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The Cellular Basis of
Why Do Cells Divide?
Cell Division is the splitting of a single cell
into 2 cells. 3 life processes occur:
Growth: this is the increase of in size.
Differentiation is the specialization in
Repair: this is the ability of an organism
to fix itself; humans repair their skin blood
vessels and bone.
Regeneration is the ability of an organism
to replace a missing body part
(like a starfish regrowing an arm).
Reproduction: When an organism is single-celled
and that cell divides, it is reproducing. This is a
form of asexual reproduction.
Bacteria and unicellular eukaryotes reproduce this way.
The arm that broke off from the starfish can also
reproduce asexually by cell division. It slowly
regrows a new body.
Asexual reproduction produces genetically
identical offspring to the parents.
Sexual Reproduction:
Sexual reproduction produces genetic varieties in
Plants and animals reproduce this way. This results in a
recombination of chromosomes through meiosis, a
specialized form of cell division.
How Do Cells Divide?
• The cell cycle is the
sequence of phases in the
life cycle of the cell
• The cell cycle has 2 parts:
Interphase (Growth and
preparation) and Cell
• Cell Division includes:
Mitosis (nuclear division)
and Cytokinesis
(cytoplasm division).
• Gap 1 – growth
• Synthesis – DNA Replication
• Gap 2 – growth and preparation for cell
division (make more organelles, etc.)
Some Terms:
• Chromatin is the fibrous form of DNA and
proteins that make up chromosomes.
– This is what is found within the nucleus of the cell
during interphase.
– It is clumped DNA.
• Once chromosomes have been replicated, they
are paired together in the form of sister
– These are identical structures that are side by
• Sister chromatids are held together by a
– This is the point of attachment.
Cell Division:
• G1, or gap 1, is characterized by growth and
• S stage, or Synthesis, is when the
chromosomes are replicated.
• G2, or gap 2, is when the cell synthesizes
organelles and other materials.
• This is the longest phase of the entire cell
cycle. The cell is in preparation for the nucleus
to divide.
Mitosis is the formation of 2 nuclei from 1.
It occurs in 4 stages:
prophase, metaphase, anaphase, telophase (PMAT)
1. Prophase:
• Chromosomes condense & become visible under the
light microscope
• Microtubules from the mitotic spindles
• The nuclear envelope & nucleolus break apart &
• Centromeres attach to the spindle fibers
2. Metaphase:
• The chromosomes move to the center of the cell
• The center of the cell is called the metaphase plate
3. Anaphase:
• Centromeres divide & the spindle fibers
pull 1 set of sister chromatids toward
opposite poles
• Once chromosomes are at opposite
poles, anaphase is over
4. Telophase:
• A nuclear envelope forms around each
set of chromosomes
• Chromosomes uncoil into chromatin
• Mitotic spindle fibers disassemble
• This is a.k.a. cytoplasm separation
• In animal cells, this begins in telophase
as the nuclei reform.
– This starts at the center of the cell and
pinches inward. This is called a cleavage
• In plant cells, this begins in anaphase
and starts in the center of the cell along
the metaphase plate and grows
– This is called the cell plate.
• Tumors: masses/clusters of cells
– Benign: non-cancerous
– Malignant: cancerous (usually uncontrolled
dividing cells)
• Metastasis: spreading of cancerous
• Treatment: surgery (removes tumor),
radiation & chemotherapy (destroys
cells by disrupting cell cycle)
• Radiation & chemo side effects: healthy
cells may die, sterility, hair loss,
What is Meiosis?
Remember that humans have 46 chromosomes (or 23
pairs) in their cells.
• This means they have 2 complete sets of
• Diploid, or 2n, is a cell that has 2 complete sets of
chromosomes (in humans, 46).
• Haploid, or 1n or n, is a cell that has only 1 set of
chromosomes (in humans, 23).
• Human’s sex cells, or gametes, are haploid.
• All human body cells are produced through
mitosis whereas the sex cells, or gametes,
are produced through meiosis.
• Gametes are sperm and egg and have only
23 chromosomes in each.
– When they fuse (at fertilization), they form
a zygote (23 + 23= 46).
– This is how each generation remains
• Meiosis is a type of cellular reproduction in
which the # of chromosomes are reduced by
½ so that the daughter cells are haploid (n).
• Homologous pairs are pairs of
– Each of the 23 chromosomes has a matching
chromosome (with 1 exception: the sex
– Sex chromosomes are X and Y.
The Phases of Meiosis:
• Prior to meiosis, a diploid cell
replicates its chromosomes
• Meiosis has 2 stages: Meiosis I and
Meiosis II. Each has 4 phases.
Homologous Chromosomes:
Karyotype: display of chromosomes in # order; informs
chromosomal # abnormality, chromosomal abnormaility &
genetic sex
Meiosis I:
1. Prophase I: chromosomes condense,
homologous chromosomes become
attached to each other, each homologous
chromosome contains 4 sister chromatid
(this is called a tetrad, meaning 4).
2. Metaphase I: homologous pairs align along
the middle of the cell.
3. Anaphase I: homologous pairs split.
4. Telophase I and Cytokinesis: Nuclei reform
and the cells split. This result is 2 haploid
cells, each with 1 complete set of
Meiosis II:
Prophase II: Spindle fibers form again &
chromosomes condense. NO tetrads; NO crossing
Metaphase II: chromatids move to the center of the
Anaphase II: chromatids are pulled to opposite
Telophase II and Cytokinesis: Nuclei reform and
cells separate. The result is 4 haploid cells.
Meiosis I 
Meiosis II 
• In human males, 4 haploid cells result
(sperm cells) but in human females,
only 1 of the 4 haploid cells forms an
egg cell. The other 3 receive no
cytoplasm and do not form gametes
(they disintegrate).
• Interphase occurs only ONCE (Meiosis
I), meaning chromosomes replicate
only 1X.
Differences between mitosis and meiosis:
• Meiosis produces daughter cells with ½ the
# of chromosomes (haploid cells: 2nn)
• Mitosis produces daughter cells with the
same # of chromosomes as parent cell
(diploid cells: 2n  2n)
• Meiosis produces daughter cells that are
NOT genetically identical to each other (the
homologous chromosomes separation is
• Mitosis produces EXACT copies of parent
• Meiosis produces 4 haploid cells; mitosis
produces 2 diploid cells.
Genetic Variation
Variation results from the recombination of
DNA (from meiosis & fertilization) and
accounts for the differences between
members of a population.
Sources of Genetic Variation:
• Random separation of homologous pairs of
• Random combination of haploid gametes
• Crossing Over (tips of homologous
chromosomes switch places) occurs during
prophase I (meiosis I)