BIO
308 - Cell Division and Life Cycles - Dr. Daley
Mitosis
and Meiosis
• Cells of organisms arise from preexisting cells.
– Zygote - original cell
• Produced by the union or fertilization of an egg and
sperm (gametes).
Cell
Division
– Provides the basis of growth.
• Also maintenance & repair
– In the formation of body cells for the nuclear
division is called mitosis.
– Nuclear
division that occurs in the production of germ cells or gametes (ova and sperm)
is called meiosis.
Stages of
Cell Division
– Mitosis (nuclear division)
– Cytokinesis (division of the cytoplasm).
Cell
Cycle
– Cell division is but one part of the life history of a
cell
– Between divisions the cell must carry out
• Various metabolic processes and grow
• This
period is called interphase
– 90% of cell cycle
– Middle to end of cell cycle - DNA replicates
• Sets the stage for mitosis.
Stages of
Mitosis
– Prophase
– The chromatin (now replicated) condenses into visible
discrete bodies called chromosomes
• Consist of two identical sister chromatids - formed during interphase
Sister
Chromatids
– These are joined together by a centromere
• Specific DNA sequence
• Bound to the centromere is the kinetochore
• The kinetochore is a disc of proteins which connects
the centromere to the spindle fibers
Stages of
Mitosis
– Prophase cont.
– Before and during prophase the centrioles replicate
and each pair migrates to opposite ends of the cell
– Network of microtubules appear between the two pairs
of centrioles and form a football shaped spindle
Stages of
Mitosis
– Prophase cont.
– Other microtubules radiate outward from the pairs of
centrioles to form asters.
– The spindle fibers, the microtubules, are dynamic and
repeatedly extend and retract.
• Spindle fibers encounters a kinetochore it binds to it
and ceases to extend and retract.
– Finally during prophase the nuclear envelope disappears.
Prophase
in Whitefish
Stages of
Mitosis
– Metaphase
– Chromatids begin to align in the center of the cell,
• Metaphase plate
– With the help of the microtubules, which are attached
to individual kinetochores, the chromatids become neatly arrayed in the center
of the cell
Metaphase
in Whitefish
Stages of
Mitosis
– Anaphase
– The centromere divides and now the two sister
chromatids are free (now called daughter chromosomes Now each daughter
chromosome is pulled to respective ends (poles) of the spindle.
– Anaphase ends when each pole has identical sets of
chromosomes (diploid number, 2N).
Anaphase
in Whitefish
Stages of
Mitosis
– Telophase
– Final stage of mitosis.
– The
spindle fibers disappear
– Nuclear envelope begins to be formed around each set
of chromosomes
– Mitosis is over but cell division is not
Telophase
in Whitefish
Cytokinesis
– Final phase of cell division
– Cytokinesis usually starts during late anaphase or
early telophase
– Now two new genetically identical cells have been
formed
Complete
Cytokinesis in Whitefish
Mitosis
Meiosis
– Begins in late interphase of the cell cycle, after DNA
replication
– Two successive nuclear divisions occur, called meiosis
I and meiosis II
• Each has a prophase, metaphase, anaphase and telophase
• Microtubules of the spindle function just as in
mitosis
Result of
Meiosis
– Four daughter cells each with 1/2 the number of chromosomes (haploid number,
(N).
Stages of
Meiosis
– Prophase I
– Replicated DNA which has formed two sister chromatids
(still considered a chromosome)
– Joined at the centromere
– Condenses into distinct structures visible with the
light microscope
Homologous
Chromosomes
– Contain the same genes in the same order
– Line up side-by-side in a process called synapsis
(tetrad of chromatids)
– During this time a network of protein and RNA is laid
down between the sister chromatids holding them precisely, so that a gene is
directly across from its sister gene
Crossing-Over
– In this process segments of the DNA are exchanged
between non-sister chromatids at points called chiasma
• X shaped structure as view with a light
microscope
Stages of
Meiosis
– Metaphase I
– Get a spindle as in mitosis.
– Homologous chromosomes (sister chromatids) pair up
with centromeres on each side of the equatorial plane (does not occur in
mitosis)
– Not all the sister chromatid pairs from an inherited
parent necessarily line up on the same side of the equatorial plane
• Source of genetic variation
Stages of
Meiosis
– Anaphase I
– Homologous chromosomes separate and move toward
opposite poles of the cell.
• Each daughter cell will receive one chromosome (pair
of chromatids) from each homologous pair
– Sister chromatids do not separate at this stage since
there is no centromere division in meiosis I
Stages of
Meiosis
– Telophase I
– Similar to mitotic telophase including cytokinesis
Meiosis I
Stages of
Meiosis
– Meiosis II
– The transition to the second meiotic division is
called interkinesis
• No DNA replication occurs
Stages of
Meiosis
– Prophase II
– The second meiotic division resembles mitosis
– The spindle reappears
Stages of
Meiosis
– Metaphase II
– The chromosomes (sister chromatids) line up along the
equatorial plate
Stages of
Meiosis
– Anaphase II
– Centromere division occurs
– Sister chromatids separate to become individual
chromosomes
• Maintaining the haploid number that was produced in
anaphase I
Stages of
Meiosis
– Telophase II
– Just as in mitosis with cytokinesis
– All different with the haploid (N) number of
chromosomes.
Meiosis
II