Cell Division
Cells must divide in order for an organism or species to survive. Cells must divide in order
to fulfill the following life-functions:
1. Growth-cells have a limit on how large they can grow, so they must divide in order to
increase an organisms size (Ex. Humans start as one cell, and are eventually composed of
trillions of cells as full-grown adults)
2. Development- cells must divide in order to increase the functionality/survivability of an
organism (Ex. Human males experience an increase in muscle tissue (increase in cells)
during puberty)
3. Repair- cells must divide in order to mend/replace damaged cells (Ex. Red blood cells
replace themselves every 120 days or so, healing a cut or broken bone, etc.)
4. Reproduction- a common form of reproduction in single-celled organisms is for the
parent cell to simply divide into two daughter cells
a. BINARY FISSION: A PARENT prokaryotic (no nucleus) cell divides, forming
two identical DAUGHTER cells
i. Some eukaryotic (has nucleus) organelles reproduce this way
(Mitochondria, Chloroplasts) REMEMBER ENDOSYMBIOTIC
THEORY!
WHY CAN’T CELL JUST KEEP GETTING BIGGER AND BIGGER?
A big part of the answer has to do with the SURFACE AREA to VOLUME ratio
1. Cells need a certain amount of contact between the external environment and the
internal environment (separated by the cell membrane) in order to adequately
exchange nutrients and waste
2. As the volume (how much space the cells occupies) increases, the cell reaches a point
at which the surface area cannot keep up, and the cell cannot take in nutrients and
expel waste at a fast enough rate to stay alive
**Notice how, as the
volume (size)
increases, the surface
area to volume
(SA:Vol) ratio goes
down
**The efficiency
requirements of the
cell place a limit on
how large it can grow,
so it must divide
All cells move through a cycle…called THE CELL CYCLE…and The Cell Cycle has two
major components:
1. Interphase- The cells perform normal cell functions; Interphase is the LONGEST
PHASE of the cell cycle
2. M-Phase- MITOSIS; the cell divides from one PARENT cell into two genetically
identical DAUGHTER cells
I-PMAT + C
InterphaseProphase-Metaphase-Anaphase-Telophase + Cytokinesis
M-Phase: MITOSIS
Interphase:
1. Growth 1 (G1) Phase: cell grows,
organelles increase in number
2. Synthesis (S) Phase: DNA is
replicated, chromosome number is
doubled
3. Growth 2 (G2) Phase: cell
continues to grow in preparation for
MITOSIS
4. ***Some cells may enter G0 phase,
where they stay indefinitely without
dividing (ex. Most nerve cells)
5.
MITOSIS (M-PHASE) ***PMAT + C***


PROPHASE

Nucleus disappears

Spindle fibers form

DNA condenses into chromosomes (sister chromatids)
METAPHASE

Sister chromatids align along the equator of the cell by attaching their centromeres to the
spindle fibers

ANAPHASE

Sister chromatids separate at the centromere and are pulled towards opposite poles of the cell
by the mitotic spindle

TELOPHASE


Chromosomes arrive at opposite poles and unwind into thin strands of DNA

Spindle fibers disappears

Nuclear membrane reappears
CYTOKINESIS involves actual splitting of the cell membrane

Animal cells – cleavage furrow occurs

Plant cells – cell plate forms
How can I tell which phase a cell is in?
1. Interphase: This is the ONLY phase where you will see the
thread-like CHROMATIN (think chromatIN is IN INterphase)
2. Prophase: This is the ONLY phase where you will see the
remainder of the NUCLEUS and the SISTER CHROMATIDS
clumped together
3. Metaphase: Think MIDDLE, here you will see the SISTER
CHROMATIDS aligned at the EQUATOR (Middle) of the cell
4. Anaphase: Think AWAY, here you will see the SISTER
CHROMATIDS being PULLED APART to opposite sides of the
cell
5. Telophase: Think TWO, here you will see the beginning of two
cell forming with the start of either a cleavage furrow (animal
cell) or cell plate (plant cell) developing
6. Cytokinesis: Near the end of telophase/cytokinesis you will see
the chromosomes de-condensing back into chromatin, and the
nucleus reforming
MEIOSIS
Meiosis: A special form of cell division utilized by sexually
reproducing organisms to produce GAMETES. Gametes from two
individuals combine to form offspring.
1. Gamete: a sex cell (sperm or egg) containing half the number of
chromosomes as a normal body cell (SOMATIC CELL)
a. Somatic cells containing the full number of chromosomes are
labelled as DIPLOID (ex. Skin cells, muscles cells, bone cells,
etc.)
b. Gametes containing half the number of chromosomes of
somatic cells are labelled as HAPLOID (ex. Sperm and egg
cells)
c. Diploid: labelled as 2n, think Di= 2 sets of chromosomes
d. Haploid: labelled as n, think Hap= half the number of
chromosomes as diploid
e. ***when a sperm and egg meet, 2 HAPLOID cells merge
to form a complete DIPLOID cell
f. EX. The skin cell of a human is DIPLOID and contains 46
total chromosomes (23 from mom and 23 from dad)…an
egg or sperm cell produced by a human is HAPLOID and
contains 23 chromosomes
****A special event called CROSSING OVER occurs in PROPHASE
1 where homologous chromosomes exchange genetic information, this
is essential from promoting genetic diversity in offspring****
HOMOLOGOUS CHROMOSOMES: set of matching
chromosomes (EX. Humans have 23 pairs of chromosomes, 23 from
mom and 23 from dad, chromosomes #1 from mom and chromosome
#1 from dad would be homologous chromosomes)
Meiosis proceeds similar to Mitosis, but there are
two rounds of division PMAT 1 and PMAT 2
PMAT1
PMAT 2
**Interkinesis is just the time in between PMAT 1 and PMAT 2
MITOSIS VS. MEIOSIS
Visit this website for some added explanations and
pictures: http://www.wikihow.com/DifferentiateBetween-Mitosis-and-Meiosis