The Cell Cycle and Mitosis
A.
B.
C.
D.
E.
F.
The eukaryotic cell cycle
Regulation of the cell cycle
Mitosis: an overview
Stages of mitosis
The spindle apparatus
Cytokinesis
A. The Eukaryotic Cell Cycle
Eukaryotic cell cycle:
– A series of stages through which most eukaryotic
cells pass during some time in their development
– In single-celled eukaryotes, the cell cycle often
represents the major mechanism for asexual
reproduction of the species
– In multicellular eukaryotes, the cell cycle is used for
cellular reproduction in embryonic cells and stem
cells
A. The Eukaryotic Cell Cycle
Key events of the cell cycle
– Interphase: a period of extensive metabolic & synthetic
activity
– During this time the DNA (chromosomes) of the cell are
replicated
– Mitosis: the nucleus of the cell divides to form two identical
nuclei, with each new nucleus having the same number of
chromosomes as the parent nucleus
– Cytokinesis: in most cases (with some exceptions), the cell’s
cytoplasm divides to form two cells, each containing one of
the new nuclei
A. The Eukaryotic Cell Cycle
Stages of the cell cycle
– G1 (gap-1) is the period before chromosomal
replication takes place
– S (synthesis) is the period when the chromosomes
are replicated
– G2 (gap-2) is the period following chromosomal
replication
– M (mitosis) is the period of nuclear division
A. The Eukaryotic Cell Cycle
G1, S, and G2 are collectively called “interphase,”
characterized by:
– The presence of a well-defined nucleus
– Individual chromosomes are not visible
– Chromatin: chromosomes are dispersed in an
uncondensed state called chromatin; “Grainy” or
“sandy ” appearance
– Nucleolus: a dark-staining spot sometimes (but not
always) seen in the interphase nucleus; The site of
ribosome synthesis
B. Regulation of the Cell Cycle
In multicellular eukaryotes, dividing cells are
found:
– In embryonic tissue;
– In certain tissues that are constantly renewed, such
as bone marrow stem cells or plant meristems;
– In damaged tissue, such as cell division in damaged
liver
B. Regulation of the Cell Cycle
Go (stationary) cells: cells that have reached
terminal differentiation and no longer divide
Cells stop dividing before they enter “S” phase;
Therefore, Go is similar in some respects to G1
except that the cell does not enter “S” phase
The cell cycle is regulated by several types of
regulatory proteins that act at two major points:
the G1 – S junction, and the G2 – M junction
C. Mitosis: An Overview
a) During G1 each chromosome is uncondensed
and in an unreplicated state.
C. Mitosis: An Overview
b) In the S phase, each chromosome is replicated.
The replicated chromosome consists of two
sister chromatids attached at the centromere.
C. Mitosis: An Overview
c) At the beginning of mitosis, the chromosomes
condense and become visible. Note that the
chromosome is still in a replicated state, with a
pair of chromatids attached at the centromere.
C. Mitosis: An Overview
d) During mitosis: the centromere splits, the
chromatids separate and become daughter
chromosomes, and the daughter chromosomes
move to opposite sides of the cell.
C. Mitosis: An Overview
e) At the end of mitosis, the chromosomes on each
side of the cell decondense and form two new,
identical nuclei.
D. Stages of Mitosis
Prophase:
– The nuclear membrane and nucleolus disperse
– The chromatin condenses to form visible
chromosomes
– The mitotic spindle apparatus begins to form
D. Stages of Mitosis
Metaphase
– The chromosomes line up along the equator of the
spindle apparatus
– Some writers use the term “prometaphase” to
represent the transitional state between prophase &
metaphase
Anaphase
– The centromeres split and the daughter
chromosomes move to opposite poles of the spindle
apparatus
D. Stages of Mitosis
Telophase
– At each pole of the spindle, the chromosomes
decondense and return to the state of chromatin
– New nuclear membranes and nucleoli form at each
spindle pole, completing the formation of two new
identical nuclei
E. The Spindle Apparatus
Composed of microtubules: fibers that are
dynamically assembled from the protein tubulin
Centrosomes:
– Found at the poles of the spindle in animal cells
– Each centrosome consists of two centrioles: short
tubulin structures that help to regulate tubulin
polymerization
– In plant cells: plant cells have “microtubuleorganizing centers” (instead of centrosomes) at each
pole
E. The Spindle Apparatus
Some fibers are attached to chromatids
– Each attached fiber is connected to the kinetochore
of the chromatid: a region near (or at) the
centromere
The movement of chromatids is accomplished by
shortening of the kinetochore fibers
– This is done by depolymerization of the microtubules
at the spindle poles
E. The Spindle Apparatus
Chemicals that inhibit microtubule formation
(such as colchicine) cause mitosis to stop at
metaphase
Colchicine and similar chemicals are used to
arrest mitosis at metaphase for the purpose of
chromosomal analysis (karyotyping)
F. Cytokinesis
In animal cells:
– Cytokinesis is accomplished by the formation of the
cleavage furrow
– A contractile ring or “girdle” of actin fibers and
myosin (a transient muscle-like assembly) forms just
inside the plasma membrane, around the equatorial
plate
– The contractile ring pulls the plasma membrane
together to pinch the cell into two cells
F. Cytokinesis
In plant cells:
– Cytokinesis is accomplished by the formation of the
cell plate
– Cell plate: A new cell wall forming at the equatorial
plate
– The cell plate enlarges until it completely separates
the cell into two cells