CH 10.2
Meiosis
Meiosis
• Meiosis is cell division that produces
gametes with half the number of
chromosomes as a body cell
Meiosis
• Diploid cell: has 2 of each kind of
chromosome
– All body cells, except gametes, are diploid
cells (such as liver or lung cells)
– When they divide, this is called mitosis
• Haploid cell: has 1 of each kind of
chromosome
– Gametes are haploid cells
– When they divide, this is called meiosis
Why Meiosis?
• Each type of organism has a specific
number of chromosomes (humans have
46 chromosomes)
• Because each parent passes along a
chromosome, there has to be some way
for offspring to have the same number
of chromosomes as their parents and
not double
Why Meiosis?
• Gametes that are produced are sperm
cells for males and egg cells for
females
• Sexual reproduction: when haploid egg
and sperm cells join together to form a
diploid zygote
• In humans, each sperm and egg cell has
23 chromosomes (which is half of 46,
the normal number of chromosomes) or
Sperm (23) + Egg (23) = Zygote (46)
Why Meiosis?
• For the most part, healthy human
zygotes cannot have more than 46
chromosomes
• Once formed, zygotes undergo mitosis
to grow and develop (asexual
reproduction)
Before Meiosis
• Homologous chromosomes are attached
to each other
– Homologous chromosomes: 2 of the same
type of chromosome
• Chromosomes copy themselves so that
they can be passed along to the
daughter cells
Before Meiosis
• Tetrad: 2 homologous chromosomes
attached together (= 4 sister
chromatids)
Genetic Variation
Genetic variation: rearrangement of
alleles that can produce completely new
combinations (this is essential for
evolution to occur)
Genetic Variation
• Crossing Over- When
the homologous
chromosomes pair so
tightly that a piece of
the chromatid breaks
off. The piece changes
place with a piece from
the other homologous
tetrad. This scrambles
the traits around =
genetic variety.
Genetic Variation
• Genetic Recombination: the
reassortment of chromosomes and
genetic material, either by crossing over
or independent assortment, which
increases genetic variety
Genetic Variation
• Both crossing over and independent
assortment are completely
RANDOM…so this is why humans can
sometimes look completely different
from other family members
Meiosis I—2 new cells are formed
Prophase I:
*Tetrads condense (2 homologous
chromosomes attached together)
*Crossing over occurs between
chromosomes
*Nuclear membrane disappears
• Centrosomes move to opposite ends of cell
to form spindle fibers
• Metaphase I: Tetrads line up in the middle
of the cell
Meiosis I
• Anaphase I: Tetrads split and each set
of sister chromatids moves to opposite
ends of the cell
• Telophase I:Cytokinesis (division of the
cytoplasm) results in 2 new cells, each
with homologous chromosomes
Meiosis II
(4 new cells are formed) *LIKE MITOSIS*
• Prophase II: Centrosomes on the
chromosomes form spindle fibers in
preparation to be pulled apart
• Metaphase II: Chromosomes (sister
chromatids) line up randomly in the middle
Meiosis II
• Anaphase II: Sister chromatids
separate and move to opposite ends of
the cell
• Telophase II/Cytokinesis: : New nuclei
are formed and four daughter cells are
produced, each containing ONE
chromosome from each homologous pair
Mistakes in Meiosis
• Nondisjunction: when homologous
chromosomes don’t separate from each
other during meiosis
• So homologous chromosomes move
together into a new gamete giving that
gamete too many chromosomes, while
the other new gamete is missing the
chromosome
Mistakes in Meiosis
• Organisms with extra chromosomes can
survive
• Organisms with too few chromosomes
usually do not survive
Mistakes in Meiosis
• Polyploidy: when organisms have more
than the normal number of chromosome
sets
– instead of 46 chromosomes, they
might have double or triple that
number
– Can be lethal for animals but not
plants (plants just usually get bigger)
Conditions that Result from
Extra Chromosomes
Examples of Nondisjunction
Turner’s Syndrome
Genotype (X=Female sex
chromosome,
Y= male sex chromosome)
XO
Trisomy X (metafemales)
XXX
Klinefelter’s syndrome
XXY
Jacob’s syndrome
XYY
Down syndrome
TRISOMY 21 (3 copies)
Patau’s syndrome
TRISOMY 13
Lethal condition in males
OY
Comparing Mitosis and Meiosis
Mitosis
DNA replicates in interphase
Meiosis
DNA replicates in interphase
ONE division of chromosomes
 Separate sister chromatids
TWO divisions of chromosomes
 Separate homologous
chromosomes (meiosis I)
 Separate sister chromatids
(meiosis II)
Makes 2 identical DIPLOID
cells
Chromosome # is identical to
parent cell
Used for replacing old cells,
growth
Makes 4 HAPLOID cells
Chromosome # is HALF of the
parent cell
Used to make gametes