Mitosis, Meiosis,
and Life Cycles
Chapter 12
KEY TERMS
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DIPLOID (2N)
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The condition of having two sets of
chromosomes per nucleus
In somatic cells of diploid organisms,
chromosomes are present in pairs
HAPLOID (N)
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The condition of having one set of
chromosomes per nucleus
KEY TERMS
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HOMOLOGOUS CHROMOSOMES
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Members of a chromosome pair that are
similar in size, shape, and genetic constitution
KEY TERMS
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CELL CYCLE
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Cyclic series of events in the life of a dividing
eukaryotic cell
KEY TERMS
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INTERPHASE
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Stage of the cell cycle between successive
mitotic divisions
Interphase
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During interphase
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cell grows and prepares for next division
DNA replicates
Interphase is divided into
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first gap phase (G1)
synthesis phase (S)
second gap phase (G2)
KEY TERMS
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MITOSIS
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Division of the cell nucleus resulting in two
daughter nuclei, each with the same number
of chromosomes as the parent nucleus
KEY TERMS
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CYTOKINESIS
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Stage of cell division in which the cytoplasm
divides to form two daughter cells
Cell Cycle
Stages of Mitosis
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Prophase
Metaphase
Anaphase
Telophase
Prophase
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Chromatin condenses into chromosomes
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Nucleolus disappears
Nuclear envelope breaks down
Mitotic spindle begins to form
At the end of prophase, each duplicated
chromosome is composed of two sister
chromatids
A Duplicated Chromosome
KEY TERMS
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SPINDLE
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Structure consisting mainly of microtubules
that provides the framework for chromosome
movement during cell division
Metaphase
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Duplicated chromosomes line up along
midplane of cell
Anaphase
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Sister chromatids separate and move to
opposite poles of the cell
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Each chromatid is now a separate
chromosome
Telophase
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Nuclear envelope forms around each set
of chromosomes
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Nucleoli reappear
Chromosomes lengthen and become
chromatin
Spindle disappears
KEY TERMS
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CELL PLATE
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Structure that forms during cytokinesis in
plants, separating two daughter cells
produced by mitosis
Cytokinesis generally takes place in
telophase
Cytokinesis
Interphase and Mitosis
(a) Interphase
(b) Prophase
(c) Metaphase
Condensing
Spindle
chromosome
microtubules
(consists of 2
sister chromatids)
(d) Anaphase
(e) Telophase
Sister chromatids Cell
New
(now called
plate nuclei
chromosomes)
move to opposite
Stepped Art
ends of cell
Fig. 12-2, p. 244
KEY TERMS
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MEIOSIS
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Process in which a 2n cell undergoes
successive nuclear divisions, potentially
producing four n nuclei
Leads to formation of spores in plants
Meiosis
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Meiosis must occur at some time in the life
of a sexually reproducing organism if
gametes are to be haploid
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Meiosis consists of two cell divisions
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Meiosis I
Meiosis II
Meiosis I
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Members of each homologous pair of
chromosomes separate and are
distributed into separate nuclei in two
daughter cells
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Chromosomes were duplicated prior to
meiosis I, so each consists of two chromatids
Meiosis II
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Chromatids separate into individual
chromosomes and are distributed into
different haploid daughter cells
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Four haploid cells form
KEY TERMS
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SYNAPSIS
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Physical association of homologous
chromosomes during prophase I of meiosis
Crossing Over
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Crossing over
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Exchange of segments of homologous
chromosomes
Synapsis and crossing over occur during
prophase I of meiosis
Crossing Over
Meiosis
Meiosis
LEARNING OBJECTIVE
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Compare and contrast mitosis and
meiosis
Mitosis
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Involves a single nuclear division in which
the two daughter cells formed are
genetically identical to each other and to
the original cell
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Synapsis of homologous chromosomes
does not occur during mitosis
Meiosis
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Involves two successive nuclear divisions
and forms four haploid cells, each with a
different combination of genes
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Synapsis of homologous chromosomes
occurs during prophase I of meiosis
Mitosis and Meiosis
KEY TERMS
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Alternation of generations
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SPOROPHYTE
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Plants alternate haploid and diploid generations
2n, spore-producing stage in plant life cycle
GAMETOPHYTE
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n, gamete-producing stage in plant life cycle
The Sporophyte
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The 2n, spore-producing stage in the life
cycle of a plant
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A diploid sporophyte plant forms haploid
spores by meiosis
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A spore divides mitotically to form a
haploid gametophyte plant
The Gametophyte
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The n, gamete-producing stage in the life
cycle of a plant
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The gametophyte produces haploid
gametes by mitosis
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Two gametes fuse to form a diploid
zygote, which divides mitotically to
produce a diploid sporophyte
Plant Life Cycle