MITOSIS / MEIOSIS
Lab 7
OBJETIVES
• Name the stages of the cell cycle and
describe their characteristics.
• Name the phases of mitosis and meiosis
and describe their characteristics.
• Identify the phases of mitosis and meiosis
from diagrams, pictures or micrographs.
• Compare the processes and end products
of mitotic and meiotic cell division.
• Describe the significance of mitotic and
meiotic cell divisions.
Be able to define the following terms in writing:
a- chromosome
l- spindle
b- chromatid
m- equatorial plane
c- centromere
n- cytokinesis
d- chromatin
o- furrow
e- nucleolus
p- cell plate
f- centriole
q- daughter cell
g- poles
h- diploid
i- crossing-over
j- haploid
k- tetrad
Cell Cycle: period between two sequential
divisions
• Interphase
– Growth (G1),
– Synthesis (S),
– Growth (G2)
• Mitotic phase
– Mitosis and
cytokinesis
Figure 3.30
Mitosis
Growth in our bodies, and in those of other
many-celled organisms, is basically a
process of increasing the number of cells.
This involves two processes; the
distribution of copies of the genetic
information from the parent cell to the two
daughter cells and the cytoplasmic
division. In this way, a body grows or
wounds are repaired. Each of the new
cells will include all the genetic information
possessed by all of the other living cells of
the body.
• Mitosis is an orderly series of events that
flow without interruption. We have artificially
divided this smooth flow into stages:
prophase, metaphase, anaphase, and
telophase. It is a convenience to be able to
classify parts of the process in this way, but
it can be misleading if we forget that there is
really no pause or interruption in the events.
• When we look at a plant or animal cell that
has been killed and stained while in the
process if mitosis, we are looking at the
stopped action.
Early and Late Prophase
• Asters are seen
as chromatin
condenses into
chromosomes
• Nucleoli
disappear
Early mitotic
spindle
Fragments
of nuclear
envelope
Pair of
centrioles
Polar
microtubules
Centromere
• Centriole pairs
separate and
the mitotic
spindle is
formed
Aster
Kinetochore
Chromosome, consisting
of two sister chromatids
Early prophase
Kinetochore
microtubule
Late prophase
Spindle
pole
Metaphase
• Chromosomes cluster
at the middle of the cell
with their centromeres
aligned at the exact
center, or equator, of
the cell
• This arrangement of
chromosomes along a
plane midway between
the poles is called the
metaphase plate
Metaphase plate
Spindle
Metaphase
Anaphase
• Centromeres of the
chromosomes split
• Motor proteins in
kinetochores pull
chromosomes
toward poles
Daughter chromosomes
Anaphase
Telophase and Cytokinesis
• New sets of
chromosomes extend
into chromatin
• New nuclear
membrane is formed
from the rough ER
Nucleolus
forming
• Nucleoli reappear
• Generally cytokinesis
completes cell division
Contractile
ring at
cleavage
furrow
Nuclear
envelope
forming
Telophase and cytokinesis
Cytokinesis
• Cleavage furrow formed in late anaphase
by contractile ring
• Cytoplasm is pinched into two parts after
mitosis ends
Meiosis
If a species is to retain the original
number of chromosomes and produce
offspring with chromosomes from both
male and female parent, then the
parents must have a mechanism to
produce cells with half the number of
chromosomes. The production of these
cell with the haploid number (one-half of
the parental number of chromosomes)
results from the special cell division called
Meiosis.
Meiosis consists of two cell divisions.
Meiosis
• Many organisms reproduce sexually. Sexual
reproduction means the formation of new
individual by a combination of two sex cells
(gametes). Gametes are product of meiosis and
they are haploid
• Meiosis is the type of the cell division that
reduces the number of chromosomes in the
daughter cells by halve.
• Meiosis is the process that includes two
consecutive cell divisions. First (Meiosis I) or
reduction. Second division (Meiosis II) or
equation division.
• During the first division of meiosis the
chromosomes line up at the equator of the
cell in pairs. Each pair separates and one
member of the pair moves to one pole; the
other member of the pair moves to the
other pole. When the two nuclei
reorganize and cytokinesis occurs, the
resultant two cells have one-half as many
chromosomes as the testis cell had.
Reduction from the diploid number of
chromosomes to the haploid number of
chromosomes has been accomplished.
Meiosis I or Reduction division
• Prophase I:
Chromosomes descondensed even
farther. Homologous chromosomes pair
and form very close contacts. As a result
of synapses, the exchange of DNA
(crossing over) between sister
chromatids may occur.
Pairs of homologous chromosomes are
called bivalents (2 chromosomes and 4
chromatides).
Meiosis I
(cont)
• Prometaphase I:
The nuclear m. dissapears.One kinetochore
forms per chromosomes some rather than
one per chromatid, and the chromosomes
attached to spindle fibers begin to move.
• Metaphase I:
• Bivalents, each composed of two
chromosomes (four chromatids) align at the
metaphase plate. (50/50 chance for the
daughter cells to get either mother or
father’s homologues)
Meiosis I
(cont)
• Anaphase I: Homologous chromosomes
separate and move to the opposite poles.
There is haploid set of chromosomes at each
pole but each chromosome has two
chromatids. Each daughter receive the
reduce number of chromosomes. For this
reason, the first division of meiosis is called
the reduction division . In this stage occurs
a misbalance between # chrom. and DNA , to
restore the balance , the second meiosis is
needed
Meiosis I (cont)
• Telophase I: Nuclear envelopes may or
may not form around the chromosomes.
Chromosomes stay condensed
• Interkinesis: This stage is analagous of
cytokinesis of mitosis. Two complete
daughter cell form. Chromosomes stay
condensed
• Each of the two cells produced as a result
of meiosis I go through a second division
(Meiosis II).
• This second division results in the
formation of four cells. During this division,
chromosomes line up on the equators of
the cells. The centromeres divide and one
of the chromatids moves to one pole; the
other chromatid moves to the other pole.
Meiosis II or Equation division
• Prophase II:
Centrosomes begin to move to oppossite
poles of the cell and microtubules cross
the cell to form the mitotic spindle.
Meiosis II (cont)
• Prometaphase:
• Microtubules attach at the kinetochores, and the
chromosomes begin moving
• Metaphase II
Spindle fibers align the chromosomes along the
equator of the cell in the metaphase plate. All
chromosomes are laying in one plane, their
sister chromatids leaning toward the opposite
poles. This organization helps to ensure that
each new nucleus will receive one copy of each
pre-S-phase chromosomes
Meiosis II (cont)
• Anaphase II: The paired sister chromatids
separate at the centromeres and move to
opposite poles of the cell. Now each
chromosome has only one DNA molecule.
(# chromosomes = # DNA molecules)
For this reason, the second meiosis division
is called equation division.
• Telophase II: Chromatids arrive to the
opposite poles of the cell, and new nucleus
reconstructed around them
Cytokinesis
• In animal cells, cytokinesis occur when
cell membrane contracts pinching the cell
into two daughter cells, each with one
nucleus.
• In plant cells, the rigid cell wall requires
the partitioning membrane to be
synthesized between the two daughter
cells.
• One diploid cell is entered meiosis.
Four haploid daughter cells emerged as
the result of the type of cell division.
• The four cells that result from these
meiotic divisions may mature into sperm
cells. Each cell has the haploid number of
chromosomes and each chromosome is
composed of a single chromatid. These
four cells are not necessarily alike in the
genetic information that they contain. Each
cell contains similar packages of
information, but the information in each
package could be quite different.