Mitosis and Meiosis
Mitosis
•  Mitosis produces two
[Prometaphase]
identical cells
–  Stages of mitosis
–  Alignment and
separation of
chromosomes
–  Cytokinesis
•  Meiosis produces
genetically variable,
haploid, cells
–  Sexual life cycle
–  Mechanics of meiosis
15 October 2012
Overview of Mitosis
G2 of Interphase
Chromatin
(duplicated)
Prophase
Early mitotic
spindle
Metaphase
Centromere
Chromosome, consisting
of two sister chromatids
DNA replication
during Interphase
Overview of Mitosis
Prometaphase
Prophase:
Chromosomes begin to
condense. Spindle
starts to form
Anaphase
Metaphase
plate
Prometaphase
Nuclear
envelope
breaks down.
Chromosomes
attach to 3
spindle
Spindle
Cleavage
furrow
Daughter
chromosomes
Centrosome at
one spindle pole
Metaphase:
Chromosomes align in
center of cell
Spindle fibers shorten at the kinetochore
Telophase and Cytokinesis
Anaphase:
Sister chromatids
separate
Telophase:
Complete set of
chromosomes at
each pole
4
Kinetochore
Mark
Chromosome
movement
Microtubule
Kinetochore
Tubulin
subunits
Motor
protein
Chromosome
5
6
1
How do the cytoplasmic
organelles divide?
Cytokinesis
Animal cells divide by
constriction
Plant cells
build a partition
•  Mitochondria (and chloroplasts) are
present in multiple copies, and
randomly segregate into the two
daughter cells.
Cleavage furrow
100 !m
1 !m
Vesicles forming
cell plate
•  Membrane bound organelles (e.g.
ER) fragment along with the nuclear
membrane and are reconstructed in
the daughter cells
Contractile ring of
microfilaments
7
The human life cycle
Key
Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Ovum (n)
•  Mitosis produces an exact copy of
the parent cell.
•  Meiosis produces reduced
(haploid) gametes, which are
genetically unique.
FERTILIZATION
MEIOSIS
Ovary
Testis
–  Used for growth and asexual
reproduction.
Gametes are the
only haploid cells
Sperm
Cell (n)
Meiosis only
occurs during
gamete
formation
8
–  Necessary only for sexual
reproduction.
Diploid
zygote
(2n = 46)
Mitosis
Figure 13.5
Multicellular diploid
adults (2n = 46)
Meiosis 1
INTERPHASE
Meiosis 2
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II AND
CYTOKINESIS
Sister chromatids
remain attached
Sister
chromatids
Chiasmata
Spindle
Cleavage
furrow
Tetrad
Chromosomes duplicate
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
Figure 13.8
Homologous
chromosomes
separate
Pairs of homologous
Tertads line up
chromosomes split up
Sister chromatids
separate
Haploid daughter cells
forming
chromosomes
are still double
Figure 13.8
2
Mitosis vs Meiosis
Not paired
Homologs pair in
meiosis 1
Mitosis vs. Meiosis
diploid
haploid
•  Mitosis
•  Meiosis
•  Conserves the number
of chromosome sets
•  Reduces the number of
chromosome sets from
two to one
•  Daughter cells
genetically identical to
their parent cell and to
each other
•  Daughter cells
genetically distinct
from parent cell and
from each other
Sister chromatids
separate in meiosis 2
3