CELL CYCLE AND CELL DIVISION

CELL CYCLEThe orderly sequence of events by

which the cell duplicates its contents and divides into two is termed as cell cycle.

The event is genetically controlled.It consists of two periods-

(a)interphase (b)mitosis.

CELL CYCLE

G A P 1 S P H A S E G A P 2

IN TE R P H A S E

K A R Y O K IN E S IS C Y TO K IN E S IS

M O R M ITO S IS P H A S E

C E L L C Y C L E

CELL CYCLE

MITOSIS

P R O P H A S E M E TA P H A S E A N A P H A S E TE L O P H A S E

K A R Y O K IN E S IS C Y TO K IN E S IS

M ITO TIC D IV IS IO N

MITOSISOccurs in somatic cells.Equational division.Duplicated chromosomes distributed

equally to the daughter cells.Consists of 4 stages –

prophase,metaphase, anaphase and telophase.

PROPHASEChromosomal material condenses into compact mitotic apparatus.Each chromosome consists of two chromatids held together at centromere.Spindle formation starts.At the of prophase nuclear membrane,nucleolus,endoplasmic reticulum &golgi bodies disappear.

METAPHASESpindle fibres attach to kinetochores of chromosome.Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.

ANAPHASECentromere split and chromatids separate.Chromatids move to opposite poles.

TELOPHASEChromosomes cluster at opposite

poles.Identity of chromosomes is lost as

discrete elements.Nuclear envelope, nucleolus, golgi

complex & E R reform.

CYTOKINESISAnimal cells-furrows in the plasma

membrane gradually deepens and ultimately joins in the center dividing the cell cytoplasm into two.

Plant cells-cell plate formation starts at the center of the cell and grows outward to meet existing lateral walls.

SIGNIFICANCE OF MITOSIS

Equal distribution of chromosome among two daughter cells.

Surface/volume ratio is maintained.Nucleoplasmic ratio is maintained.Causes growth of organ.Replaces worn-out cells.

Early prophase

Prophase

Late prophase

Transition to metaphase

MetaphaseAnaphase

Telophase

DONE

Interphase

MEIOSISOccurs in germ cells.Involves two sequential cycles that is meiosis I and meiosis II.There is only a single cycle of DNA replication.Meiosis I is initiated after is S phase.Meiosis involves pairing of homologous chromosomes and recombination between them.Four haploid cells are formed called germ cells.

MEIOSIS

Prophase IMetaphase IAnaphase ITelophase I

Prophase IIMetaphase IIAnaphase IITelophase II

MEOSIS-I MEOSIS-II

MEIOSIS I

L E P TO TE N E Z Y G O TE N E P A C H Y TE N E D IP L O TE N E D IA K E N S IS

M E IO S IS

LEPTOTENEChromosomes become

gradually visible due to compaction of chromosomes.

ZYGOTENEPairing of homologous chromosomes take place by the process called synapsis.The complex structure formed due to pairing is called synaptonemal complex and the chromosomes is called a bivalent or tetrad.

PACHYTENECrossing over(exchange of genetic material) occurs between two non sister chromatids.recombination nodules appear at the site of crossing over.Enzyme recombinase involves in crossing over.Crossing leads to recombination of genetic material between two chromosomes.

DIPLOTENERecombined homologous chromosomes tends to separate from each other except at the site of cross over.Chromosomes look X shaped called chiasmata.

DIAKINESISTerminalisation of chaismata.Chromosomes are fully condensed.Meiotic spindle is assembled to separate homologous chromosomes.Nucleolus disappear and nuclear envelope breaks down.

METAPHASE IBivalent chromosomes arranged at equatorial plate.Spindle fibres from opposite poles attach to to the pair of homologous chromosomes.

ANAPHASE IHomologous chromosomes

separate, while sister chromatids remain associated at their centromere.

TELOPHASE I

Nuclear membrane and nucleolus reappear.Two cells are formed.Chromosomes under go dispersion.

PROPHASE II OF MEIOSIS II

Chromosomes become compact.Nucleolus and nuclear membrane disappear.

METAPHASE IIChromosomes align at the equator.Microtubules from opposite poles of the spindle get attach to the kinetochores of sister chromatids.

ANAPHASE II

Centromere of each chromosome splits and chromatids(chromosome) move towards opposite poles of cell.

TELOPHASE II

∗Chromosomes are enclosed by nuclear envelope.

∗Nucleolus reappear .

SIGNIFICANCE OF MEIOSIS

Conservation of specific chromosome number of each species is achieved.Increase genetic variability in organisms which are important for evolution.