Useful for B. Sc. Botany Syllabus of Dr. Babasaheb Ambedkar Marathwada University

1. CHROMOSOMES
Dr. ANJALI NAIK
Dr. A.S. Naik
Dept.of Botany
SBES College of Science

2. HISTORICAL ASPECTS
• A stainable threadlike nuclear
component having special
organisation, individuality and
function.
• Their presence was first
demonstrated in the
eukaryotic cell by E.
Strasburger in 1875 and were
termed as chromosomes by W.
Waldeyer in 1888.
• Greek chromasoma means
“coloured bodies” (chroma =
colour; soma = body)
• Marked affinity for basic dyes

3. CHROMOSOME-GENERAL
— Chromosomes are capable
of duplication and
maintaining their
morphologic and
physiologic properties
through successive cell
divisions.
— During reproduction of an
organism , they are passed
on to the next generation
through the gametes.
Besides, they play an
important role in variation,
mutation and evolution.

4. Chromosome Types
— MAIN TYPES
— Autosomes: which control
all somatic characteristic of
an organism and
universally present in all
eukaryotic-organisms
— Allosomes: chromosomes
which control some
specialized characteristics
of an organism e.g. sex
chromosomes, may or may
not be found in all
organisms.

5. CHROMOSOME NUMBER
— Chromosome Number:
— The number of chromosomes varies from species to
species but it remains constant for a particular species.
— Each somatic cell has 2 sets- maternal and paternal= 2
sets-diploid (-ploid=set):2n
— Sex cells or gametes—which contain half the number of
chromosome set found in somatic cell—are referred to
as haploid cells (n).
— The whole collection of chromosomes in the nucleus of
an organism is referred to as chromosome complement

6. Plant species Chromosome number (2n)
Onion 16
Mold 16
Carrot 20
Tomato 24
Rice 24
Maize 20
Tobacco 48
Haplopappus gracilis 4
Crepis capillaris 6
Arabidopsis thaliana (wild mustard) 2
Ophioglossum (pteridophyte) >1200

7. Chromosome Size
— The size of chromosome of a cell shows a remarkable
variation depending upon the stage of cell division-
longest and thinnest during interphase but progressive
decrease in size associated with an increase in thickness
from prophase onwards.
— The size of mitotic metaphase chromosome of various
plants and animals varies from 0.5µ to 32µ in length and
0.2 µ to 3.0 µ in diameter.
— size of chromosome may vary in the different tissues
within a single organism.
— Salivary gland chromosomes-endomitosis, large chms.

8. Condensation-Decondensation cycle of
chromosomes.

9. Chromosome shapes/categories
Centromere position:
— Metacentric-look ‘V’-shaped
during anaphasic
movement.
— sub-metacentric-look ‘L’-
shaped in anaphase.
— Acrocentric- look ‘J’-shaped
in anaphase.
— Telocentric chromosome

11. Structure of
chromosomes
Parts of
Chromosomes:
Pellicle and matrix
(2) Chromonemata
(3) Chromomeres
(4) Centromere
(5) Secondary
constrictions
(6) Satellite

12. Parts of Chromosomes
— Pellicle: thin, achromatic outer
membrane.
— Matrix: jelly like substance
enclosed by pellicle.
— Chromonemata: two identical,
spirally coiled threads of 800A0
thickness first observed by
Baranetzky in 1880.
— Chromatids: Thick, duplicated
chromonemata seen at metaphse.
— Kinetochores: :specialized multi-
protein complex called
kinetochore to which spindle
fibers (microtubules) attach. 2
kinctochores facing in opposite
direction. Ball/cup shaped.

13. Parts of Chromosomes-contd.
— Chromomeres: are regions of
the superimposed coils, each
corresponds to a genetic
locus.
— Primary constriction or
centromere: disc-shaped
protein structure 0.20 to
0.25mμ
— Secondary constrictions mark
the locations at which the
nucleoli organized-NOR
— Satellites: The part of the
chromosome which is present
beyond the secondary
constriction is called satellite
body or trabant.

14. Euchromatin and Heterochromatin
— Chromosomes may be identified by
regions that stain in a particular
manner when stained.
— Chemical techniques are used to
identify certain chromosomal regions
so that they form chromosomal bands.
— For example, darker bands are
generally found near the
centromeres or on the ends
(telomeres) of the chromosome,
while other regions do not stain
as strongly.
— The position of the dark-staining or
heterochromatic region or
heterochromatin.
— Light staining are euchromatic region
or euchromatin.

15. EUCHROMATIN AND HETEROCHROMATIN
— Heterochromatin is classified into two groups:
(i) Constitutive and (ii) Facultative.
— Constitutive heterochromatin remains
permanently in the heterochromatic stage,
i.e., it does not revert to the euchromatic
stage.
— In contrast, facultative heterochromatin
consists of euchromatin that takes on the
staining and compactness characteristics of
heterochromatin during some phase of
development.

16. DNA Tertiary Structure
•DNA DOUBLE HELICAL STRUCTURE COILS ROUND
HISTONES.
•DNA BOUND TO HISTONES FORMS
NUCLEOSOMES (10nm FIBRES)
•NUCLEOSOMES CONTAIN 146 NUCLEOTIDES
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Biochemistry for Medics

17. NUCLEOSOME MODEL
— The basic structural unit of chromatin, the nucleosome, was described by
Roger Kornberg in 1974.
— Two types of experiments led to Kornberg’s proposal of the nucleosome
model.
— First, partial digestion of chromatin with micrococcal nuclease (an enzyme
that degrades DNA) was found to yield DNA fragments approximately 200
base pairs long.
— In contrast, a similar digestion of naked DNA (not associated with protein)
yielded a continuous smear randomly sized fragments.
— These results suggest that the binding of proteins to DNA in chromatin
protects the regions of DNA from
nuclease digestion, so that enzyme can
attack DNA only at sites separated by
approximately 200 base pairs.

18. Nucleosomes
any of the repeating globular subunits of
chromatin that consist of a complex
of DNA and histone
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Biochemistry for Medics

19. Structure of nucleosome core
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Biochemistry for Medics

20. Compaction of DNA in a eukaryotic chromosome
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Biochemistry for Medics

21. Supercoil = coil over coil
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Biochemistry for Medics

22. — Electron microscopy revealed that chromatin fibers have a beaded
appearance, with the beads spaced at intervals of approximately
200 base pairs.
— Thus, both nuclease digestion and the electron microscopic studies
suggest that chromatin is composed of repeating 200 base pair unit,
which were called nucleosome.
— “BEADS ON STRING”

23. — Detailed analysis of these nucleosome core particles has
shown that they contain 146 base pairs of DNA wrapped
1.75 times around a histone core consisting of two
molecules each of H2A, H2B, H3, and H4 (the core
histones).
— One molecule of the fifth histone H1, is bound to the
DNA as it enters and exists each nucleosome core
particle.
— This forms a chromatin subunit known as
chromatosome, which consist of 166 base pairs of DNA
wrapped around histone core and held in place by H1 (a
linker histone)

25. Nucleosome-contd. 3D
structure

27. DNA PACKAGING PATTERN

28. Functions of Chromosomes
— 1. They contain hereditary information in the form of
genes and act as hereditary vehicle.
— 2. They control division, growth, metabolism and
differentiation of cell.
— 3. The ploidy of chromosomes determines the expression
of gametophyte or sporophyte generation.
— 4. Sex chromosomes determine sex of the individuals.
— 5. Crossing over and aberrations of chromosomes
introduce variations in population.

29. Special Types of Chromosomes
— Giant Chromosomes:
— (a) Polytene chromosome (Salivary gland
chromosome)
— (b) Lampbrush chromosomes

30. Salivary gland chromosomes
— First discovered by E.G. Balbiani (1881) in
salivary gland cells of larvae of Chironomus
(Dipteran insect). cells are unable to undergo
mitosis.
— Multi-stranded giant chromosomes formed
by somatic pairing of homologous
chromosomes and their repeated replication
without nuclear division (i.e. endoduplication
or endomitosis).
— After staining polytene chromosomes show
alternate dark and light bands all along their
length at places polytene chromosome bear
swellings called chromosome puffs while the
larger swellings are called Balbiani rings.

31. Lampbrush Chromosomes
• Flemming (1882) discovered them in
Salamander oocyte.
• Lamp brush chromosomes found in
the form of paired homologous
chromosomes joined at places called
chiasmata.
• Each chromosome has along axis and
lateral loops.
• The axis consists of two chromatids
and bears alternating dark granules of
chromo-meres and inter-chromomeric
regions.
• From chromo-meres lateral loops
extend. Each loop consists of a DNA
covered by RNA and proteins.