Lesson #2: The Cell Cycle Flashcards
(13 cards)
The Cell Cycle
Cells reproduce through a
continuous sequence of
growth and division known as
the cell cycle.
The cell cycle consists of two
main stages:
Growth stage called
Interphase
Division stage
Interphase
The longest stage of the cell cycle.
Divided into 3 phases:
Gap 1 phase (G1)
S phase
Gap 2 phase (G2)
G1 phase
The first part of interphase
Rapid growth and metabolic activity
S phase
The second part of interphase
DNA duplication
G2 phase
The third part of interphase
Centrioles replicate and cell prepares for division
Cell Division
The division stage involves two processes:
Mitosis
Cytokinesis
Mitosis
Although mitosis is a continuous
process it is divided into 4 phases
These phases are characterized by a
particular arrangement of the
chromosomes within the cell and by
the appearance or disappearance of
other cell structures.
Prophase
The first phase of mitosis
During prophase the chromatin (DNA) coils into
chromosomes
The nuclear membrane and nucleolus disappear
Centrioles migrate to the poles of the cell and spindle
fibers begin to form
Metaphase
The second phase of mitosis
The spindle fibers attach to the centromeres of the
replicated chromosomes
The chromosomes are guided to the center of the cell
by the spindle fibers
Anaphase
The third stage of mitosis
The centromere splits apart and chromatids are
pulled to opposite poles of the cell by the spindle
fibres shortening
Telophase
The fourth phase of mitosis
The chromatids have reached the poles of the cell
and begin to unwind and become less visible.
Spindle fibers breakdown and disappear
The nucleolus reappears and the nuclear membrane
begins to form around each set of genetic
information at the poles
Mitotic
Index
The mitotic index is a measure of
how fast cells are dividing can be
calculated from micrographs.
The mitotic index of cells is often
used in the diagnosis and
progression evaluation of cancer.
Mitotic index is calculated as:
Mitotic index= number of cells in mitosis
total number of cells
Cytokinesis
The final separation of the cytoplasm
and the formation of two new
daughter cells
In animals the indentation of the
membrane surrounding the two cells
forms and deepens. This is called a
cleavage furrow
In plant cells a cell wall and
membrane form to separate the two
daughter cells. This is called a cell
plate.
Control of
the Cell
Cycle
The cell cycle in controlled by a group
of proteins known as cyclins.
Cyclins interacts with other proteins
called CDK’s (cyclin dependent
kinases) to form enzymes that direct
cells through the cell cycle.
Cyclins are divided into four types
based on their behaviour in vertebrate
and yeast cells.
Types of Cyclins
Cyclin A – Also known as S cyclins
Induce DNA replication.
Levels stay high throughout S phase,
G2, and the first part of mitosis.
Cyclin B – Also known as M cyclin
Influences the formation of mitotic
spindles and the alignment of sister
chromatids along the spindles.
Cyclin D – Also known as G1 cyclin
Coordinate cell growth and start a
new cell cycle.
Levels gradually rise and fall
throughout the cell cycle.
Influenced by cell growth and
growth regulating signals from
outside the cell
Cyclin E – Also Known as G1/S cyclins
Start the initial processes of DNA
replication and promote early events
in mitosis.
Cyclin Action
Cyclins activate cyclin dependent kinases (CDKs), which
control cell cycle processes through phosphorylation.
When a cyclin and CDK form a complex, the complex will
bind to a target protein and modify it via phosphorylation.
The phosphorylated target protein will trigger some
specific event within the cell cycle.
After the event has occurred, the cyclin is degraded and
the CDK is rendered inactive again.
