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Cellular Reproduction
Why is there a limit on cell size?
If a cell continues to grow, the surface
area of the membrane might not be able
to transport enough nutrients and waste.
 Transport of substances within the cell is
also more difficult in larger cells.

What is the Cell Cycle?
Once a cell reaches its size limit, it has to
stop growing or divide.
 Cells reproduce by a
cycle of growing and
dividing called the
Cell Cycle.

Stages of the cell cycle
Interphase – Cell grows, carries out
normal cell functions, and replicates
(makes copies of) its DNA, and prepares
for mitosis
 Mitosis – Cell division: nucleus and DNA
divide
 Cytokinesis – Cytoplasm divides, splitting
into 2 complete identical cells

What happens during interphase?

Cell grows and develops, duplicates its
DNA, and prepares for division
Stages of Interphase

G1 (Gap1) – period immediately after a
cell divides. Cell grows, carries out normal
cell functions, and prepares to replicate
DNA. (Some cells do not divide again)

S (Synthesis) – DNA is copied

G2 (Gap2) – Cell prepares for mitosis.
Microtubules for cell division are formed,
DNA is checked for mistakes
Chromosomes and Chromatin

Chromosomes – Condensed form of DNA,
the genetic material that is passed from
generation to generation of cells.

Chromatin- Relaxed form of DNA during
interphase, which allows for DNA to
replicate
How do prokaryotes divide?

Binary Fission
 (it’s
the same method as mitosis.)
Chromosome structure

Sister chromatid- each half
of the X-shaped
chromosome is an identical
copy of that chromosome’s
DNA.

Centromere- the structure
at the center where the
sister chromatids are
attached
PROPHASE
Chromatin condenses into
X-shaped chromosomes
(containing 2 identical
sister chromatids).
 The nucleolus disappears.
 The nuclear membrane
disintegrates and the
 The spindle apparatus
begins to form.

METAPHASE

Sister chromatids are pulled by motor
proteins along the spindle apparatus
towards the middle of the cell

Sister chromatids
line up along the
equator
ANAPHASE
Chromatids are pulled
apart
 The microtubules of the spindle apparatus
shorten, which pulls at the centromere of
each sister chromatid, causing them to
separate into 2 identical chromosomes.
 • The chromosomes move toward the
poles of the cell (opposite sides)

TELOPHASE
The last stage of mitosis is telophase.
 Chromosomes arrive at the poles and
begin to relax or decondense
 2 new nuclear membranes begin to form
 Nucleoli reappear
 Spindle apparatus breaks down and parts
are recycled to build the cytoskeleton
 Nucleus has divided, but cell division is not
yet complete

Cytokinesis

Cytoplasm and
all other cell
contents divide,
resulting in 2
identical cells
How does a cell regulate its cycle?

Proteins (cyclins and CDKs) regulate the
cell cycle by controlling the start and end
of various activities within the cell

Quality control checkpoints monitor the
cycle and can stop it if something goes
wrong. Ex. Check DNA for mistakes after
replication
How is cancer a result of
uncontrolled mitosis?

Cancer is uncontrolled growth and division
of cells – a failure in the regulation of the
cell cycle.

Cancer cells can crowd out normal cells

Carcinogens are agents that are known to
cause cancer
Apoptosis

Apoptosis is programmed cell death

Examples:
 cells
between fingers and toes during development
of hands and feet
 cells in the leaves that fall in autumn
 damaged cells (like sunburned skin)
Stem Cells

Embryonic Stem Cells – after a sperm fertilizes an egg,
the cells divide repeatedly until there about 150 cells
which have not become specialized yet. Each can
develop into many various types of cells. Embryonic
stem cell research is controversial because of ethical
concerns about the source of the cells.

Adult Stem Cells – found in various tissues, might be
used to repair the same kind of tissue in which they are
found. They might be able to develop into different types
of cells, or used in treatments of diseases. Less
controversial because the cells can be obtained with the
consent of their donors.