bio 178 lecture 22 meiosis
Post on 22-Dec-2015
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TRANSCRIPT
Reading
• Chapter 12
Quiz Material
• Questions on P 240
• Chapter 12 Quizzes on Text Website (www.mhhe.com/raven7)
The Sexual Life cycle
• Alternation of Generations
All sexual life cycles alternate between a haploid and diploid state. In some organisms the diploid stage is dominant and in others the haploid stage is dominant.
This is a consequence of mitosis and meiosis:
• Mitosis
Maintains chromosome number.
• Meiosis
Halves chromosome number.
Genetic Variation in the Sexual Life Cycle
• Maternal and Paternal Chromosomes
If there was no exchange of genetic material between the homologues each gamete would receive an entire homologue that is either maternal or paternal.
• The Process of Crossing OverPurpose
Genetic Variation
Synapsis
Pairing of chromosomes along their length (after DNA replication).Homologues attached to nuclear envelope.
Spermatocyte Synaptonemal Complex
http://7e.devbio.com/article.php?ch=19&id=188
Green = SCP 1
Blue = DNA
Red/Yellow = Centromere
Top = Normal
Bottom = SCP 3 deficient
Genetic Variation in the Sexual Life CycleCrossing Over• DNA is exchanged between the non-sister chromatids at the chiasmata.
• Recombination nodules break and rejoin homologous chromatids.
http://www.blc.arizona.edu/courses/181summer/10.html
Post Crossing Over
• Synaptonemal complex breaks down.
• Homologues are now joined only by common centromeres and chiasmata.
• Homologues released from nuclear envelope.
Meiosis• Cell Divisions
2 divisions after the chromosomes have replicated:
Meiosis I
Separation of homologous chromosomes.
Meiosis II
Separation of sister chromatids.
• Ploidy
Diploid Haploid
Meiosis I• Prophase I
Chromosomes finish condensing.
Synapsis and crossing over occur.
Centrioles (animal cells) move to opposite poles and spindle fibers form between them.
Nuclear envelope breaks down.
Chiasmata move to the ends of the chromosomes ( terminal chiasmata).
Meiosis I (Cntd.)• Metaphase I
Homologous chromosomes align on the metaphase plate, attached to the kinetochore microtubules. These are attached only to the outward facing kinetochores.
Independent assortment - The homolgue orientation is random.
Meiosis I (Cntd.)• Anaphase I
Homologues separate and are pulled to opposite poles.
• Telophase I
Homologues reach opposite poles.
Nuclear envelopes may form.
• Cytokinesis
May occur.
Meiosis II
• Prophase II
There may be a brief interphase, but NO DNA SYNTHESIS occurs.
Centrioles move to opposite poles and spindle fibers form.
If the nuclear envelope reformed it breaks down.
• Metaphase II
Chromosomes align at the metaphase plate. Kinetochore microtubules are attached to both sides of the centromere.
Meiosis II (Cntd.)• Anaphase II
Sister chromatids separate and are pulled apart ( chromosomes).
• Telophase II
Chromosomes reach opposite poles.
Nuclear envelopes form.
• Cytokinesis
Results in 4 daughter cells.