Slide 1

CELL DIVISION CELL CYCLE: Cell cycle includes interphase and cell division.

INTERPHASEINTERPHASE: Preparatory period for cell division.

Consists of 3 stages: a. G1(Gap1)stage b. S(synthesis)stage c.G2(Gap2)stages G1(GAP1) STAGE It takes few hours to many years or even absent. Chromosomes become thin,elongated and visible. Cells are metabollically active and synthesize structural protien,enzyme and organalles. S(SYNTHESIS) STAGEIt takes 7 hours. Replication of DNA. Characteristic X shaped configuration of chromosomes. G2 (GAP2) STAGE:

It takes 5 hours.Further thickening and condensation of chromosomes. Spindle appartus is synthesised in this stage. CELL DIVISIONTwo types: A.Direct AmitosisB.Indirect : Mitosis Meiosis MITOSIS SITE:All somatic cell.Initial stages of germ cell. STAGES:ProphasePrometaphaseMetaphaseAnaphaseTelophasePROPHASEChromosomes become shorten and thicken.Chromosome splits into 2 chromatids which are attached to centromere.Centriole separate and go to the opposite pole.Microtubules form central spindle and asters.Fig. of PROPHASE

PROMETAPHASE

It is late prophase.Disappearance of nucleoli.Nuclear membrane disitegrate.

Fig. of PROMETAPHASE METAPHASE

Chromosomes are allinged on equatorial.

Chromosomes attached with central spindle.

Fig. of METAPHASE IMPORTANCE OF METAPHASE

In karyotyping studies,Colchicine arrest the cell division at metaphase stage,because it prevents the formation of microtubules of the spindle.

ANAPHASE

Longitudinal splitting of centromere.New chromosome arranged transversely.Each new chromosome move to the pole.Appearance of cleavage furrow.Fig. of ANAPHASE

TELOPHASE

Lengthening of new chromosomes.Reappearance of nuclear membrane around new chromosome.Nucleoli and centriole reappear.Cytoplasm divides and two identical daughter cells of equal size are formed.Each contain 46 chromosomes.Fig. of TELOPHASE

PURPOSE OF MITOSIS Increase the number of cell.MEIOSIS SITE: Mature germ cells PURPOSE: a.parent cell gives rise to 4 daughter cell b.Each germ cell contain a haploid number of chromosome c.Exchange of genetic material MEIOSIS

STAGES2 stagesMeiosis I - reduction divisionMeiosis II equational divisionStages of Meiosis IProphase I# Leptotene# Zygotene# Pachytene# Diplotene# DiakinesisMetapahse IAnaphase ITelophase I

Prophase I (Leptotene)Long complex phase differing considerably from mitotic prophase

LeptoteneChromosome at first thin, begin to shorten and thickerChromosome become visible and thread likeShow characteristics beads throughout their length

Prophase I (Zygotene & Pachytene)ZygoteneShortening, thickening of chromosome continuesChromosome form homologous pairSynapsis conjugation pairing occurred

PachyteneSpiralized, shortening and thickening of chromosomeTetrad formation, chiasmata formation Crossing over

Prophase I (Zygotene & Pachytene)

Prophase I (Diplotene)Crossing over obvious in this stage due to separation of fusion point more fusion points more exchange of genetic material At this stage primary oocyte become diplotene by 5th month in utero and remain in this state until the period of ovulation

Prophase I (Diakinesis)The remaining chiasmata finally breakdown and the chromosome still remain in bivalent, become shorter and thickerThe bivalent pairs move away from each otherDisappearance of nucleoli and nuclear envelopeSpindle and aster formation as like mitosisMarked by dissolution of nuclear envelope and movement of bivalent chromosome towards equatorial plate

Metaphase I & Anaphase IMetaphase I Homologous bivalent chromosome align in the equator plane of the spindle microtubules

Anaphase IThe centromere do not splitThe paired chromatid separating to move towards the poles of the spindleCleavage furrow

Metaphase I & Anaphase I

Telophase IChromosome now reduced to haploid number Formation of two daughter cell which contain haploid number of chromosome

MEIOSIS II

This commences after only a short interval of meiosis1.This second meiotic division is more like a mitosis,but unlike mitosis,the separating chromatids are dissimilar genetically. Each of the daughter cell of the first meiotic division enters straight in Metaphase2. METAPHASE IIChromosomes allign in the new equatorial planes of the spindle.

ANAPHASE IICentromeres divide.Chromatids separate New chromosomes form and migrate to the opposite poles of the spindle.

TELOPHASE IIChromosomes reextendNuclear envelop reappear.Nucleoli reappear.Beginning of membranous vescicle .Cytoplasm divides.Four gametes are formed from the two cells of first meiotic division.Each contain haploid number of chromosomes.