Meiosis
I. Purpose of Meiosis
A.
Meiosis produces sex cells (gametes) with half the genetic
information as the parent cells.
1. Cells that contain a double set of chromosomes are called
diploid (2n)
a. All somatic cells (body cells) contain 2 sets of
chromosomes
b. In humans 2n = 46
2. Cells that contain a single set of chromosomes are called
haploid (n).
a. Gametes (egg or sperm) contain 1 set of chromosomes
b. In human egg or sperm n = 23
3. Germ cells in the testes or ovaries produce gametes.
a. Oogeneis is the production of egg cells which are larger,
non-motile cells
b. In most vertebrate animals only 1 viable egg is produced,
the three other polar bodies are not able to be fertilized.
c. Spematogenesis is the production of sperm cells which
are smaller and have flagella.
B. Meiosis allows for genetic diversity.
1. Each pair of chromosomes can line up in 2 different ways.
a. If a cell has 2 pair of chromosomes there are 22 or 4
possible combinations
b. In humans there are 223 possible or 8,388,608 possible
combinations
2. Chromosomes can also exchange genetic information during
Crossing-Over .
a. During metaphase I of meiosis, chromosmomes arrange
themselves in groups of 4.
b. Information from homologous (like) chromosomes can be
exchanged.
c. Crossing-over allows for even more genetic
combinations.
Crossing Over
Mom’s
Dad’s
Sister chromatids
Homologous chromosomes
II. The Phases of Meiosis
A. In meiosis, cells divide twice after a single DNA duplication.
B. Meiosis I – Separates homologous (like) chromosomes.
1. Prophase I
a. Chromosomes coil tightly & are visible
b. Nuclear membrane & nucleolus disintegrate
c. Spindle forms
d. Synapsis (joining) of homologous chromosomes occurs
making tetrads (groups of 4 chromosomes)
2. Metaphase I
a. Tetrads become aligned in the center of the cell attached to
spindle fibers
b. Chromosomes in tetrad exchange fragments by a process
called Crossing-Over.
3. Anaphase I – Homologous chromosomes separate
4. Telophase I
a. May not occur in some species
b. Cytokinesis occurs producing 2 cells
c. In females, the 2nd cell or first polar body dies due to uneven
splitting of the cytoplasm
C. Meiosis II – Separates homologues (sister chromatids)
1. Prophase II
a. Cells are called secondary spermatocytes or oocytes
b. DNA is not copied before cell divides
c. Chromatids attach to spindle fibers
2. Metaphase II
chromosomes become aligned in the center of the cell
attached to spindle fibers
3. Anaphase II
a. Sister chromatids (homologues) separate randomly
b. Called independent assortment
4. Telophase II
a. Cytokinesis occurs producing 4 cells in males called
spermatids
b. Spermatids mature and form flagellum to become sperm
c. Cytokinesis in females produces a 2nd Polar Body (that
dies) and an ootid
d. Ootids mature to become ovum or egg
III. Asexual and Sexual Reproduction
A.
B.
C.
D.
Evolution is the slow process of change in living populations
over time
Variations are differences that occur due to crossing-over
among members of a sexually reproducing population
Variations are important to the survival of individuals in a
population (some must survive to reproduce)
Asexually reproducing organisms rarely show variations
because the organisms have identical genes.
Map of the Y
chromosome
The End!