cell cycle and cell division
TRANSCRIPT
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.