Cell division – meiosis

Cell Division – Cell Division – MeiosisMeiosis

Sexual reproduction in Sexual reproduction in eukaryotes takes place by the eukaryotes takes place by the

fusion of gametes and they fusion of gametes and they maintain their ploidy. How is maintain their ploidy. How is

this possible?this possible?

Asexual and Sexual Asexual and Sexual Reproduction - DifferencesReproduction - Differences

• Asexual or vegetative reproductiono Takes place only through mitosis which

involves duplication of cells. o Produces a genetically identical

offspring for example a new plant originating from a shoot or root

o This method is rapid and allows the spread of the organisms to a large extent.

Asexual and Sexual Asexual and Sexual Reproduction - DifferencesReproduction - Differences

• Sexual reproductiono Takes place only in eukaryotes.o A new individual is formed by the fusion of two

gametes which are haploid cells.o The new individual so formed is a single cell – zygote

and is diploid having two sets of chromosomes.o Gametes usually come from two different parents

Female – eggMale –sperm

o Meiosis increases the genetic diversity as it changes the genetic information.

Chromosomal CharactersChromosomal Characters• Ploidy – Number of sets of chromosomes in a single cell

• Depending on the number of sets of chromosomes the cells are known as o Haploid (n) – Those which have a single set of chromosomeo Diploid (2n) - Those which have two sets of chromosomeso Triploid (3n) - Those which have three sets of chromosomes

• Many plants and higher organisms are diploid (2n) however their gametes are haploid (n).

• In case of human beings –• Diploid (2n) – 23 pairs or 46 chromosomes• 22 pairs of homologus chromosomes are known as Autosomes• One pair of sex chromosomes

o Female-sex chromosomes are homologous (XX) o Male-sex chromosomes are non-homologous (XY)

An Outline of MeiosisAn Outline of Meiosis

• The term meiosis was coined by J.B Farmer and J.B Moore in 1905.

• It is alternatively known as reduction division.

• Nuclear division takes place in two different phases which are:1. Meiosis 1 (reduction) –

which results into two haploid cells

2. Meiosis 2 (Division) – Each haploid cell divides into two producing 4 haploid cells.

Image reference - http://www.daviddarling.info/encyclopedia/M/meiosis.html

InterphaseInterphase

o The preparatory stage of meiosis and mitosis is

named as interphase and is similar in both. o It is divided into three phases:

A. Growth 1 (G1) phase: An active period marked by the synthesis of

various proteins and enzymes needed for cellular

growth. DNA appears like a single long coiled molecule.

InterphaseInterphase

A. Synthesis (S) phase: Replication of the genetic material takes place. Each chromosome divides into two sister

chromatids. RNA transcription is low. Protein synthesis is low.

B. Growth 2 (G2) phase: This stage is absent in meiosis

Meiosis I- Prophase IMeiosis I- Prophase I• Four stages of prophase I are as follows:

Leptotene (leptonema)This stage is of a short duration during which

condensation and coiling of chromosomes takes place.

Chromosomes become visible as long slender threads bearing numerous bead-like nucleosomes (chromomeres).

Chromosomes are arranged in a linear fashion along their lengths.

The nuclear envelope and the nucleolus are prominently visible.

The thin chromosomes are scattered in the nucleus.

Meiosis I- Prophase IMeiosis I- Prophase I• Zygotene (zygonema )

Pairing of homologus chromosomes known as synapsis takes place.

Pairing or synapsis takes place in a zipper like fashion and may start at the centromere, chromosomal ends or at any other position.

Thus pairing is highly specific and exact.The paired chromosomes are called Bivalent or

tetrad chromosome.The chromosomes at this stage are shorter

thicker and more distinct.

Meiosis I- Prophase IMeiosis I- Prophase I• Pachytene (Pachynema)

– Within a tetrad consisting of homologous pair of chromosomes, the paternal and maternal chromatids are non-sister to one another.

– The non- sister chromatids take part in crossing over.

– Non-sister chromatids randomly exchange segments over regions of homology.

– The exchange of information between the non-sister chromatids results in a recombination of genes

Crossing OverCrossing Over • Crossing over consists of a mutual exchange of

equal quantity (segments) of chromosomal material between two non-sister chromatids.

• It involves the following events: The non-sister chromatids, taking part in crossing

over, break simultaneously at the identical points (i.e. at homologous points).

The broken segments are of equal lengths. The broken segments may rejoin or there may be an

exchange of the segments between the non-sister chromatids.

Crossing OverCrossing Over During crossing over, the genes located on the

segments are exchanged between the two chromatids.

It results in the recombination of genes (characters).

Crossing over does not take place between sister chromatids.

The point where the non sister chromatids exchange is known as chaisma.

Crossing OverCrossing Over

Image reference - http://www.accessexcellence.org/RC/VL/GG/comeiosis.php

Significance of Crossing OverSignificance of Crossing Over

• The gametes produced through meiosis receive a new combination of characters (genes).

• Therefore individuals with new combination of characters are produced in each generation.

• This forms the genetic basis for variations and plays important role in evolution.

Meiosis I- Prophase IMeiosis I- Prophase I• Diplotene (Diplonema)• Two major events involved in this stage are:

1. Repulsion of homologous chromosomes Homologous chromosomes of a pair start repelling each

other. Homologous chromosomes begin to separate and uncoil. The non-sister chromatids which are involved in the cross-

overs are still held together at the points of crossing over.

2. Terminalization The separation and uncoiling of the homologues begins at

the centromeres and proceeds towards the ends. This causes progressive shifting of the chiasmata towards

the ends of the chromatids. This is called terminalization of chiasma.

Meiosis I- Meiosis I- DiakinesisDiakinesis• This is the last phase of Prophase-I. • Chromosomes are still in the pairs and in

contact with each other by terminal chiasma. The chromosomes become shorter, thicker and more prominent.

• The nucleolus and nuclear envelope disappear completely.

• The pairs of chromosomes are seen distributed in the nucleoplasm.

Meiosis I- Prophase IMeiosis I- Prophase I

Image reference – http://www.pinkmonkey.com/studyguides/subjects/biology-edited/chap6/b0606302.asp

Meiosis I- Metaphase IMeiosis I- Metaphase I• Spindle apparatus showing two types of spindle

fibers is formedContinuous fibers extending from pole to pole and Chromosomal fibers extending from pole to the

equator of the spindle body. • The homologous chromosomes, still in pairs, are

arranged at the equator of the spindle. • Maternal homologues of all the pairs face toward

one pole while the paternal homologues face the opposite pole.

• The chromosomal fibers connect the centromeres of all the homologues to the pole on their respective side.

Meiosis I- Anaphase IMeiosis I- Anaphase I• Shortening of the chromosomal fibers• The sets of maternal and paternal chromosomes

separate (segregate) and start moving towards opposite poles.

• The cell starts elongating in preparation for division into two.

Image reference –http://www2.estrellamountain.edu/faculty/farabee/biobk/biobookmeiosis.html

Meiosis I- Telophase IMeiosis I- Telophase I• The two sets of chromosomes reach

the opposite poles.• The chromosomes, each with two

chromatids and one centromere, become thin and long.

• A nucleolus reappears. • Each nucleus is haploid as it has

received only one set of chromosomes.

• Hence M-I is called reduction division.• Cytokinesis leads to creation of two

daughter cells.• Microtubules disappear and a new

nuclear membrane surrounds each haploid set.

Image reference - http://www2.estrellamountain.edu/faculty/farabee/biobk/biobookmeiosis.html

Meiosis-IIMeiosis-II

• Interkinesis : The time interval between M-I and M-II is called interkinesis.

• The stages of Meiosis – II are similar to mitosis.

• The two nuclei formed after M-I divide during M-II and produce four haploid nuclei.

Meiosis-II - Prophase-IIMeiosis-II - Prophase-II• The chromosomes

become shorter, thicker and distinct.

• Each chromosome has two sister chromatids which are joined by a centromere.

• The nuclear envelope and the nucleolus disappear by the end of Prophase-II.


Meiosis-II Meiosis-II • Metaphase-II

The bipolar spindle body is formed. Chromosomes are arranged along the equators in

such a way that their chromatids are facing the opposite poles.

The centromere of each chromosome is connected with both the poles by chromosomal fibers.

• Anaphase-II : During early anaphase-II, the centromere of each

chromosome divides longitudinally into two. Therefore each chromosome is divided into two

halves (chromatids) or daughter chromosomes.During late anaphase-II, the two sets of daughter

chromosomes are pulled away from each other and move towards the opposite poles.


Meiosis-II – Telophase -IIMeiosis-II – Telophase -II• The sets of chromosomes reach the opposite

poles and a new nucleus is organized at each pole.

• In all, four daughter nuclei are formed. • Each nucleus has half the number of

chromosomes as compared to the original mother nucleus.

• These nuclei also differ from each other in the structure and characters of chromosomes. This is because of the crossing over during prophase-I.

• Cytokinesis : • This is the division of the cell cytoplasm. It

follows the nuclear division and may be successive or simultaneous.


Significance of MeiosisSignificance of Meiosis• Facilitates stable sexual reproduction

• Recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the offspring.

Comparison between Mitosis Comparison between Mitosis and Meiosisand Meiosis

Mitosis Meiosis

Occurs in somatic cells. Occurs in reproductive cells.

Consists of only one nuclear division.

Consists of two nuclear divisions M-I and M-II.

Cytokinesis takes place only once.

May take place only once (simultaneous type) or twice (successive type).

Two daughter cells are formed.

Four daughter cells are formed.

Dividing cells can be haploid or diploid.

Dividing cells are diploid.


Mitosis Meiosis

Does not involve either pairing of homologous chromosomes or crossing over.

Pairing of homologous chromosomes and crossing over occur during Prophase-I.

It is an equational division. It is a reduction division.

Original characters of the chromosomes are maintained in the daughter cells.

Chromosomal characters are altered due to "crossing over" causing recombination of genes.


Mitosis Meiosis

Daughter cells are similar to each other and also to the original mother cell.

Daughter cells differ from each other as well as from the original mother cell.

Helps in growth and body repairs.

Helps in the sexual reproduction and regulation of chromosome number in the life cycle of sexually reproducing organism.

Summary of the classSummary of the class• Meiosis is a reductional division which

helps in sexual reproduction.• During he first part of meiosis the

chromosome number is reduced to half.• The second part of meiosis is similar to

mitosis during which the haploid cell duplicates.

• The end result of meiosis is 4 haploid cells.• Meiosis leads to genetic diversity.

• Thank you

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Cell division – meiosis