A general account of chromosomes- morphology, types of chromosomes based on the position of centromere, chemical composition, C - value

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