Cell Division, Chapter Notes, Class 11, Biology PDF Download

Cell Division 

INTRODUCTION

W.Flemming at first studied mitotic division in Salamander.
Strasburger discovered meiosis.
Strasburger gave name prophase, metaphase, anaphase, telophase.
Meiosis name was given by Farmer and Moore.
It is important for development, regeneration and reproduction.
Hormone cytokinin increases rate of cell division.

TYPES OF DIVISION
Amitosis
Mitosis
Meiosis

Amitosis (No spindle, No chromosome)

It is the simplest mode of cell division at first described by REMAK (1841).
This type of division starts with elongation of nucleus.
Nucleus becomes dumbbell shaped, and get divided into two daughter nucleus.
Nucleur division is followed by the division of cytoplasm its results in formation of two daughter cells.
In this division, no spindle formation and no distinct chromosome formation occurs. Nuclear
evelope remains intact. The daughter cells are approximately the two equal halves of a parental cell.

e.g.PPLO, Blue-green algae, Bacteria and Eukaryotic cells. Examples are yeast-budding occurs by amitosis.
Ameoba multiple fission occurs by amitosis.
Paramecium division of meganucleus.
Mammals-growth of foetal membranes (amnion, chorion, allantois, yolk sac)
Division of mitochondria and chloroplasts.

MITOSIS

Mitosis was discovered by Flemming in 1879 in animal cell and in plants cells by Strasburger in 1875.
Mitosis is a cell division in which parent cell divide to form two daughter cell, in which number of
chromosome, amount of DNA, number and types of gene are equal to parent cell.
It occurs in somatic cell (n, 2n, polyploid any).
It is called indirect division.
Mitosis results in increase in number of cells in the body.
Mitosis cell division can be divided into two phases.
G Interphase G Division phase or M phase

Interphase / Cell cycle

Interphase is phase between two cell division. group into three sub stages G1 S and G2.
In this phase cell prepares itself for division so, called preparatory phase.
It is longest phase of cell cycle. It get completed approx. in 19-22 hours.
It is also called resting phase. (previously)
In this phase, cell remain metabolically extremely active.
In this phase synthesis of protein, enzyme, DNA and RNA takes place
Centrosome (Centriole) duplicates into two. Thus two centrosome (4 centriole) are formed.
Cell cycle can remain arrested only G1 phase. Then G1 is called as G0 phase. G0 phase found in cells of permanent tissue.

Cell cycle is running by a group of special proteins “Cyclins and Cdks (MPF), (Nurse, T-Hunt & Hartmann 2001 studies on sacchromyces (Baker yeast))

Three sub stages of interphase :
G1 phase [6-15 hr]- Most variable phase for duration.
Cell growth occurs in this phase, so most probably it is longest stage of interphase.
Mitochondria, Chloroplasts (plants), Lysosomes, Ribosome, Endoplasmic Reticulum Golgi Complex,
Vacuoles etc. are produced.
Structural and functional proteins are formed.
Nucleolus produces rRNA, mRNA and tRNA. (RNA synthesis may inhibited by actinomycin).
Metabolic rate of the cell becomes very high.
It may be called pre DNA synthesis phase.
S-phase [6-10 hr)- (Longest phase in human interphase)
Replication of DNA takes place. (DNA get doubled)
Protein molecules called histones are synthesized that cover each strand of DNA.
Centrosome (Centriole) replicate is late S-phase G2-phase [3 - 12 hr]
Tubulin protein synthesis start for spindle formation.
This phase may be called post DNA synthesis phase.
Cell division involves enormous expenditure of energy thus cell stores ATP in G2 phase
After G2 phase cell enters in division or M-phase

CAUSE OF MITOSIS

Kern plasm theory : Hertwig, says that mitosis occurs due to disturbance in karyoplasmic index (KI) of cell.



Small cell  less cytoplasm  High KI  No division it show nucleus control the activity of cytoplasm and no division occur.

Large cell  More cytoplasm  Low KI  Division occur. It cause to loose control of nucleus on cytoplasmic metabolism so large cell divide into two Surface area volume Ratio : It says that when cell grows in size its volume increases but surface area remain less so it affect metabolic activity of cell which result into division of cell.

Note : Above two hypothesis regarding the division of cell are completely discarded because new concept give the genetic control of cell division.

CELL DIVISION CONTROL

A cell reproduces by performing an orderly set sequences of irreversible events, In which it duplicates It’s contents & then divides into two, these events are known as cell cycle.

Molecular biologists, identifying the biomolecules, that control or drive the cell cycle, many biologists, some of whom worked with invertebrate or frog egg’s others with yeast cell or cell culture.
Scientists concluded that the activity of enzymes, known as cyclin dependant kinases. (Cdk’s) regulates the cell cycle.
They are activated when they combined with key protein called cyclin.
Kinase is an enzyme that removes a phosphate group from ATP & add to another protein.

1 a kinase enzyme combines with cyclin & this moves the cell cycle forwardly.

S-kinase is capable of starting the replication of DNA after it combined with S-cyclin (G1 - Cyclin). After some time S-cyclin is destroyed & S-kinase is no longer active.

M-kinase is capable of turning on mitosis after it has bind with M-cyclin, (G2-cyclin).

The detail of cell cycle varied from organism to organism & different time in an organism. However certain characteristics are universal component of cell cycle control.

Genes

CDC2, CDC28-designated in Budding yeast cdc2, cdc-28, designated in fission yeast

Division Phase

In this phase nuclear and cytoplasmic division takes place.
Karyokinesis- Nuclear division.
Cytokinesis-Cytoplasmic division.

Karyokinesis

It is divided into four phases –
G Prophase G Metaphase G Anaphase G Telophase

Prophase

It is the longest phase of karyokinesis.
Chromosomes appear as pairs of chromatids joined by centromere (chromatin condensation start).
The nuclear membrane disintegrate and disappear into the cytoplasm.
Nucleolus start disappearing.
Each centrioles Separates and start to move towards the opposite pole of the cell.
Around each centriole astral rays are formed in the cytoplasm. due to (gelation of protein).
Anastral mitosis - In plants, centrioles are absent and no asters are formed. Mitosis without asters is known as anastral mitosis.
Amphiastral Mitosis - In animals, the asters are present and the mitosis is described as amphiastral, or astral mitosis.

Metaphase

The chromosome arrange at the equatorial plane.
Each centromere is joined by two chromosomal fibre or kinetochore spindle one from each pole.
Some other fibres of the spindle extend from one pole to the other pole. These are known as continous fibres or non kinetochore

spindle

Centromere lies at equator and arms remain directed towards poles.
Chromosomal fibre have polarity i.e. + end at equator and – end at the pole.
In metaphase each chromosome splits lengthwise upto the centromere (division of matrix of chromosome). Thus replicated chromatids clearly visible at metaphase stage.

Anaphase (Smallest stage)

The fibres which occur in between the chromosomes are called interzonal/non kinetochore spindle fibres.
In early anaphase interzonal fibres (small and contracted) appears at equator of cell.
Centromere of each chromosome splits lengthwise (division of centromere).
Number of chromosome becomes double in cell during mitotic anaphase.
Interzonal fibres expands and they push chromosomes towards the oppsite poles.(Pushing)
Chromosomal fibres contract and they pull chromosome towards opposite poles. (Pulling)
By pulling and pushing mechanism chromosomes rapidily move towards the opposite poles.
Approximately 30 ATP are required to carry a chromosome to pole. Chromosomes reach at poles in late anaphase.
At this phase cytokinesis process starts.
Telophase (Reverse prophase)
The daughter chromosomes with their centromere at the poles begin to uncoil and lengthen. They aggregate together to form a mass at the poles.
The nucleolus reappear.
New nuclear membrane develops around the chromosomes from the elements of the E.R.
Spindle and astral fibre are absorbed in the cytoplasm.
Thus two daughter nuclei are formed and they have the appearance of the interphase nuclei.
 

Cytokinesis

After nuclear division (Karyokinesis) cytoplasm get divided into two (more or less) equal parts, this results in formation of two daughter cells from a single parent cell.
The cytoplasmic division differs in animal and plant cells.
Cytokinesis starts in late anaphase.
In animals cytokinesis occurs by constriction & furrow formation. Microtubules and microfilaments arrange on equator to form midbody.
Contraction occurs in midbody and Plasma membrane starts constricting to form contratile ring.
Thus a furrow forms form the out side to inside in cell. Furrow deepens continuosly and ultimately a cell divides into two doughter cells.
In animals cytokinesis occurs in centripetal order.

Mid body

Cytokinesis in plants : Takes place by cell plate formation because constriction is not possible due to presence of the rigid cell wall.
Phragmoplast Cell Plate
Many golgi vesicles arrange themselves on equator to form phragmoplast. ER and Fragmets of spindle fibres also collect on equator. Collectively this structure is known as Cell plate.
Golgi vesicles secrete calcium and magnesium pectate. Further cell plate is modified into middle

lamella.

In plants, cytokinesis occurs in centrifugal order (cell plate formation is form centre to periphery).

SIGNIFICANCE OF MITOSIS

Identical gene composition In mitotic cell division daughter cell contain the same number of chromosomes as the parent cell.
The daughter cells carry the same hereditary information as is in the parental cells.
There is no variation in genetic information.
It gives a genetic stability within a population.

Growth

Mitotic cells division is responsible for growth in an organism.

Cell Replacement

Mitotic cell division is responsible for the replacement of lost cells , healing of wound.
Regeneration and Asexual reproduction
In many lower animals mitotic cell division is responsible for regeneration and asexual reproduction.

MODIFICATION OF MITOSIS

Cryptomitosis of Promitosis :- It is a primitive type of mitosis. In this type of divsion, nuclear membrane does not disappear but remain intact throughout the division.
All the changes of karyokinesis occurs inside nucleus even the formation of spindle (Called as intranuclear spindle). eg. some protozoans (Amoeba) during Binary fission.
Dinomitosis :- Dinomitosis founds in dinofilagellates, which are mesokaryotes. In mesokaryotic cells histones are absent. Because of this, the chromosomes fail to condense properly and hence are not distinctly visible during cell division.
Nuclear membrane persists throughout the cell division and so spindle formed is intranuclear type.
Free nuclear division :- Karyokinesis is not followed by cytokinesis so such divisions lead to coenocytic condition. eg. endosperm, fungi of phycomycetes group.
Endomitosis :- This is duplication of chromosomes without division of nucleus. Endomitosis leads to polyploidy. i.e. Increase in number of sets of chromosome.
Colchicine induces polyploidy in plants. Colchicine is a mitotic poison as it arrest the formation and arrangement of spindle fibres.
Endoreduplication :- Endoreduplication is a modification of endomitosis. The polytene chromosomes form by process of endorduplication. In endoreduplication, the chromatids (DNA) replicate but do not get seprated. This process is also known as polyteny.
Mustard gas and Ribonucleases are also mitotic poisons.

MEIOSIS (TERM BY FARMER & MOORE)

It occurs in male and female germ cells, such cells are called meiocytes.
In this type of division parent cell divides to produce four daughter cell (genetically different).
It is a reduction division in which the number of chromosomes (genetic matter) is reduced to half i.e. diploid number (2n) becomes haploid (n).
It is responsible for transmission of hereditary information from generation to generation.

It is divided into two main phases.
Meiosis I : Heterotypic division or reduction division.
It leads to reduction in chromosome numbers. Division of chromosome does not occurs in meiosis-I and only segregation of homologus chromosomes takes place.
Meiosis II : Homotypic division or equational division.
It does not leads to any change in chromosome number.
Meiosis II is just like mitosis. Division of centromere occur in meiosis II.
In Meiosis, division of nucleus takes place twice but division of chromosome occurs only once.
Prophase - I (Longest and complex step of meiosis)

It is divided in five sub - stages :

G Leptotene G Zygotene G Pachytene
G Diplotene G Diakinesis

Leptotene

The volume of the nucleus is increased and chromatin condense to from chromosome.
Chromosomes becomes distinct, long and uncoiled. (Longest and Thinest)
On chromosomes contain a series of beaded structure appear called chromomere.
The centrosome divides into two.
The ends of all chromosomes are directed towards cetriole on one side of nucleus forming bouquet.
At this stage orgamism shows a peculiar type of orientation of chrmosomes - animals show bouquet

type while plants synizesis type.
Zygotenes
Two homologous chromosomes approach each other and being to pair, this pairing is called synapsis.
Each pair consists of a maternal chromosomes ( the chromosomes of the mother) and a paternal
chromosomes (the chromosomes of the father). The pair so formed are known as bivalents.
The chromosomes become thicker and shorter.
There develops a structure in between homologous chromosomes, Which is termed as synaptonemal complex. Synaptonemal complex is composed of three thick lines of, DNA and proteins.
According to Mosses (1956) synaptonemal complex helps in pairing and chiasmata formation.
About 0.3% DNA is synthesized in Zygotene substage.This DNA is used in chromosome pairing or
synapsis. (Zyg. DNA)
The centrosome move to the opposite poless.
Pachytene
Each individual chromosome of each bivalent begins to split longitudinally into two similar chromatids.

As a result, each bivalent now contains four chromatids. This stage is called tetrad stage.
Due to increased attraction, homologous chromosomes tightly coil around each other.
Both the chromatids of a chromosome are called sister chromatids and each chromosome is called Dyad.
Nonsister chromatids of homologous pair develops recombination nudules and exchange their parts
i.e. crossing over. Crossing over was discovered by Morgan and Castle in Drosophila.
Endonuclease first breaks the nonsister chromatids at the place of recombination nodule.
Nonsister chromatids reunite after exchanging their parts (by Ligase) As the result of crossing over
cross like structures - chiasmata (discovered by Janseen) form in bivalent.

Diplotene

In this stage, the homologous chromosomes repel (desynapsis) each other. The two homologous chromosomes thus separate from each other. So chiasmata become visible.
By the end of this stage, the chiasmata begin to move alone the length of the chromosomes from the centromere towards the end. This displacement of chiasmata is termed as terminalization.
Diplotene (Dictyotene) may last long up to month or year (12 to 15 years in Human Female)

Diakinesis

Terminalization is completed in this stage.
The bivalents tend to repel each other and migrate to the periphery of the nucleus just inside the nuclear membrane.
The nucleolus disappears.
The nucleus membrane also begins disintegrate and disappear.
Spindle fibres make their appearance in the cytoplasm.

Metaphase-I

The spindle fibres get well developed are of three types
 Chromosomal / Kinetochore microtubules
 Continous / non kinetochore microtubules (pole to pole)
 Interzonal / non kinetochore microtubules (between the chromosomes)
The chromosomes move towards the equator and finally they orient themselves on the equator.
The centromeres lies towards the poles and the arms towards the equator.
At this stage each chromosome have two chromatids.

Anaphase-I

Chromosomal fibre contracts and interzonal fibre expands. So homologous chromsomes segregate from each other and move towards the opposite poles.
Anaphase-I is characterised by segregation or disjunction of homologous chromosomes. Division of centromere is absent.

Unlike the condition in mitosis, the two chromatids of each chromosomes do not separated in meiosis-I because centromere does not splited.
Anaphase-I is responsible for reduction in chromosome number in daughter cells.

Telophase-I

The haploid number of chromosomes after reaching their respective poles, become very long and uncoil.
The nuclear membrane and the nucleolus reappear and thus two daughter nuclei formed are haploids.

Cytokinesis-I

In animals - by constriction and furrow (successive type)
In plants mostly cytokinesis absent.
In plants, all the four daughter cells are produced simultaneously. Group of four cells produced simultaneously is called tetrad. (Tetrad of pollen grains).

Interphase-II

It is period in between meiosis-I and Meiosis-II. It may or may not be present. It is generally present
in animal cell. There is no s-phase and no DNA synthesis. So called Interkinesis.

MEIOSIS-II

In this division no change in number of chromosome takes place (Haploid to haploid).

Karyokinesis
Like meiotic-I, it is divided into four phases.

Prophase - II
The chromosomes appear distinct with two chromatids.
Each centrosome divides into two resulting in the formation of two centrosome. Then each moves to opposite pole. They produce asters. Spindle fibres are formed between the asters.
The nucleus membrane and nucleolus disappear.

Metaphase - II
The chromosomes get arranged on the equator similar to mitosis.

Anaphase II
The two chromatids of each chromosomes are separated by the divisions of centromere and are attached to the spindle fibres.
The separated chromatids becomes daughter chromosomes and move the opposite poles due to the contraction of the spindle fibres.

Telophase II
During this stage, the daughter chromosomes again form chromatin thread.
The nuclear membrane surrounds each group. The nucleolus reappears.
Spindle fibre disappears.

Cytokinesis-II
After this there is cleavage of cell membrane in animal cells or cell wall formation in plant cells.
Each daughter cell of meiosis-I produces two haploid cells at the end of meiotic II
Hence this leads to the formation of four daughter cells (gametes). genetically different from eachother.
Significance of Meiosis
Gametes are produced by meiosis.
In meiotic cell division number of chromosome get reduced to half. When male and female gametes fuse, it again leads to (2n) number of chromosomes in the zygote. In this way by meiotic cell division number of chromosome is maintained constant in a sexually reproducing organism.
During crossing over exchange of genes takes place in between maternal and paternal (two) chromosomes, this exchange of genes leads to variation. Variation is the raw material for evolution.

Type of Meiosis

Zygotic or Initial meiosis : When the meosis in life cycle of an organism occurs in zygote cell. eg.
in Thallophyta.
Sporic meiosis or Intermediate Meiosis : Meiosis takes place during spore formation eg. all the plants except thallophyta.
Gametic or Terminal Meiosis : Meiosis during the gamete formation. Eg. Animals.