L1. Mitosis & Meiosis

Two types CELL DIVISION 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) 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 46chr. 4N ITOTIC (1 hr.)

46chr. 2N Human studies show that the complete cycle lasts for 12 to 24 hrs of which 1hr involves mitosis

The Cell Cycle 

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

STAGES OF MITOSIS INTERPHASE

PROPHASE 1

METAPHASE - 2

TELOPHASE - 4

ANAPHASE - 3

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 Cell membrane Barr body

Nucleus Nuclear membrane

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

centriole centromere

chromatid

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

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

ANAPHASE The 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 The nuclear membrane is formed. Chromosomes unwind & stain lightly & 2 identical interphase cells are formed.

INTERPHASE

MITOSIS (after interphase)

Prophase 46, 4N

46, 2N + 46, 2N

Metaphase Anaphase Telophase

46, 2N 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 METAPHASE

•Bivalent chromosomes move to equatorial plane 46 4n ANAPHASE

•One member of each bivalent pair moves to either pole – disjunction – no splitting of chrs. leading to the fomation of 2 unidentical cells 23 2n TELOPHASE

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

MEIOSIS •Resembles mitotic division.

II

•No DNA replication in interphase •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

METAPHASE

Splitting of chrs. 23 n 4 unidentical cells with 23 n chrs.

ANAPHASE

TELOPHASE

MEIOSIS I (after interphase) Prophase (crossing over) 46, 4N

23, 2N + 23, 2N

Metaphase Anaphase (disjunction without splitting) Telophase

MEIOSIS II (without interphase)

Haploid, Unidentical Prophase 23, 2N

Metaphase Anaphase (splitting)

23, N + 23N Telophase 23, N 4 unidentical cells from 1 cell

(Like mitosis)

GAMETOGENESIS Spermatogonium \ Oogonium ( 46 2n)

MEIOSIS I

Primary gametocyte (46 4n) Synapsis

Reduction division

Chiasma formation

PROPHASE

Disjunction (no ANAPHASE splitting of chrs.) Secondary gametocyte (23 2n-haploid – unidentical)

4 Gametes (23 n – unidentical)

No DNA replication

MEIOSIS II Disjunction (splitting of chrs.) 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 Mitosis requires one division

MEIOSIS Meiosis requires two divisions

Two diploid daughter cells Four haploid daughter cells result from mitosis 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 Occurs only in the for growth and repair. reproductive organs for the production of gametes.

In the mitotic cell cycle the interphase consists of all the following phases except a.G1 b.Mitotic c.S (DNA synthesis) d.G2 Prophase of mitosis consists of all of the following execpt a.Visibilty of chromsomes b.Degenration of nuclear membrane c.Duplication of centriole d.Formation of mitotic spindle All are true for metaphase of mitosis except a.Chromosomes have reached their maximum contraction b.Spindle of microtubules is formed c.Chromsomes reach the equatorial plane d.Disjunction (splitting) of chromsomes takes place

Which of the following is a major characteristic of meiosis I a.Splitting of centromere b.Reducing the amount of DNA to 1N c.Achieving the haploid number of chromosomes d.Producing primordial germ cells During meiosis large segments of DNA are exchanged. What is the process called? a.Synapsis b.Nondisjunction c.Crossing over c.Disjunction

SHORT QUESTIONS ON LECTURE – I MITOSIS & MEIOSIS 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

10)Give the difference between mitosis & meiosis 11)Draw a diagram showing the stages of gametogenesis