Cellular Reproduction Part II
• Ploidy (N) describes the number of copies of
chromosomes a cell contains. Haploid (1N), Diploid
(2N), Triploid (3N), Tetraploid (4N).
• Humans are diploid organisms, each cell has 2 sets of
• In order to conserve the diploid requirement for
humans, sex cells must be haploid (1N). That way when
two sex cells join they create a diploid organism.
• Meiosis is the process of creating the haploid sex cells
called gametes.
• If meiosis didn’t occur, when our sex cell combine they
would create a tetraploid zygote, which would be
incompatible for human life and not survive.
Human Spermatogenesis
Humans Oogenesis
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Results in one
diploid primary
oocytes and a
polar body in
• Results in two
diploid primary
in males.
Crossing Over: exchange of genetic
material between homologous
chromosomes. Occurs during Prophase
I. Responsible for genetic diversity.
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
• Results in 4 haploid secondary spermatocyes in males.
• Results in 1 haploid secondary oocyte and 3 polar bodies
in females.
• Occurs in
anaphase II, when
the sister
chromatids are not
correctly. This
results in one n+1
gamete, one n-1
gamete, and two
normal haploid
• If the n+1 gamete
gets fertilized it
will result in
trisomy, If the n-1
gamete gets
fertilized it will
result in
Chromosomal Diseases
• Monosomy
– Most monosomy zygotes are incompatible with life and
spontaneously abort.
– Turner’s Syndrome: XO
• Trisomy’s:
– 21: Down’s syndrome
– XXY: Kleinfelter’s syndrome
– XYY syndrome
• Meiosis occurs in sex cells to create gametes.
• It is two events of cell division with only one even of DNA
synthesis which results in haploid gametes.
• In human men it results in 4 sperm. Men constantly
produce sperm through meiosis once they reach puberty.
• In human females it results in 1 oocyte and 3 polar bodies.
Females create all the oocytes they will ever have in while
they are a fetus. The oocytes are rested in the primary
oocyte phase until puberty, then each ovulation cycle they
mature 1 oocyte to the secondary oocyte stage.
• The division events have the same stages as mitosis.
(prophase I, metaphase I, anaphase II, telophase I,
interkinesis, prophase II, metaphase II, anaphase II,
telophase II, cytokinesis)
• The gametes are all unique.
• Nondisjunction is an error in separating chromosomes
(anaphase), it can happen during meiosis II, and results in
monosomy or trisomy diseases.

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