The cell cycle involves four main phases - G1, S, G2, and M. In M phase (mitosis), the cell undergoes nuclear division to form two identical daughter cells each with a full copy of the genome. Mitosis is divided into prophase, prometaphase, metaphase, anaphase and telophase where the chromosomes are aligned and separated between the two cells. Cytokinesis then divides the cytoplasm and cell membrane, completing cell division to form two daughter cells.

1. Cell cycle and Mitosis
Cell cycle
 In order for a living organism to grow
their individual cells must:
1. Increase in size
2. Make exact replicas of all their genetic
material, and
3. Go through a process of division
resulting in two daughter cells each
with one complete copy of the entire
genome

2. Mitosis
 The eukaryotic cell of higher
organisms, ranging from yeast to
humans, has its genetic material
(DNA) packaged into a membrane
bound nucleus
 In eukaryotic cells, the process of cell
proliferation, the cell cycle, is
characterized by four distinct phases
as illustrated in the following
diagrams:

3. Cell cycle

5. Cell cycle cont…
1. Gap 1 (G1) Phase - Cell are active
and growing.
 Cells are receptive to signals to begin
DNA synthesis
2. Synthetic (S) Phase - Cells are
actively replicating DNA
3. Gap 2 (G2) Phase - Cells are
actively preparing
for mitosis.

6. Cell cycle cont…
 Cells contain twice the normal amount
of DNA
4. G0 - Cells in G0 have exited mitosis
and are quiescent (inactive).
 These resting cells may be reactivated
and enter the G1 phase upon receipt
of the appropriate stimuli.

7. Mitosis
 Mitosis a nuclear division produces two
identical daughter cells by replicating and
dividing the original chromosomes during
prophase, prometaphase, metaphase,
anaphase, and telophase
 This is followed by the division of the cell
referred to as cytokinesis
 Interphase is often included in discussions of
mitosis, but interphase is technically not part
of mitosis, but rather encompasses stages
G1, S, and G2 of the cell cycle

8. Mitosis
 The parent cell duplicates its
chromosomes, providing both daughter
cells with a complete copy of genetic
information
 When an eukaryotic cell divides into
two, each daughter or progeny cell
must receive:

9. Mitosis cont..,
1. A complete set of genes (for diploid
cells, this means 2 complete genome,
2n).
2. A pair of centrioles (in animal cells).
3. Some mitochondria and, in plant cells,
chloroplasts as well.

10. Mitosis cont…
4. Some ribosomes, a portion of the
endoplasmic reticulum, and perhaps other
organelles.
 There are so many mitochondria and
ribosomes in the cell that each daughter cell
is usually assured of getting some.
 But ensuring that each daughter cell gets
two (if diploid) of every gene in the cell
requires the greatest precision.

11. Mitosis cont…
The answer:
1. Duplicate each chromosome
during the S phase of the cell
cycle.
2. This produces dyads, each made
up of 2 identical sister chromatids.
 These are held together by a ring
of proteins called cohesins.

12. Human chromosomes

13. Mitosis cont…
3. Condense the chromosomes into a
compact form.
 This requires ATP and a protein
complex called condensin.
4. Distribute these equally between the
two daughter cells.
5. Separate the sister chromatids.
 Steps 3-5 are accomplished by
mitosis.

14. Mitosis cont…
 It distributes one of each duplicated
chromosome (as well as one centriole)
to each daughter cell.
 It is convenient to consider mitosis in 5
phases: Prophase, Metaphase,
Anaphase and Telophase.
 When a cell is not engaged in mitosis
it is said to be in interphase.

15. Interphase

16. Interphase

17. 1. Prophase
 Chromatin in the nucleus begins to
condense and becomes visible in the
light microscope as chromosomes.
 The two centrosomes of the cell, each
with its pair of centrioles, move to
opposite "poles" of the cell.
 Some fibers cross the cell to form the
mitotic spindle.
 The chromosomes become shorter and
more compact.

18. Prophase

19. 2. Prometaphase (Late prophase)
 The nuclear envelope disintegrates
because of the dissolution of the laminins
that stabilize its inner membrane.
 A protein structure, the kinetochore,
appears at the centromere of each
chromatid.
 With the breakdown of the nuclear
envelope, spindle fibers attach to the
kinetochores as well as to the arms of the
chromosomes.

21. Prophase/prometaphase

22. 2. Prometaphase cont…
 For each dyad, one of the
kinetochores is attached to one pole,
the second (or sister) chromatid to
the opposite pole.
 Failure of a kinetochore to become
attached to a spindle fiber interrupts
the process.

23. Metaphase
 At metaphase all the dyads have
reached an equilibrium position midway
between the poles called the
metaphase plate.
 The chromosomes are at their most
compact at this time.

24. Metaphase

25. 4. Anaphase
 The sister kinetochores suddenly
separate and each moves to its
respective pole dragging its attached
chromatid (chromosome) behind it.
 Separation of the sister chromatids
depends on the breakdown of the
cohesins that have been holding them
together.

26. Anaphase

27. 4. Anaphase cont…
 It works like this:
1. Cohesin breakdown is caused by a
protease called separase (also known
as separin).
2. Separase is kept inactive until late
metaphase by an inhibitory chaperone
called securin.

28. 4. Anaphase cont…
3. Anaphase begins when the anaphase
promoting complex (APC) destroys
securin (by tagging it for deposit in a
proteosome) thus ending its inhibition
of separase and allowing Separase to
break down the cohesins.

29. Metaphase/anaphase

30. 5 Telophase
 A nuclear
envelope reforms
around each
cluster of
chromosomes
and these return
to their more
extended form.

31. Telophase

32. 6. Cytokinesis
 Mitosis is the process of separating the
duplicates of each of the cell's
chromosomes.
 It is usually followed by division of the
cell.
 Thus a special term, cytokinesis, for the
separation of a cell into two.

33. Mitosis animation