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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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C

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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

E

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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E

N

V

E

L

O

E

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E

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V

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L

O

E

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P

O

R

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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

.

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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P

O

R

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P

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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

D

N

A

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Chromatin

D

N

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Chromatin

D

N

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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

N

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

N

A

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D

N

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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

N

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

-

N

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

N

A

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The nucleolus

N

U

Nucleolus is a dense heterogeneous structure,

C

,

metaphase, reappear during telophase. This structureconsists of nucleolar or anizer re ion (DNA for rRNA

L

E

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,

U

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

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E

O

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S

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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

L

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

E

L

L

C

Y

L

E

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InterphaseC

E

L

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

C

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

E. 0

lifespan of the organism.

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C

E

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L

C

Y

L

E

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Mitosis

M

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.

O

S

S

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M

I

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S

S

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M

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S

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S

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S

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Division of the cytoplasm

M

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

M

First meiosis separates homologous chromosomes and

the second meiosis separates the chromatids. During the

E

prophase of the first meiosis there is an exchange of alleles

between the chromatids of homologous pair of duplicated

O

.

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

E

O

S

S

M i i

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Meiosis

M

During separation of the homologous chromosomes,

points of interchange temporarily remain united, and the

E

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

O

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

M

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

O

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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I

S

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S