Meiosis Stages NEET Notes | EduRev

Biology Class 11

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MEIOSIS

"Term meiosis" was proposed by Farmer and Moore. Meiosis produces genetically different type of cells. All the four daughter cells produced by meiosis are genetically different from each other and also differ from the mother cell.

Meiosis includes 2 division of cell (i) Meiosis - I (ii) Meiosis - II

  • Meiosis I :- Heterotypic division or reduction division. It leads to reduction in chromosome numbers. Division of chromosome does not occurs in meiosis-I and only segregation of homologous chromosomes takes place.

  • Meiosis II :- This is a homotypic division or equational division. It does not leads to any change in chromosome number.

  • Meiosis II is just like mitosis. Division of centromere occurs during meiosis II.

  • In meiosis, division of nucleus takes place twice but division of chromosome occurs only once.

Interphase – same as in mitosis Stages of meiosis I 

1. Prophase – 

I : This is longest and most complex stage of meiosis. Prophase I is classified in five sub-stages :

(a) Leptotene r Chromatin threads condense to form chromosomes. Chromosomes are longest & thinest.

  •  On Chromosomes, bead like structures are present i.e. chromomeres. All the chromosomes in nucleus remain directed towards centrioles, so group of chromosomes in nucleus appears like a bouquet in animal cell. (Bouquet stage). At this stage organism shows a peculiar type of orientation of chromosomes– animals show bouquet type while plants synthesis type.

Meiosis Stages NEET Notes | EduRev

(b) Zygotene or Synaptotene – Zygotene is characterized by pairing of homologous chromosomes (Synapsis). Pairs of homologous chromosomes are called Bivalents. There develops a structure in between homologous chromosomes, Which is termed as synaptonemal complex. Synaptonemal complex is composed of three thick lines of DNA and proteins. According to Mosses (1956) synaptonemal complex helps in pairing and chiasmata formation.

  • About 0.3% DNA is synthesized in Zygotene sub-stage. This DNA is used in chromosome pairing or synapsis. (Zyg. DNA)

Meiosis Stages NEET Notes | EduRev

(c) Pachytene (Thick thread) – Due to increased attraction, homologous chromosomes tightly coil around each other.

Meiosis Stages NEET Notes | EduRev

  • Both chromatids of each chromosome become distinct.

  • Each chromosome in a bivalent at pachytene stage has two chromatids, as a result of which  bivalent really consists of four chromatids and is called tetrad. Both the chromatids of a chromosome are calledsister chromatids.

  • Bivalent is called tetrad and each chromosome is called Dyad.

  • Nonsister chromatids of homologous pair develops recombination nodules and exchange their parts i.e. crossing over. Crossing over was discovered by Morgan and Castle in Drosophila.

Theories of crossing over–

(i) Copy choice theory ⇒ Belling, Laderberg.

(ii) Breakage first theory ⇒ Muller

(iii) Strain theory ⇒ Darlington

(iv) Breakage and reunion theory ⇒ Stern and Hotta

  • Breakage and reunion theory is appropriate theory for crossing over which is explained with the help of a most accepted model (Hybrid DNA model) of Holiday and Whitehouse.

Meiosis Stages NEET Notes | EduRev

Meiosis Stages NEET Notes | EduRev

  •  Endonuclease first breaks the nonsister chromatids at the place of recombination nodule.

  •  Nonsister chromatids reunite after exchanging their parts by Ensome Ligase As the result of crossing over cross like structures–chiasmata (discovered by Janssen) form in bivalent.

  •  Crossing over is an enzyme mediated process and the enzyme involved is called recombinase (Endonuclease + ligase)

  •  Number of chiasmata (singular- chiasma) per bivalent depends on the length of chromosome.

(d) Diplotene – The beginning of dipotene is recognised by dissolve of synalptonemal complex (desynapsis) Homologous chromosomes start repulsing each other so chiasmata become visible. Due to repulsion (desynapsis) chiasmata begin to move along with length of chromosome and towards the end of chromosome. This event is called terminalization. 

  •  In diplotene, chiasmata move toward the end of chromosome in a zip like manner.

  •  Diplotene (Dictyotene) may last long up to month or years (12 to 15 years in human Female)

  •  According to modern scientists chiasmata are not the cause but are only the result of crossing over.

Meiosis Stages NEET Notes | EduRev

(e) Diakinesis – Terminalisation completes.

Homologous chromosomes remain attach to each other only at the ends. Centrioles move towards the opposite poles.

  •  Nuclear membrane and nucleolus disappear.

Meiosis Stages NEET Notes | EduRev  

Meiosis Stages NEET Notes | EduRev

2. Metaphase I : Bivalents arrange on equator (congression) of cell to form metaphase plate. Three types of spindle fibres appear in the cell.

(i) Chromosomal/ Kinetochore Spindle fibres

(ii) Supporting/ Continuous Spindle fibres

(iii) Interzonal Spindle fibres.

3. Anaphase I : 

  •  Chromosomal fibre contracts and interzonal fibre expands. So homologous chromosomes segregate from each other and move towards the opposite poles.

  •  Anaphase I is characterised by segregation or disjunction of chromosomes. Division  of centromere is absent. Anaphase I  is responsible for reduction in chromosome number in daughter cells.

4. Telophase I : Just like the telophase of mitosis Due to formation of nuclear embrane conduction of cell is called as dyad of cell.

Cytokinesis I : 

  •  In animals by constriction & furrow formation (successive).

  •  In most of the plants cytokinesis does not occur after meiosis I.

  •  In plants, all the four daughter cells are produced simultaneously. Group of four cells produced simultaneously is called tetrad. (Tetrad of pollen grains, ).

  •  Gap between meiosis I and meiosis II is called Interkinesis. Preparations of meiosis II occur during interkinesis.

It can't be termed as interphase because replication of DNA is absent in interkinesis.

Meiosis – II– Just like mitosis.

Types of Meiosis :-

  •  Zygotic or Initial meiosis :- When the meiosis in life cycle of an organism occurs in zygote cell. eg. in Thallophyta.

  •  Sporic meiosis or Intermediate Meiosis :- Meiosis takes place during spore formation.

Eg. all the plants except thallophyta.

  •  Gametic or Terminal Meiosis :- Meiosis during the gamete formation. Eg. Animals.

Significance of Meiosis :-

(1) The meiosis maintains constant number of chromosomes in sexually reproducing organisms.

(2) By crossing over, meiosis provides an opportunity of exchange of genes (variations) & thus meiosis is raw material for process of evolution.

AMITOSIS

  •  Name 'Amitosis' was given by Remake and detail of amitosis is given by Flemming. It is most primitive type of cell division. Condensation of chromosomes not occurs in amitosis. Chromosomes are not visible during division.

It is a process of division without recognizable chromosomes. Amitosis does not involve the formation of spindle. Division of nucleus is direct. i.e. without sequential changes (prophase, metaphase, anaphase & telophase). In amitosis, division of cytoplasm and nucleus occur simultaneously  by the constriction. In amitosis division may be equal or unequal. Amitosis is fastest cell division which may complete in 20–30 minute.

Amitosis is cell division of prokaryotes. But exceptionaly also occurs in some eukaryotes. eg. yeast–budding occurs by amitosis.

  •  In amoeba multiple fission occurs by amitosis.

  •  In Paramecium division of meganucleus.

  •  In mammals–growth of foetal membranes (amnion, chorion, allantois, yolk sac)

  •  Division of mitochondria and chloroplasts.

 

S.No.

Mitosis

Meiosis

1.GENERAL

1.

The division occurs in smetio cells.

It occurs in reproductive; cells.

2.

Both diploid and haploid cells stow mitosis.

If ciosis is found only in diploid cells.
 (or in 4n, 6n. 8n)

3.

It is a single division.

It is a clodde division.

i.

Mitosis produces two cells

Ifeiosis produces four cells.

S.

It dees not inrmdice variation.

ihiosis introduces variations die to gene exchange.

6.

Hunter of chrcmoscme same as mother cells.

Chrcmoscme number reduced {halved) .

7.

It is respired fear growth, ngair and healing.

Meiosis involved in only sexual reproduction.

 2 . PROPHASE

8.

UisairpLer.

Etcphase - I is amputated.

9.

Bouquet stage and synapsis absent.

Bouqiet stage (sinizsis in plants) and synapsis occurs.

10.

Crossing over and chiasma not seen.

'Crossing over and chiasna formation occurs.

 3. METAPHASE

11.

Chromosome remain as monovalent and has two chromatics

Chranosame as bivalet and four dmmatids.

12.

Centromeres are present over the equator or metaphase plate while aims facing towards the poles.

Centrcneres projects towards tie poles.

 

4 . ANAPHASE

13.

Csitromere dervides diring anaphase.

Division of centrarere is absent in anaphase - I

14.

Chromosomes are single chranatided.

'Chrcmosome has two chrcmatid3.

15.

The two chromatids of a chrcmoscme separate and become as daughter chromosomes.

Qiromatids do not separate in anaphase-E

16.

Chromosomes moving towards the opposite poles are similar

'Imnasies are dissimilar.

 5 . TELOPHASE

17.

It is ai essential in which nucleous  and MM reappears

Telcphase-I may be absent.

18.

Nucleolus appears.

Nucleoli are not formed in telqchase-I

 6. CYTOKINESIS

19.

Cytokinesis takes place.

Not ampul sory (simultensous after both the divisions in plants)

20.

Eanghter oells posses same genetic constitution

All ficur caughter cells arE differ.

 

S.No.

Mitosis

Meiosis - II

1.

An interphase occurs prior to mitosis.

Interphase may or may not present prior to meiosis-II if present then called interkinesis.

2.

S-phase present.

S-phase absent.

3.

Daughter cells formed after mitosis resemble to mother cell.

Differ from mother cell, quantitatively and qalitativiy.

4.

Longer

Shorter.

 

SPECIAL POINTS

  • Robert Sovian change :- Conversion of a metacentric chromosome in to two telocentric chromosomes.

  •  According to recent research, cell wall is considered as dynamic, porous and capable of recognizing signals of cells.

  •  Symplast :- Path in plant body, which is made up of living components or cytoplasm (or through plasmodesmata) is called as symplast.

  •  Apoplast :- Path in plant body, which is made up of nonliving spaces is called as apoplast.

  •  Intermediate filaments has size/diameter in between microfilaments and microtubules. These fillaments form basket like structure around the nucleus.

  •  Cadherins called as cell adhesion molecule.

  •  Human myelin has about 20% protein & 79% lipids.

  •  Spectrin :- (Responsible for structural integrity & biconcave shape of RBC membrane) deficiency or mutation in spectrin makes humans anaemic, leading to abnormality in shape of RBC which become spherical instead of concave.

  •  Gangliosides :- The most complex glycolipid are gangliosides.

(i) A variety of membrane protein may present in or on the membrane are called as cell adhesion molecule (CAMs). eg. 'Cadherins (in cell adhesion), integrins and selectins (work in inflammatory response).

  •   Gorter and Grendel (1926) :- Suggested that plasma membrane consists of double layer of lipid molecule.

  •   Carrier protein are mainly involved in facilitated diffusion and active transport but rarely some carrier proteins involved in simple diffusion. Carrier (tunnel) proteins may be following type

(1) Uniporter carrier proteins:-When a carrier transports only single solute from one side of membrane to other side.

(2) Symporters :- When carriers transport one solute which depends on transport of simultaneously second solute in same direction.

(3) Antiporter carrier protein:-When carrier transports one solute which depends on simultaneously second solute in the opposite direction.

Meiosis Stages NEET Notes | EduRev

  •  Active transport is always facilitated by carrier proteins or pumps. Although passive transport may or may not involve carrier protein. Ion channels are never involved in active transport. 

  •  Freeze etching E.Microscopy :- In this technique first of all, living cell is frozen in liquid nitrogen at temperature –196°C in the presence of antifreeze (or cryoprotectant). Frozen block is sliced with a knife/blade and studied under E.M. * For the structure of membrane, membrane skeleton fence model proposed by (Kusumi 1993).

  • Hydrogenosome is a recently discoverd eukaryotic cell organelle in Nyctothermus ovalis. This is a double membranous cell organelle which can produce ATP only by substrate level phosphorylations because cytochromes (ETS) and TCA cycle is absent. This organelle produces high amount of hydrogen by anaerobic process. Thus named as hydrogenosome.

  • Endosome (receptosome) is also recently discovered cell organelle which consists of tubules and vesicles and involved in intracellular traffic.

  • In certain plants (Lilium) during leptotene, all chromosomes of a cell are densely clumped to one side and rest of the nucleus has no chromatin material. This event is called synthesis. Its significance is subject of future research.

  •  During the leptotene stage each chromosome is attached to nuclear envelope by attachment plate which is made up of by specific proteins. At this time, chromosomes directed to centrosome and appear as Bouquet.

  •  Chiasma formation is the result of crossing over, this is explained by F. A. Janssen with chiasma type theory (or one plane theory)

  •  Protein transferrin which works in iron transport helps in mitosis division.

Satellite DNA :- Repeatitive sequence used to know relation among species because two most important features of satellite DNA are

(i) Remarkable relative uniformity within the same species.

(ii) Great variability between closely related species.

  • Genes for 5-s r-RNA presents separately than NOR.

  • In active state of mitochondria, (metabolically busy state) number of cristae is high and matrix is small. (condensed state)

  • When mitochondria, metabolically relax, than size of matrix is large and less number of cristae this is called orthodox state of mitochondria.

  • Metabolically active cell have more dense/developed E.R.

  • Porin proteins form pores in outer mitochondria and bacterial membranes.

  •  Erythrocyte membrane in human contain about 40% phospholipids and 52% proteins.

  •  Generally root tip of onion used for study of mitosis (2n=14)

  • Chromosome doubling without division of cell can induce by colchicine. This event is called C-mitosis. 

  • Telomere is concern with ageing of organisms.

  • Chloroplast and mitochondria not included in endomembrane system of eukaryotic cell.

  • Membrane of human RBC Contain about 60% Proteins 39% lipids and 1% carbohydrates.

  • Differential staining of cromatin net is called heteropycnosis. Ex. Barrbody.

 

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