MeiosisMeiosisGamete ProductionGamete Production
Mr. YoungMr. Young
Cells do not increase in size. They increase in number.
• Meiosis = process of nuclear division that reduces the chromosome number by half in new cells (gametes)
• cells undergo interphase, grow, copy DNA just like mitosis, but in meiosis the cells divide twice
• two Diploid (2n) cells divide twice to produce four Haploid (1n) cells
• 2 stages = Meiosis I and Meiosis II
Meiosis• Interphase• Meiosis I
– Prophase I– Metaphase I– Anaphase I– Telophase I & Cytokinesis (results in 2 haploid cells)
• Goes directly into Meiosis II (no interphase, no dna copying)– Prophase II– Metaphase II– Anaphase II– Telophase II & Cytokinesis (results in 4 haploid cells)
Meiosis
• Creates four haploid cells (gametes)
• Two nuclear divisions occur without DNA being copied in between
Crossing Over• Crossing over - portions of
chromatids break off and attach to adjacent chromatids on homologous chromosome
• Crossing over allows for the random mixing of genes (genetic recombination) which adds genetic variety to a species so that no two individuals are exactly the same.
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.
Prophase I:Tetrad formation/
crossing over
Crossing Over
Anaphase I
Telophase II
Metaphase I
Telophase IBecause of crossing over, every gamete receives a unique set of genetic information.
Crossing over animation
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__unique_features_of_meiosis.html
Stages Of Meiosis• Meiosis resembles mitosis except that it
is actually two divisions not one• These divisions are called Meiosis I and
Meiosis II• Meiosis I results in haploid cells with
chromosomes made up of two chromatids• Meiosis II is essentially mitosis on haploid
cells
Stages Of Meiosis - Meiosis I• Prophase I - The beginning phase -
– DNA which was unraveled and spread all over the nucleus is condensed and packaged
– Homologous chromosomes (each made of two identical chromatids) come together and form tetrads (4 chromatids)
– Crossing over, in which chromatids within tetrads exchange genetic material, occurs
• Metaphase I - Middle stage - Tetrads line up along the equator of the cell
Stages Of Meiosis - Meiosis I• Anaphase I - One copy of each
chromosome still composed of two chromatids moves to each pole of the cell
• Telophase I - End stage - New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two haploid daughter cells
• NOTE: There is NO interphase between meiosis I and meiosis II.
• Why is this important?
• So that each new cell will only have half (haploid) the number of chromosomes as the original parent cell.
• Meiosis II is not preceded by the copying of DNA
Stages Of Meiosis - Meiosis II• Prophase II - Cells do not go into interphase
between meiosis I and II, thus chromosomes are already condensed
• Metaphase II - Chromosomes line up at the equator of the two haploid cells produced in meiosis I
• Anaphase II - Chromosomes made up of two chromatids split to make chromosomes with one chromatid which migrate to the poles of the cells
• Telophase II - Cytokinesis and reformation of the nuclear membrane in four haploid cells each with one set of chromosomes made of one chromatid
Interphase
Mother cell Stages Of Meiosis: Meiosis I
Meiosis II
Prophase I:Tetrad formation/
crossing overMetaphase I
Telophase I
Prophase I:Condensing
Chromosomes
Anaphase I
Telophase I
Stages Of Meiosis: Meiosis II
Metaphase II
Anaphase II
Telophase II
The products of mitosis are 2 diploid cells with identical chromosomes.
The products of meiosis are 4 haploid cells each with a unique set of chromosomes.
Prophase II
Let’s look at Meiosis Animations!!
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__how_meiosis_works.html
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__stages_of_meiosis.html
Gametes Are Haploid• Gametes = haploid reproductive cells (sperm, egg cells) one
set of chromosomes (1n)• Gametes must have half the genetic material of a normal cell• If the genetic material in the gametes was not halved, when
they combined the zygote would have more genetic material than the parents
• Meiosis is specialized cell division resulting in cells with half the genetic material of the parents
“Putting It All Together” - Fertilization
Spermatogenesis
• Spermatogenesis - diploid reproductive cell divides through meiosis to form 4 sperm cells
Oogenesis
• Oogenesis - Production of mature egg cells through meiosis.
• Only one of the four develops into a mature egg cell, the other 3 are called ‘polar bodies’ and degenerate
What Meiosis is About
Meiosis allows the creation of unique individuals through sexual reproduction.
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