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Cellular Reproduction
and the Cell Cycle
What do all cells require to survive?
• A complete set of genetic instructions
– produce required molecules
– direct life processes
• Genetic instructions are coded in
the DNA of cells
Why do cells divide?
• Growth
• Development
• Repair
Cell Cycle
• Activities of a cell from one cell
division to the next
– Cell grows, adding more
cytoplasmic constituents
Why?
– DNA is replicated
Why?
– cell divides into two identical
daughter cells
Essential Features of Cell Division
1. Transmit a complete copy of
genetic information (DNA)
2. Transmit materials necessary for
cell to survive and use genetic
information
Two Fundamental Types of Cells
(organisms):
• Prokaryotic
• Eukaryotic
Prokaryotic Cell
• no nucleus – genetic material
(DNA) in cytoplasm
• no membrane-bound organelles
• cell division is called binary fission
• example: bacteria
Prokaryotic Cell Cycle
• Prokaryotic chromosome a circular loop
1. chromosome attaches to one point
on plasma membrane
2. chromosome is replicated
– replicated chromosome attached
to plasma membrane at a different
nearby point
3. cell elongates – new plasma
membrane is added between
between chromosomes, pushing
them towards opposite ends of cell
4. plasma membrane grows inward
at middle of cell
5. parent cell is divided into two
identical daughter cells
Cell wall
Chromosome
Plasma membrane
Eukaryotic Cell
• membrane-bound organelles, including
a nucleus
• genetic material (DNA) contained within
the nucleus
• cell division of somatic cells called
mitotic cell division
• examples: fungi, protists, plants,
animals
Chromosomes
• In Eukaryotic cells, the genetic information
that is passed from generation to the next is
carried by chromosomes.
• Chromosomes:
– threadlike structure within the nucleus
containing the genetic information that is passed
from one generation of cells to the next.
• Each cell has a specific number of
chromosomes
– Fruit flies = 8 chromosomes
– Humans = 46 chromosomes
Chromosomes
• Chromosomes are not visible in cells until cell
division
– Prior to cell division the DNA and protein molecules
that make up the chromosomes are spread throughout
the nucleus.
– During cell division the chromosomes condense into
compact, visible structures.
• Chromosomes are copied before cell division and
consist of identical “sister” chromatin.
Chromosomes
• A
– Centromere
• B
– Sister chromatids
Limitations on Cell Growth
• If a cell is larger then it demands more on the cell’s
DNA.
– DNA Overload:
• When a cell is small the information stored in the DNA is able
to meet the cells needs, but as a cell increases in size the DNA
in not copied so there is an “information crisis”.
• It will also have trouble moving enough nutrients
and wastes across the cell membrane.
– Exchanging Materials:
• The rate at which exchange occurs depends on the surface area
of the cell, which is the total area of the cell. A cells waste
production depends on the amount used by the cell. When a cell
is larger it is producing a higher number of waste and
exchanges waste for nutrients at a lower number.
Division of the Cell
• Before a cell gets too large, and begins
undergoing problems, a cell will divide
forming 2 daughter cells in cell division.
• Cell division solves the problem of
increasing size by reducing cell volume.
Each daughter cell has an increased ratio of
surface area to volume, allowing efficient
exchange of materials with the environment.
Before Cell Division
• Cells Replicate:
– Prior to cell division a cell will make copies of
its DNA.
• This replication solves the problem of information
storage because each daughter cell will have their
own complete set of genetic information.
• Cell division also solves the problem of increasing
size by reducing cell volume, which allows for
efficient exchange of materials with the environment.
The Cell Cycle
• Cell Cycle:
– Series of events that cells go through as they
grow and divide
– During the cell cycle, a cell….
• grows
• prepares for division
• divides to form 2 daughter cells which begin their
own cell cycle
The Cell Cycle
• 4 phases of the Cell Cycle
– M Phase:
• Mitosis and cytokinesis
– S Phase:
• Chromosome replication or synthesis
– G1 and G2 Phase:
• “Gap” phases, occur between M and S Phase
• Period of intense growths and activity
The Cell Cycle
•
•
•
•
A: G1 Phase
B: S Phase
C: G2 Phase
D: M Phase
Events of the Cell Cycle
• Interphase:
– G1, S, and G2 Phase
– G1 Phase:
• Cells do most of their growing. The cells will
increase in size and synthesis new proteins and
organelles.
– S Phase:
• Chromosomes are replicated and the synthesis of new
DNA molecules takes place. Key proteins associated
with the chromosomes are also synthesized.
– G2 Phase:
• Shortest of the 3 phases. Organelles and molecules
required for cell division are produced
Events of the Cell Cycle
• After the cell has gone through the 3 phases
of Interphase the cell will be ready to go
through M Phase (cell division).
What is Mitotic Cell Division?
• Division of somatic cells
(non reproductive cells) in
eukaryotic organisms
• A single cell divides into two
identical daughter cells
(cellular reproduction)
=> Maintains chromosome ploidy of cell
Ploidy – refers to the number of pairs of
chromosomes in cells
• haploid – one copy of each
chromosome
– designated as “n”
• diploid – two copies (= pair) of each
chromosome
– designated as “2n”
Each species has a characteristic
number of chromosomes:
Prokaryotes – one chromosome
Crayfish – 200 chromosomes
Human – 46 chromosomes
=> 23 pairs of chromosomes
Diploid organisms receive one
chromosome from female parent
(= maternal) and one chromosome
from male parent (= paternal)
A “matched” pair of maternal and
paternal chromosomes are called
homologues
Structure of a eukaryotic chromosome
• unreplicated chromosome
arm
arm
centromere
Prior to cell division:
• chromosomes (DNA) are replicated
(duplicated)
• duplicated chromosome
– attached at their centromeres
– as long as attached, known as
sister chromatids
duplicated
chromosome
sister
chromatids
daughter
chromosomes
Eukaryotic Cell Cycle
2 major phases:
• Interphase (3 stages)
– DNA uncondensed (= chromatin)
• Mitotic cell division (4 stages)
– DNA condensed (= chromosomes)
Mitotic Cell Division
2 major processes:
• mitosis – nuclear division
=> preserves diploid number of
chromosomes
• cytokinesis – cytoplasmic division
=> cell divides into two daughter cells
Mitosis
4 phases:
1st – Prophase (3 major events)
2nd – Metaphase
3rd – Anaphase
4th – Telophase and Cytokinesis
Prophase
• 3 major events
i) chromosomes condense
ii) spindle fibers form
iii) chromosomes are captured by
spindle
Chromosomes Condense
• Recall that chromosomes were
duplicated during interphase
• => each chromosome consists of
2 sister chromatids attached to
each other at the centromere
Mitotic Spindle Forms
• spindle fibers are specialized
microtubules
• spindle fibers radiate out from
centrioles, forming the “aster”
• centrioles occur in pairs, and are
duplicated during interphase
• one pair of centrioles migrates to
one pole of cell, the other pair
migrates to opposite pole of cell
Spindle Captures Chromosomes
• When spindle fibers are fully formed
nuclear envelope disintegrates and
nucleolus disappears
• Spindle fibers attach to chromosomes
at the kinetochore, a structure located
at the centromere
• other spindle fibers do NOT attach
to chromosomes, but retain free
ends that overlap at cell’s equator
=> “free spindle fibers”
• function of spindle fibers is to
organize division of sister chromatids
into daughter cells
chromatin
nucleus
nucleolus
centrioles
condensing
chromosomes
Metaphase
• chromosomes align along
equator of the cell, with one
kinetochore facing each pole
centrioles
chromosomes
spindle fibers
Anaphase
• sister chromatids separate
• spindle fibers attached to
kinetochores shorten and pull
chromatids poleward
• free spindle fibers lengthen and push
poles of cell apart
V-shaped chromatid
free spindle fibers
Telophase
• spindle fibers disintegrate
• nuclear envelopes form around both
groups of chromosomes
• chromosomes revert to their extended
state
• nucleoli reappear
•Telophase
chromosomes
decondensing
pinching of cell
membrane at equator
nuclear envelope
reforming
nucleolus reappears
Cytokinesis occurs, enclosing each
daughter nucleus into a separate
cell
cytokinesis
Cytokinesis
• Animal cells:
– microfilaments attached to plasma
membrane form a ring around
equator of cell
– ring contracts, like a drawstring,
dividing the cytoplasm
• Plant cells:
– stiff cell wall makes pinching
impossible
– Golgi complex buds off vesicles
filled with carbohydrate
– vesicles line up at equator and
fuse, producing a structure
called the cell plate
– cell plate becomes new cell wall
between the two cells
Mitotic Cell Division
Functions:
• Growth, maintenance, repair of body
tissues
• Forms the basis of
Asexual Reproduction
Cell Cycle Regulators
• Internal Regulators: proteins that respond to
events inside the cell
– Cyclin: protein that regulates the timing of the cell
cycle in eukaryotic cells
– Many other proteins have been discovered that also
assist in the cell cycle timing.
– There are also proteins that check the cell before it can
enter into the next part of the cell cycle.
• Ex: A cell cannot enter mitosis until the chromosomes are
replicated.
Cell Cycle Regulators
• External Regulators: proteins that respond
to events outside the cell
– External regulators direct cells to speed up or
slow down the cell cycle.
– Growth regulators: stimulate the growth and
division of the cell.
Uncontrolled Cell Growth
• Cell growth is constantly regulated to
prevent uncontrolled cell growth.
– Cancer: disorder in which some of the body’s
own cells lose the ability to control growth.
– Cancer cells do not respond to the signals that
regulate the growth in most cells. This constant
growth causes masses call tumors.
Uncontrolled Cell Growth
• What causes the loss of growth control?
– Cancer cells have a defect in a gene called p53,
which halts the cell cycle until chromosomes
have been properly replicated.
– Damage to p53 causes the cell to lose the
information needed to respond to signals that
ensure orderly growth.