This document discusses the morphology and types of chromosomes. It begins by defining what a chromosome is and where they are located in prokaryotic and eukaryotic cells. It then describes the different structural features of chromosomes visible under a light microscope, including chromatids, centromeres, secondary constrictions, telomeres, and satellites. It explains the different types of chromosomes based on centromere position, number of centromeres, size, and composition. The key differences between heterochromatin and euchromatin are also summarized.

1. MODULE - 2
Morphology and types of
chromosomes
Pillai Aswathy Viswanath
Dept.of Botany
Assumption college
Chenganacherry

2. Introduction
 A chromosome is an organised structure
of DNA and protein that is found in the
nucleus of eukaryotic cells.
 In prokaryots they localised in the
cytoplasam
 Chromosome is a single dsDNA in coiled
and condensed form
 Chromosome other wise it called as
chromatin

3.  The difference is that chromatin is less
condensed extented DNA
 While chromosomes are highly condensed
DNA
 DNA is condensed with the help of
histones protein in eukaryotes and
polyamines in prokarotes

5.  There are two kinds of eukarotic
chromosomes,namely autosomes and sex
chromosomes
 Autosomes carry the genes which control
somatic or non sexual characters and
sex chromosomes contain the genes
which control sexual characteristics
 The somatic chromosome number is the
number of chromosomes found in somatic
cell and is represented by 2n (Diploid)
 The gametic chromosome number is half
of the somatic chromosome numbers and
represented by n (Haploid).

6.  Haploid cells have only one copy of each
chromosome. In animals, gametes (sperm
and eggs) are haploid.
 Diploid cells have two homologous copies
of each chromosome.
 Both the copies are ordinarily identical in
morphology, gene content and gene order
and hence known as homologous
chromosomes.
 Each pair of chromosomes made up of two
homologs.
 Homologous chromosome is inherited from
separate parents; one homolog comes from
the mother and the other comes from the
father.

8. Chromosome number
 Normally all individual of a species have the
same chromosome number.
 The number of chromosomes varies from
species to species.
 Sometimes,changes may occur in the somatic
number.Such changes are called ploidy
changes
 Ploidy refers to the number of basic
chromosome sets
 for eg. A diploid has 2 sets where as a
hexaploid has 6 sets.

9.  The basic chromosome number x,also
called the monoploid number is the
number of different chromosomes that
make up a single complete set.
 Generally somatic cells contain two
copies of each chromosome except the
sex chromosomes.

10. Chromosome size
 The size of the chromosome shows a
remarkable variation depending upon the
stage of cell division.
 longest and thinnest during interphase and
hence not visible under light microscope.
 smallest and thickest during mitotic
metaphase.
 Chromosome size is not proportional to the
number of genes present on the
chromosome.

11. Chromosome morphology
 Mitotic metaphase is the most suitable
stage for studies on chromosome
morphology.
 The outer covering or sheath of a
chromosome is known as pellicle, which
encloses the matrix.
 Within the matrix lies the chromatin.
 The chromosome morphology changes during
cell division.
 During Interphase: the chromosomes
remain in form of chromatin .They are
thin, coiled, elastic, thread-like structures

12.  As cells enter mitosis, their chromosomes
become highly condensed
 Prophase: distinct thread like structures
called chromatid
 Metaphase and anaphase: they become
fully condensed and take the shapes
eukaryotic nuclear chromosomes.
 This cyclic change in shape and size of
chromosomes during cell cycle is called
chromosomal cycle

14.  In mitotic metaphase chromosomes, the
following structural features can be seen
under the light microscope.
 1. Chromatid
 2. Centromere
 3. Secondary constriction
 4. Telomere
 7. Matrix
 8. Satellite

15. Chromatid
 Each metaphase chromosome appears to be
longitudinally divided into two identical
parts each of which is called chromatid.
 Chromatids of a chromosome appear to be
joined together at a point known as
centromere.
 Two chromatids making up a chromosome
are referred to as sister chromatids.
 The chromatids of homologous
chromosomes are known as nonsister
chromatids.

16.  A chromosome consists of two chromatids
and each chromatid consists of thread like
coiled structures called chromonema (plural
chromonemata).
 Matrix
The mass of acromatic material which
surrounds the chromonemata is called
matrix.
The matrix is enclosed in a sheath which
is known as pellicle.

17. Centromere (primary constriction)
 Chromosome has a constriction point
called the centromere which divides the
chromosome into two arms.
 The short arm of the chromosome is
labeled the "p" arm.
 The long arm of the chromosome is
labeled the "q" arm.
 Centromere usually not located exactly in
the center of the chromosomes and in
some cases,is located almost at the
chromosome’s end

19. Based on the position of centromere,
chromosomes are called:
 i. Metacentric (centromere median).
 ii. Sub-metacentric (centromere is
submedian),
 iii. Acrocentric (centromere subterminal
and capped by telomere),
 iv. Telocentric (centromere terminal),

20. Metacentric chromosome
 The centromere is located in the centre
of chromosomes, i.e. the centromere is
median.
 The centromere is localized
approximately midway between each end
and thereby two arms are roughly equal
in length.
 Metacentric chromosome take V shape
during anaphase.

21. Submetacentric chromosome
 Centromere is located on one side of
the central point of a chromosome.
 Centromere is submedian giving one
longer and one shorter arms.
 Submetacentric chromosome may be J
or L shaped during anaphase.

22. Acrocentric chromosome
 The centromere located close to one end
of chromosomes.
 The centromere is more terminally placed
and forms very unequal arm length (The
"acro-" in acrocentric refers to the Greek
word for "peak").
 The p (short) arm is so short that is hard
to observe, but still present.
 Acrocentric chromosome may be rod shape
during anaphase.

23. Telocentric chromosome
 Centromere located at one end of
chromosome (at terminal part of
chromosome) lies at one end.
 Telocentic chromosome may be rod
shape during anaphase.

25. Based on the no. Of centromere
 Acentric ( without cetromere)
 monocentric (one centromere);
 dicentric (e.g. in wheat, maize etc.)
 polycentric (with few distributed
centromer e.g. Luzula. Ascaris

26. Kinetochore
 A complex of proteins associated with
the centromere of a chromosome during
cell division, to which the microtubules
of the spindle attach.
 The structure of kinetochore is complex
and is seen during late prophase.
 Kinetochore proteins serve as motor
proteins for the poleward anaphasic
movement of chromosome

27.  Kinetochore serve as a site for the
attachment of spindle fibers during cell
division.

28. Fuction of centromer
 Plays a role in the differentiation of
chromosomal arms
 Plays a vital role in the correct
distribution of daughter chromosomes to
the corresponding daughter cells in
mitosis
 Serves as an attachment site for
chromosomal spindle so that the
daughter chromosomes are pulled apart
to opposite poles

29. Secondary constriction
 In some chromosome addition to
centromere / primary constriction, one or
more constrictions in the chromosome are
present termed secondary constrictions.
 They are always constant in their positions
,number and extent among the members of
species.
 Secondary constrictions are of two types :
NOR (nucleolus organizer region)
Joint

30.  The NOR (nucleolus organizer region) are
specialized for the organization of
nucleolus
 It is best known as the site of ribosome
biogenesis ( contains the genes which code
for 18S and 28S rRNA
 The joints sometimes develop due to
breaking and fusion of chromosome
segments.
 Represents the site of fusion and fission of
chromosome segments

32. Satellite
 A Satellite chromosome or SAT
Chromosome is a chromosome segment
that is separated from the main body of
the chromosome by secondary
construction

33. Telomeres
 The terminal ends of chromosomes are
called telomeres.
 A telomere is a short repeated DNA
sequence (GC rich) complexed with
proteins.
 They are synthesized separately and
later add to the chromosomal tips
 They play critical roles in chromosome
replication and maintenance of
chromosomal length.

34.  They are highly stable and do not fuse
or unite with telomeres of other
chromosomes due to polarity effect.
 Any broken end of a chromosome is
unstable and can join with a piece of
any other chromosome.
 But the telomeres impart stability to
the chromosome, which retains its
identity and individuality through cell
cycle and for many cell generations.

35. Composition of chromosomes
 The material of which chromosomes are
composed is called chromatin.
 N.Fleming introduced the term chromatin in
1879.
 Chromatin was classified into two groups by
cytologists on the basis of its affinity to basic
dyes like acetocarmine or feulgen reagent at
prophase.
 The darkly stained regions were called
heterochromatin, while lightly stained regions
were called euchromatin.
 This differential staining capacity of different
parts of a chromosomes is known as
‘heteropycnosis’

37. Heterochromatin?
 Heterochromatin is the tightly packed form
of chromatin present in the cells of
eukaryotes.
 It is usually present at the periphery of
the nucleus.
 Due to its highly packed nature, it is
visible during the staining of DNA of a
cell.
 Also, this intensely stained DNA has two
types; they are the constitutive and
facultative heterochromatin.

38. a) Constitutive :- It is present in all cells at
identical positions on both homologous
chromosomes of a pair.
Constitutive heterochromatin is basically
responsible for forming the centromere or
the telomere while attracting signals for
both gene expression and repression.

39. b) Facultative:- It varies in state in
different cell types, at different stages
or sometimes, from one homologous
chromosome to another.
 Facultative heterochromatin becomes
repetitive under special signals or
environments;
 otherwise, it stays quiet with a highly
condensed structure.

40. Euchromatin?
 Euchromatin is the loosely-packed DNA
structures in the cells.
 Usually, they are present towards the
inner core of the nucleus.
 Euchromatin is present in both prokaryotes
and eukaryotes.
 In fact, euchromatin is the only type of
chromatin present in the prokaryotic
genetic material.
 Moreover, its loosely packed structure
causes less visibility during the DNA
staining, unlike heterochromatin.

41.  The uncondensed nature of euchromatin
is mainly due to the loose wrapping of
histone proteins around the DNA
strand.
 Therefore, the access of DNA is easy
to initiate the DNA transcription.
Moreover, euchromatin contains the
most active genes of an organism.
 It is because euchromatin participates
actively in the transcription of DNA
into mRNA.

43. What is the difference between
heterochromatin and euchromatin?
 Heterochromatin and euchromatin are two
varieties of chromatin present in living
organisms.
 The key difference between heterochromatin
and euchromatin is that the heterochromatin is
the highly packed form of chromatin in the
nucleus while euchromatin is the loosely packed
form of chromatin in the nucleus.
 Heterochromatin is inactive while euchromatin
is active.
 Consequently, heterochromatin contains more
dna, while euchromatin contains less dna.

44.  Heterochromatin is less abundant.
 But, around 90% of the total human
genome is euchromatin.
 A further difference between
heterochromatin and euchromatin is
that heterochromatin is only present in
eukaryotes, but, euchromatin is present
in both prokaryotes and eukaryotes.

45. Questions
 Morphology of chromosomes?
 Types of chromosomes?
 Hetrochromatin?
 Euchromatin?
 Difference?
 Terms? Like Kinetochore,Chromatid,etc.