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Cell Cycle
Cells are the basic unit of life.
Just like you and me, as they get older they grow
bigger. This triggers the cell to divide.
First, let’s talk about the stages of the cell
cycle…
G1 Phase
Or the Growth 1 Phase is the first event in
the life cycle of a cell.
G1 Phase
During this phase:
• the cell is metabolically active with
chemical reactions mostly in the
cytoplasm
• Cellular respiration (makes ATP)
• Produces protein
• At the end organelles are duplicated
S Phase
Or the Synthesis phase.
Synthesis=to put parts together to equal a
whole.
During this phase:
• DNA is replicated in the nucleus
G2 Phase
Or Growth 2 Phase is a second period
of growth after the DNA has been
replicated.
(SO the cell has TWICE the amount of
genetic material it needs)
During this phase:
• Cell resumes making proteins and
other molecules
Interphase
Interphase =G1,S,G2
Cells spend most of their life in interphase
During Interphase:
• The cell appears normal
• Nucleus is visible
• DNA is loosely organized (bowl of spaghetti)
– This is called Chromatin
M phase
Or Mitosis Phase
During Mitosis:
• The nucleus divides in a series of steps
to form two nuclei
• The nuclei, duplicated organelles and
cytoplasm are separated in cytokinesis.
Stages of Mitosis
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
Prophase
The FIRST step in Mitosis.
During Prophase:
• DNA condenses to form
chromosomes
– To make a chromosome the
DNA is wrapped around
proteins called histones
• Chromosome looks like a X
Prophase
Chromosomes:
• Each half of the “X” is called a chromatid.
• These chromatids are held together in the
center of the “X” in a region called the
centromere.
Prophase
• Microfiliments in the cytoskeleton begin
to break down
• In animals, Centrioles move to the
opposite sides of the cell and form
spindle fibers toward the center of the
cell
• In plants, spindle fibers form at the
opposite ends of the cell in regions called
asters
Prophase
By the end of prophase, the nuclear
membrane has been broken down and
the nucleolus disappears.
This is the longest phase of Mitosis
Metaphase
The second phase of mitosis
During Metaphase:
• Spindle fibers attach to the centromeres
on each chromosome
• Chromosomes are lined up down the
middle of cell.
Anaphase
The third phase of mitosis
During Anaphase:
• The chromatids are pulled apart at the
centromere by spindle fibers
• Spindle fibers pull the chromatids to
opposite sides of the cell
Telophase
The last phase in mitosis
During Telophase:
• Chromatids arrive at the opposite poles
of the cell
• The nuclear membrane forms around
each new set of chromosomes
• DNA uncoils and the nucleoli reappear
Telophase
• Once the nuclear membrane
completely forms, the new nuclei
appear
• Each nuclei contains a complete
set of chromosomes identical to
the parent cell
• Microfiliments reform the
cytoskelton and move the
duplicated organelles to
opposite sides
Just Remember…
ProphaseMetaphaseAnaphaseTelophase
Cytokinesis
The division of the cytoplasm
• In animals, the cytoplasm
pinches in from the top form
the cleavage furrow
• The cleavage furrow continues
to constrict towards the
center until pinching off the
two cells.
Cytokinesis
• In plants, a cell plate forms in the middle
of the cell which turns into the new cell
wall.
Controlled Cell Growth
• Cell growth and division is controlled by
internal and external factors
• These can tell a cell to divide or to
continue to grow
• Example: Human growth hormone which
tells bone cells to grow and divide
Uncontrolled Cell Growth
• If uncontrolled by internal or external
factors cells will divide continuously.
• Example: Cancer
– Causes cells to form masses called tumors
– Malignancy is when a cancer cell breaks
away and forms a new mass
Uncontrolled Cell Growth
• Cancer cells use up nutrients and
oxygen and do not provide any function
for the organism
• The tumor can add stress to surrounding
cells
Causes of Cancer
• A carcinogen is something that causes
cancer.
– Causes a mutation in the genetic material
(DNA) which can prevent the cell from
responding to external and internal signals.
• Examples: smoking, exposure to
excessive radiation from the Sun,
pollution, and some viruses.