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Reproduction
and
Meiosis
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Reproduction
• Reproduction—the process of producing
offspring
– Asexual—one parent, offspring are
genetically identical to parent
• Budding, fission, fragmentation, parthenogenesis
– Sexual—two parents, offspring genetically
different from parents
• Parents produce gametes
– Females=egg
– Males=sperm
• Two gametes combine=fertilization; produces first
cell called the zygote
• Offspring will have traits of both parents
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Homologous Chromosomes
• Each species has a characteristic number
of chromosomes
– Humans=46; dogs=78; fruit flies=8
• Chromosomes come in pairs—homologous
chromosomes
– Chromosomes with the same size, shape, and
carry the codes for the same genes
– Each chromosome in a homologous pair comes
from one of the parents
• In humans, 23 chromosomes come from mom and
23 from dad
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Meiosis
• Form of cell division that produces 4
daughter cells with half the number of
chromosomes as the parent cell
• 2 distinct stages
– Meiosis I
• Prophase I, Metaphase I, Anaphase I,
Telophase I
– Meiosis II
• Prophase II, Metaphase II, Anaphase II,
Telophase II
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Meiosis I
• Prophase I
– Chromosomes condense
– Homologous chromosomes pair up
– Crossing over
• Chromatids exchange genetic material
• Increases genetic variability
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Meiosis I
• Metaphase I
– Homologous chromosomes
line up along equator of cell
• Anaphase I
– Homologous chromosomes
move apart, pulled by
spindle fibers
• Telophase I
– Cytoplasm divides, resulting
in two cells with the same
number of chromosomes as
parent
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Meiosis II
Prophase
II
Metaphase
Telophase
II
Anaphase
II
II
4 different
haploid cells
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Results of Meiosis
Gametes (egg & sperm)
form
Four haploid cells with
one copy of each
chromosome
One allele of each gene
Different combinations
of alleles for different
genes along the
chromosome
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Mitosis vs Meiosis
Mitosis
Meiosis
2
Number of
divisions
1
Number of
daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Body cells
Sex cells
When
Throughout life
At sexual maturity
Growth and
Repair
Sexual Reproduction
Genetically
identical?
Chromosome #
Role
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Gamete Production
• Gametes have half the number of
chromosomes of the organism
– Sperm (23 chromosomes) + Egg (23
chromosomes) = Human (46 chromosomes)
• Diploid (2n)
– A cell with two of each chromosome, which
are in pairs {FULL set}
• Haploid (n)
– A cell with only one of each chromosome
{half set}
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Gamete Production: Sperm
• Occurs in the testes
• Two divisions produce
4 spermatids
• Spermatids mature
into sperm
• Men produce about
250,000,000 sperm
per day
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Gamete Production: Egg
• Occurs in the ovaries
• Two divisions produce
3 polar bodies that
die and 1 egg
• Polar bodies die
because of unequal
division of cytoplasm
• Immature egg called
oocyte
• Starting at puberty,
one oocyte matures
into an ovum (egg)
every 28 days
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