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Chapter 11
Meiosis and Sexual
Reproduction
Section 2
Meiosis
Chapter 11 Section 2:
Meiosis
Key Vocabulary
Terms
Adapted from Holt Biology 2008
Meiosis
A special process of cell division that results in
haploid sex cells
The total number of chromosomes decreases to
half the original number
Crossing-over
The exchange of genetic material between
homologous chromosomes during meiosis
Independent Assortment
The random distribution of pairs of genes on
different chromosomes to the gametes
Mitosis
In eukaryotic cells,
a process of cell
division that forms
two new nuclei,
each of which has
the same number
of chromosomes
Tetrad
The four
chromatids in a pair
of homologous
chromosomes that
come together as a
result of synapsis
during meiosis
Homologues
Something
homologous
Genetic Variation
Genetic diversity in a
population or species
as a result of new
gene combinations
(e.g. crossing over of
chromosomes),
genetic mutations,
genetic drift, etc.
Sperm
The male gamete (sex cell)
Ovum
The female gamete: A mature egg cell
Chapter 11
Section 2:
Meiosis
Notes
Adapted from Holt Biology 2008
Stages of Meiosis
During meiosis,
a diploid cell
divides twice to
form four haploid
cells.
Meiosis I
Meiosis I separates
homologous
chromosomes,
producing two
haploid cells (N
chromosomes, 23
in humans).
Meiosis I (PMAT1)
• Prophase I – Homologous chromosomes pair
– Chromatids exchange genetic material
(crossing-over)
• Metaphase I – Homologous pairs of
chromosomes align at the cell equator
• Anaphase I – Homologous pairs separate,
chromatids do not.
• Telophase I – two new haploid cells result
(one set of chromosomes)
Prophase I
Chromosomes
condense, and the
nuclear envelope
breaks down.
Homologous
chromosomes pair.
Chromatids exchange
genetic material.
Metaphase I
Homologous pairs
of chromosomes
align at the cell
equator
Anaphase I
Homologous
chromosomes
separate. The spindle
fibers pull the
chromosomes of each
pair to opposite poles
of the cell without the
chromatids separating
at their centromeres.
Each chromosome is
still made of 2
chromatids.
Telophase I
The cytoplasm divides
and two new cells are
formed. Both cells
have one
chromosome from
each pair of
homologous
chromosomes.
Meiosis II (PMAT II)
No more homologous chromosomes
Meiosis II
A separation of identical chromatids of
each of the two haploid cells created in
meiosis I, giving birth to four haploid cells.
Prophase II
The disappearance of
the nucleoli and the
nuclear envelope again
as well as the shortening
and thickening of the
chromatids. Centrioles
move to the polar
regions and arrange
spindle fibers for the
second meiotic division
Metaphase II
The centromeres
attach to spindle
fibers from the
centrosomes
(centrioles) at each
pole.
Anaphase II
The centromeres are
cleaved, allowing
microtubules to pull the
sister chromatids apart.
The sister chromatids
by convention are now
called sister
chromosomes as they
move toward opposing
poles.
Telophase II
Uncoiling and
lengthening of the
chromosomes and the
disappearance of the
spindle. Nuclear
envelopes reform and
cleavage or cell wall
formation eventually
produces a total of four
daughter cells, each with
a haploid set of
chromosomes.
Meiosis II (PMAT II)
• Prophase II – new spindle forms
• Metaphase II – individual chromosomes
align at the cell equator
• Anaphase II – chromatids separate
• Telophase II – four genetically different
cells result
11-2 Meiosis (SKIP)
• Compare Mitosis and Meiosis
Mitosis
Purpose
Location
Resulting cells
Meiosis
Genetic Variation
Why is genetic
variation in a
population
beneficial?
(See the
advantages of
sexual reproduction
on page 1.)
Genetic Variation
Three parts of sexual
reproduction that contribute
to variety:
•Crossing-over
•Independent assortment
•Random fertilization
Crossing-over
• Occurs during prophase I of meiosis.
• Sister chromatids of homologs exchange
genetic material.
• After crossing over, do sister chromatids
on homologous chromosomes still have
identical genetic material?
Independent Assortment
• Selection of one trait will not determine
the selection of another.
• Alignment of homologous chromosomes
during metaphase I is random.
• A pair of chromosomes can line up in two
possible ways.
• Resulting gametes are all different.
Meiosis
The red chromosomes don't
influence the blue ones.
Random Fertilization
• The union of egg and sperm is random.
• In humans, there are 70 trillion different
possibilities.
– 223 x 223
11-3 Life Cycles
• Diploid Life Cycles – Diploid phase is
dominant in humans
Diploid
Diploid
Haploid
Diploid
11-3 Life Cycles
• Spermatogenesis – the formation of sperm
11-3 Life Cycles
• Oogenesis – the formation of an ovum
11-3 Life Cycles
• Haploid Life Cycles
• – fungi: haploid phase is dominant
11-3 Life Cycles
• Alternation of Generations – plants and multicellular
protists