Bio 178 Lecture 22
Meiosis
http://homepage1.nifty.com/scilla/sonota/ityou/ityou.html
Reading
•
Chapter 12
Quiz Material
•
Questions on P 240
•
Chapter 12 Quizzes on Text Website
(www.mhhe.com/raven7)
Outline
• Sexual Reproduction
 The Sexual Life Cycle
 Meiosis
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.
The 3 Types of Sexual Life Cycle
Unique Features of Meiosis
McGraw-Hill Video
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 Over
Purpose
Genetic Variation
Synapsis
Pairing of chromosomes along their length (after DNA
replication).
Homologues attached to nuclear envelope.
Synaptonemal Complex
Synaptonemal Complex
(Enzymes)
http://www.ucl.ac.uk/~ucbhjow/bmsi/synapto.html
Spermatocyte Synaptonemal Complex
Green = SCP 1
Blue = DNA
Red/Yellow =
Centromere
Top = Normal
Bottom = SCP 3
deficient
http://7e.devbio.com/article.php?ch=19&id=188
Genetic Variation in the Sexual Life Cycle
Crossing 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.
Comparison of Meiosis and Mitosis
McGraw-Hill Video
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).
Prophase I
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.
Metaphase I
Independent Assortment
McGraw-Hill Video
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.
Anaphase I
Telophase I
Meiosis II
There may be a brief interphase, but NO DNA
SYNTHESIS occurs.
• Prophase II
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.
Prophase II
Metaphase II
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.
Anaphase II
Telophase II
?
Biodidac.
http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=197D&File_type=GIF
Sources of Genetic Variation in Meiosis
• Crossing over
• Independent assortment