1. Meiosis
Genetic Variation

2. Homologous Chromosomes
homologous chromosomes- a pair of chromosomes that code for the same trait (one from mom one form dad)
23 pairs in humans

3. Karyotype
A karyotype - is an ordered display of the pairs of homologous chromosomes from a cell

4. 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

5. 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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6. Interphase The same as mitosis Fig. 13-4
Figure 13.4 Describing chromosomes

7. Prophase I Longest phase (two main things happen in prohase one)
Synapsis
Crossing over

8. Synapsis Synapsis – homologous chromosomes pair up
Sister chromatids connnect to other homologous sister chromatids to form a tetrad.
Tetrad
(Synapsis)

9. Crossing Over
Crossing over – homologous chromosomes exchange DNA while connected as tetrads.
Chiasma -the point at which they connect to exchange DNA **

10. 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)

11. 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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12. 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

13. 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) **

14. Telophase II and Cytokinesis
Fig. 13-8d
Telophase II and
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Figure 13.8 The meiotic division of an animal cell
Sister chromatids
separate
Haploid daughter cells
forming

15. Mitosis vs. Meiosis

17. 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
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18. Independent Assortment
The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

19. Crossing Over Crossing over produces recombinant chromosomes
(new combinations
of chromosomes),
which combines
genes inherited
from each parent

20. 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 **

21. You should now be able to:
Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid
Describe the events that characterize each phase of meiosis
Describe three events that occur during meiosis I but not mitosis
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