4. nucleus
TRANSCRIPT
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man n vers y o c ence ec no ogy
Faculty of Dentistry
Histology & Cell Biology08 01 112
Dr. Al-Moutassem Billah Khair
e uc ear e o ogy
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NUCLEUS
N
UThe nucleus is an essential part of protoplasm, present
in almost all livin cells o animals and lants
C (Eukaryotes). A notable exception is the humanerythrocyte, which lacks a nucleus.
E
*The nucleus is a spherical or ovoid body with a
diameter of roughly 1-3 m and its location depends on
Uthe cell type (central, peripheral, basal, superficial).
*A cell may contain more than one nucleus. It is derived
from a mother cell by cell division (mitosis or meiosis). It
contains the structures controlling there production and
unc on ng o e ce ; ese are .
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NUCLEUSN
UThe nucleus contains, in addition to the DNA (DNA is less
than 20% o nucleus mass also a lar e uantit of roteins
C called nucleoproteins ( DNA-binding proteins & nuclear matrix proteins) and some RNA (newly synthesized m, t,
E
rRNA).
The nucleus contains the entire human karyotype, which
Uconsists of 46 chromosomes. Chromosomes are discrete
collections of genes and nuclear proteins. Genes determine
most of an organisms characteristics, but not all genes arelocated on nuclear chromosomes (e.g. mitochondria have a
.
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NUCLEUSNU
C *Nuclear Envelope
E
**Nuclear Pores
***Chromatin
U****Chromosomes
*****Nucleolus
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The nuclear envelope
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N
The nuclear envelope separates the entire of the
nucleus from cytoplasm. It consists of two layers of plasma
V structure), which represent a specialized part of theendoplasmic reticulum.
E
L
The nuclear envelope is 7-8 nm thick and contains
several hundred nuclear pores.
OThe outer membrane often is continuous with the
membrane of the RER, and is studded with ribosomes. The
Eendoplasmic reticulum and like much of it. On the inner
aspect of the envelope there is a fibrous lamina bound to
membrane proteins and linked with condensed peripheral
chromatin. This structure does not block nuclear pores.
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V
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The nuclear pores
P The nuclear pores are openings in nuclear envelope,
at the margins of which the inner and outer nuclear
O
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macromolecules synthesized in the nucleus to pass into
the cytoplasm and vice versa.
R
Nuclear pores are annuli with 60 nm diameter and
each annulus consists of eight peripheral subunits withlarge dense body in the center. The pores not open all
times but are bridged by a diaphragm of protein.
physiological state of nucleus, and it is increased with the
increasin of nucleus activit . That is mean the are not
stable structures and they are in dynamic state.
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R
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ChromatinExcept during cell division, the chromosomes, each
make u a se arate len th of the DNA com lement exist
D in a complicated strands. Chromatin is composed mainlyof coiled strands of DNA bound to basic proteins. The
N
chromatin basic unit is nucleosome, which consists of a
core of four types of histones surrounded by DNA. Two
microscopes:
I. Heterochromatin tends to be clumped around the periphery of the
A nucleus but also forms irregular clumps throughout the nucleus. Itrepresents that DNA which is not active in RNA synthesis.
-. .
represents that DNA which is active in RNA synthesis.
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Chromatin
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A
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Chromatin
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Chromatin
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Chromosomes
Chromosomes are flexible rod-like structures that
D chromatin of the cell nucleus reverts to the condensed
state. Each s ecies of lant and animal has a constant
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number of chromosomes. The chromosomes in different
species of organisms differ in shape as well as in number.The set of chromosomes included in diploid cells called
karyotype.
A In human cells, there are 46 chromosomes comprising23 homologous pairs, the members of each pair having the
same eng o an co ng or e same pro e ns.
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D
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A
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D
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Chromosomes
The human karyotype contains 22 pairs of
autosomes and a pair of sex chromosomes. The female
D chromosomes (homomorphic), while the male karyotypecontains a pair of morphological different sex
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chromosomes designated X & Y (heteromorphic). One of
X chromosomes in female consists of heterochromatinan s oca e near e nuc ear enve ope. s
chromosome is known as Barr body, which can be used
to determine the sex and some chromosomal
A abnormalities.
Each member of a homologous pair of mitotic
chromosome is identical in length, centromere location
and banding pattern.
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Chromosomes
D
N
A
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Chromosomes
These airs are classified b Deniver s stem accordin to their len thand centromere position in eight groups:
1. Group A: three pairs of the largest metacentric chromosomes (1-3).
D
2. Group B: two pairs of large submetacentric chromosomes (4-5).
3. Group C: seven pairs of medium-sized metacentric chromosomes
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4. Group D: three pairs of medium-sized acrocentric chromosomes
(13-15).
5. Group E: three smaller pairs (16-18), 16 is metacentric, while 17 &
18 are submetacentric.
A. - .
7. Group G: two pairs of small acrocentric chromosomes (21-22).
8. Group H: includes in female two medium submetacentric X
chromosomes, while in male a medium submetacentric X
chromosome and a very small acrocentric Y chromosome.
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Chromosomes
D
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A
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The nucleolus
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U
Nucleolus is a dense heterogeneous structure,
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,
metaphase, reappear during telophase. This structureconsists of nucleolar or anizer re ion (DNA for rRNA
L
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transcription), fibrillar region (primary rRNA) and
granular region (ribosome subunits). Heterochromatin is
Oattached to the nucleolus but its function unknown.
The nucleoli especially are found in embryonic cells,
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n ce s g y act ve n prote n synt es s, an n rap ygrowing malignant tumors. They are the sites of
S
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C CELL CYCLEE
L
The cell cycle is arbitrarily defined as the period from
the formation of a cell by division to the point when that cell
Litself divides. [ It is the life cycle of an individual cell ].
Most types of cell never divide again after they have grown
C
an ecome spec a ze , w e some unspec a ze ce s
retain the ability to divide. The length of the cell cycle is
ver variable.
Y
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E
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CELL CYCLEC
E
LThe cell cycle consists of two periods a relatively short
dividin eriod and a nondividin eriod.
L I. Interphase is the period between mitotic divisions. It isnot a resting period, but a period during which the
C
metabolic activities of the cell is intense. During
interphase the nucleus is involved in protein synthesis,
Y.
II. Cell division is a nuclear division followed by
c to lasmic division. Nuclear divisions are of two t es
Ldepending on whether the number of chromosomes
present in each of the daughter cells is the same as the
Enumber in the parent cell, or is half that number.
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C
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Y
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InterphaseC
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This period consists of:
I. First gap or G1 phase; this is usually much longer than
L
. 1- ,
cells grow and perform their specialized functions.
II. S- hase durin which DNA s nthesis occurs in each
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chromosome and histones of the daughter chromatids is
also produced.
YIII. Second gap or G2 phase; is relatively short and is the
period in which cells prepare for mitotic division.
LThe duration of S, G2 and M phases is relatively constant, each
of about several hours whereas the G1 is highly variable, in some cases
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lifespan of the organism.
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C
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Y
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Mitosis
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os s s v s on o e nuc eus o pro uce wo nuc e containing identical sets of chromosomes (replicative
division . Mitosis takes lace when an or anism rows, and
I when tissues are repaired. Also, can occur in asexualreproduction.
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S
S
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Division of the cytoplasm
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v s on o e cy op asm, or cy o nes s , usua yfollows nuclear division. In cytokinesis the cell organelles
become evenl distributed between the dau hter cells.
I Cytokinesis may, in some situations, result in theformation of two cells with unequal amount of cytoplasm.
O
In other circumstances, mitosis may occur in the absence
of cytokinesis as in the formation of binucleate or
S
.
In animal cells the cell membrane begins to invaginate at the
S
.
circumference forms a furrow in the cytoplasm which then extends to
divide the cytoplasm into two cells. A ring of microfilaments is
present just beneath the surface of the cleavage furrow and
cytokinesis occurs as a result of its contraction.
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Meiosis
M
Meiosis is division of the nucleus to produce four
daughter nuclei, each containing half the number of
Echromosomes of the original nucleus (reductive).
Meiosis normally precedes the formation of gametes,
I
O
however , so the chromosome number remains constant
from generation to generation. In sexual reproduction,
S
,
number of the new cell (zygote) is doubled by the
amalgamation of two nuclei.
SMeiosis involves two successive divisions of the
nucleus. Both the divisions superficially resemble mitosis.
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Meiosis
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First meiosis separates homologous chromosomes and
the second meiosis separates the chromatids. During the
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prophase of the first meiosis there is an exchange of alleles
between the chromatids of homologous pair of duplicated
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.
In prophase of 1st meiosis, the first feature is that the
chromosomes a ear lon slender & ver thin then the
S homologous chromosomes approach, line up and pair
point for point except for the X-Y combination. The
Ssecon eature s ca e crossover an cons sts o t einterchange of chromatid segments between two paired
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M i i
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Meiosis
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During separation of the homologous chromosomes,
points of interchange temporarily remain united, and the
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s ape nown as c asma. ur ng e c asma s age,
blocks of genes are exchanged between homologouschromosomes. In the meantime se aration continues and
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the two members of each pair become oriented on the
spindle. In subsequent stages the members migrate to the
Sopposite poles of the cell.
After the 1st meiosis has been completed, each daughter
Sthus has 23 double-structured chromosomes. So, the
amount of DNA in each dau hter cell e uals that of a
normal somatic cell.
M i i
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Meiosis
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or y a er e me os s, e ce eg ns smaturation division. The time period between two division
known as interkinesis, and characterized b no DNA
Esynthesis. The 23 double-structured chromosomes divide
at the centromere, and each of the newly formed daughter
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ce s rece ves c romat s. e amount o n t ese
cells is now half that of the normal somatic cell.
S a) to provide for genetic variability through the processes
of crossover and random distribution of homolo ous
Schromosomes to the daughter cells;
b) to provide each germ cell with both a haploid number
of chromosomes and half the amount of DNA of a normal
somatic cell.
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