Cell structure and functions

CELL STRUCTURE AND FUNCTIONS.

DR. ABHIMANYU CHATURVEDIM.D.S

1ST YEAR

I.T.S CENTRE FOR DENTAL

STUDIES AND RESEARCH

DEPARTMENT OF CONSERVATIVEDENTISTRY

& ENDODONTICS

SEMINAR NUMBER 1

Contents:

Introduction

History

Types of cells

Anatomy of cells:- Prokaryotic

Eukaryotic

Difference between cells of Plants and Animals.

Components of a cell (Structure & Function) :-

Plasma Membrane

Cytosol

Cell organelles

Cell inclusions

Cell Division:- Mitosis

(Somatic cell division)

Reproductive Cell Division

Fertilization

Meiosis

Transcription

Translation

Specialized Cells of Human Body

Cells and Ageing

Cells of dental interest

Cell Death

Conclusion

References

CELL STRUCTURE & FUNCTION

I. CELL : THE BASIC UNIT OF LIFE:-

• Cell is the basic unit of life, & the structural & functional unit of an organism.

• It is the smallest unit capable of independent existence & performing the essential functions of life.

• All life begins as a single cell. Many organisms are made of a single cell e.g. Amoeba, Chlamydomonas,

Acetabularia, Bacteria, yeast, etc.

• A multi-cellular organism is made of many cells. Here, cells are the building blocks of the body or basic units

of body structure.

Robert Hooke -An English Mathematician & Physicist.

Robert Hooke, in 1665 took a piece of cork of Spanish Oak and prepared thin slices

which was then observed under a microscope.

Honey – Comb structure was observed with a number of box-live compartments,

each having a pore & separated from others by diaphragms.

He called these compartments Cellulae (singular- cellula), now known as Cells

(Latin – cella, meaning hollow space or compartments).

This is mentioned in his book Micrographia, Chapter: ‘Observe XVII

HISTORY : DISCOVERY OF CELL:-

Antonie Philips van Leeuwenhoek:-

He was a Dutch tradesman & scientist.

Father of Microbiology.

He was the first so observe & describe & sketch single celled organism which he

called animalcules, which are now referred to as ‘micro organisms’.

He observed bacteria (1976), protozoa, spermatozoa (1676), Red blood cells,

banded pattern of muscle fibres (1682), the infusoria (protists) (1674).

Robert Brown (1831):-

A Scottish Botanist & Paleobotonist.

Discovered preserve of nucleus in cells of orchid root

Felix Dujardin (1835):-

A French Biologist.

Discovered living semi-fluid substance of cells & called it

Sarcode

.

Matthias Jakob Schleiden (1838):-

A German botanist.

Found all plant cells to have a similar structure – Cell wall, a

clear jelly-like substance & a nucleus.

Theodor Schwann (1838):-

A German physiologist.

Discovered that animal cells lacked cell-wall.

He contributed to the cell theory:-

All living things or organisms are made of cells & their

products.

Defined cell as membrane enlocked, nucleus containing

structure.

Purkinje (1839) & Von Mohl (1838, 1846):-

Renamed Sarcode, or jelly-like substance of cells as

PROTOPLASM (Greek:- protos – first, plasma - form)

• Cell membrane was discovered by

SCHWANN but was provided with a

name by NAGELI & CRAMER (1855).

• Rudolph Virchow (1855) , a

German doctor & biologist was

the first so accept the work of

Robert Remak, who showed the

origins of cells was the division

of pre-existing cells.

• On the basis of organisation of DNA, cells are of 2

types :-

• Prokaryotic

• Eukaryotic

Prokaryotic cells

Characteristics:-

• Nuclear material:- DNA is naked & lies variously

coiled in the cytoplasm, a.k.a GENOPHORE,

NUCLEAR BODY OR NUCLEOID, or prochromosome.

• Additional small circular DNA entities called

plasmids maybe present.

• Nuclear components absent.

• Cell wall presents in bacteria & cyanobacteria but

absent in mycoplasma or PPLOs (pleuro-

pneumonia like organisms).

1) Single stranded flagella/fimbriae may be present.

2) The photosynthetic thylakoids are not organised into chloroplasts & lie freely in cytoplasm.

3) Membrane lined cell organelles like mitochondria, E.R, Golgi apparatus, lysosomes, microtubules,

microfilaments and centrioles are absent.

4) Complex gas vacuoles present.

5) Ribosomes are 70s.

6) One-Envelope system

7) Cyclosis absent

8) Mitotic spindles not formed during cell division.

9) Sexual reproduction absent.

10) Low DNA content

11) Transcription and Translation occur in cytoplasm.

12) Respiratory enzymes usually lie in close contact with cell membrane.

13) Endocytosis & Exocytosis absent.

14) Nitrogen fixation occurs only in some prokaryotes.

Eukaryotic Cells:-

1) Has organised nucleus.

2) Membrane covered cell organelles present.

3) Cell wall present in case of plants, fungi & protists (without

muramic acid.)

4) Flagella if present are 11 stranded with differentiation of

axoneme & sheath.

5) The nucleus is differentiated into nuclear envelope,

chromatin, one or more nucleoli & nucleoplasm .

6) Nuclear DNA is linear, associated with histones, extra

nuclear DNA is commonly circular.

7) Most of the cell DNA lies within the nucleus, a small

quantity is found in the plastids & mitochondria.

8) Transcription occurs in nucleas whereas translation occurs

in cytoplasm.

9) Respiratory enzymes present in cytoplasm & mitochondria.

10.Endocytosis & Exocytosis quite

common.

11.Cyclosis present.

12.Ribosomes are of 80s, 70s ribosomes

occur in plastids and mitochondria.

13.True or Sap vacuoles found.

14.Thylakoids, if present, grouped inside

chloroplasts.

15.Sexual reproduction present.

16.Spindle apparatus produced during

nuclear division.

17.Nucleus contains more than one

chromosome.

EUKARYOTIC CELL

Right: Colorized micrograph of a prokaryotic cell of the

bacterium.

Left: Colorized micrograph of a eukaryotic cell of the

green algae

PLANT CELL ANIMAL CELL

1. Cell wall present.

2. Has a definite form.

3. Larger in size.

4. Cannot change shape.

5. Cannot change position.

6. Plastids present.

7. Chloroplasts(chlorophyll containing

plastids) present.

8. A large center vacuole present

9. Nucleus lies on one side in

peripheral cytoplasm.

10.Nucleus is elliptical.

11.Fewer mitochondria.

12.Plants do not bust if placed in

Hypotonic solution due to cell wall.

1. Cell wall absent

2. Definite form less common

3. Smaller in size

4. Can often change shape.

5. Many cells can change position

6. Plastids absent.

7. Absent.

8. Many small vacuoles present.

9. Nucleus usually in centre.

10.Nucleus rounded.

11.Numerous mitochondria.

12.Unless contractile vacuole present,

animal cells will burst.

13. Centrioles absent.

14. Spindle apparatus nuclear division is anastral.

15. Golgi apparatus consists of a number of distinct or

unconnected units called DICTYOSOMES.

16. Cell cannot do phagocytosis.

17. Lysosomes are rare.

18. Glyoxysomes maybe present

19. A plant cell produces all the materials needed by it.

20. Crystals of inorganic substances occur inside cells.

21. Reserve food is generally starch & fat.

22. A tissue fluid does not bathe cells.

23. Adjacent cells may be connected through

plasmodesmata.

24. Cytokinesis occurs by cell plate.

13. Centrioles present.

14. Spindle is amphiastral.

15. Golgi apparatus is either localised or consists of a well

connected single complex.

16. It can ingest materials through phagocytosis.

17. Lysosomes present.

18. Absent.

19. An animal cell cannot synthesize certain amino acids,

fatty acids, vitamins & coenzymes.

20. Usually do not occur.

21. Usually glycogen & fat.

22. Tissue fluid having Nacl bathes cells.

23. Cells connected through a number of cell junctions.

24. Cytokinesis takes place by cleavage.

PLANT CELL ANIMAL CELL

COMPONENTS OF A CELL:-

1.Plasma Membrane

2.Cytosol

3.Organelles

4.Nucleus

5.Cell Infusions

1)Plasma Membrane:-

i. Also called cell membrane / plasma lemma / bio membrane.

ii. Quasifluid (acts partially like fluid and partially like solid),

elastic, pliable & film-like thin partitions over and inside

cytoplasm.

iii. Average is 75 A (50-100A).

iv. Selectively permeable for solutes but semi permeable for water.

v. Dynamic in nature. Any injured part of Membrane is repaired

within no time.

vi. Appear trilaminar or tripartite under electron microscope.

Composition:-

a) Lipids (20-79%)

b) Proteins (20-70%)

c) Carbohydrates (1-5%)

d) Water (20%)

a. Lipids:-

• Phosphoglycerides or phospholipids.

• Lipid molecules are amphiatic or

amphipathic i.e, they posses both polar

hydrophilic & non polar hydrophobic

ends.

• Hydrophilic region is the head &

hydrophobic part contains 2 tails of fatty

acids.

• Hydrophobic tail usually occur towards the

centre of members resulting in formation

of a lipid bilayer .

b.Proteins :-• They can be fibrous or globular, structural,

carrier, receptor or enzymatic.

• These also posses both polar & non polar side

chains.

• Polar hydrophilic linkages are towards outer

side.

• Non polar hydrophobic linkages are either

kept folded inside or used to established

connections with hydrophobic part of lipids.

c) Carbohydrates:-

• These are branched or unbranched

oligosaccharides

e.g.- hexose, fucose,

hexoamine, sialic acid etc.

FLUID – MOSAIC MODEL:-

1. The plasma membrane has 2 layers (a bilayer) of phospholipids (fats with phosphorus

attached) which at body temperature is like vegetable oil.

2. Membrane is not solid but Quasifluid.

3. Protein molecules occur at places both inside ( intrinsic, integral proteins) and on outer

side of lipid bilayer (extrinsic, peripheral protein )

4. Protein icebergs in a sea of lipids.

5. Integral proteins pass into lipid bilayer & establish hydrophobic bonds with lipid

molecules. Some run through out the lipid bilayer called tunnel protein or

transmembrane proteins.

6. These tunnels form channels for passage of water & water soluble substances, which

posses selective properties for passage of different ions .

7. Proteins are held by both polar & non-polar side chains.

8. The extrinsic proteins are located more so on cytosolic face than

on external face ( e.g. spectrin)

9. Extrinsic proteins are attached covalently to phospholipid head

or non-covalently to transmembrane protein.

10.Proteins provide structural & functional specificity to

membranes

11.Proteins may shift laterally hence providing flexibility &

dynamism to membrane.

12.Proteins may function as enzymes, permeases, carriers.

13.Some lipids & extrinsic proteins present on outer side possess

small carbohydrate molecule to form glycolipids &

glycoproteins. They constitute glycocalyx or cell coat.

14.Conjugated oligosaccharides function as :-

a) Recognition centres.

b) Sites of attachments.

c) Antigens.

d) Provide negative charge to outer surface.

FUNCTIONS OF CELL MEMBRANE:-

1. Compartmentalisation.

2. Protection from injury

3. Providing organic connections between adjacent cells

(plasmodesmata & gap junctions)

4. Providing selective permeability barrier.

5. Transporting solutes.

6. Responding to external stimuli.

7. Energy transduction.

8. Secretory, excretory & waste products thrown out by

exocytosis.

Membrane transport:-

Occurs by:-

a. Passive transport

b. active transport

c. bulk transport

Passive transport :-

• No energy needed to move particles.

• Transport according to concentration gradient.

1. PASSIVE DIFFUSION OR TRANSPORT ACROSS CELL MEMBRANE:-

• Cell membrane plays a passive role.

a) NEUTRAL SOLUTES AND LIPID SOLUBLE SUBSTANCES:-

• Movement through simple diffusion along

concentration gradient, from higher concentration to

lower concentration.

b) Open channel transport :-

Channels in form of tunnel proteins.

Water channel or aquaporins allow water & water

soluble gases (CO2 & O2 ) to pass e.g.:- osmosis.

Filtration is diffusion under pressure through a

membrane.

c. Facilitated diffusion:-

• Occurs through agency of gated ion channels &

permeases.

• Energy not required.

• Transport along concentration gradient.

i) Ion channels :-

Highly specific.

A specific channel for each ion.

Do not move in dissolved state.

Most ion channels are gated.

Depending upon stimulus required for opening the ion

channels gated, they are of 3 types:-

Voltage gated.

Mechanical gated.

Ligand gated.

Movement is according to concentration gradient.

Rate of passage is high.

ii. PERMEASES:-

Function as facilitated pathway for movement of

substances.

Rate of transport in stereospecific.

Saturation effect recorded.

iii. CONTRASPORT:-

Often occurs against concentration gradient.

It’s a membrane transport that accompanies active

transport of some substances. Eg:- glucose (with Na+)

2.ACTIVE TRANSPORT:-

Uphill movement of materials across membrane

where solute particles more against chemical

concentration electro-chemical gradient.

Energy in form of ATP required.

Sodium, potassium, hydrogen, calcium, iodine,

chlorine etc. are some of ions transported.

Carrier proteins-embedded proteins change shape to

open & close passage across membrane.

3) BULK TRANSPORT:-

a) ENDOCYTOSIS:-

i. Pinocytosis.

ii. Phagocytosis.

b) EXOCYTTOSIS:

PINOCYTOSIS or POTOCYTOSIS:

• Minute detachable vesicle formed of fluid matter and substances

dissolves ion it of diameter 100-200 nm.

• Also called cell drinking.

• As soon as solute or ligand particle form complexes with receptor

sites, plasma membrane invaginates.

• Invagination deepens and gets pinched off as a vesicle called

pinosome.

• Pinosome migrates inwards and if digestion is involved then

lysosomes are required.

PHAGOCYTOSIS:-

Also called cell eating.

Detachable vesicles formed

around solid matter, called

phagosomes, by

invagination of plasma

membrane.

Phagosome diameter is 1-2 um.

Fusion with lysosomes to

produce digestive vacuole.

Digested food diffuses in

cytoplasm.

B. EXOCYTOSIS:-

Exportation of substances from

cell via plasma membrane to

extra cellular fluids.

Important mainly in 2 cells:-

(a)Nerve cells for neurotransmitter

release.

(b)Secretory cells- e.g. – insulin

3) CYTOPLASM:-

(A) Cytoplasmic matrix or Cytosol (Hyaloplasm).

• Clear fluid part of cytoplasm which can exist in either

sol and/or gel. (plasmasol, Plasmagel).

Functions:-

1. Providing raw materials to cell organelles for

functioning.

2. Exchange of materials between cell organelles.

3. Biosynthesis of fats, nucleotides, some carbohydrates,

proteins, coenzymes etc.

4. Catabolic activities:- glycolysis, anaerobic respiration,

& pentose pathway type of respiration occurs in

cytosol.

5. Distribution of various materials inside cell via

cytoplasmic streaming.

4) Cell organelles:-

Endomembrane System:-

Endoplasmic reticulum.

Golgi complex

Lysosomes

Vacuoles.

ENDOPLASMIC RETICULUM:-

Discovered by Porter & Thompson (1945).

It’s a 3-D, complicated & inter connected

system of membrane–lined channels running

through cytoplasm, called Cisterns.(Flat

interconnected sac-like).

Types:-

Smooth E.R.:- Ribosomes absent.

A.K.A. Agranular E.R.

Rough E.R. :- Ribosomes present.

A.K.A. granular E.R.

Structure:-

1. Cisternae :- Flat interconnected sac-

like.

2. Vesicles :- oval, rounded sacs,

A.K.A. MICROSOMES

3.Tubules:- Tube-like extension

connected to either cisternae or vesicles.

Functions:-

Provides large surface area for various physiological activities.

Provides mechanical support to colloidal cytoplasmic matrix.

Provides quick intracellular transport.

Provides precursors of different secretory substances to Golgi apparatus.

Gives rise to vacuoles.

Rough E.R stores newly formed / synthesized molecules & forms glycoprotein when sugar group is added.

Smooth E.R is site of synthesis of fatty acids, phospholipids and steroids.

Smooth E.R as SARCOPLASMIC RETICULUM stores Ca++ for release during muscle contraction.

Detoxification of various chemicals.

GOLGI COMPLEX (APPARATUS):-

Made up of smooth membrane saccules or cisternae, a

network of tubules with vesicles and vacuoles.

First seen by George in 1867 but named after Camillo

Golgi who in 1898 recognized its reticular structure.

Functions:-

Secretion - All glandular cells depend upon Golgi

complex for concentrating & packaging their products

inside a soluble protein coat.

Formation of glycoproteins & glycolipids.

Formation of sialic acid and galactose.

Fat transport.

Mediation of hormones produced by endocrine glands.

Formations of lysosomes & secretory vesicles.

LYSOSOMES:- Small vesicles bounded by a single

membrane containing hydrolytic enzymes

in form of minute crystalline or semi

crystalline granules of 5-8 nm.

Are called suicide bags.

Work in acidic pH (pH=5)

Formed by Golgi complex.

Functions :- Intracellular Digestion :- food obtained via

phagocytosis is digested by Lysosomes.

Extracellular digestion by release of enzymes via

exocytosis.

Defence- lysosomes of leucocytes devour foreign

proteins, bacteria & toxic substances.

Autophagy:- In metamorphosis of many animals, certain

embryonic parts are digested for growth of other parts.

Autolysis:-

Formation of Thyroxine- active hormone thyroxin in

thyroid is formed through hydrolysis of thyroglobulin

by lysosomes.

VACUOLES:-

Are non-cytoplasmic areas present inside

cytoplasm separated by specific membrane.

Sap vacuoles– Fluid filled vesicles separated

by tonoplasts.

Contractive vacuoles– occur in algal cells &

protistans.

Food vacuoles – Formed by fusion of

phagosomes & a lysosome.

Air vacuoles–

. Pseudo vacuole.

. Gas vacuoles.

MITOCHONDRIA:-

• Power house of cell.• Contains 2 membranes and 2 chambers, outer &

inner.

• Contains enzymes that catalyse chemical

reactions of cells that generate ATP.

Function:-• Are miniature biochemical factories where

respiratory substrate and food are completely

oxidized to CO2 & water. Energy liberated is initially

stored in form of reduced coenzymes & prosthetic

groups.

• Inner chamber has enzymes for synthesise of

fatty aids.

• Synthesise of many amino acids.

• May store & release calcium when required.

RIBOSOMES:-

(Palade Particle)• Are naked ribonucleo proteins protoplasmic

particles (RNP) to length of 200-340 A & diameter of

170-240 A.

• Function As sites of protein or polypeptide

synthesis.

• Are protein factories.

• Sub spherical in outline.

• Membrane absent.

• Found freely in cytosol and attached to E.R via

SRP protein.

Function:-1. Protein synthesis.

2. Free Ribosome synthesizes protein for use inside

cell.

3. Attached Ribosome synthesize protein for

transport.

NUCLEUS:-

A specialized double membrane bound

protoplasmic body containing all the genetic

information for controlling cellular

metabolism & transmission to the posterity.

Its the largest cell organelle.

Found in the region of maximum metabolic

activity. Commonly situated in the geometric

centre of cell .

Spherical or oval in shape.

a. Nuclear Envelope:-• separates nucleus from cytoplasm.• Made up of lipoprotein & trilaminar

membrane.

• Has pores or perforations which control

passage of substances to inside or outside of

nucleus.

b.Nucleoplasm :-• its transparent semifluid & colloidal

substances filling the nucleus.

• Contains nucleosides & enzymes.

c. Nuclear matrix:-

• A network of fine fibrils of acid proteins

functioning as scaffold for chromatin.

d. Chromatin :-• Hereditary DNA - protein fibrillar

complex.

e. Nucleolus :-• Naked, round or slightly irregular

structure attached to chromatin at the

NOR (nuclear organiser region).

• Has 4 components :-

I. Amorphous matrix.

II. Granular portion (proteins & RNA,2:1)

III. Fibrillar portion (nucleonema)

IV. Chromatin portion.

• Principal site for development of

ribosomal RNA.

• Stores nucleoproteins.

I. Chromatin:-• Hereditary material.

• Bear genes.

• Contains genetic information required for

growth & development.

II. Nucleus controls cell metabolism.

III. Formation of ribosomes in nucleolus.

IV.Directs cell differentiation.

V. Replication of nucleus essential for cell

replication.

FUNCTIONS:-

CHROMOSOMES:

• Rod shaped or thread-like condensed chromatin

fibres which are hereditary vehicles as they store &

transmit coded hereditary information.• There are 2 chromosome halves or chromatids

attached to each other by CENTROMERE.

Chromosomes are of 4 types (based on

position of centromere) :-(a) Telocentric – (centromere terminal at area of

telomere)

(b) Acrocentric – Centromere inner to telomere.

(c) Submetacentric – Centromere submedian(d) Metacentric – Centromere median.

• Chromosomes contain a coiled filament called

Chromonema.

Functions:-

Contains genes. All hereditary information

located in genes.

Control synthesis of structural proteins thus

helping in cell division & cell growth.

Control cellular differentiation.

Can replicate themselves or produce their carbon

copies for passage to daughter cells & next

generation.

Produce nucleoli for synthesis of ribosomes.

Form a link between offspring &

parents.

Determine the sex of individual.

By process of crossing over, they

introduce variations.

Mutations are produced due to change

in gene Chemistry.

CYTOSKELETON:-

• Extremely minute, fibrous & tubular

structures which form the structural

frame-work inside cell.

• 3 main types of protein filaments:-

i. Microfilament.

ii. Intermediate filaments.

iii. Microtubules.

Function:-

a. Maintain the integrity of cells.

b. Cause Cyclosis.

c. Responsible for change in plasma

membrane during endocytosis &

exocytosis.

FLAGELLA & CILIA:-

Fine hair like movable protoplasmic processes of cells

capable of producing a current in fluid medium for

locomotion & passage of substances.

Flagella are longer (100-200 um) but fever.

Cilia are smaller (5-20 um ) but numerous .

Function:-

1) Help in locomotion in flagellate & ciliated organisms.

2) Create current for obtaining food from aquatic

medium.

3) In land animals, cilia of respiratory tract help in

eliminating dust particles in incoming air .

4) Can function as sensory organs .

5) Internal transport - passage of eggs in oviduct, passage

of excretory substances in kidneys.

CENTROSOMES & CENTRIOLES:-

CENTROSOMES:

Dense area of cytoplasm with radiating microtubules.

Contribute in building mitotic spindles.

CENTRIOLES:

Minute submicroscopic microtubular subcylinders with a configuration of 9 triplet fibrils and ability to form

their own duplicates without having DNA and a membranous covering.

FUNCTIONS:

Help in cell division by forming microtubules-

organising centres (MTOCs).

Centrioles can be transformed into basal bodies which

give rise to cilia and flagella.

CELL INCLUSIONS:-

They are non-living substances present in cells

Also called Ergastic Bodies.

Maybe present in soluble or insoluble state, can be organic or

inorganic in nature.

3 categories:

a) Reserve food.

b) Excretory or Secretory products.

c) Mineral matter.

Melanin: present in skin, hair, eyes and meninges for

protection against UV rays.

Glycogen: present in liver, skeletal muscles for quick

energy, break down occurs in liver into glucose.

Triglycerides: present in fat cells, broken down to utilize

ATP.

CELL DIVISION:-

It’s a mean of cell multiplication or formation of new cells from pre-existing cells.

It occurs in 3 ways:-

a) Amitosis

b) Mitosis

c) Meiosis.

In prokaryotes, the cell division is known as Binary Fission.

The cell cycle is divided into 2 major phases:-

a) Interphase

b) Mitotic phase.

INTERPHASE:

Latin – inter – between, Greek – phasis – aspect.

It’s a series of changes that takes place in a newly

formed cell and its nucleus before it becomes

capable of division again, hence it’s called

intermitosis.

It has G1, S, and G2 phases.

G1 phase:

It’s the longest phase (post mitotic gap

phase).

Cell is metabolically active and grows

continuously.

RNA and protein are synthesized.

Nucleotides, amino acids for histone

synthesis and energy rich compound

are formed.

Cell organelles increase in number.

S-PHASE:-

Synthetic phase.

Chromosomes replicate

For this their DNA molecules function as templates

and form carbon copies.

DNA content doubles and duplicate sets of genes

are formed.

New chromatic fibres are formed.

Also known as INVISIBLE PHASE of M-STAGE

since chromosomes prepare for equitable

distribution later on.

Subunits of kinetochores are synthesized.

Centrosomes begin to divide.

G2 PHASE:-

Pre-mitotic phase.

Synthesis of DNA stops.

Formation of RNA and protein

continues

Preparation of cell to undergo cell

division.

MITOSIS:-

Greek – mitos – thread or fibril.

It’s that type of division in which chromosomes replicate

and become equally distributed both quantitively and

qualitively into 2 daughter nuclei so that the daughter

cells come to have the same number and type of

chromosomes as are present in parent cell.

First observed by Strasburger in plant cells (1870).

Boveri and Fleming (1879) in animal cells.

Mitosis occurs in formation of somatic body cells and

hence often called somatic cell division.

Consists of 2 steps:-

a) Karyokinesis

b) Cytokinesis.

KARYOKINESIS:

Indirect nuclear division.

a) PROPHASE: (Greek – pro – first, phasis - stage)

- Early prophase: nucleus becomes spheroidal.

- Viscosity of cytoplasm increases.

- DNA molecules condense to form elongated chromosomes.

Middle Prophase:-

- Chromosomes shift towards periphery, hence a clear

central area is seen.

- Chromosomes shorten and thicken.

- Chromatid threads seen.

- Nucleoli found to be attached to one or more

chromosome.

Late Prophase:-

- Fine fibres start appearing around

nucleus.

- Nucleoli degenerates.

- 2 asters (centriole pairs and astral

rays) come to lie in area of future

spindle poles.

- Spindle poles are organised without

asters in plant cells.

- 2 spindle poles begin to get

connected by fine fibres.

Prometaphase:-

- Nuclear Envelope degenerates.

- Differentiation between cytoplasm

and nucleoplasm disappears.

- E.R and Golgi complex disorganize.

- Spindle apparatus fully organized.

b) METAPHASE:-• Greek: meta- after or second, phases – stage

• Discontinuous fibres coming from 2 spindle poles get connected

to 2 centromere surfaces or kinetochores of each chromosomes by

means of corona having molecular motors.

• Chromosome fibres tighten, which brings chromosomes to

equator of spindle known as Congression.

METAPHASE

c) ANAPHASE:-

• Greek: Ana – up, phasis – stage.

• 2 sub stages, A and B.

ANAPHASE A:-

Centromere of each chromosome divides into 2 so that each

chromatid comes to have its own centromere.

2 chromatids repel and separate to become daughter

chromosomes.

Daughter chromosomes move towards poles of spindle along

their tractile fibrils’ path.

ANAPHASE B:-

At end of anaphase, 2 groups of chromosomes are formed, one

at each pole of spindle.

Spindles elongate.

Spindle fibres disappear from near the poles but remain intact

near the middle.

TELOPHASE:-Greek: telos – end, phasis – stage.

Cytoplasmic viscosity decreases.

Nuclear envelope reappears and

encloses chromosomes.

Chromosomes resume chromatic

fibres.

Nucleoli reappears.

Mitotic spindle breaks up.

CYTOKINESIS (D-PHASE):Greek: kytos – hollow or cell, kinesis –

movement.

Division of protoplast of a cell into 2

daughter cells after nucleus division or

karyokinesis, so that each daughter cell

comes to have its own nucleus.

Mitochondria and plastids undergo

division by cleavage or fission.

Cleavage furrow forms around the

centre of a cell, progresses inwards and

separates cytoplasm into 2 and usually

equal portions.

REPRODUCTIVE CELL DIVISION:• In sexual reproduction, each new organism is produced by union and fusion of 2 different

germ cells, one produced by each parent.

• Gametes (germ cells) are the secondary oocytes produced in the female gonads

(ovaries).

• Sperm, produced in the male gonads (testes).

FERTILIZATION:• The union and fusion of gametes is called Fertilization, and thus the cells are called

ZYGOTE.

• Somatic cells contain 23 pairs of chromosomes that is 46 chromosomes.

• A gamete has 23 chromosomes.

• 2 chromosomes that makes up a pair are homologous chromosomes or Homologs.

• These homologs appear similar except the sex chromosome that is the X & Y

chromosome.

• In females, the homologous pair of chromosomes consists of 2X chromosomes.

• In males, the pair consists of X & Y chromosomes.

• The other 22 pairs of chromosomes are called Autosomes.

MEIOSIS• Greek: meioum or meio – to lessen.

• It’s a double division which occurs in a diploid

cell (or nucleus) & gives rise to 4 haploid cells (or

nuclei) each having half the number of

chromosomes as compared to parent cells.

KARYOKINESIS:

MEIOSIS 1:-

Prophase 1:-

a) Lepotene: Greek: leptos – slender, tainia – band,

Nema – thread.

Nucleus enlarges.

Chromatin fibres become visible.

b) Zygotene: Greek: zygon – yoke or tied, tainia –

band.

Synaptonemal Complex: it’s a ladder like

structure with transverse protein filaments connecting

the 2 lateral elements.

The complex formed by a pair of synapsed

homologous chromosomes are called Bivalent or

Tetrad.

Pachytene: Greek: pachys – thick, tainia –

band.

Crossing over occurs. It’s a process of

exchange of genetic material or chromatid

segments between 2 homologous

chromosomes.

d) Diplotene: Greek: diplos, tainia – band.

Nucleoprotein fusion complex of the

synapsed chromosomes dissolve partially, the

remaining points where chromosomes are

attached is called Chiasmata.

Diakinesis: Greek: dia –

through, kinesis – movement.

Chiasmata shifts towards

end of chromosomes.

This phenomenon is called

Terminalisation.

Nuclear envelope

disintegrates.

METAPHASE 1:-• A colourless bipolar spindle apparatus appears in

region of degenerated nucleus. It consists of fine fibres.

• The fibres converge towards 2 ends called Poles.

• Each chromosome gets detached to spindle pole by

means of tractile fibrils.

ANAPHASE 1:-• The homologs break their connection and separate

out. This process is called Disjunction, and separated

chromosomes are called Dyads.

• Chromosomes move towards spindle poles along path

of their tractile fibrils.

TELOPHASE 1; CYTOKINESIS; MEIOSIS

2:-

These are similar to those that occur in

mitosis.

The net effect of reduction division is that

each resulting daughter cell contain the

haploid number of chromosomes.

SUMMARY OF MEIOSIS:

TRANSCRIPTION & TRANSLATION

Transcription is the process by which the genetic information encoded in DNA is copied onto a strand of RNA.

Three forms of RNA are made from the DNA template:

• Messenger RNA (mRNA), which directs synthesis of a polypeptide chain

• Ribosomal RNA (rRNA), which comes together with ribosomal proteins to make up ribosomes

• Transfer RNA (tRNA), which binds to amino acids during translation. Each tRNA can bind specifically to many of the amino acids.

TRANSCRIPTION:

RNA synthesis involves separation of the DNA strands and synthesis of an RNA molecule in the 5' to 3' direction by RNA polymerase, using one of the DNA strands as a template.

In complementary base pairing, A, T, G, and C on the template DNA strand specify U, A, C, and G, respectively, on the RNA strand being synthesized.

When RNA synthesis is complete, mRNA leaves the nucleus and enters the cytoplasm, where translation occurs.

TRANSLATION Just as a DNA molecule provides the template for making an

mRNA, so mRNA provides a template for synthesizing a protein.

Translation is the process whereby the nucleotide sequence in a molecule of mRNA specifies the amino acid sequence for a protein molecule.

In the mRNA molecule, each set of three consecutive nucleotide bases is called a codon and specifies one amino acid.

KEY EVENTS OF TRANSLATION

SUMMARY OF

TRANSLATION

SPECIALIZED CELLS OF HUMAN BODY

SPECIALIZED CELLS OF HUMAN

BODY

NERVE -Brain, Spinal Cord, Peripheral Nerves

EPITHELIAL - Skin, lumen Of Bodies

EXOCRINE - Secrete Products Through Ducts

ENDOCRINE - Secrete Directly Into Blood Stream

BLOOD - RBC,WBC.

106

CELLS AND AGING:-

Cellular aging is the result of a progressive decline in cellular function

and viability caused by genetic abnormalities and the accumulation of

cellular and molecular damage due to effects of exposure to exogenous

influences.

Aging is a regulated process influenced by a limited number of genes.

1. DECREASED CELLULAR REPLICATION:-

Normal human fibroblasts when placed in tissue culture have limited

division potential.

After a fixed number of divisions, all somatic cells become arrested in

a terminally non-dividing state known as Senescence.

One probable mechanism in human cells is that with each cell

division, there is incomplete replication of chromosome ends

(telomere shortening) which ultimately results in cell cycle arrest.

2. ACCUMULATION OF METABOLIC AND GENETIC DAMAGE:-

Cellular life span is determined by a balance between damage resulting from

metabolic events occurring within cell and counteracting molecular responses

that can repair the damage.

One group of potentially toxic products of normal metabolism are Reactive

Oxygen Species.

They cause covalent irredification of proteins, lipids and nucleic acids.

This maybe an important cause of Senescence.

Free radicals may have deleterious effects on DNA leading to breaks and

genome instability thus affecting all cellular functions.

CELL DEATH:-

1. NECROSIS.

2. APOPTOSIS.

1. NECROSIS:

Focal death along with degradation of tissue by hydrolytic enzymes liberated by cell.

Accompanied by inflammatory reaction.

Osmosis occurs resulting in swelling of cell.

The cell then proceeds to blebbing and this is followed by Pyknosis in which in which

nuclear shrinkage transpires.

The Karyorrhexis occurs in which the nucleus is dissolved into the cytoplasm.

The second path of necrosis is shown to occur after apoptosis and budding where nuclear

breaks into fragments known as Karyolysis.

CAUSES:-

Hypoxia.

Chemical and physical agents.

Microbial agents.

Immunological injuries.

MORPHOLIGICAL TYPES OF NECROSIS:-

• Coagulative necrosis.

• Liquefactive necrosis.

• Caseous necrosis.

• Fat necrosis.

• Fibrinoid necrosis.

APOPTOSIS:-

It’s a pathway of cell death that is induced by a tightly regulated suicide program in which the

cells activate enzymes that degrade the cell’s own nuclear DNA and cytoplasmic proteins.

Initiation of Apoptosis occurs principally signals from 2 distinct pathway.

a) Intrinsic or mitochondrial pathway.

b) Extrinsic or Death receptor initiated pathway.

INTRINSIC PATHWAY:-

• The mitochondria releases cytochrome C into the cytosol which binds to the

protein called apoptosis activating factor – 1 thus creating wheel – like

hexamer called APOPTOSOME.

• Apoptosome is able to bind caspase – 9 which in turn amplifies the death

signal.

EXTRINSIC PATHWAY:-

• Death receptors are members on TNF receptor family which can deliver

the apoptic signals.

• The best known death receptors are TYPE 1 TNF RECEPTOR.

• When death receptor ligand binds to the receptor site, active caspase – 8

enzyme is triggered which mediates the execution phase of apoptosis.

THE EXECUTION PHASE:

• Caspase once activated, cleave an inhibitor of cytoplasmic

DNAse and makes it active.

• This enzyme introduces the characteristic cleavage of DNA

into nucleosome sized particles/pieces.

• Structural components of nuclear matrix is degraded and

fragmentation of nuclei promoted.

• Cell shrinkage occurs.

• Pyknosis occurs.

• Cell membrane shows buds.

• Cell breaks into several vesicles called APOPTIC BODIES.

STEM CELLS

• A cell that has the ability to

continuously divide and produce

progeny cells that differentiate into

various other types of cells or tissues.

• These have the potential to treat

many diseases, including Parkinson's,

Alzheimer's, diabetes and cancer.

Classification of stem cells On the basis of origin

- Embryonic stem cells

- Somatic/ Adult/ Post-natal/ Mesenchymal stem cells

On the basis of source

- Autologous ( Obtained from the same individual)

- Allogenic

- Xenogenic

- Syngenic/ Isogenic (Having the same or closely similar genotypes.)

On the basis of potency

- Totipotent

- Pluripotent

- Multipotent

- Unipotent

117

Sources of stem cells in human dental pulp

• Dental pulp stem

cells (DPSC)

•Stem cells from Human Exfoliated deciduous teeth

(SHED)

•Stem cells from apical papillae (SCAP)

•Periodontal ligament stem cells (PDLSC)

•Dental follicle progenitor cells

Niche – The specialized microenvironment

housing adult stem cells and transient

amplifying cells.

Two types:-

The cervical loop of rodent incisor for epithelial stem cells (EpSC).

A perivascular niche in adult dental pulp for Mesenchymal stem cells (MSC)

Use of stem cells for tooth formation in vitro

and ex vivo. A tooth germ can be created in vitro

after co-culture of isolated epithelial and

Mesenchymal stem cells. This germ could be

implanted into the alveolar bone and finally

develop into a fully functional tooth.

Use of stem cells for tooth formation

Construction of a bioengineered

tooth. The

association of tooth-derived stem

cells with defined

scaffolds in the presence of growth

factors allows the

creation of tooth specific constructs

such as crown and

root of missing parts of an injured

tooth. These

biological constructs could be used

in dental clinics as

substitutes for metal implants,

crowns and restorative

dental materials.

Construction of a bioengineered tooth

DRAWBACKS:

• The use of animal cells for human diseases is

restricted by immune rejection risks.

• A reliable source of Epithelial Stem cells is to be

determined.

• Isolating autologous stem cells requires a source of

easily accessible cells without need for a surgery.

• Procedure is costly, time consuming and

incompatible with the treatment of extensive tooth

loss.

CELLS OF DENTAL INTEREST AND SIGNIFICANCE:

AMELOBLASTS:

• These are the cells that deposit enamel.

• They are 4-5 micrometer in diameter and about 40 micrometer in

length.

• They secret the matrix protein and are responsible for creating and

maintaining an extracellular environment favorable to mineral

deposition.

• Enamel formation begins at early crown stage of tooth development

and involves the differentiation on inner enamel epithelium first at tips

of cusp outlines formed in the epithelium.

• The secretory end of ameloblasts ends in a 6 sided pyramid-like

projections known as TOMES’ PROCESS.

FUNCTION:

Amelogenesis- formation of enamel

Functional stages in the life cycle of

Ameloblasts

PRESECRETORY STAGE:

1. Morphogenic stage

2. Differentiation stage.

SECRETORY STAGE

MATURATION STAGE:

1. Transitional phase

2. Maturation proper.

CLINICAL CONSIDERATIONS

STAGE OF AMELOBLAST

Formative stage

Maturative stage

Protective stage

Desmolytic stage

DEFECT

Matrix formation is affected, enamel hypoplasia.

Hypo calcification of enamel.

Enamel resorbed or covered by a layer of cementum.

Failure of tooth eruption.

ODONTOBLASTS:

• Most distinctive cells of dental pulp.

• Are arranged in a PALISADE PATTERN in crown of mature tooth.

• Forms a layer lining the periphery of the pulp and have a process

extending into the dentin.

• Are columnar in shape and measure approx. 50 nm in height (in

crown of fully developed tooth.)

• In mid portion of pulp, they are more cuboid, and in the apical part,

more flattened.

• FUNCTION:

• Dentinogenesis: formation of dentin

• They also secret tertiary dentin when irritated.

FIBROBLASTS:

• These cells are greatest in number in the pulp.

• Stellate in shape.

• With age, the fibroblasts become flattened, spindle

shaped with dense nuclei.

• FUNCTION:

• They make the structural fibers and the ground

substances.

• They are responsible for collagen fiber formation.

UNDIFFERENTIATED ECTOMESENCHYMAL CELLS:

• These represent the pool from which connective tissue cells of

the pulp are derived.

• Depending upon the stimulus, these cells may give rise to

odontoblast & fibroblasts.

• Found throughout the cell rich area & pulp core & often are

related to blood vessels.

• Appear as large polyhedral cells possessing a large, lightly

stained, centrally placed nucleus.

CEMENTOBLASTS:

• Soon after HERTWIG’S EPITHELIAL ROOT SHEATH becomes interrupted

and ectomesenchymal cells from the inner portion of dental follicle

then can come into contact of predentin; infiltrating dental follicle

cells receive a reciprocal inductive signal from the dentin and/or the

surrounding HERs cells transform into ameloblasts.

• They form follicle cells around root.

• FUNCTION:

• CEMENTOGENESIS: formation of cementum.

• They synthesize collagen and protein polysaccharides which make

up the organic matrix of cementum.

CEMENTOBLASTS 142

OSTEOBLASTS:

• Are mononucleated cells.

• Arise from pluripotent stem cells.

• Are plump, cuboidal cells (when very active) or slightly

flattened cells.

• Are responsible of production of organic matrix of bone.

• They cover periodontal surface of alveolar bone.

CEMENTOCLASTS:

• Resemble osteoclasts.

• Occasionally found in normal functioning periodontal ligaments.

• Are mononucleated cells and cause resorption of cementum.

OSTEOCLASTS:

• Are large and multinucleated cells formed by

fusion of precursor cells similar to circulating

monocytes.

• These are cells that resorb bone.

• Occupy the bays in bone (HOWSHIP’S LACUNAE)

or surround the ends of a bone spicule.

• Resorption of bone occurs in 2 stages:

First the mineral is removed from the narrow zone

of/at the bone margin, then the recognizable

exposed organic matrix is disintegrated.

CONCLUSION

All living things are made of cells.

Cell is the structural and functional unit of all living things.

Cell biology research has reached great heights, whereby it has

become possible to maintain, grow and manipulate cells outside

the living organism by genetic engineering techniques which have

been applied in numerous fields, e.g.: Research, Biotechnology and

Medicine.

REFERENCES

• Trueman’s Elementary Biology volume 1.

• Ten Cate’s – Oral Histology, 6th edition.

• Orban’s – “Oral histology and embryology”,11th edition.

• Robbins and Cotran’s Pathologic Basis of Disease, 8th edition.

• http://www.nature.com/cdd/journal/v16/n1/pdf/cdd2008150a.pdf

• Santos A. Susin; Daugas, E; Ravagnan, L; Samejima, K; Zamzami, N; Loeffler, M; Costantini, P; Ferri, KF et al. (2000). "Two Distinct Pathways Leading to Nuclear Apoptosis". Journal of Experimental Medicine 192 (4): 571–80. doi:10.1084/jem.192.4.571. PMC 2193229. PMID 10952727