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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].