becomes thick
and filamentous.

12. Centromere:

14. Introduction:
 The centromeric DNA is normally in a heterochromatin state.
 The centromere is the part of a chromosome that links sister
chromatids.
 During mitosis, spindle fibers attach to the centromere.

15. Kinetochore::
 Kinetochore: disc-shaped protein structure, found on the
centromere of a chromatid.
 The kinetochore links the chromosome and spindle mitotic spindle
during mitosis.

16. Depending on the position of the centromere, chromosomes
divided into four categories:
1. Metacentric (V)
2. Sub-metacentric (L)
3. Acrocentric (J)
4. Telocentric (i)

17. Metacentric:
 If the centromere is present near about in the middle of the
chromosome, then it is called as metacentric.
 Amphibians possess such chromosomes.
 During anaphase movements, the chromosomes bend at the
centromere, so that metacentric chromosomes are V-shaped.
Human chromosomes
1 and 3 are metacentric…….

18. Sub-metacentric:
 The centromere located near the
centre of the chromosome (Not
exact centre).
 Sub metacentric chromosomes
are L-shaped at anaphase.
 Majority of human
chromosomes are sub
metacentric.

19. Acrocentric:
 These chromosomes possess the centromeres near end of
chromatids forming a long arm and a very short arm .
 Acrocentric chromosomes are J-shaped at anaphase, i.e., having
arms of unequal length.
 Example. Grasshoppers.
Human chromosomes
13,15,21,22 are acrocentric
chromosomes

20. 3. Telocentric:
 When centromere is at the terminal or proximal position, then the
chromosome is called telocentric.
 Chromosome is rod-shape or i shape at anaphase.
.
These type of chromosomes
are rare.

21. Acentric Chromosomes:
 If centromere is lacking, the chromosome is termed as acentric.
 Acentric fragments are commonly generated by chromosome-
breaking events, such as irradiation.

22. Telomeres

23. Telomeres
TTAGGG
Shelterin proteins

24. Due to each cell division, the telomere ends become shorter.

25. Introduction:
 The sequences at the ends of eukaryotic chromosomes, called
telomeres, play critical roles in chromosome replication and
maintenance.
 A telomere is a region of repetitive nucleotide sequences at each
end of a chromosome.
 It prevent fusion of chromosomes with neighbouring
chromosomes.

26.  The telomeres themselves are protected by a complex of
shelterin proteins.
 This sequence of TTAGGG is repeated approximately 2,500
times in humans

27. Replication of telomeres:
 During chromosome replication, the enzymes that duplicate DNA
cannot continue their duplication all the way to the end of a
chromosome.
 so in each duplication the end of the chromosome is shortened.
 In humans, average telomere length declines from about 11
kilobases at birth, in old age the size is less than 4 kilobases.


28.  Over time, due to each cell division, the telomere ends become
shorter.
 The ends of linear chromosomes cannot be replicated by the
normal action of DNA polymerase
 But the ends of the telomere is replicated by an enzyme,
Telomerase reverse transcriptase.

29.  Most prokaryotes, having circular chromosomes rather than
linear, do not have telomeres.
 Maintenance of telomeres appears to be an important
factor in determining the lifespan and reproductive
capacity of cells.
 So telomeres responsible of aging and cancer.

30. In complete replication of chromosome due to :
The end-replication problem:

31. The end-replication problem:
 Replication fork is made continuously and is called the leading
strand.
 The other strand is produced in many small pieces called
Okazaki fragments, each of which begins with its own RNA
primer, and is known as the lagging strand.

32.  In most cases, the primers of the Okazaki fragments can be easily
replaced with DNA and the fragments connected to form an
unbroken strand.
 When the replication fork reaches the end of the chromosome,
however, there is (in many species, including humans) a short
stretch of DNA that does not get covered by an Okazaki
fragment—essentially, there's no way to get the fragment started
because the primer would fall beyond the chromosome end.
 Also, the primer of the last Okazaki fragment that does get made
can't be replaced with DNA like other primers.
 Not require for exams