Meiosis..Edited By:

Gift Thamsanqa Lubisi (201209380)

Introduction..• A process of reduction division in which the number of

chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

• Diploid – 2 sets of chromosomes. It is the term used to refer to a cell that contains both sets of homologous chromosomes.• Haploid – 1 set of chromosomes.

it is the term used to refer to a cell that contains only a single set of chromosomes.

Meiosis

• Sex cells divide to produce gametes (sperm or egg).

• Gametes have half the # of chromosomes.

• Occurs only in gonads (testes or ovaries).

Male: spermatogenesis

Female: oogenesis

• Meiosis is similar to mitosis with some chromosomal differences.

Homologous chromosome….Homologous - term used to refer to chromosomes that

each have a corresponding chromosome from the opposite-sex parent.

Homologous chromosomes are similar in shape, position and information, but not identical to each other.

Homologous chromosome.

Important definitions..• Meiosis is a process of reduction division in which the

number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

• Tetrad - structure containing 4 chromatids that forms during meiosis.

• crossing-over - process in which homologous chromosomes exchange portions of their chromatids during meiosis

Crossing over..

Crossing over..

• Formation of a tetrad while going through the process of crossing-over

• This occurs during prophase I of meiosis.

Introduction..

• Meiosis have mainly two division..

Meiosis-I.

Meiosis-II.

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Phases of Meiosis

Cells undergo a round of DNA replication, forming duplicate chromosomes.

Interphase I

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Phases of Meiosis

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

There are 4 chromatids in a tetrad.

MEIOSIS I Prophase I

Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Synapsis

and

Crossing

Over in

Action

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Phases of Meiosis

Spindle fibers attach to the chromosomes.

Tetrads line up in the middle: metaphase plate.

MEIOSIS I Metaphase I

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Phases of Meiosis MEIOSIS I Anaphase ISpindle fibers pull the

homologous chromosomes toward opposite ends of the cell.

Tetrads split into single chromosomes.

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Phases of Meiosis MEIOSIS I Telophase I and Cytokinesis

Nuclear membranes form.

Cell separates into two cells.

The two cells produced have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

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Phases of Meiosis

Meiosis IIThe two cells produced by meiosis I now enter a second meiotic

division.

Unlike meiosis I, neither cell goes through chromosome replication.

Each of the cell’s chromosomes has 2 chromatids.

Telophase-1..

Meiosis-II

• No interphase..

• Similar to Mitosis…

Prophase-II..

• Cells have one set of sister chromatids• Nuclear membrane breaks down• Centrioles separate.• Each of the daughter cells forms a spindle,

and the double stranded chromosomes moves towards the equator..

Metaphase-II..

• Sister chromatids line up at the middle of the cell..

Anaphase-II..

• The centromeres of sister chromatids finally separate.

• The sister chromatids of each pair

move toward opposite poles.

Now its individual chromosomes.

Telophase-II..

• Cells start to split in two• Each cell has one set of chromosomes—it is a haploid• Nuclei form at opposite

poles of the cell and

cytokinesis occurs.• After completion of

cytokinesis there are

four daughter cells.• All are haploid (n)..

Common non disjunction disorder..• Down’s Syndrome – Trisomy 21• Turner’s Syndrome – Monosomy 23 (X)• Kleinfelter’s Syndrome – Trisomy 23 (XXY)• Edward’s Syndrome – Trisomy 18

A Comparison of Mitosis and Meiosis

• Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell

• Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell

• The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-9MITOSIS MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Homologouschromosomepair

Chromosomereplication

Parent cell

2n = 6

Chromosomereplication

Replicated chromosome

Prophase

Metaphase Metaphase I

Anaphase ITelophase I

Haploidn = 3

Daughtercells of

meiosis I

AnaphaseTelophase

2n 2n

Daughter cellsof mitosis

n n n n

MEIOSIS II

Daughter cells of meiosis II

SUMMARY

Meiosis

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, andtelophase

Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion

Four, each haploid (n), containing half as many chromosomesas the parent cell; genetically different from the parentcell and from each other

Produces gametes; reduces number of chromosomes by halfand introduces genetic variability amoung the gametes

Mitosis

Occurs during interphase beforemitosis begins

One, including prophase, metaphase,anahase, and telophase

Does not occur

Two, each diploid (2n) and geneticallyidentical to the parent cell

Enables multicellular adult to arise fromzygote; produces cells for growth, repair,and, in some species, asexual reproduction

Property

DNAreplication

Number ofdivisions

Synapsis ofhomologouschromosomes

Number ofdaughter cellsand geneticcomposition

Role in theanimal body

Fig. 13-9a

MITOSIS MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Chromosomereplication

Homologouschromosomepair

Chromosomereplication

2n = 6

Parent cell

Prophase

Replicated chromosome

Metaphase Metaphase I

Anaphase ITelophase I

Haploid n = 3

Daughter cells ofmeiosis I

MEIOSIS II

Daughter cells of meiosis II

nnnn

2n2n

Daughter cellsof mitosis

AnaphaseTelophase

Thank You. . .

Reference list:• Gretchen, G. (2012). Meiosis slideshare. Available from:

http://www.slideshare.net/gunthergretchen/biologa-meiosis [Accessed 06 March 2014].

• Guest21d4fb. (2010). Meiosis slideshare. Available from: http://www.slideshare.net/guest21d4fb/meiosis-2 [Accessed 06 March 2014].

• Patel, S. (2013). Meiosis slideshare. Available from: http://www.slideshare.net/GIFTTHAMSANQA1663/savedfiles?s_title=meiosis-28422102&user_login=Lionheart009 [Accessed 06 March 2014].

• Swan, J. (2011). Meiosis slideshare. Available from: http://www.slideshare.net/jayswan/chapter-13-presentation-6504581 [Accessed 06 March 2014].