The document discusses chromosomes, detailing their structure, types, and key components such as centromeres, telomeres, and chromatin. It highlights the significance of chromosome number and morphology, as well as the chemical organization of nucleoproteins. Additionally, it explores abnormalities in chromosomes and introduces the concept of karyotypes as a representation of a species' chromosome set.

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

2. CHROMOSOMES
( Chromo = colour ; Soma = body)
• The term chromosome – coined by Waldeyer
(1888).
• At the time of cell division, chromatin
become condensed into chromosomes.
• Chromosome number – constant for a
species.
.

3. CHROMOSOME MORPHOLOGY
• Studied during Metaphase and Anaphase -
periods of maximum condensation -
chromosomes become thick and filamentous.
• Each chromosome has two chromatids –
attached to each other by a clear zone –
Centromere (Kinetochore or primary
constriction).

4. • Each chromatid – a single linear DNA
molecule with its associated proteins known
as chromonema (Plural- Chromonemata).
• Chromonemata of thin chromosomes of
mitotic and meiotic prophase have
alternating thick and thin regions

5. • The thick, bead-like structures –
Chromomeres – accumulation of chromatin
material – tightly folded regions of DNA
correspond to the units of genetic function in
the chromosome.
• The thin region in between the chromomeres
– interchromomeres.

6. CHROMOSOMES….
Image: http://selfstudy.co

7. CENTROMERE
(KINETOCHORE OR PRIMARY CONSTRICTION)
• Region of chromosomes that becomes attached
to the mitotic spindle
• Contain specific DNA sequences with special
proteins bound to them forming a disc-shaped
structure.
• Function – to provide a centre of assembly for
microtubules for chromosomal movement
during mitosis.

8. CHROMOSOME
Image:https://www.quora.com
https://learn.mindset.africa
https://onlinelibrary.wiley.com

9. • Monocentric - one centromere - most of the
chromosomes.
• Diffuse centromere (Holocentric
chromosomes) – microtubules attached along
the length of the chromosomes.
• E.g. In species of Juncaceae and Cyperaceae.

10. Fig. a : Monocentric chromosomes (in the upper
part)
Fig.b : Chromosomes with diffuse centromeres
(in the lower part)

11. Acentric and Dicentric chromosomes
• In some chromosomal abnormalities,
chromosomes breaks and fuse with
other ones, producing acentric
(chromosomes without centromere) or
Dicentric chromosomes (with two
centromeres).

12. Dicentric chromosome
Image:Steven M. Carr (2013) https://www.mun.ca

13. Types of Chromosomes based on shape
Shape - determined by the
position of the centromere.
4 Types :
i. Telocentric:
Centromere is terminal –
rod shaped
ii. Acrocentric:
Centromere near one end –
a very short arm and a very
long arm
iii. Submetacentric:
Centromere near the centre
– two unequal arms – ‘J’ or
‘L’ shaped.
iv. Metacentric - Centromere in
the centre – 2 equal arms –
‘V’ shaped . Image:https://www.embibe.com

14. Secondary constriction
• Some chromosomes may have a secondary
constriction at any part of the chromosomes.
• Certain secondary chromosomes play a vital role
in the formation of the nucleolus – Nucleolar
organizers or Nucleolar Organizing Regions
(NOR) Nucleoli form around the chromosomal
regions that contain 5.8 S, 18 S and 28 S rRNA
genes. They contain the genes for the pre-RNA.

15. Secondary constriction
Image:https://www.topperlearning.com

16. TELOMERE
• Extreme tip of the chromosome.
• It protects the end of the chromosome from
deterioration and prevents other chromosomal
segments to be fused with it.
• A region of repetitive nucleotide sequences at each
end of a chromosome .
• In most organisms, these repeats are maintained by
the enzyme, telomerase.
• Telomeres assure the physical integrity of
chromosome ends.

17. Human telomeres on metaphase chromosomes
using digital fluorescence microscopy.
Image:https://www.utsouthwestern.edu

18. SATELLITES
• Present in some chromosomes.
• A round body separated from the rest of the
chromosomes by a delicate chromatin filament ,
which may be long or short.
• Chromosome with satellite - SAT chromosomes.
• For a particular chromosome, the size and shape
of the satellite and the filament are constant.

19. SATELLITE
Image:https://biology.homeomagnet.com

20. KARYOTYPE
• It is the characteristic chromosome set of a
species.
• This include the characters by which a set of the
chromosomes of a species is identified such as
the number of chromosomes, relative size,
position of the centromere, length of arms,
secondary constriction, satellite etc.
• Diagrammatic representation of the karyotype is
known as ideogram.

21. Human Karyotype (Male)2n = 46
(23 pairs of homologous chromosomes ;
22 pairs of autosomes & 1 pair of allosomes (sex
chromosomes –XY)
Image: https://www.mun.ca

22. Ideogram of human chromosome
Image :https://commons.wikimedia.org

23. Chemical composition/ Chemical organization
of Nucleosome / Nucleoproteins
Chromatin - a viscous , gelatinous substance that
contains DNA, RNA, basic proteins – Histones and
non- histone (more acidic) proteins.
• Histones and DNA are present in a 1:1 ratio.
• The content of RNA and non – histone proteins is
variable in different chromatin.
• The non- histone proteins are heterogeneous – vary in
different tissues - include RNA and DNA polymerases
and other regulatory proteins.

24. i) DNA
• The most important chemical component of
chromatin – measured in picogram.
• Each species has a characteristic content of
DNA which is constant in all individuals of that
species – C- value.
• Nuclei contain a constant amount of DNA – all
the cells in a diploid organism contain the
same DNA content , 2C.

25. • Since gametes are haploid, they
have half the DNA content, 1C.
C – value Paradox (Gall, 1981):
There is no correlation between
the C values of species and their
evolutionary complexity.
Eg. : The Salamander ,
Ambystoma has 84 picogram
(pg) of DNA where as man has
only about 3pg of DNA per
haploid genome.

26. ii) Histones
• Small proteins having high content (10 to 20 %)
of basic amino acids Arginine and Lysine.
• Bind tightly to DNA
• Five histones –H2A, H2B, H3, H4. – very similar
in different species and are present twice every
200 base pairs of DNA.
• The fifth histone, H1 is present only once per
200 base pairs of DNA and varies between
species and within tissues of the same species.

27. • Histones play an important role in
maintaining the structure and functional
integrity of chromatin. They also protect DNA
from the action of nucleases.
iii) Non- histone proteins
• Include all the enzymes and factors that are
involved in DNA replication, transcription and
in the regulation of transcription.
e.g. DNA polymerase

28. THANK YOU