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Meiosis. Box 55. Meiosis results in cells with ½ the # of chromosomes, so they can combine in sexual reproduction , which results in an embryo with the normal amount of chromosomes. - PowerPoint PPT Presentation
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Fall 2005 – @ K. Malone
Meiosis• To solve this problem, the cells in the gametes (eggs
and sperm), undergo Meiosis instead of Mitosis, when they divide.
• Why Meiosis?
• Most cells in our body are Diploid (“di” = 2)• Diploid = have 2 sets of chromosomes, 2n
(one from mom, one from dad)• Sex cells, or gametes, are Haploid • Haploid = have 1 set of chromosomes, 1n
Box 41
Box 42
Box 53
Box 54
Meiosis results in cells with ½ the # of chromosomes, so they can combine in sexual
reproduction, which results in an embryo with the normal amount of chromosomes.
Box 55
Fall 2005 – @ K. Malone
Chromosome Number
• An example.
• Fruit Flies (Drosophila), have 8 chromosomes total – 4 from dad fruit fly, 4 from mom fruit fly.
Diploid number?2n = 8
Haploid number?
1n = 4
Box 43
Homologous chromosomes= 2 sets of similar chromosomes, one from mom, one from dad
Box 56
Fall 2005 – @ K. Malone
Phases of Meiosis
• How are haploid (1n) gamete cells produced from diploid (2n) cells?
• By Meiosis (my-OH-sis)
• Meiosis = process of reduction cell division where the # of chromosomes per cell is cut in ½ by the separation of homologous chromosomes
Homologous = same chromosome, it just comes from different parents (mom or dad)
Meiosis involves 2 distinct divisions, 1. Meiosis I 2. Meiosis II Results of Meiosis? = cells with a haploid (1n)
number of chromosomes
Box 45
Box 46
Box 57
Box 58
Box 59
Fall 2005 – @ K. Malone
Meiosis I
Chromosome duplicationChromosome duplication
2 sister chromatids2 sister chromatids
4 sister chromatids 4 sister chromatids form a Tetradform a Tetrad
Interphase I Prophase I
Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes
Box 60
Prophase I = Each chromosome pairs up with its matching homologous chromosome forming a tetrad
Box 61
Tetrads= in prophase I, chromosomes duplicate and form a structure with 4 chromosomes (tetrad)
Box 62
Fall 2005 – @ K. Malone
During prophase I of Meiosis, when the tetrads form, a phenomenon called “Crossing Over” can occur.
Owww! My
arm!
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Who me?
Hey hot stuff, wanna dance?
I think he likes
me!
You got chocolate in my peanut
butter!
You got peanut butter
in my chocolate!
Help!Crossing Over = results in the exchange of genetic information between homologous chromosomes
Box 63
Fall 2005 – @ K. Malone
Meiosis I
Interphase I Prophase I Metaphase I
Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes
Prophase I = Each chromosome pairs up with its corresponding homologous chromosome to form a tetrad
Anaphase I = Fibers pull the homologous chromosomes towards opposite ends of the cell (“A” for apart)
Box 64
Metaphase I = Spindle fibers attach to the chromosomes, then they line up in the middle (“M” for middle)
Box 65
Anaphase I
Fall 2005 – @ K. Malone
Meiosis I
Interphase I Prophase I Metaphase I Anaphase I Telophase and Cytokinesis
Prophase I = Each chromosome pairs up with its corresponding homologous chromosome to form a tetrad
Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes
Anaphase I = Fibers pull the homologous chromosomes towards opposite ends of the cell (“A” for apart)
Metaphase I = Spindle fibers attach to the chromosomes, then they line up in the middle (“M” for middle)
Telophase I and Cytokinesis = Nuclear membranes form, the cell separates into 2 new cells
Box 66
Fall 2005 – @ K. Malone
Meiosis IIThe 2 new cells now enter a 2nd round of meiotic division
No more chromosome duplication
prophase II
Prophase II = of meiosis I results in 2 haploid daughter cells, each with ½ the # of chromosomes as the original cell Box
67
Fall 2005 – @ K. Malone
Meiosis II
prophase II metaphase II
Prophase II = of meiosis I results in 2 haploid daughter cells, each with ½ the # of chromosomes as the original cell
Metaphase II = chromosomes line up in the middle Box
68
Fall 2005 – @ K. Malone
Meiosis II
anaphase II
1
2
3
4
telophase IIprophase II metaphase II
Metaphase II = chromosomes line up in the middle
Prophase II = of meiosis I results in 2 haploid daughter cells, each with ½ the # of chromosomes as the original cell
Anaphase II = sister chromatids separate and move to opposite ends of the cell
Box 69
Telophase II and Cytokinesis = of meiosis II results in 4 haploid (1n) daughter cells
Box 70
Fall 2005 – @ K. Malone
Prophase I
Anaphase I
Telophase I&
Cytokinesis
Prophase II
IP
MA
T
P
MAT
I then …PMAT…..PMATI then …PMAT…..PMAT
Telophase II&
CytokinesisAnaphase II Metaphase II
4 daughter cells4 daughter cellswith haploid # (1n)with haploid # (1n)
Humans = 2n = 46
9292
92 92
46
46
464623
Flow Chart for MeiosisFlow Chart for Meiosis
Interphase I
Metaphase I
Fall 2005 – @ K. Malone
Gamete Formation
• Why do we even have Meiosis?• To make gametes (eggs and sperm).• Male gametes = sperm• Female gametes = eggs
Why do we need eggs and sperm?Why do we need eggs and sperm?
Skip Box 59
End of Chapter 10 and Chapter 11 Section 11-4
Box 71
For For Sexual ReproductionSexual Reproduction, where egg , where egg and sperm unite to make an embryoand sperm unite to make an embryo
Box 72