Cell Biology Lecture No. 7: Mitosis & Cell Cycle Control I
Wednesday January 30 , 2013
The Function Of The Cell Cycle:
-The cell cycle is the essential mechanism by which all living things reproduce and pass on genetic
information to the next generation of cells. It is a process that ensures DNA in each chromosome is
faithfully replicated to produce 2 copies and that replicated chromosomes be accurately distributed
(segregated) to 2 genetically identical daughter cells. The cell cycle is also involved in coordinating
growth (increasing cell mass) in preparation for division.
The Cell Cycle & Its Phases:
-The cell cycle has four principle phases: G1, S, G2, and M. In proliferating cells, G1 is the period
between the "birth" of a cell following mitosis and the initiation of DNA synthesis, which marks the
beginning of the S phase. At the end of the S phase, cells enter G2 containing twice the number of
chromosomes as G1 cells. The end of G2 is marked by the onset of mitosis, during which numerous
events leading to cell division occur. The G1, S, and G2 phases are collectively referred to as interphase,
the period between one mitosis and the next. Most non-proliferating cells in vertebrates leave the cell
cycle in G1, where most cells arrest in the G0 state. G1 phase is by far the longest for most cells, while
G2 phase contains cells with double their genetic information (visible with a Hoechst stain, which is
proportional to the amount of DNA content). A post-mitotic state refers to cells (like neurons) which
remain terminally differentiated (divided during embryogenensis and stay in G0 forever).
M Phase (Mitosis):
-Mitosis (M phase) is further subdivided into stages: Prophase, metaphase, anaphase, and
Telophase/cytokinesis. During prophase, the nuclear envelope breaks down, microtubules form the
mitotic spindle apparatus, and chromosomes condense. At metaphase, attachment of chromosomes to
microtubules via their kinetochores is complete (align at metaphase plate). During anaphase,
microtubule motors and shortening of spindle microtubules pull the sister chromatids toward opposite
spindle poles. After chromosome movement to the spindle poles, chromosomes decondense, and cells
reassemble nuclear membranes around the daughter cell nuclei and undergo cytokinesis (cell division).
It is highly important that chromosomes are segregated accordingly in order to prevent aneuploidy
(abnormal number of chromosomes).
Cytological Features Of Cycling Cultured Human HeLa Cells:
-HeLa cells are flat during interphase, but as cells undergo mitosis, they round up and divide.
Subsequently, they flatten out again. The type of microscopy featured in the diagram, is phase contrast
light microscopy (notice the “halo” effect produced in the images).
The Cell Cycle In Budding Yeast: -By examining a form of budding yeast, S. cerevisiae, we can infer cell cycle stages of eukaryotic cells
simply by observing the size of the bud. The larger the bud, which emerges at the end of the G1 phase,
the farther along in the cycle the cell is. Daughter cells are born smaller than mother cells and must
grow to a greater extent in G1 before they are large enough to enter the S phase. START is the point in
the cell cycle after which cells are irreversibly committed to undergoing a cell cycle. Budding yeast spend
a great deal of time in G1 phase, similar to mammalian cells.
The Cell Cycle In Fission Yeast:
-Fission yeast (like S. pombe) are different than budding yeast as they undergo elongation instead of bud
formation. The completion of cytokinesis in fission yeast is not the pinching of a cell membrane, but the
formation of a wall (known as a septum) between parent and daughter cells. Fission yeast have a shorter
G1 phase in comparison to budding yeast, but have longer G2 and M phases. Both budding and fission
yeast are important as experimental models since they are easier to observe (mammalian cells are
relatively indistinguishable during interphase) and they are organism that are easier to genetically
manipulate. Temperature-sensitive mutants (with defective proteins) in yeast cultures were important
for figuring out genes encoding proteins vital for cell cycle regulation (e.g. cdc gene).
Concept Of The Cell Cycle Control System:
-Cell cycle control is a system based on cyclically-active kinases (enzymes sticking phosphates on amino
acids like serine, threonine and tyrosine). Progression of the cell cycle is highly regulated with
checkpoints that ensure the cell is ready for each consecutive phase (because the cell cycle is not