3. Introduction:
Discovery of Nucleus by Ernest Rutherford ( 1871-1937)
Nucleus theory given in 1910, An atom’s mass is mostly in the nucleus and
it has a positive charge.
Prominent and Characteristics Features
Eukaryon means True nucleus
Basis of eukaryote- membrane bounded nucleus
Imp function: -Physically separates DNA from the cytoplasm’s complex
metabolic machinery
- Nuclear membrane serve as boundry
4. Definition:
The nucleus is the genetic control centre of a eukaryotic
cell.
In most cells, there is only one nucleus. It is spherical, and
the most prominent part of the cell, making up 10% of the
cell’s volume.
It has a unique structure and function that is essential for
the cell.
5. Components of Nucleus:
• 1. Nuclear Envelope – pore riddled
• 2. Nucleoplasm – Fluid interior portion
• 3. Nucleolus – Dense cluster of RNA &
Proteins
• 4. Chromatin – all DNA+ Proteins
Average diameter of nucleus is 6 µm,
which occupies around 10% of cell
volume.
7. Nuclear Membrane:
Also known as nuclear envelope or nucleolemma.
Separates the nuclear material from cytoplasm.
Consists of two lipid bilayers: Outer membrane, Inner
membrane
The nuclear envelope is a double-layered membrane
perforated with pores, which control the flow of
material going in and out of the nucleus. The outer
layer is connected to the endoplasmic reticulum,
communicating with the cytoplasm of the cell. The
exchange of the large molecules (protein and RNA)
between the nucleus and cytoplasm happens here.
8. Function Of Nuclear Membrane:
Shape and Stability: Helps the Nucleus From Collapsing
Compartmentalizing: Separates The Nuclear Material From
Cellular Material
Regulation Of Substances: Allow The Exchange Of Materials
Communication: Develops A Chemical Connection Between
Nucleus And Cell
9. The Nuclear Pore:
Most distinctive feature of NE
Small cylindrical channels- direct contact cytosol
& Nucleoplasm
Readily visible – freeze fracture microscopy (a
specimen is frozen rapidly and cracked on a plane
through the tissue)
Mammalian nucleus – 3000 to 4000 pores
Inner & outer membranes fused
Structural complexity – control transport of key
molecules
10. Function Of Nuclear Pore:
Exchange of materials between nucleus and cytoplasm
Passive diffusion of low molecular weight solutes
Efficient passage through the complex it requires
several protein factors.
11. Nucleoplasm:
A jelly-like (made mostly of water) matrix within the nucleus
Just like the cytoplasm found inside a cell, the nucleus contains nucleoplasm,
also known as karyoplasm
All the other materials “float” inside
Helps the nucleus keep its shape and serves as the median for the transportation
of important molecules within the nucleus
The nucleoplasm is a type of protoplasm that is made up mostly of water, a
mixture of various molecules, and dissolved ions
It is completely enclosed within the nuclear membrane or nuclear envelope
12. Nucleolus:
Ribosome factory
Large, prominent structures
Doesn’t have membrane
Most cells have 2 or more
Directs synthesis of RNA
The nucleolus takes up around 25% of the volume of the nucleus.
This structure is made up of proteins and ribonucleic acids (rna). Its
main function is to synthesis ribosomal RNA(rrna) and combine it
with proteins.
13. Function of nucleolus:
Site for transcription ( cell makes RNA copy from
fragment of DNA)
Association of ribosomes
Synthesis of ribosomes
Synthesis of RNA
14. Chromosome:
Chromosome means: (chroma - colour, some - body)
A chromosome is a thread-like self-replicating genetic structure containing
organized DNA molecule package found in the nucleus of the cell.
Chromosomes are seen during metaphase stage of mitosis when the cells are
stained with suitable basic dye and viewed under light microscope.
E. Strasburger in 1875 discovered thread-like structures which appeared during
cell division.
In all types of higher organisms (eukaryote), the well organized nucleus contains
definite number of chromosomes of definite size and shape.
15. H. G. Waldeyer coined the term chromosome first time in 1888.
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).
The two copies of chromosome are ordinarily identical in
morphology, gene content and gene order, they are known as
homologous chromosomes.
16. Chromosomes are of two types:
Autosomes: that control characters other than sex characters or carry
genes for somatic characters.
Sex chromosomes (Gonosomes): Chromosomes involved in sex
determination.
Humans and most other mammals have two sex chromosomes X &
Y, also called heterosome.
Females have two X chromosomes in diploid cells; males have an X
and a Y chromosome.
In birds the female (ZW) is hetero-gametic and male (ZZ) is homo-
gametic.
17. The size of chromosome is normally measured at mitotic metaphase
and may be as short as 0.25 µm in fungi and birds, or as long as 30
µm in some plants like Trillium.
Each chromosome has two arms - p (the shorter of the two) and q
(the longer).
Chromosome shape is usually observed at anaphase, when the
position of primary constriction (centromere) determines chromosome
shape.
This constriction or centromere can be terminal, sub-terminal or
median in position.
19. Chromosome Morphology:
Mitotic metaphase is the most suitable stage for studies on
chromosome morphology.
The chromosome morphology changes during cell division.
Chromosomes are thin, coiled, elastic, thread-like structures during
the interphase.
As cells enter mitosis, their chromosomes become highly condensed
so that they can be distributed to daughter cells.
In mitotic metaphase chromosomes, the following structural features
can be seen under
20. Chromatid:
Each metaphase chromosome appears to be longitudinally divided into two
identical parts each of which is called chromatid.
Both the chromatids of a chromosome appear to be joined together at a point
known as centromere.
The two chromatids of chromosome separate from each other during mitotic
anaphase (and during anaphase II of meiosis) and move towards opposite poles.
Since the two chromatids making up a chromosome are produced through
replication of a single chromatid during synthesis (S) phase of interphase, they
are referred to as sister chromatids.
In contrast, the chromatids of homologous chromosomes are known as non-
sister chromatids.
21. Two types of chromatids
1) Euchromatin which undergoes the normal process of condensation and
decondensation in the cell cycle.
2) Heteochromatin which remain in a highly condensed state throughout the
cell, even during interphase.
Chemical composition of chromatid :
DNA= 20-40 %- most important chemical constituent of chromatin.
RNA=05-10 %-associated with chromatin as; Ribosomal RNA-( rRNA)
Messenger RNA- (mRNA) Transfer RNA- (tRNA).
PROTEINS=55-60%-associated with chromatin as,
I. Histones : - Very basic proteins +ve charged at neutral PH, constitute about 60%
of total protein, almost 1:1 ratio with DNA.
II. Non-Histones : - They are 20% of total chromatin protein
22.
23. Centromere (Primary constriction)
• Centromere is the landmark for identification of chromosome.
• Each chromosome has a constriction point called the centromere (Synonym:
Kinetochore), which divides the chromosome into two sections or arms.
• The short arm of the chromosome is labeled the "p" arm. The long arm of the
chromosome is labeled the "q" arm.
Telomere
• The two ends of a chromosome are known as telomeres, they play critical roles
in chromosome replication and maintenance of chromosomal length.
• The telomeres are highly stable and telomeres of different chromosomes do not
fuse.
• The telomeric region of chromosome is made up of repetative sequence of T and
G bases.
24. Secondary constriction
• In some chromosome addition to centromere / primary
constriction, one or more constrictions in the chromosome are
present termed secondary constrictions.
Satellite
• The chromosomal region between the secondary constriction and
nearest telomere is called as satellite and chromosomes that
possess this region called as satellite chromosome or sat
chromosome.
• A small chromosomal segment separated from the main body of
the chromosome by a secondary constriction is called Satellite.
25. Size of the chromosome:
• The size of the chromosome varies from stage to stage of cell
division.
• The chromosomes are the longest and thinnest during interphase
(resting stage) and hence not visible under light microscope.
• Chromosomes are the smallest and thickest during mitotic
metaphase.
• Chromosome size is not proportional to the number of genes
present on the chromosome.
• The location of the centromere on each chromosome gives the
chromosome its characteristic shape.
26. Types of chromosome based on
centromere:
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.
27. Submetacentric Chromosome
Centromere is located on one side of the central point of a chromosome.
Centromere is sub median giving one longer and one shorter arms.
Submetacentric chromosome may be J or L shaped during anaphase.
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.
28. Telocentric Chromosome
Centromere located at one end of chromosome (at terminal part of
chromosome) lies at one end.
Telocentric chromosome may be rod shape during anaphase.
According to the number of the centromere the eukaryotic chromosomes
may be:
Acentric: without any centromere
Mono centric: with one centromere
Dicentric : with two centromeres
Polycentric: with more than two centromeres
29.
30. Special type of chromosome:
1. Giant chromosomes: These were first discovered by E. G. Balbiani in 1882. They are made
up of several dark staining regions called “bands”.
It can be separated by relatively light or non-staining “interband” regions.
The bands in Drosophila giant chromosome are visible even without staining, but after
staining they become very sharp and clear.
These chromosomes are also known as “Polytene chromosome”, and the condition is
referred to as “Polytene”
2. Lampbrush Chromosome: These were first observed by W. Flemming in 1882. It was
given this name because it is similar in appearance to the brushes used to clean lamp
chimneys in centuries past.
These are found in oocytic nuclei of vertebrates (sharks, amphibians, reptiles and
birds)as well as in invertebrates (Sagitta, sepia, Ehinaster and several species of
insects).
31. 3. Accessory chromosomes:- In many species some chromosomes are
found in addition to normal somatic chromosomes.
These extra chromosomes are called accessory chromosomes or B-
chromosomes or supernumerary chromosomes.
These chromosomes are broadly similar to normal somatic chromosomes
in their morphology
For instance, presence of several such chromosomes often leads to
reduction in vigour and fertility in males. Origin of these chromosomes in
most species is unknown.
4. Isochromosomes:- An isochromosome is the one in which two arms are
identical with each other in gene content and morphology.
Every isochromosome is metacentric. The attached ‘x’ chromosome of
Drosophila is a classical example of an isochromosome. However its
origin is uncertain.
32. 5. Allosomes / sex chromosomes:- Chromosomes differing in
morphology and number in male and female are called allosomes.
They are responsible for determination of sex.
eg: X and Y chromosomes in human beings and Drosophila.
Chromosomes which have no relation with determination of sex and
contain genes which determine somatic characters of individuals are
called autosomes and are represented by letter ‘A’.
33. Variation in chromosome number:
Organism with one complete set of chromosomes is said to be
euploid (applies to haploid and diploid organisms).
Aneuploidy - variation in the number of individual chromosomes
(but not the total number of sets of chromosomes).
The discovery of aneuploidy dates back to 1916 when Bridges
discovered XO male and XXY female Drosophila, which had 7
and 9 chromosomes respectively, instead of normal 8.
34. Nullisomy - loss of one homologous chromosome pair.
(e.g., Oat )
Monosomy - loss of a single chromosome (Maize).
Trisomy - one extra chromosome. (Datura)
Tetrasomy - one extra chromosome pair.
35. Chromosomal Aberrations:
The somatic (2n) and gametic (n) chromosome numbers of a
species ordinarily remain constant.
This is due to the extremely precise mitotic and meiotic cell
division.
Somatic cells of a diploid species contain two copies of each
chromosome, which are called homologous chromosome.
Each chromosome of a genome contains a definite numbers and
kinds of genes, which are arranged in a definite sequence.
36. Sometime due to mutation or spontaneous (without any known
causal factors), variation in chromosomal number or structure do
arise in nature. - Chromosomal aberrations.
Chromosomal aberration may be grouped into two broad classes:
1. Structural
2. Numerical
37. Structural Chromosomal Aberrations:
Chromosome structure variations result from chromosome breakage.
Broken chromosomes tend to re-join; if there is more than one break, re-
joining occurs at random and not necessarily with the correct ends.
The result is structural changes in the chromosomes.
Chromosome breakage is caused by X-rays, various chemicals, and can also
occur spontaneously.
There are four common type of structural aberrations:
1. Deletion or Deficiency
2. Duplication or Repeat
3. Inversion
4. Translocation.
38. Consider a normal chromosome with genes in alphabetical order: a b c d
e f g h i
1. Deletion: part of the chromosome has been removed:
a b c g h i
2. Dupliction: part of the chromosome is duplicated:
a b c d e f d e f g h i
3. Inversion: part of the chromosome has been re-inserted in reverse order:
a b c f e d g h i
4. Translocation: parts of two non-homologous chromosomes are joined:
If one normal chromosome is a b c d e f g h i and the other chromosome is
u v w x y z, then a translocation between them would be
a b c d e f x y z and u v w g h i.
39. A New Chromosome Model
• Chromosomes and their function have been well known for more than 100
years. The three-dimensional architecture of chromosomes however, still a
matter of intense discussion.
• In general, metaphase chromosomes consist of the following elements: A pair
of two chromosome arms of either equal or unequal length which splits into
two chromatides during metaphase.
• There is one obligatory constriction which defines the “centromere” and a
facultative constriction which represents the “satellite region.” The
chromatides are terminated by telomeres.
G. Wanner and H. Formanek
August 22, 2000
Munich, Germany
Case study: 1
40. •The structural compound of chromosomes, the chromatin,
consists of equal amounts of DNA, histones and
nonhistone proteins (Earnshaw, 1991).
• Chromatin consists of “euchromatin,” which condenses
(and stains intensely) during mitosis, and
“heterochromatin,” which remains condensed during
interphase also (Passarge, 1979; Traut, 1991a).
41. Methodology:
• Human and plant chromosomes were prepared for high-resolution scanning
electron microscopy as described (Martin et al., 1994, 1996).
• Controlled decondensation of (unfixed) metaphase chromosomes was achieved
by treatment with citrate buffer (60 mM, pH 7.2) for 60 min at room temperature.
• For proteinase treatment chromosomes were first fixed with glutaraldehyde
(2.5% in 75 mM cacodylate buffer) and then treated with proteinase K (0.1–1
mg/ml) for 30 min at 37°C.
• For separate visualization of DNA, chromosomes were stained for 30 min at
20°C with 1% zirconiumchloridoxide in 1% hydrochloric acid.
42. • Proteins were separately visualized after staining for 12 h at 60°C with 20%
aqueous silver nitrate solution.
• The colloidal silver solution was prepared in the following way: 0.5 g silver nitrate
dissolved in 1.5 ml water was slowly added to 25 ml of an aqueous solution of
0.25% tannic acid and 2% sodium carbonate by continuous stirring at room
temperature.
• A slight precipitate forms within 24 h which is removed by centrifugation (Lea,
1891; Gmelin, 1971). After the specimens were washed with aqua dest. (three
times for 5 min at 2°C) they were dehydrated through a graded acetone series (20–
100%) and critical point dried with liquid CO2. Chromosome spreads were
preselected with a light microscope.
43. Result
• From their investigations it is obvious that there are two dominant
structural elements in metaphase chromosomes: coiled chromomeres
and parallel matrix fibers.
• Since 1929 these heterochromatic structures have been called
“chromomeres” During metaphase they can be observed only (if at
all) in the centromeric and satellite regions. They become visible best
after controlled decondensation of metaphase chromosomes by
proteinase K treatment.
44. •According to our ultrastructural investigations, they
propose a new model for the three-dimensional structure
of chromosomes.
•The model can simply explain the enormous variety of
chromosome morphology in plant and animal systems by
varying only a few cytological parameters.
45. The Structure of the Nucleus Studied by Electron
Microscopy in Ultrathin Sections with Special
Reference to the Chromonema-An Advocation of
"Subchromonema" and "Protochromonema"
• The nucleus was the first intracellular structure discovered and
was originally described by Franz Bauer in 1802 and later
popularized by Robert Brown.
• The purpose of study to obtain a precise knowledge concerning the
structure of the nucleus, especially (a) the finest structure of the
chromosome, (b) the chromosomal constrictions and (c) various
nucleolar structures found in resting nuclei.
Kyoto, Japan Shigeyasu Amano et. al
March 30, 1956
Case study: 2
46. Methodology:
• Plasma cells, monocytes and lymphocytes in mice were utilized in
this study. Referring to the procedure to get ultrathin sections and
electron microphotographs. (Dohi, 1955)
Observations:
• Chromonema and subchromonema in the nucleus of plasma cell
• Chromosome, chromonema and subchromonema in the mitotic
nucleus of lymphocyte
• Chromonema and subchromonema in the resting nuclei of
lymphocytes.
47. Result:
• In their previous phase microscope studies, various interkinetic nuclei were
described, in which the chromonema structure is either (a) uncoiled completely.
The cells belonging to the latter group (b) such as lymphocytes, monocytes and
plasma cells in mice, referring especially to their chromonema structure as seen in
ultrathin section by an electron microscope.
• The proto chromonemata (coiled chromatin thread within a single chromosome)
belong to the morphological limit capable of being observed by an electron
microscope in ultrathin sections.
• observation was confirmed also with a structure of the more or less loosely coiled
chromosomes found in mitotic lymphocytes in prophase.
• The size and structure of the chromonema and subchromonema in mitotic and
interkinetic stages were studied.
48. Reference:
Genome 4 by T. A. Brown
Life Science (Fundamental and practice) by Pranav Kumar and
Usha meena
B D Singh
