Cell DivisionBy- Dr. Saurabh

Anand ThawraniGuided by- Dr. R.S. Rathor Dr. Rashmi Phulari

Contents• Introduction• Types of cells• Types of cell divison• Cell cycle• Mitosis• MeiosisMeiosis IMeiosis II• Applied aspects

IntroductionWhat is it?

The process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division: a vegetative division, whereby each daughter cell is genetically identical to the parent cell (mitosis), and a reproductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half, to produce haploid gametes (meiosis).

• Cell division is an inherent property of living organisms. It is a process in which cells reproduce their own kind.

• The growth, differentiation, reproduction and repair take place through cell division.

• Rudolf Virchow(1858) suggested ”Omnis cellula e cellula” means every cell is derived from pre existing cell.

• There are two types of cell division namely Mitosis and Meiosis.

Types of cell• The cell (from Latin cella, meaning "small room") • The basic structural, functional, and biological unit

of all known living organisms.• A cell is the smallest unit of life that

can replicate independently, and cells are often called the "building blocks of life".

• Prokaryote is a single-celled organism that lacks a membrane-bound nucleus (karyon), mitochondria, or any other membrane-bound organelle.

• Eukaryote is any organism whose cells contain a nucleus and other organelles enclosed within membranes.

Types of cell division

Cell cycle•The cell cycle is the sequence of events or changes that occur between the formation of cell and its division into daughter cells.•It has a nondividing, growing phase called Interphase and dividing phase called mitotic or M-phase.

Interphase ( inter – between, phases – aspect) :metabolically active phase

• Longest phase in the cell cycle• Cell grows in size• Doubles cytoplasmic organelles• Duplicates chromosomes• Three stages: G1, S, G21. G1: cell growth2. S: synthesis of DNA (chromosome

duplication)3. G2: cell growth and preparation for

division

G1 Phase or GAP 1 Phase :• Period in the cell cycle during interphase,

after cytokinesis and before the S phase.• The cell is metaboically active,• There is great amount of protein and

enzymes synthesis, • There is NO DNA REPLICATION

S-Phase or Synthesis Phase :• Period in the cell cycle during interphase,

between G1 phase and the G2 phase.• DNA synthesis or replication occurs.

G2 Phase or GAP 2 Phase: • The final, and usually the shortest phase during interphase• Cell undergoes a period of rapid growth to prepare for M phase.• The nucleus is well defined, bound by a nuclear envelope and

contains at least one nucleolus.• At the end of this phase is a control checkpoint (G2 checkpoint) to

determine if the cell can proceed to enter M phase and divide. • G2 checkpoint prevents cells from entering mitosis with DNA

damaged since the last division, providing an opportunity for DNA repair and stopping the proliferation of damaged cells so that the G2 checkpoint helps to maintain genomic stability

G 0 Phase (Quiescent stage) : • Period in the cell cycle where cells do not divide further and exist in

a quiescent state. • This usually occurs in response to a lack of growth factors or

nutrients.• Cells in this stage remain metabolically active but no longer

proliferate. This is a very common phase for most mammalian cells. Cells that are permanently in the G 0 phase are called post mitotic cells.

Mitotic or M-phase• The most dramatic period of the cell

cycle, involving a major reorganisation of virtually all components of the cell.

• Since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division.

• It includes two important processes that occur simultaneously.

Karyokinesis (division of the nucleus)

Cytokinesis (division of the cytoplasm)

• Resulting in two daughter cells.

Mitosis(Gr. Mitos – thread, osis – stage)• Walter Flemming (1882) studied mitotic cell

division in animal cells and coined the term mitosis.• Mitosis is a type of cell division in which a parental

cell produces two similar daughter cells that resemble the parental cell in terms of chromosomal number. Hence called Equational cell division (homotypic cell division).

• This maintains constant number of chromosomes in each cell of successive generation.

• It occurs in somatic cells of the body. Also called somatic cell division.

• Mitosis occurs in two stages viz., Karyokinesis and Cytokinesis.

Karyokinesis (Karyon – nucleus, kinesis – movement)The division of nuclear material.

• Occurs in four stages1. Prophase (Gr. Pro – before, phases –

appearance)2. Metaphase (Meta – after, phase –

appearance)3. Anaphase (ana – up, phases -

appearance)4. Telophase (Telo – end, phases –

appearance)

Prophase (Gr. Pro – before, phases – appearance)• It is the longest phase.• The chromatin is organized into distinct

chromosomes by coiling or spiralization.• The centrioles develop into asters and

move towards the opposite poles of the cell to establish the plane of cell division.

• Spindle apparatus begin to appear• Nucleolus and Nuclear membrane

disintegrate and disappear• The chromosomes are set free in the

cytoplasm.

Centromere : It is the primary constriction in chromosome to which the spindle fibres attach during mitotic and meiotic division. It appears as a constriction when chromosomes contract during cell division. After chromosomal duplication, which occurs at the beginning of every mitotic and meiotic division, the two resultant chromatids are joined at the centromere.

Spindle fibres : It is a group of microtubules that extend from the centromere of chromosomes to the poles of the spindle or from pole to pole in a dividing cell.

Sister chromatids : During S phase of the cell cycle the DNA is replicated and an identical copy of the chromatid is made. These identical copy of chromatids are called sister chromatids.

Kinetochore : These are disc shaped structures present on the sides of centromere.

Metaphase (Meta – after, phase – appearance)

• Spindle fibres are completely formed• The chromosome become short and thick

with two distinct chromatids each• All the chromosomes move towards the

centre of the cell and arrange in the equatorial plane, right angles to the position of asters to form metaphasic plate

• Chromosomes are attached to spindle fibres at their centromeres

Metaphase plate : The plane of the equator (a plane that is

equally distant from the two spindle poles) of the spindle into which chromosomes are positioned during metaphase

Anaphase (ana – up, phases - appearance)

• The centromere of all the chromosomes undergo longitudinal splitting and the chromatids of each chromosome separate to form daughter chromosomes

• The daughter chromosomes move toward the opposite poles from the equator by the activity of spindle fibres

Telophase (Telo – end, phases – appearance)• During this, the events of prophase will be reversed• The daughter chromosomes reach the opposite poles• The chromosomes undergo despiralization to form

long, thin thread like structures called chromatin• Nucleolus and nuclear membrane reappears• The spindle fibres disappear

Cytokinesis (cyto – cell, kinesis – movement)The Division of cytoplasm

• A cleavage furrow develops in the middle of the cell in centripetal direction due to the contraction of microtubules.

• It occurs till the edges of the plasma membrane meet.

• They fuse to form a separate membrane.

Overview

Significance of Mitosis:• It maintains genetic stability within the population of cells derived from same parental cell• It helps the growth and tissue repair• It helps in the replacement of dead and worn out cells• It is a means of reproduction in lower organisms

SUMMARY• Type of cell division which takes places during

growth, repair and replacement.• Takes place in somatic cells.• Basis of asexual reproduction.• Daughter cells identical in chromosome

number and genetic make-up at the parent cell.

• Ensures consistency of genetic information in nuclei of cells of one individual.

• Chromosomes don’t arrange themselves along equator in homologous pairs during metaphase.

• Chromatids separate at anaphase.

MEIOSIS• Derived from the Greek word meaning 'lessening'.• The term meiosis (originally spelled "maiosis")

was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905

• This is a special method of cell division, occurring in maturation of the sex cells, by means of which each daughter nucleus receives half the number of chromosomes characteristic of the somatic cells of the species.

• It is also called reductional division• Two successive stages namely Meiosis I and

Meiosis II

• Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.• Meiosis involves pairing of homologous

chromosomes and recombination between them.• Four haploid cells are formed at the end of

meiosis II.• Homologous Chromosomes : Homologous

chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis and contain the same genes at the same loci but possibly different genetic information, called alleles, at those genes.

Sequence Of EventsMeiosis I

(reductional division)

Karyokinesis IProphase I

a) Leptoteneb)Zygotenec)Pachytened)Diplotenee)Diakinesis

Metaphase IAnaphase ITelophase I

Cytokinesis I

Meiosis II (mitotic meiosis)Karyokinesi

s IIProphas

e IIMetaphase II

Anaphase II

Telophase II

Cytokinesis II

MEIOSIS IIt is reductional division in which a diploid parental cell produces two haploid daughter cells. Hence it is called reductional division

Prophase I; longest phase (5 stages)a) Leptotene: (bouquet stage)• The chromatin condenses to form chromosomes• The chromosomes appear as long, thin and thread like

structures. They undergo coiling and become short and thick.• Each chromosome has two chromatids that are not distinctly

visible.• Each chromosome shows bead like structures called

chromomeres.• The telomeric ends of all the chromosomes converge towards

one side of nuclear membrane; therefore they appear horse shoe shaped. This stage is called Bouquet stage.

• Centrioles form into asters and keep moving towards opposite poles.

• Spindle apparatus begin to appear

b) Zygotene (Zipper stage):• Pairing of homologous chromosomes takes place called

synapsis. The pair is called bivalent.• The chromosome continues to undergo condensation and

asters keep moving towards opposite poles.

c) Pachytene (Tetrad stage):• The chromosomes become more short and thick• Each bivalent shows four chromatids called tetrad• In this stage the exchange of genetic material takes place

between the non sister chromatids of homologous chromosomes.

• This process is called genetic crossing over.• This results in genetic recombination which is responsible

for variations.• The region at which the crossing over occurs is called

chiasma

d) Diplotene:• The chiasma move towards the tips of

chromosomes as the homologous chromosomes of bivalent start moving apart. This event is called Terminalisation

e) Diakinesis:• The chromosomes at this stage appear

thick, short and distinct• The tips of some chromosome show

chiasma• Nucleolus and Nuclear membrane

disappear• Chromosomes set free in the cytoplasm

Synapsis: Pairing of the homologous chromosomes is called Synapsis

Bivalent: A paired unit formed of homologous chromosomes consisting of a paternal and a maternal chromosome is called Bivalent

Tetrad: Each homologous chromosome pair (bivalent) shows four chromatids called tetrad

Crossing over: Exchange of identical parts between the nonsister chromatids of homologous chromosomes is called Crossing over.

Chiasma: These are the regions on homologous chromosomes at which crossing over occurs

Metaphase-I• Chromosomes are arranged in the equatorial

region with their centromeres towards the poles and arms towards the equator.

Anaphase-I• The Centromeres do not under go longitudinal

splitting• The chromosome of each homologous pair move

towards opposite poles by the activity of spindle fibres. This is called separation or disjunction of chromosomes.

Telophase-I• The homologous chromosomes separate and reach the opposite

poles• The nuclear membrane reappears around the chromosomes at

each pole• The spindle fibres disappear• Cytokinesis occurs

Cytokinesis 1- It is the division of cytoplasm.

Interkinesis-The Interphase after the first meiotic

division is called Interkinesis.It may be present or absent between

meiosis-I and meiosis-II.If present it may be short or in some cases

telophase-I directly enters to prophase-II.It is similar to Interphase except for

the absence of replication of DNA.

Meiosis II Occurs soon after meiosis-I. There is no duplication of chromosomes.Hence called mitotic meiosis

Prophase-II• The chromosomes start condensing again• Spindle apparatus begin to appear• The nuclear envelope and nucleolus disintegrate

and disappear

Metaphase-II• The Chromosomes arrange in the equatorial

region at right angles to the asters• The Spindle fibres connect to the centromere.

Anaphase-II• The centromeres of all the chromosomes undergo

longitudinal splitting.• The chromatids of each chromosome separate and

they move towards opposite poles.

Telophase-II• The chromosomes arrive at the poles and undergo

decondensation to become thin and long chromatin fibres.• A nuclear envelope is formed.• Nucleolus also appears• The spindle fibres disappear

Overview (Meiosis I)

Overview (Meiosis II)

Significance of Meiosis• It helps to restore diploidy and

maintain the constant number of chromosomes for a species.

• Meiosis produces new combination of chromosomes and genes by crossing over and by a random distribution of paternal and maternal chromosomes to daughter cells.

SUMMARY• Location - gonads.• Reduction division - results in halving number chromosomes.• Cell division involved in gamete production.• Four daughter cells formed. Male - all four develop. Female -

three abort, one develops.• Two divisions occur.• Chiasma formation leading to crossing over occurs prophase I

resulting in a mixing of genetic material of the chromosomes.• Metaphase I - bivalents arrange themselves on equator of

cell. Results in further variation as random which may position themselves.

• Anaphase I - chromosomes separate.• Daughter cells formed all genetically different from each

other and parent cell

Applied aspectsDefective Mitotic Division• It usually results in the production of

uncontrolled growth of cells or defective cells.

• Uncontrolled Mitosis• If the genes that regulate the cell cycle

are damaged or mutated, cell divides abnormally or may lead to uncontrolled cell growth.

• Cause may be due to Harmful ray: nuclear radiation, UV ray, Viruses, Carcinogenic chemicals: food additives, benzo (α) pyrene in cigarette smoke.

Micronuclei• Micronuclei are extranuclear cytoplasmic bodies. • They are induced in cells by numerous genotoxic agents that damage

the chromosomes.• The damaged chromosomes, in the form of acentric chromatids or

chromosome fragments, lag behind in anaphase when centric elements move towards the spindle poles.

• After telophase, the undamaged chromosomes, as well as the centric fragments, give rise to regular daughter nuclei. The lagging elements are included in the daughter cells, too, but a considerable proportion is transformed into one or several secondary nuclei, which are, as a rule, much smaller than the principal nucleus and are therefore called micronuclei.

• Bigger micronuclei result from exclusion of whole chromosome following damage to the spindle apparatus of the cell, whereas smaller micronuclei result from structural aberrations; causing chromosomal fragments.

Defective Meiotic Division• Defective meiotic divisions usually result in the

production gametes with chromosomal numerical abnormalities

• Down Syndrome - trisomy of chromosome 21• Patau Syndrome - trisomy of chromosome 13• Edward Syndrome - trisomy of chromosome 18• Klinefelter Syndrome - extra X chromosomes in

males - i.e. XXY, XXXY, XXXXY, etc.• Turner Syndrome - lacking of one X chromosome

in females - i.e. X0• Triple X syndrome - an extra X chromosome in

females• XYY Syndrome - an extra Y chromosome in

males.

References•  http://www.biologyreference.com/Gr-Hi/History-of-Biology-

Cell-Theory-and-Cell-Structure.html#ixzz4FChB53a6• Journal of Oral and Maxillo Facial Pathology Vol. 12 Issue 1

Jan - Jun 2008. Clinico-pathological correlation of micronuclei in oral squamous cell carcinoma by exfoliative cytology .Devendra H Palve, Jagdish V Tupkari.

• Vander’s human physiology 8th edition• Kahel-frotcher- atlas of human anatomy 10TH edition.• Color Atlas of Cytology, Histology, and Microscopic

Anatomy4th edition, revised and Enlarged Wolfgang Kuehnel, M.D.Professor Institute of Anatomy Universität zu Luebeck Luebeck, Germany

• Ganong’s Medical Physiology - 21 Edition• Textbook of Medical Physiology 11th edition Arthur C. Guyton,

M.D.

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