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Cell Division
Mitosis and Meiosis
Cell Cycle
• Encompasses the time between the creation
of a new cell and that cell’s division.
• Cell Division: the splitting of one cell into two.
– The process that makes growth and reproduction
possible for any organism.
– Each division different depended upon if the cell is
eukaryotic or prokaryotic
• Two major phases
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– 1- Interphase: Preparation for cell division.
• Three phases:
– G1 phase (Growth 1): Cell growth – Cell organelles are formed within
the cell.
– S phase (Synthesis): DNA is synthesized
– G2 phase (Growth 2): Second period of cell growth, during which the
cell prepares for the division.
Example: Some cells, including many nerve cells, are
programmed never divide. These cells are said to be in
G0 or resting phase.
– 2- Mitosis: Division of Nucleus
• Four phases of mitosis (Prophase, Metaphase, Anaphase,
Telophase)
• Cytokinesis: division of cytoplasm and cell membrane
Cell Cycle Control
• All of the cell cycle are controlled by
checkpoints
• There are three checkpoints:
Checkpoints
Occurs at
Details
G1
The end of the
phase
If conditions are not suitable for replication, the cell will
not proceed to S phase but will instead enter a resting
phase G0
G2
The end of the
phase
If conditions are not suitable, transition to the M phase
will be delayed.
If DNA is damaged, cell division will be delayed to allow
time for DNA repair
M
Between
metaphase and
anaphase stages
of mitosis
If the chromosomes are aligned properly and ready for
division, the cell will proceed from metaphase to
anaphase, during which it will divide. If the chromosome
are not aligned properly, the anaphase stage will be
delayed
Continue…
• Triggers at each checkpoint assess the cell’s
readiness to proceed to the next stage.
• Checkpoints makes sure proper number of
chromosomes and type of chromosomes &
organelles
Example of Checkpoint
• Example: Malignant cancer are deadly, in part,
because they undergo unregulated cell division, which
enables them to spread rapidly throughout the body.
Scientists have discovered one reason behind this
uncontrolled growth: a defective p53 gene. Proteins
produced by the p53 gene assess the cell’s DNA for
damage at the G1 checkpoint. If the DNA is intact, cell
division proceeds. If the DNA is damaged, however,
the p53 proteins halt cell division until the DNA is
repaired or the cell is destroyed. If the p53 gene itself
has been damaged, as in the case of cells that are
cancerous the G1 checkpoint will fail and a malignant
cancer cell may develop.
Other Cell Division Controls
• Density-dependent inhibition: When a certain
density of cells is reached, growth of the cells
will slow or stop because there are not
enough raw materials for the growth and
survival of more cells.
– Example: Cancer cells can lose this inhibition and
grow out of control.
• Growth Factors: Some cells will not divide if
certain factors are absent.
Continue with Other Cell Division
Control
• Cyclins: is a protein that acccumlates during
G1, S, G2, of the cell cycle
• Protein Kinase: is a protein that control other
proteins through the addition of phosphate
groups.
Chromosomes
• In eukaryotic cells, DNA and associated
proteins are wrapped together in packages
called chromosomes.
• DNA in eukaryotic cells is wrapped around the
proteins to form a complex called chromatin
• Throughout the cell’s life, the chromatin
becomes is loosely packed within the nucleus.
– Chromatin can not been seen by humans.
– Think a rubber band ball.
Continue…
• During cell division, however, the chromatin becomes
highly condensed and folds up to form condensed
chromosomes. (This is when we can see it).
• DNA is always replicated, or copied before becoming
condensed .
• The x shape associated with chromosomes actually
represents a replicated chromosome consisting of two
identical sister chromatids joined at the centromere
Example: Prokaryotes do not
have chromosomes.
Prokaryotic DNA exist in a
single loop
Chromosome Number
• Refers to the number of chromosomes within
each cell of an organism.
• Most animals possess two nonidentical version of
every chromosome. These are known as
homologous chromosomes.
• Homologous chromosomes have the same size,
shape, and function but may have slightly
different versions of most genes, the basic unit of
hereditary information.
Continue…
• Cells with two sets of every chromosomes
between their homologous chromosomes are
diploid (2n), while cells with one set of every
chromosome are haploid (1n)
– Diploid: Somatic (Body) Cells
– Haploid: Gamete (Sex) Cells
Human Chromosome Number
• Humans has 46 chromosomes or 23 pairs
• 2n indicates diploid
– 2n= 46 (2 sets of 23 chromosomes)
• 1n indicates haploid
– 1n=23 (1 set of 23 chromosomes)
• Egg and sperms are haploid
• The union of sperm and egg that occurs during
fertilization restores the chromosomes number
of the resulting embryo to 2n = 46
Mitosis
• Is the method of eukaryotic cell division that
produces two genetically identical cells.
• All cells in an organisms, except for sperm and
eggs, are produced by the process of mitosis.
• Mitosis progresses along five stages:
Prophase, Metaphase, Anaphase, Telophase,
and Cytokinesis
Prophase
• Duplicated chromosomes condense and become visible
as distinct sister chromatids.
• Nuclear envelope breaks down
• Centromeres move toward the poles of the cell.
• The mitotic spindle, which is made of microtubules
attaches to a specialized structure called the
kinetochore, located at the centromere of each
replicated chromosomes.
Metaphase
• Replicated chromosomes align at the equator,
or metaphase plate, of the cell.
• M&Ms (Metaphase Middle)
Anaphase
• The sister chromatids separate and are moved
toward opposite poles of the cell by the
spindle.
• As this happens, the cell begins to elongate
toward the poles.
Telophase
• Mitotic spindle breaks down.
• Nuclear envelope forms at each end of the
cell, and the chromosomes within begin to
unfold into chromatin.
Cytokinesis
• The cytoplasm and organelles are evenly divided
between the two new cells during cytokinesis,
completing the process of cells division.
• Plants and animals cells differ in cytokinesis
– Plants, a cell plate is formed as vesicles containing cell
membrane materials fuse together along the equator of
the cell.
– Animals, a ring of microfilaments contracts in the center
of the elongated cell, producing a cleavage furrow that
eventually pinches off the two cells.
Example of cytokinesis
Plant Cell
Animal Cell
Cell Cycle
Binary Fission
• Occurs in prokaryotes because have a single
double-stranded loop of DNA.
• Occurs in four steps
– 1. DNA is replicated
– 2. Cell doubles in size
– 3. Cell membrane grows into the center of the cell,
between the two circles of DNA, dividing the cell in
two.
– 4. Two cell seperate, and a cell wall forms around each
new cell.
Meiosis
“Me” likes Sex (Cells)
• The method of cell division that takes place in
sexually reproducing organisms specifically for
the creation of gametes– sperm and egg cells.
• Production four haploid cells, each genetically
different
• Meiosis requires two rounds of cell division.
Continue…
• Meiosis I: Homologous pairs of each chromosome
join and might exchange genetic material. The
homologous chromosomes are pulled to opposite
poles in the cell, at which point the cell separates,
resulting in two cells. Each cell contains half the
chromosome number of the original diploid cell.
Each chromosome remains in the duplicated state
and is made up of two sister chromatids
Continue…
• Meiosis II: The second stage of meiosis follows
similar steps as mitosis in the creation of two
more cells. Chromosomes do not replicate
between Meiosis I and Meiosis II.
– The result is four haploid cells genetically different
from one another.
Prophase I
• The most important events in prophase I are synapsis
and crossing over
– Synapsis: occurs when the two homologous
chromosomes condense and combine to form
complexes called tetrads
• Crossing over is the exchange of genetic material that
takes place between these homologous chromosomes
along several junctions known as chiasmata (place
where crossing over occurs)
Metaphase I
• The tetrads align along the metaphase plate of
the cell.
Anaphase I
• The homologous chromosome of each tetrad and
are pulled toward opposite poles of the cell by the
spindle.
• The side of the cell toward which a homologous
chromosome is pulled a random, depending only
on the orientation of the tetrad.
• The independent assortment of chromosome for
each cell is result of this random mix of
chromosomes derived from that organism’s parent
Telophase I
• Identical to telophase in mitosis.
• The cell continues to elongate, and the mitotic
spindle breaks down.
• A new nuclear envelope forms at each end of
the cell the chromosomes within unfold into
chromatin
Example
• Crossing over and the independent
assortment of chromosomes during meiosis
are two forces that help to produce genetic
variation . By independent assortment alone,
a single human can produce more than 8
million genetically different gametes. When
crossing over is also considered, the possible
number of genetically different is nearly
limitless.
Cytokinesis I
• Cytokinesis is very similar to mitosis divide
cytoplasm and organelles.
• Two genetically different haploid cells have
been produced.
• Each chromosome is still in the duplicated
state and is made up of two sister chromatids.
• Because crossing over during prophase I, the
sister chromatids are no longer identical.
Meiosis II
• Meiosis II occurs right after Cytokinesis I
• There is no Interphase (therefore No DNA
Replication)
Prophase II
• Chromosome condense within haploid cell
condense, and the spindle attaches to the
kinetochore of each chromosome.
• The nuclear envelope breaks down and the
centrosomes move toward the poles of the
cell.
Metaphase II
• Chromosomes align along the center of the
metaphase plate
Anaphase II
• The sister chromatids separate and are moved
toward opposite poles of the cell by the
spindle.
• The cell begins to elongate toward the poles
Telophase II
• The cell continues to elongate and the mitotic
spindle breaks down.
• A new nuclear envelope forms at each end of
the cell and the chromosomes within may
unfold into chromatin.
Cytokinesis II
• The cytoplasm and organelles are divided
between the two cells, completing the process
of cell division.
• By the end of this stage, four genetically
different haploid cells have been produced.
Sex Cells (Gametes)
• Meiosis produces four genetically different
haploid cells.
– Males haploid cells are sperms
– All four sperm can be used in sexual reproduction.
– Females haploid cells are 1 egg and 3 polar bodies
– Only the 1 egg can be used in sexual reproduction
– The three polar bodies will be recycled back into
the body.
Example
• The process of meiosis results in four genetically different
haploid cells. In animals, these haploid cells develop into
gametes, a sperm in males and an egg in females.
Fertilization is the process by which a sperm and egg fuse
together. The resulting zygote is diploid, with half the
chromosomes coming from the mother and other half
coming from the father. The processes of meiosis and
fertilization both account for the genetic variation found in
animals of the same species. Meiosis is responsible for
creating gametes whose genetic material varies from that
of the parent. Fertilization then combines the genetic
material of the two parents to produce the genetic material
of the offspring
Life Cycles
• Life Cycle is the sequence of events that make up
the reproductive cycle of an organisms.
• Alternation of generations: Plants sometimes
exist as a diploid organism and other times as a
haploid cell.
– Two haploid gametes combine to form a diploid
zygote, which divides mitotically to produce.
• Sporophyte: undergoes under meiosis to produce a haploid
spore
• Gametophytes: Mitotic division leads to production of
haploid multicellular organisms.
– Produces haploid gametes, which form diploid zygotes
Diagram of Alternation of Generation
Human Life Cycle
• The only haploid cells present in this life cycle
are gametes formed during meiosis.
• Two haploid gametes combine during
fertilization to produce a diploid zygote.
• Mitotic division then leads to formation of the
diploid multicellular organisms.
• Meiotic division later produces haploid
gametes.
Example of Human Life Cycle
Life Cycle of a Fungi
• Fungi are haploid organisms with the zygote being
the only diploid form.
• Like humans, the gametes for fungi are haploid
(n), and fertilization yield a diploid zygote.
• Instead of dividing by mitosis, the zygote divides
by meiosis to form a haploid organisms.
• Gametes are formed by mitosis, not meiosis—the
organism is already haploid, before forming the
gametes.
Life Cycle of a Fungi
• We will discuss more about the life cycles as
we get into the individual kingdoms.