Meiosis
How does the process of meiosis assure genetic diversity? Why is meiosis necessary for sexually reproducing
organisms?
1. Meiosis: cell division that results in haploid gametes; used for sexual reproduction: sperm and egg each carry one set
of genetic information; when combined, a new "genome" is created so that all offspring carry traits of both parents;
uses same four phases as mitosis with the following differences:
A. Occurs with two "sessions" of divisions continuously but with no replications of chromosomes between Meiosis
I and Meiosis II
1. Homologous Chromosomes: chromosomes that contain different "versions" of genes for the same traits; one came
from mother, its matching homolog came from the father
2. Diploid Cells: have two copies of every chromosome, one set that came from each parent; females received an X
chromosome from both parents so are XX; males receive an X chromosome from the mother and a Y chromosome from
the father so males are XY.
3. Haploid Cells: have one copy of each chromosome; also known as sex cells or gametes; male and female gametes
must fuse (fertilization) to form a diploid zygote that will undergo mitosis to develop into an organism
B. In Meiosis I: all chromosomes make copies of themselves. This doubles the number of chromosomes in the cell;
while each single chromosome is still attached to its copy, each is referred to as a chromatid.
http://upload.wikimedia.org/wikipedia/en/thumb/3/31/Single_and_double_chromosomes.png/150pxSingle_and_double_chromosomes.png
1. Prophase I, homologous chromosomes from each parent pair up to form two attached sets of chromatids called a
tetrad; there are many variations in the way that they could line up
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/meiosis3.gif
2. In Metaphase I, each tetrad lines up and attaches to a single spindle fiber; crossing over may occur to provide
additional genetic variation among offspring
Figure 1 Crossing over allows new combinations of genetic traits
A.
A.
B.
http://www.accessexcellence.org/RC/VL/GG/images/comeiosis.gif
B. http://www.emc.maricopa.edu/faculty/farabee/BIOBK/Crossover.gif
3. In Anaphase I, tetrads are separated so that sister chromatids are still attached and go to different new cells.
4. In Telophase 1, cells may finish cytokenesis, or may stay only partially divided.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/Telophase1.gif
C. In Meiosis II: (begins in same two cells just created but there is NO REPLICATION OF CHROMOSOMES
BEFORE MEIOSIS TWO!)
1. Prophase II, chromosomes do not replicate!
2. Metaphase II, sister chromatids line up
metaphase II looks like metaphase as
recombinations of chromosomes
on individual spindle fibers;
seen in mitosis, except for possible
3. Anaphase II, sister chromatids are
each cell now has only half as many
Prophase I.
separated into each new cell so that
chromosomes as the original cell in
http://www.emc.maricopa.edu/faculty/fara
bee/BIOBK/meiosis4.gif
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/meiosis5.gif
4. Telophase II, each of the four cells completes reforming nuclei and cytokenesis separates the four new haploid cells.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/meiosis6.gif
Meiosis diagrams
©1992, 1994, 1997,
2007, by M.J. Farabee,
Use for educational
encouraged.
adapted from: Text
1998, 2000, 2001,
all rights reserved.
purposes is
2. Genetic
(variations) can come
random arrangement of
combinations of sperm
for great genetic diversity
increase the probability
reproductive success.
recombinations
from crossing over,
tetrads, and random
and eggs. This allows
which would
of the species
3. Oogenesis: meiosis that produces eggs (ova); one will be viable, and three tiny polar bodies, all with correct # of
chromosomes but only one ovum, is large enough to survive and be fertilized and have enough nutrients to be divided
many times before implantation into the uterine wall; the three tiny polar bodies will dissolve
http://upload.wikimedia.org/wikipedia/commons/thumb/4/44/Gray5.svg/250px-Gray5.svg.png
4. Spermatogenesis: meiosis that produces sperm; all four are tiny and all four are capable of fertilizing an egg (ovum)
http://upload.wikimedia.org/wikipedia/commons/thu
mb/8/86/Sperm-egg.jpg/350px-Sperm-egg.jpg
5. Nondisjunction Mutations: improper separation of
sister chromatids may result in a cell having one too
many chromosomes (trisomy) or not having one of a
certain chromosome (momosomy); the more
commonly seen conditions are shown here:
A. Trisomy 21, Down Syndrome (3 of the 21st pair
of chromosomes)
B. Trisomy 23, Klinefelter Syndrome: XXY (two female chromosomes and one male)
After Meiosis I, there are two cells, each with the same number of chromosomes. After Meiosis II, there are three small
polar bodies and one large ovum; each has half the original number of chromosomes
C. Monosomy 23, Turner Syndrome: XO (only one X, no other sex chromosome)
6. Karyotype: a picture of an individual's chromosomes so that the above types of mutations might be seen; a fetus's
cells may be collected by:
A. Amniocentesis: long needle withdraws fluid around fetus; some cells from the fetus are floating in the fluid; fetal cells
are then tested using DNA analysis to search for damaged chromosomes
B. Chorionic Villi Testing: scraping a few cells from villi of the placenta which connect fetus to mother in the uterus,
then making an analysis of the DNA to look for damaged genes
7. Ultrasound pictures can be taken using sound waves while fetus is still in the uterus to determine gross structural
abnormalities; safest procedure; only sound waves are used;
This 32 week old fetus can be
seen yawning in this 4D ultrasound image
http://upload.wikimedia.org/wikipedia/commons/3/39/3dultrasound.png