●To study the structure of chromosomes. ● To understand the concepts of linkage and crossing over. ● To understand structural and numerical chromosomal aberrations.

1. Cytogenetics
Dr. Jagadisha T.V., M. Sc., PGDGT., PhD
Assistant Professor
Department of Life Sciences (Genetics)
Ph.-No: 8892698143/9449442521,
E-mail:jagadisha.tv@kristujayanti.com
ORCID ID: https://orcid.org/0000-0002-0596-7830
Research Gate Profile: https://www.researchgate.net/profile/Jagadish-T-V

2. COURSE TITLE CREDITS TOTAL
NO. OF
HOURS
OBJECTIVES
21GEN2L431
CYTOGENETICS
4 60 • To study the structure of chromosomes
• To understand the concepts of linkage and crossing
over
• To understand structural and numerical
chromosomal aberrations
21GEN2L231
CYTOGENETICS
PRACTICAL
2 60 • To understand the biology of basic development
• To understand the mechanisms of development
from genes to the formation of organism
• To understand genetic control of determination of
anterior-posterior and dorsal-ventral axes
• To understand how mutation causes developmental
disorders

3. Unit /Topic No. of hours
Unit 1
Chromosomes
14
Unit 2
Linkage
10
Unit 3
Crossing over
14
Unit 4
Numerical Chromosomal
Aberrations
10
Unit 5
Structural Chromosomal
Aberrations
12

4. UNIT-1:CHROMOSOMES
The study of chromosomes and their role in inheritance and genetic disorders.
It involves the analysis of the number, structure, and behavior of
chromosomes within cells.
Chromosomes are thread-like structures found in the nucleus of every cell,
and they carry the genetic information in the form of DNA.
CYTOGE
NETICS

5. Structure:
•Linear DNA molecules associated with proteins called histones.
•These DNA-protein complexes are highly organized and form a compact
cell's nucleus.
Chromatid:
• During cell division (mitosis and meiosis), each eukaryotic chromosome consists
identical sister chromatids held together by a region called the centromere.
Centromere:
•The centromere is a specific region on the chromosome where sister chromatids
attached. It plays a crucial role in the movement and segregation of chromatids
cell division.
Primary Constriction (Centromere Placement):
•The primary constriction is the location on the chromosome where the
located. It divides the chromosome into two arms:
• short arm (p) and
• long arm (q).
EUKARYOTIC
CHROMOSOME

6. Secondary Constrictions:
•Additional constrictions - secondary
constrictions.
•These regions are essential for the
certain structures, such as the nucleolus
organizer regions (NORs) responsible for
ribosome synthesis.
Sat Bodies (Nucleolar Organizer Regions):
•Sat bodies or nucleolar organizer regions
specific regions on some chromosomes
play a role in the formation of the
where ribosomal RNA is synthesized.
Telomeres:
• Repetitive DNA sequences found at the
eukaryotic chromosomes.
•They protect the chromosome from
and ensure proper replication and stability
during cell division.

7. Chromatin
• Chromatin has a compact organization in which
most DNA sequences are structurally inaccessible
and functionally inactive.
• Within this mass is the minority of active
sequences.
• It is Mad up of DNA + Protein structure.
• Basic Unit of Chromatin is Nucleosome.

9. Nucleosome
• Nucleosome is fundamental unit of the Chromatin.
• The nucleosome contains about 200 base pairs
(bp) of DNA, organized by an octamer of small,
basic proteins into a beadlike structure.
• The protein components are histones.
DNA wrapped around
the Protein (Histone)
HISTONE
PROTEIN

10. Eukaryotic Genome Organization
• Genome refers to the complete hereditary information that is required to build and maintain an
organism.
• The eukaryotic genome is composed of DNA.
• Genome represents the sum total of all genes and intergenic regions in the DNA.
• There are about 3.4 billion (340 crore) base pairs in the human genomic DNA.
• The DNA along with a group of proteins called histones together forms a nucleoprotein complex
called chromatin.
• Chromatin is a mesh-like structure present in the nucleus of a non-diving cell.
• During cell division, chromatin is condensed into discreet units called chromosomes. This is to
ensure equal distribution of the genetic material to daughter cells.
• There are 23 pairs of chromosomes in the nucleus of a human cell. Therefore, there are 23 pairs of
DNA helices in the human cell.

11. Nucleosome:
• Under EM, chromatin appears as a string of beads. The bead like structures are called
nucleosomes
• Nucleosome is the fundamental organizational unit of chromatin.
• Nucleosome is composed of DNA and proteins called histone.
• Histones are basic protein as they are rich in lysine and arginine residues. The alkaline nature of
histone proteins helps them to bind to the phosphate backbone of DNA though ionic and hydrogen
bonds.
• There are 5 types of histone namely H2A, H2B, H3, H4 and H1
• Each nucleosome consist of an octameric core particle wrapped around by DNA.
• The octameric unit is composed of 2 copies of following histone proteins: H2A, H2B, H3 and H4.
• There are 8 protein molecules per nucleosome core.

12. • Each octameric unit is wrapped by 146 bps of DNA.
• Adjacent octameric units are linked by linker DNA of about 20-40 bps.
• H1 histone protein is bound to the linker DNA outside of the octamer. It helps to tighten the interaction
between the octamer and the 146 bp DNA.
• The diameter of the nucleosome is about 10 nm.
Solenoid – 30 nM Fibre
• Nucleosomes undergo condensation to form helical structure called as a solenoid.
• The solenoid structure also called as a 30 nM fiber due to its diameter.
• It is also called as solenoid as the nucleosomes are arranged in a helical pathway.
• In the solenoid, the H1 histone proteins faces the centre of the helix.

13. Coils and Supercoils:
• The solenoid fibres are condensed into structures
called as loops.
• The loops are arranged in a radial manner around a
central structure called as the chromosome scaffold.
• The chromosome scaffold is composed of scaffold
proteins called as condensin, topoisomerase II and
kinesin family member 4.
• The diameter of the loops is about 300 nm.
• The DNA is attached to the chromosome scaffold
through nucleotide sequences called as Scaffold
Attaching Regions (SAR)
• The loops undergo further coiling to formed supercoiled
structures with a diameter of about 700 nm.

15. EM of Chromatin: ‘String of beads structure’

16. Structure of Nucleosome
H1

18. Chromosome
Supercoil
Coil of loops
Solenoid
Nucleosome
DNA

19. 1%
25%
25%
15%
44%
Components of human genomic DNA
Introns and regulatory
Unique noncoding DNA
Repetitive DNA not
related to transposons
Repetitive DNA including
transposons
Exons
Genome = sum of all genes
Exome = sum of all exons

20. • Nucleosomes are an invariant component of euchromatin and
heterochromatin in the interphase nucleus And of mitotic
chromosomes.
Nucleosome
Euchromatin is a lightly packed form of chromatin that is
enriched in genes , and is often (but not always) under
active transcription .
Heterochromatin is densely packed form of chromatin.

23. Character Euchromatin Heterochromatin
Definition
Euchromatin is a more
lightly packed DNA that is
characterized by less
enormous staining and
DNA sequences that are
transcriptionally active or
might become
transcriptionally-active at
some point during growth.
Heterochromatin is a firmly packed
or condensed DNA that is
characterized by enormous stains
when stained with nuclear stains
and transcriptionally inactive
sequences.
DNA conformation
The DNA is compressed
and unfolded to form a
beaded structure.
The DNA is condensed
and folded with the histone
proteins.
Transcription It is transcriptionally-active. It is transcriptionally-inactive.

24. Staining
It is lightly stained under
nuclear stains.
It is darkly stained under
nuclear stains.
Genes
The genes found in this
are either already active
or will be active during
growth.
The genes found in this are
usually inactive.
DNA content
Euchromatin consists of
less amount of DNA lightly
compressed with the
histone proteins.
Heterochromatin consists
of more amount of DNA
tightly compressed with the
histone proteins.
Found in
Euchromatin is present in
both prokaryotes and
eukaryotes.
Heterochromatin is present
only in eukaryotes.

25. Content in
genome
It forms a more
significant part of the
genome. In humans, it
is approximately 90-
92% of the genome.
It forms a smaller part of the
genome. In humans, it is
approximately 8-10% of the
genome.
Location
Euchromatin is found
in the inner body of
the nucleus.
Heterochromatin is found
towards the periphery of the
nucleus.
Heteropycnosis
Euchromatin doesn’t
indicate
heterozygosis.
Heterochromatin indicates
heterozygosis.
Function
Euchromatin allows
the transcription and
variation of the gene
to occur within the
Heterochromatin maintains
the structural integrity of the
genome and allows the
regulation of gene

26. Replicative
It is an early
replicative and
replicates earlier than
heterochromatin.
It is a late replicative and
replicates later than
euchromatin.
Genetic
processes
It is affected by
various genetic
processes.
Heterochromatin is not
affected by genetic
processes.
Types
It consists of a single
type; constitutive
euchromatin.
It consists of two types;
constitutive and facultative
heterochromatin.
Examples
All the chromosomes
in the genome except
the heterochromatin
are examples of
Telomeres and
centromeres, one of the X
chromosomes, genes 1, 9,
and 16 of humans, Barr
bodies, are some examples

27. SPECIAL CHROMOSOMES
Introduction:
•Some cells at certain particular stages contain large nuclei with giant or large
sized chromosomes.
•The giant chromosomes are the polytene and lampbrush chromosomes.
•Polytene chromosomes were discovered by Balbiani which are formed due to
polyteny.
•Lampbrush chromosomes are a special form of chromosome found in the
growing oocytes (immature eggs) of most animals, except mammals.

28. POLYTENE CHROMOSOMES:
Introduction:
This special type of chromosome is observed by Balbiani in salivary glands of the
Chironomus larvae of Dipteran insects.
Since they were discovered in the salivary glands, they were also called salivary
gland chromosomes.
The present name polytene chromosome was suggested by Kollar due to the
occurrence of many chromonemata(DNA) in them.
Thus, some cells of Drosophila, Chironomus and mosquitoes become very large
having high DNA content.
Polyteny of giant chromosomes is achieved by replication of the DNA several times
without nuclear division and the resulting daughter chromatids do not separate but
remain aligned side by side.

32. LAMPBRUSH CHROMOSOME
:
Introduction:
Lampbrush chromosomes were first observed in Salamander(amphibian) oocytes in
1882.
He coined the name because the chromosomes look like the brushes which were
used for cleaning the glass chimneys of old fashioned paraffin or kerosene lamps.
This type of chromosome was observed by Flemming in 1882.
Lampbrush chromosomes occur in the diplotene chromosomes bivalents of most in
animal oocytes.
It is also found in spermatocytes of several species, a giant cell of Acetabularia, and
even in plants.
These chromosomes are even larger than the polytene chromosomes.

34. LAMPBRUSH
CHROMOSOMES

35. S.no. Lampbrush chromosomes Polytene chromosomes
1.
They were firstly observed by
Flemming.
They discovered by Balbiani
2.
These are found in yolk-rich primary
oocytes of Amphibians like Newt
(Triturus), spermatocytes of many
animals, and the giant nucleus of
Acetabularia.
They were observed in the cell of salivary
glands of Chironomus larvae of Dipterian
Insect. These are also found in
malpighian tubules, endosperm,
antipodal cells and salivary glands of
Drosophila.
3.
They are found In permanent diplotene
stage of meiosis.
They are found in the permanent
prophase stage.
4. The size up to 5.9 mm (5900pm).
The size of polytene chromosomes is
2000pm.
5.
Special Characteristic: The axis of the
lamp-brush is composed of DNA and a
matrix of RNA. Proteins in its lateral
loops help In synthesis of RNA and
Special Characteristic: They become
giant due to endomitosis or
endoduplication. Large swellings are
found on some places of each strand that
are called puffs (Balbiani rings). In puffs

36. STRUCTURE

38. B CHROMOSOMES
I. Are mainly or entirely heterochromatic
II. (i.e. largely non-coding), but some contain sizeable euchromatic
segments
III. (e.g. such as the B chromosomes of maize )
IV. Supernumerary, accessory chromosomes of maize)

39. B-CHROMOSOMES
Chromosomes are thread-like structures comprising nucleic acids and
proteins found within a live cell’s nucleus.
Their chief function is to convey genetic information in the form of genes.
They occur in females and are the primary cause of Turner syndrome, which
affects development in females.
found in all vertebrates, from fish to humans, and have been found to have
significant roles in several pathways of the cell, but little is known about their
regulation.
A B-chromosome is one that has a complete set of genes.
This is the default state for all cells, as most human cells do not have a B-
chromosome. However, in certain cases, a cell may have an extra B-
chromosome.

43. SIGNIFICANCE
•They increase the cell volume and DNA content so it has multiple copies of
genes that allow a high level of gene expression.
•For example, in Drosophila melanogaster, the polytene chromosomes of the
larval salivary glands help produce a large amount of adhesive mucoprotein
•Polytene chromosomes are large chromosomes that have thousands of DNA
strands.
•They provide a high level of function in certain tissues such as the salivary
glands of insects.

44. SIGNIFICANCE
Lampbrush chromosomes are chromosomes with lateral loops that produce
a large number of mRNAs and non-coding RNAs
 These transcripts are used during oogenesis and at the early stages of
embryogenesis
Help in the synthesis of proteins and yolk materials for the egg.