MEIOSIS & Fertilization
Objective: E1 Recognize the
significance of
meiosis to sexual
reproduction,
understand how
gametes are
formed, &
recognize the
process of
fertilization.
Homologous Chromosomes

Homologous refers to chromosomes
that each have a corresponding
chromosome from the opposite-sex
parent.

Humans have 23 pairs of homologous
chromosomes.
22 pairs of autosomes
1 pair of sex chromosomes
Number of Chromosomes
Diploid
cells contain both
sets of chromosomes
Haploid cells contain a
single set of chromosomes
Organisms that reproduce Sexually are
made up of two different types of cells.
1.
Somatic Cells are “body” cells and
contain the normal number of
chromosomes ….called the “Diploid”
number (the symbol is 2n). Examples
would be … skin cells, brain cells, etc.
2.
Gametes are the “sex” cells and contain
only ½ the normal number of
chromosomes…. called the “Haploid”
number (the symbol is n)….. Sperm cells
and ova (eggs) are gametes.
Meiosis
is describe as reduction division
because the number of
chromosomes per cell are cut in
half through the separation of
homologous chromosomes.
Diploid (2n)  Haploid (n)
Meiosis
Has 2 cell divisions
(called Meiosis I and Meiosis II)
with only one duplication of
chromosomes.
So, 1 diploid cell becomes 4 haploid
cells.
In prophase I of meiosis, each chromosome pairs
with its corresponding homologous chromosome to
form a
Tetrad
(shown below)
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Crossing Over
-Process in which homologous chromosomes exchange
portions of their chromatids during Prophase I of
meiosis
- Creates variation (diversity) in the offspring’s traits
Tetrad
nonsister chromatids
chiasmata: site
of crossing over
variation
Gametogenesis & Fertilization
A. Cytokinesis of meiosis differs in males
and females
1. Male – Spermatogenesis
a. Cytokinesis is even
b. All 4 gametes survive
Gametogenesis & Fertilization
2. Female – Oogenesis
a. Cytokinesis is uneven
b. Most of the cytoplasm goes to 1
cell = egg
c. The other 3 (very small) = polar
bodies
3. Fertilization
◦ Egg + sperm = fertilized egg (zygote)
 n + n = 2n
- Fertilization restores the diploid number
Question:
A cell containing 20 chromosomes
(diploid) at the beginning of meiosis
would, at its completion, produce cells
containing how many chromosomes?
Answer:
10 chromosomes (haploid)
Fertilization
Sperm + Ovum (egg)
fertilization
Zygote
?
Cell Division Review:
spindle
A fiber
B
centrioles
Mitosis or Meiosis?
Etymology
Haploid: haplo- (Gk. HAPLOS, single) + oid (shape or form)
 Diploid: Diplo- (Gk. DIPLOOS, double) +
-oid (shape or form)
