THE CELL CYCLE
CELL DIVISION
A characteristic of living organisms is the ability to reproduce—this occurs through cell division.
Cell division can result in…
• a new organism (in the case of unicellular organisms)
• a multicellular organism that develops from a single cell.
• the renewal and repair of damaged or older cells.
CELL DIVISION
• Cell division is just one part of the cell cycle. The cell cycle is the entire life of a cell from when it forms to when it divides.
• For cell division to be successful, genetic material must be copied with fidelity and the entire
genome as well as organelles must be distributed to two daughter cells.
• Organisms that reproduce sexually have two
types of cells—somatic cells and reproductive
cells (gametes). Somatic cells have twice as
many chromosomes as gametes do.
CHROMOSOME STRUCTURE
• Once duplication of genetic material has occurred, each chromosome has two sister chromatids.
• These contain identical genetic material and are attached to each other by protein complexes known as cohesins (the attachment is known as sister chromatid cohesion).
• The Kinetochore is a structure of proteins associated with specific
sections of DNA. These are found in the centromere and help with
movement of chromosomes
THE CELL CYCLE
THE CELL CYCLE
The cell cycle has two main phases:
1) The mitotic phase (M) takes up about 10% of the cycle.
2) Interphase (G
1, S, and G
2) takes up about 90% of
the cell cycle.
MITOSIS
G2 OF INTERPHASE
• Nucleus fully intact with nuclear membrane.
• Two centrosomes present (will contain centrioles in animal cells).
• Chromosomes not visible.
PROPHASE
• Chromatin condenses.
• Mitotic spindle begins to form.
• Centrosomes migrate.
PROMETAPHASE
• Nuclear membrane begins to disassemble.
• Chromatin continues to condense.
• Each chromatid has a kinetochore.
• Some microtubules attach to kinetochores.
• Nonkinetochore microtubules interact with each
other.
METAPHASE
• Longest step (lasts app 20 minutes).
• Centrosomes at opposite poles.
• Chromosomes line up at metaphase plate.
ANAPHASE
• Shortest step.
• Cohesion proteins are cleaved. Chromatids are now referred to as chromosomes.
• Chromosomes move to opposite poles (at about 1 mm/min).
• Cell elongates.
TELOPHASE
• Two daughter nuclei form.
• Chromosomes unravel.
CYTOKINESIS
• Cleavage furrow forms. This is created by
contractile ring of actin microfilaments associated with myosin.
• In plant cells, a cell plate forms from vesicles
derived from the Golgi apparatus (these also
contain material for building new cell wall).
THE MITOTIC SPINDLE
• The mitotic spindle begins to form during prophase.
• It is made of microtubules and associated proteins.
• It grows in length by adding subunits of tubulin.
• Mitotic spindles start at centrosomes.
Centrosomes replicate during interphase and
migrate to opposite poles during prophase and
prometaphase.
BINARY FISSION VS MITOSIS
• In prokaryotic cells, chromosome begins to replicate at origin of
replication. As chromosome is copied, one version of DNA
migrates to opposite pole of cell.
• While this is occurring, the cell is also elongating.
• When replication of DNA is complete AND cell is
approximately twice its original
length, division of cell occurs.
CELL CYCLE CONTROL SYSTEMS
The cell cycle control system…
• is a cyclically operating set of molecules in the
cell that both trigger and coordinate key events in the cell cycle.
• Involves checkpoints which contain signals that tell the cell to proceed with the cell cycle or enter a period of stasis. These signals are transmitted by signal transduction pathways. STOP is the
default mode of the cell. A cell must receive a signal to proceed.
• Has checkpoints at G
1, G
2, and M.
CELL CYCLE CONTROL POINTS
CELL CYCLE CONTROL POINTS
• G
1is seemingly the most important checkpoint.
Because of this, it is also referred to as the restriction checkpoint.
• If a cell goes through the G
1checkpoint, it usually continues on through S, G
2and M phases. If it
does not receive the signal during the G1
checkpoint, it enters a non-dividing state known
as G
O.
CELL CYCLE CONTROL
• Cell cycle control molecules (which are mostly proteins) pace the events of the cell cycle.
• Protein Kinases: enzymes that activate (or inactivate) other proteins by phosphorylating them. Many protein kinases are present at a
relatively constant level; however, they are in an inactive form. These kinases can be activated by having a protein known as a cyclin attached to it.
These are known as cyclin-dependent kinases (Cdks).
• The Cdks activity cycles with its cyclin partner.
Cyclin production is controlled by growth factors.
CELL CYCLE CONTROL: AN EXAMPLE
• Maturation-promoting factor (MPF) is a cyclin-Cdk complex that triggers the cell to move past G
2checkpoint.
• Cyclins associated with this complex rise during S and G
2phases, binding with MPF complex and activating it.
• MPF complex then phosphorylates a variety of
proteins which serve to initiate mitosis. Some proteins exist in the nuclear lamina and they stimulate break down of nuclear envelope. Others may contribute to condensation of chromosomes and spindle production.
• MPF also sets in motion a series of reactions that
actually degrades its own cyclin.
CELL CYCLE CONTROL
REGULATION OF THE CELL CYCLE: CYCLINS AND CYCLIN DEPENDENT KINASES (CDKS)
During G1, conditions in the cell favor degradation of cyclin, and the Cdk
component of MPF is
recycled.
5
4. During anaphase,
cyclin of MPF is degraded.
Stops M phase. Cell enters G1.
Accumulated cyclin combines with Cdk, producing enough MPF to pass the G2 checkpoint initiating mitosis.
2
Synthesis of cyclin in late S, and
through G2. Cyclin is protected from
degradation now and accumulates.
1
Cdk
G2 Cdk checkpoint
Cyclin MPF
Cyclin is degraded Degraded Cyclin
G1
G2 S M
G1
G1 S G2 M S G2 M
MPF activity Cyclin
Time (a) Fluctuation of MPF activity and
cyclin concentration during the cell cycle
MPF phosphorylates various proteins initiating Mitosis
3 M
