Two types 1.Mitosis - Cell division by which body grows or replaces dead\injured tissue.

- Occurs in somatic cells. - Each cell division gives rise to 2 identical daughter cells with chromosomes of the parent cell.(46chr.2n)

CELL DIVISION

2.Meiosis – Cell division for reproduction. - Occurs in reproductive cells only.

- Each cell division gives rise to 4 nonidentical reproductive cells or gametes with haploid chromosomes (23chr.n)

oocyte

spermatocyte

MITOTIC CELL CYCLE

Human studies show that the complete cycle lasts for 12 to 24 hrs of which 1hr involves mitosis

46chr. 4N

46chr. 2N

ITOTIC

(1 hr.)

Interphase (90% of cycle)• G1 phase~ growth • S phase~ synthesis of DNA • G2 phase~ preparation for cell division

Mitotic phase• Mitosis~ nuclear division• Cytokinesis~ cytoplasm

division

The Cell Cycle

STAGES OF MITOSIS

INTERPHASE

PROPHASE - 1

METAPHASE - 2

ANAPHASE - 3 TELOPHASE - 4

INTERPHASE

INTERPHASE• The cell is not actively (mitotically) dividing but the chromosomes are metabolically active and replication of DNA takes place.

•Chromosomes are not seen but the inactive X chromosome in the female is seen as a compact mass near the nuclear membrane as Barr body

•As soon as the chromosomes are visible the cell has entered the 1st stage of mitotic division – the prophase

Barr body

Nucleus

Cell membrane

Nuclear membrane

PROPHASE•The chromosomes appear as a pair of strands – the chromatids – held together by the centromere – its position determines its classification.

•Premetaphase – Centriole duplicates and starts moving to either pole. Nuclear membrane disappears.

centriole

centromere

chromatid

METAPHASE

•Spindle of microtubules is formed by the centrioles

•Chromosomes move to the equatorial plane of the spindle and get attached by their centromeres

•Chromosomes have reached their maximum contraction and are easily studied under the microscope

Spindle

ANAPHASEThe centromeres split (disjunction) and the daughter chromosomes are pulled to either pole by the contracting actin filaments of the spindle.

TELOPHASE

•Division of the cytoplasm –cytokinesis – takes place in the equatorial plane and 2 cells with cell membranes are form

INTERPHASE

The nuclear membrane is formed.

Chromosomes unwind & stain lightly & 2 identical interphase cells are formed.

MITOSIS (after interphase)

46, 4N

46, 2N + 46, 2N

46, 2N

Prophase

Metaphase

Anaphase

Telophase

2 identical cells from 1 cell

MEIOSIS

•The reproductive cell – Spermatocyte \ Oocyte undergoes 2 divisions - Meiosis I and Meiosis II

•Each division has the stages of prophase, metaphase, anaphase & telophase

•In meiosis I the prophase is prolonged and the chromosomes are reduced from 46 4n to 23 2n (reduction division)

•In meiosis II the division occurs as in normal mitotic division

MEIOSIS I – PROPHASE STAGES

Leptotene “thin threads”- become visible due to condensation of chromatin

Zygotene “paired threads”- homologous chr. pair called bivalent chr. Process is called synapsis.

Pachytene “thick threads”- thickening of chr. & separation of chromatids to form tetrads.

Diplotene “2 threads”- homologous pairs separate but remain bound at the chiasmata (cross over) where exchange of genetic material & chromatid segments takes place

Diakinesis “moving through”- separation of chr. Nuclear membrane disappears & spindle forms.

MEIOSIS I

•Bivalent chromosomes move to equatorial plane

•One member of each bivalent pair moves to either pole – disjunction – no splitting of chrs. leading to the fomation of 2 unidentical cells

•Each cell has haploid (23) chr. & 2n DNA. Hence called reduction division.

METAPHASE

ANAPHASE

TELOPHASE

46 4n

23 2n

23 2n

•No DNA replication in interphase

METAPHASE

ANAPHASE

TELOPHASE

MEIOSIS II•Resembles mitotic division.

•Splitting of chrs. leads to formation of 2 cells of 23 n chrs.

•Thus 4 unidentical cells of 23 n chrs. formed from 2 unidentical cells of 1st meiotic division

23 2n

23 n

Splitting of chrs.

4 unidentical cells with 23 n chrs.

MEIOSIS I (after interphase)

Prophase (crossing over)

Metaphase

Anaphase (disjunction without splitting)

Telophase MEIOSIS II (without interphase)

46, 4N

23, 2N + 23, 2N

Haploid, Unidentical

23, 2N

Prophase

Anaphase (splitting)

Metaphase

23, N + 23N

23, N

Telophase

(Like mitosis)

4 unidentical cells from 1 cell

GAMETOGENESIS

Spermatogonium \ Oogonium ( 46 2n)

Primary gametocyte (46 4n)

MEIOSIS I

MEIOSIS II

Synapsis

Chiasma formation

Disjunction (no splitting of chrs.)

PROPHASE

ANAPHASE

Disjunction (splitting of chrs.)

Reduction division

Secondary gametocyte (23 2n-haploid – unidentical)

4 Gametes (23 n – unidentical)

No DNA replication

ANAPHASE

Abnormal disjunction of chromosomes leads to abnormalities like trisomy or monosomy.

Common examples –

Trisomy 21 – Down’s syndrome

Trisomy 18 – Edward’s syndrome

Trisomy 13 – Patau’s syndrome

Monosomy – 45X – Turner’s syndrome - the only monosomy compatible with life

MITOSIS MEIOSISMitosis requires one division

Meiosis requires two divisions

Two diploid daughter cells result from mitosis

Four haploid daughter cells result from meiosis

Daughter cells are genetically identical to parental cells

Daughter cells are not genetically identical to parental cells

Occurs in all somatic cells for growth and repair.

Occurs only in the reproductive organs for the production of gametes.

In the mitotic cell cycle the interphase consists of all the following phases excepta.G1 b.Mitotic c.S (DNA synthesis)d.G2

Prophase of mitosis consists of all of the following execpta.Visibilty of chromsomesb.Degenration of nuclear membranec.Duplication of centrioled.Formation of mitotic spindle

All are true for metaphase of mitosis excepta.Chromosomes have reached their maximum contractionb.Spindle of microtubules is formedc.Chromsomes reach the equatorial planed.Disjunction (splitting) of chromsomes takes place

Which of the following is a major characteristic of meiosis Ia.Splitting of centromereb.Reducing the amount of DNA to 1Nc.Achieving the haploid number of chromosomesd.Producing primordial germ cells

During meiosis large segments of DNA are exchanged. What is the process called?

a.Synapsisb.Nondisjunctionc.Crossing overc.Disjunction

1)Draw a diagram of the mitotic cell cycle

2)What are the phases in the interphase and what do they denote?

3)What are the phases of mitosis and give their significance in short

4)In which cells does mitosis and meiosis occur?

5)At what stage is karyotyping done and why?

6)Draw diagrams showing the stages of mitosis and meiosis

7)What is the chromosomal no. & amount of DNA before & after mitosis & meiosis

8)What are the stages of prophase of meiosis I & what is the significance of chiasmata formation

9)What is the difference between anaphase of mitosis & meiosis & what is its significance

SHORT QUESTIONS ON LECTURE – I MITOSIS & MEIOSIS

10)Give the difference between mitosis & meiosis

11)Draw a diagram showing the stages of gametogenesis