MEIOSIS
Cellular Reproduction of Reproductive
Cells
Vocab you should know…
 Meiosis: cell division where one body cell
produces four gametes, each containing
half the number of chromosomes as a
parent’s body cell
 Homologous Chromosomes: paired
chromosomes w/ genes for the same traits
arranged in the same order
 Gametogenesis: process by which male &
female gametes are formed
 Spermatogenesis: process by which male
gametes form
More Vocab…
 Oogenesis: production, growth &
maturation of an egg (ovum)
 Zygote: diploid cell formed when a
sperm fertilizes an egg
 Crossing Over: exchange of genetic
material b/t nonsister chromatids
from homologous chromosomes
during prophase I of meiosis; results
in new allele combinations
More Vocab…
 Genetic Recombination: major source of
genetic variation among organisms caused by
reassortment or crossing over during meiosis
 Nondisjunction: failure of homologous
chromosomes to separate properly during
meiosis; results in gametes with too many or
too few chromosomes
 Fertilization: fusion of male & female gametes
 Independent Assortment: random distribution
of the pairs of genes on different chromosomes
to the gametes
More Vocab…
 Gamete: male & female sex cells; sperm &
eggs
 Tetrad: 4 chromatids in a pair of
homologous chromosomes that come
together as a result of synapsis during
meiosis
 Polar Body: a short-lived product of the
formation of gametes by meiosis
 Gene: short segment of DNA containing
instructions for a single trait
More Vocab…
 Synapsis:
the pairing of homologous
chromosomes during meiosis
 Homologue: something homologous
 Ovum: a mature egg cell (female
reproductive cell)
 Spermatid: immature sperm cell that
has almost completed development
 Spermatazoa: mature male
reproductive cell
Cell Cycle for Reproductive
Cells
 Interphase
 Meiosis I: prophase I, metaphase I,
anaphase I, telophase I
 Cytokinsis I / interkinesis
 Meiosis II: prophase II, metaphaseII,
anaphase II, telophase II
 Cytokinesis / Cytokinesis II
Cell Cycle of a Reproductive Cell
 Interphase – long first stage of cell grow &
DNA duplication (same as in mitosis)
 Meiosis I – similar to mitosis, results in
production of 2 2n (diploid) cells
 Interkinesis or Cytokinesis I – division of
cytoplasm to create 2 separate cells
 Meiosis II – similar to mitosis also, except
that 2 cells are undergoing it at the same
time, resulting in 4 n (haploid) cells
 Cytokinesis II – division of cytoplasm,
similar to mitosis, except results in 4 cells
Interphase
 Just as in mitosis, prior to division,
the cell must duplicate all of its
contents, including its DNA and must
grow in size in order to be able to
accommodate all of the duplicate
material
 Humans = 46 chromosomes, after
interphase = 46 x 2 = 92
Meiosis I
There are four stages to
meiosis I
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I
Prophase I
DNA coils into chromatids
Sister chromatids pair up
Spindle fibers appear
Nucleolus & nuclear membrane
disappear
 **synapsis occurs: each pair of
sister chromatids lines up next to its
homologue forming a tetrad (the two



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pairs of sister chromatids join together instead
of a pair of just a pair of sister chromatids)
Prophase I
Prophase I cont’d
 In the tetrad, chromosomes are
aligned lengthwise, so genes of one
chromosomes are adjacent to the
same genes on the other
chromosome
Prophase I – crossing over
 During synapsis, chromosomes twist
around each other
 Sometimes they twist so tightly they
exchange genetic material = crossing over
 In humans this happens 2-3 times for every
pair of chromosomes
 Crossing over can result in new traits w/in
the organism = genetic recombination
 After prophase I = 92 chromosomes
Crossing Over
Metaphase I
 Tetrads line up randomly at the
equator
 Spindle fibers from the poles
(centrioles) attach to the
centromere of each pair of the
homologous chromosomes
 After metaphase I = 92
chromosomes
Metaphase I
Metaphase I
Anaphase I
 Homologous chromosomes (sister
chromatids) move to the poles, each
pair of the tetrad to the opposite pole
 Independent assortment = the
random separation of homologous
chromosomes resulting in genetic
variation (this is what is responsible
for the differences in individuals)
 After anaphase I = 92 chromosomes
Anaphase I
Independent Assortment
Telophase I
 Homologous chromosomes reach
the opposite poles
 Spindle fibers break down
 Nuclear envelopes forms around
each set of chromosomes, nuclei
reappear
 After telophase I = 92
chromosomes, 46 in each new
nucleus
Telophase I
Interkinesis or Cytokinesis I
 Interkinesis, like cytokinesis, divides
the cells cytoplasm resulting in 2 2n
(diploid) cells, each having a
complete set of DNA
 Meiosis I starts w/ 1 2n cells, at the
end of Meiosis I, we have 2 2n cells
 Each new cell has 46 chromosomes
Interkinesis
Meiosis II
 After Meiosis I is complete, the two cells go
directly into Meiosis II
 ***No Interphase*** this means no
duplication of chromosomes
 Meiosis II has four phases
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Prophase II
Metaphase II
Anaphase II
Telophase II
 Cytokinesis II – division of cytoplasm of
both cells to create 4 separate cells
Prophase II
 Nuclear membrane breaks down,
nucleoli disappears
 Spindle fibers form
 Chromatin condenses
 Sister chromatids pair up
 After prophase II = 46
chromosomes in each cell
Prophase II
Metaphase II
Pairs of sister chromatids
move to equator in each
cell
After metaphase II = 46
chromosomes in each cell
Metaphase II
Anaphase II
Chromatids separate &
move towards opposite
poles in each cell
After anaphase II = 46
chromosomes in each cell
Anaphase II
Telophase II
 Nuclear membranes form around all 4
sets of chromosomes
 Spindle fibers break down in all cells
 Nucleoli appear in the nucleus of all 4
cells
 Chromatids uncoil into chromatin
 After telophase II = 46 chromosomes
in each cell, 23 in each new nucleues
Telophase II
Cytokinesis II
 Cleavage furrow deepens until
it pinches off each cell, creating
4 new cells
 All 4 cells are haploid (n)
(contain 1 set of chromosomes)
 After cytokinesis II = 23
chromosomes in each new cell
Cytokinesis II
Gametes & Gametogenesis
 Meiosis results in the production of
gametes
 In humans:
 Females: oogenesis
 Males: spermatogenesis
Oogenesis
 Diploid reproductive cell divides meiotically
to form 1 mature ovum (egg)
 But…meiosis results in the formation of 4
cells…how is it that we only get 1 mature
ovum?
 Ova are very large, so when cytokinesis
occurs, one cells get the majority of the
cytoplasm, this becomes the ovum
 the other 3 cells do not receive enough
cytoplasm to survive, they are called polar
bodies & will degenerate (disappear)
Oogenesis
Spermatogenesis
Diploid reproductive cell
divides meiotically to form 4
haploid spermatids
Spermatids develop into
mature spermatozoa
Spermatogenesis
Fertilization
 The process of fusing a male and
female gamete to produce offspring
 Only occurs b/t two gametes, haploid
(n) cells
 Results in an offspring that has
diploid (2n) cells (n x n = 2n)
Fertilization
Cancer
 Uncontrolled cell division that may be
caused by environmental factors &/or
changes in enzyme production in the
cell cycle
 Basically, when the internal timing of
a cell causes it to repeatedly divide at
a faster than normal rate
Meiosis Quiz
1. What happens to the DNA of a
reproductive cell during interphase?
2. Synapsis occurs during which phase
of Meiosis (be sure to note I or II)?
3. What does synapsis result in?
4. During what phase does crossing
over occur in?
Meiosis Quiz cont’d
5. What is crossing over?
6. The random separation of
homologous chromosomes resulting
in genetic variation is ____ _____?
7. Interkinesis/Cytokinesis I results in
the formation of 2 ________ cells.
8. Cytokinesis II results in the
formation of 4 _______ cells.
Meiosis Quiz Cont’d
9. Oogenesis results in the formation of
_____ mature egg cells & 3 ______
______.
10.Spermatogenesis results in the
formation of ________ spermatids.
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Bio%20101%20Lectures/Meiosis/mei
osis.htm