Mitosis, Meiosis,
and Life Cycles
Chapter 12
LEARNING OBJECTIVE 1
•
Distinguish between a haploid cell and
a diploid cell
•
Define homologous chromosomes
KEY TERMS
•
DIPLOID (2N)
•
•
•
The condition of having two sets of
chromosomes per nucleus
In somatic cells of diploid organisms,
chromosomes are present in pairs
HAPLOID (N)
•
The condition of having one set of
chromosomes per nucleus
KEY TERMS
•
HOMOLOGOUS CHROMOSOMES
•
Members of a chromosome pair that are
similar in size, shape, and genetic constitution
LEARNING OBJECTIVE 2
•
Identify the phases in the cell cycle
•
Describe the main events of each
phase
KEY TERMS
•
CELL CYCLE
•
Cyclic series of events in the life of a dividing
eukaryotic cell
KEY TERMS
•
INTERPHASE
•
Stage of the cell cycle between successive
mitotic divisions
Interphase
•
During interphase
•
•
•
cell grows and prepares for next division
DNA replicates
Interphase is divided into
•
•
•
first gap phase (G1)
synthesis phase (S)
second gap phase (G2)
KEY TERMS
•
MITOSIS
•
Division of the cell nucleus resulting in two
daughter nuclei, each with the same number
of chromosomes as the parent nucleus
KEY TERMS
•
CYTOKINESIS
•
Stage of cell division in which the cytoplasm
divides to form two daughter cells
Cell Cycle
S
(Synthesis phase)
G1
(First gap phase)
G2
(Second gap
phase)
M PHASE
(Mitosis and cytokinesis)
Fig. 12-1, p. 242
LEARNING OBJECTIVE 3
•
Describe the events that occur in each
stage of mitosis
Stages of Mitosis
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
Prophase
•
Chromatin condenses into chromosomes
•
•
•
•
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
Centromere
region
Microtubules
Kinetochore
Sister
chromatids
Fig. 12-3, p. 245
KEY TERMS
•
SPINDLE
•
Structure consisting mainly of microtubules
that provides the framework for chromosome
movement during cell division
Metaphase
•
Duplicated chromosomes line up along
midplane of cell
Anaphase
•
Sister chromatids separate and move to
opposite poles of the cell
•
Each chromatid is now a separate
chromosome
Telophase
•
Nuclear envelope forms around each set
of chromosomes
•
•
•
Nucleoli reappear
Chromosomes lengthen and become
chromatin
Spindle disappears
KEY TERMS
•
CELL PLATE
•
•
Structure that forms during cytokinesis in
plants, separating two daughter cells
produced by mitosis
Cytokinesis generally takes place in
telophase
Cytokinesis
Nucleus
Vesicles gather Small vesicles Eventually one New cell walls
on cell's
fuse, forming
large vesicle (from vesicle
midplane
larger vesicles
exists
contents)
Plasma Cell
membrane wall
Cell plate
forming
Cell plate
forming
New plasma
membranes
(from vesicle
membranes)
Fig. 12-4, p. 247
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
ends of cell
Fig. 12-2, p. 244
(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
LEARNING OBJECTIVE 4
•
Explain why meiosis is needed at some
point in the life cycle of every sexually
reproducing organism
•
Describe the events that occur during
meiosis I and meiosis II
KEY TERMS
•
MEIOSIS
•
•
Process in which a 2n cell undergoes
successive nuclear divisions, potentially
producing four n nuclei
Leads to formation of spores in plants
Meiosis
•
Meiosis must occur at some time in the life
of a sexually reproducing organism if
gametes are to be haploid
•
Meiosis consists of two cell divisions
•
•
Meiosis I
Meiosis II
Meiosis I
•
Members of each homologous pair of
chromosomes separate and are
distributed into separate nuclei in two
daughter cells
•
Chromosomes were duplicated prior to
meiosis I, so each consists of two chromatids
Meiosis II
•
Chromatids separate into individual
chromosomes and are distributed into
different haploid daughter cells
•
Four haploid cells form
KEY TERMS
•
SYNAPSIS
•
Physical association of homologous
chromosomes during prophase I of meiosis
Crossing Over
•
Crossing over
•
•
Exchange of segments of homologous
chromosomes
Synapsis and crossing over occur during
prophase I of meiosis
Crossing Over
Sister
chromatids
Crossing over
Kinetochores
Fig. 12-6, p. 249
Meiosis
Meiosis
(a) Prophase I
(b) Metaphase I
(c) Anaphase I
(d) Telophase I
Cell plate
Each
chromosome
consists of
two
chromatids
Homologous
chromosomes
Spindle
microtubules
New nuclei
Fig. 12-5 (a-d), p. 248
(e) Prophase II
(f) Metaphase II
Chromosomes
(each with two
chromatids)
(g) Anaphase II
Spindle
microtubules
(h) Telophase II
New
nuclei New cell walls
and plasma
membranes
Fig. 12-5 (e-h), p. 249
LEARNING OBJECTIVE 5
•
Compare and contrast mitosis and
meiosis
Mitosis
•
Involves a single nuclear division in which
the two daughter cells formed are
genetically identical to each other and to
the original cell
•
Synapsis of homologous chromosomes
does not occur during mitosis
Meiosis
•
Involves two successive nuclear divisions
and forms four haploid cells, each with a
different combination of genes
•
Synapsis of homologous chromosomes
occurs during prophase I of meiosis
Mitosis and Meiosis
Synapsis (pairing of
homologous chromosomes)
Chromosomes
Parent cell
(2n = 4)
Parent cell
(2n = 4)
Prophase I
Prophase
Nuclear
envelope
Mitosis
1st
meiotic
division
Nuclear
envelope
Prophase II
2nd
meiotic
division
Two daughter cells (each 2n = 4)
identical to parent cell
Mitosis
Four daughter cells (each n = 2)
genetically different from parent cell
Meiosis
Fig. 12-7, p. 250
LEARNING OBJECTIVE 6
•
Define alternation of generations
KEY TERMS
•
Alternation of generations
•
•
SPOROPHYTE
•
•
Plants alternate haploid and diploid generations
2n, spore-producing stage in plant life cycle
GAMETOPHYTE
•
n, gamete-producing stage in plant life cycle
The Sporophyte
•
The 2n, spore-producing stage in the life
cycle of a plant
•
A diploid sporophyte plant forms haploid
spores by meiosis
•
A spore divides mitotically to form a
haploid gametophyte plant
The Gametophyte
•
The n, gamete-producing stage in the life
cycle of a plant
•
The gametophyte produces haploid
gametes by mitosis
•
Two gametes fuse to form a diploid
zygote, which divides mitotically to
produce a diploid sporophyte
Plant Life Cycle
Gametophyte (n)
(multicellular
haploid organism)
Mitosis
Spores (n)
Mitosis
Gametes (n)
Meiosis
Fertilization
Zygote (2n)
Mitosis
Sporophyte (2n)
(multicellular
diploid organism)
Fig. 12-8, p. 251
Animation: The Cell Cycle
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Animation: Comparing
Mitosis and Meiosis
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