MEIOSIS

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MEIOSIS
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I. Cell Division and Reproduction
A. Asexual reproduction - a single parent
gives rise to one or more
individuals
1. Offspring are clones (genetically
identical)

B. Sexual reproduction - involves the
combination of genetic material from two
parents
1. Each parent contributes one gene for
each trait

2. Chromosome number stays the same from
generation to generation

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C. Cells of sexually reproducing organisms have
a characteristic number of chromosomes
1. Diploid (2n) - sets of two matching
chromosomes
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a. Such chromosome pairs are called homologous
chromosomes
(1) Homologue - each member of a pair
(2) Alleles - different forms of a gene
for a particular trait may occur in
homologous chromosomes

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b. Somatic (body) cells are diploid
(Y)
(X)
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2. Haploid (n) - single set of chromosomes
D. Gametes (germ cells) - haploid reproductive
cells
1. Produced by special gamete-producing
structures


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HUMAN MALE
HUMAN FEMALE
vagina
penis
testis (where sperm originate)
ovary (where eggs develop)
FLOWERING PLANT
ovules inside ovary (where eggs originate)
anther (where sperm originate)
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2. During gamete formation, the diploid number is
reduced to haploid by meiosis
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3. Haploid gametes unite in fertilization
restoring the diploid chromosome number
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II. Overview of Meiosis
A. Meiosis - cell division in which the
chromosome number is reduced from
diploid and gametes are formed
B. Overall process
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1. Chromosomes duplicate during interphase
2. Two consecutive cell divisions - meiosis I and
meiosis II
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III. Stages of Meiosis
TELOPHASE I
PROPHASE I
METAPHASE I
ANAPHASE I
A. Prophase I
1. Synapsis - replicated chromosomes
form tetrads
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TELOPHASE I
PROPHASE I
METAPHASE I
ANAPHASE I
B. Metaphase I - tetrads are aligned at the
spindle equator
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TELOPHASE I
PROPHASE I
METAPHASE I
ANAPHASE I
C. Anaphase I - tetrads split and sister
chromatids move toward the spindle poles
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TELOPHASE I
PROPHASE I
METAPHASE I
ANAPHASE I
D. Telophase I - each daughter cell has a haploid
chromosome set
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PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
E. Meiosis II - sister chromatids separate
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IV. Meiosis and Genetic Variation
A. Crossing over - the exchange of
corresponding segments of homologous
chromosomes during prophase I
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B. Metaphase I alignments
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1. 2n chromosome combinations are possible in
gamete (n haploid number)
2. 223 combinations possible in humans (8,388,
608)
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C. Random fertilization
1. Fertilizations in humans 223 x 223
combinations possible in zygote (64
trillion)
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V. Formation of Gametes
A. Gamete formation in plants
1. Gamete- producing bodies and
spore-producing bodies alternate
during the life cycles of plants
(alternation of generations)
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multicelled sporophyte
mitosis
zygote
Diploid
meiosis
fertilization
Haploid
spores
gametes
multicelled gametophytes
mitosis
2. Spores - haploid cells produced by meiosis
that give rise to a multicellular individual
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B. Gamete formation in animals
multicelled body
mitosis
zygote
Diploid
meiosis
fertilization
Haploid
gametes
1. Gametes are the only haploid cells
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2. Spermatogenesis - formation of sperm cells
secondary spermatocytes (haploid)
spermato- gonium (diploid male reproductive cell)
primary spermatocyte (diploid)
spermatids (haploid)
MEITOSIS I, CYTOPLASMIC DIVISION
MEIOSIS II, CYTOPLASMIC DIVISION
GROWTH
a. Results in production of four spermatids
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secondary spermatocytes (haploid)
spermato- gonium (diploid male reproductive cell)
primary spermatocyte (diploid)
spermatids (haploid)
MEITOSIS I, CYTOPLASMIC DIVISION
MEIOSIS II, CYTOPLASMIC DIVISION
GROWTH
b. Spermatids develop a tail to become
mature sperm
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3. Oogenesis - formation of egg cells in
females
three polar bodies haploid)
first polar body haploid)
primary oocyte (diploid)
oogonium (diploid reproductive cell)
secondary oocyte haploid)
ovum (haploid)
MEITOSIS I, CYTOPLASMIC DIVISION
MEIOSIS II, CYTOPLASMIC DIVISION
GROWTH
a. Results in one large ovum and three polar
bodies
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VI. Comparing Meiosis and Mitosis (Figure 10.10)
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MITOSIS
MEIOSIS Occurs in most types of Occurs in
formation of eukaryotic cells
gametes in eukaryotic cells
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MITOSIS MEIOSIS No
pairing of Homologous
chromosomes homologous
pair in synapsis crossing chromosomes
over may occur
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MITOSIS
MEIOSIS Chromosome number is Chromosome
number is maintained
reduced from diploid to
haploid

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MITOSIS MEIOSIS One
division Two divisions
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MITOSIS MEIOSIS Two
daughter cells Four daughter cells
areare produced
produced
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MITOSIS
MEIOSIS Daughter cells are
Daughter cells contain identical to each other
varying combinations of and to parent
cell chromosomes