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Meiosis
Genetic Variation
Homologous Chromosomes
• homologous chromosomes- a pair of
chromosomes that code for the same trait (one
from mom one form dad)
• 23 pairs in humans
Karyotype
• A karyotype - is an ordered display of the pairs
of homologous chromosomes from a cell
•
•
•
•
Chromosomes Types
sex chromosomes - are called X and Y
Human females (XX) homologous pair
Human males (XY)homologous pair
autosomes -The 22 pairs of chromosomes that
do not determine sex (determine everything
else)
XY
XX
Meiosis
• Meiosis – creates four haploid (n) gametes
(egg or sperm) (two divisions)
• Egg and sperm unite to form a diploid (2n)
zygote. (develops into an embryo)
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Fig. 13-4
Interphase
• The same as mitosis
Prophase I
• Longest phase (two main things happen in
prohase one)
• Synapsis
• Crossing over
Synapsis
• Synapsis – homologous chromosomes pair up
• Sister chromatids connnect to other
homologous sister chromatids to form a tetrad.
Tetrad
(Synapsis)
Crossing Over
• Crossing over – homologous chromosomes
exchange DNA while connected as tetrads.
• Chiasma -the point at which they connect to
exchange DNA
• **
Metaphase I
• In metaphase I, tetrads line up at the metaphase plate
• Independent assortment – homologous chromosomes
line up randomly on any side (ex. All of mom’s
chromosomes don’t line up on one side)
Anaphase I
• In anaphase I, tetrads separate (homologous
chromosomes are separated)
• Sister chromatids remain attached at the
centromere and move as one unit toward the
pole
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Telephase I
• The cell is divided into two halves with sister
chromatids in each half. They are no longer identical
because of crossing over.
• Cytokinesis usually occurs simultaneously, forming
two daughter cells (not identical). 2 cells are now
present.
Cleavage
Furrow
Meiosis II (repeat)
• Meiosis II is very similar to mitosis:
• Prophase II – spindle fibers and kinetichores
form
• Metaphase II – chromatids line up
• Anaphase II – chromatids are separated into
individual chromosomes
• Telophase II and cytokinesis – each cell splits,
and nuclei reform (4 unidentical haploid cells
are now present)
• **
Fig. 13-8d
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
Sister chromatids
separate
Haploid daughter cells
forming
Mitosis vs. Meiosis
Creating Variation
• The behavior of chromosomes during
meiosis and fertilization is responsible for
most of the variation that arises in each
generation
• Three mechanisms contribute to genetic
variation:
– Independent assortment of chromosomes
– Crossing over
– Random fertilization
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Independent Assortment
• The number of combinations possible when
chromosomes assort independently into gametes is
2n, where n is the haploid number
Crossing Over
• Crossing over
produces recombinant
chromosomes
(new combinations
of chromosomes),
which combines
genes inherited
from each parent
Random Fertilization
• Random fertilization adds to genetic variation
because any sperm can fuse with any egg
• An egg and sperm can make about 70 trillion
diploid combinations
• **
You should now be able to:
1. Distinguish between the following terms:
somatic cell and gamete; autosome and
sex chromosomes; haploid and diploid
2. Describe the events that characterize
each phase of meiosis
3. Describe three events that occur during
meiosis I but not mitosis
4. Name and explain the three events that
contribute to genetic variation in sexually
reproducing organisms
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings