11.1 How
Prokaryotic Cells Divide
A. Prokaryotic cells (bacteria) lack a nucleus and other
membranous organelles.
B. The Prokaryotic Chromosome
1. Prokaryotic chromosome contains DNA and associated proteins, but much less
protein
than
eukaryotic chromosomes.
2. Chromosome consists of nucleoid
(an irregularly-shaped region, electron-dense, and not enclosed by a membrane).
3. Chromosome, when stretched out,
is a circular loop attached to the inside of the plasma membrane;
about 1000
times the length of the cell.
C. Binary Fission
1. Binary fission of prokaryotic cells produces two
genetically identical daughter cells by division (fission).
2. Before cell division, DNA is replicated
so two chromosomes are attached inside plasma membrane.
3. Following DNA replication, the
two chromosomes separate as a cell lengthens and pulls them apart.
4. When cell is approximately twice
its original length, the plasma membrane grows inward, a new cell
wall forms
dividing the cell into two approximately equal daughter cells.
5. Generation times of Escherichia
coli is 20 minutes; most bacteria need up to one hour to a day.
11.2
Eukaryotic Chromosomes and the Cell Cycle
A. Eukaryotic Chromosomes
1. DNA in chromosomes of eukaryotic cells is associated with proteins; histone
proteins organize chromosomes.
2. When cell is not undergoing
division, DNA in nucleus is a tangled mass of threads called chromatin.
3. At cell division, chromatin
becomes highly coiled and condensed and now visible as chromosomes.
4. Each species has a
characteristic number of chromosomes (2n).
a. Diploid
(2n) number includes two sets of chromosomes of each type.
1) Found in all the non-sex cells of an organism’s body (with a few
exceptions).
2) Examples include humans (46), crayfish (200), etc.
b. Haploid
(n) number contains one of each kind of chromosome.
1) In the life cycle of many animals, only sperm and egg cells have the haploid
number.
2) Examples included humans (23), crayfish (100), etc.
5. Cell divisionin
eukaryotes involves nuclear division (Karyokinesis) and cytokinesis
(division of the cytoplasm).
a. Somatic
(body) cells undergo mitosis for development, growth, and repair.
1) This nuclear division leaves the chromosome number constant.
2) A 2n nucleus replicates and divides to provide daughter nuclei that are also
2n.
b. A
chromosome begins cell division with two sister chromatids.
1) Sister chromatids are two strands of genetically identical
chromosomes.
2) At the beginning of cell division, they are attached at a centromere.
3) The centromere is a region of constriction on a chromosome
where sister chromatids are attached.
B. The Mitotic Spindle
1. Centrosomes are believed responsible for organizing the
spindle.
2. The centrosome is the main
microtubule organizing center of the cell.
3. The centrosome has divided before
mitosis begins.
4. Each centrosome contains a pair
of barrel-shaped organelles called centrioles; plant cells
lack centrioles.
5. The spindle contains many fibers,
each composed of a bundle of microtubules.
6. Microtubules are
made of the protein tubulin.
a.
Microtubules assemble when tubulin subunits join, disassemble when tubulin
subunits become free,
and form interconnected filaments of cytoskeleton.
b.
Microtubules disassemble as spindle fibers form.
C. Mitosis in Amimal Cells
1. Mitosis (karyokinesis) is divided into five phases: prophase,
prometaphase, metaphase,
anaphase,
and telophase.
2. Prophase
a. Nuclear
division is about to occur because chromatin condenses and chromosomes become
visible.
b. The
nucleolus disappears and the nuclear envelope fragments.
c. Already
duplicated chromosomes are composed of two sister chromatids held together by a
centromere.
1) This configuration in diagrammatic drawings gives accurate chromosome
number.
2) Chromosomes have no particular orientation in cell at this time.
3) Specialized protein complexes (kinetochores) develop on each side of
centromere for future
chromosome orientation.
d. Spindle
begins to assemble as pairs of centrosomes migrate away from each other.
e. Short
microtubules radiate out from the pair of centroiles located in each controsome
to form starlike asters.
3. Prometaphase
a. At this
time, the spindle consists of poles, asters, and fibers that are bundles of microtubules.
b. Important
event during prometaphase is attachment of chromosomes to the spindle and their
movement
as they align at the metaphase plate (equator) of the spindle.
c. The
kinetochores of sister chromatids capture kinetochore spindle fibers.
d.
Chromosomes move back and forth until they are aligned at the metaphase plate.
4. Metaphase
a.
Chromosomes, attached to kinetochore fibers, are aligned at the metaphase
plate.
b.
Non-attached spindle fibers, called polar spindle fibers, can
reach beyond the metaphase plate and overlap.
5. Anaphase
a. Two
sister chromatids of each duplicated chromosome separate at centromere.
b. Daughter
chromosomes, each with a centromere and single chromatid, move to opposite
poles.
1) Polar spindle fibers lengthen as they slide past each other.
2) Kinetochore spindle fibers disassemble at the kinetochores; this pulls
daughter chromosomes to poles.
6. Telophase
a. Spindle
disappears.
b.
Chromosomes decondense and return to chromatin; the nuclear envelope reforms
and nucleoli reappear.
c.
Cytokinesis is nearly complete.
D. Mitosis in Cells
1. Plant meristematic tissue in tips of roots and shoots of stems retains
ability to divide throughout life.
2. Stages are exactly same as in
animal cells.
3. Although plant cells have a
centrosome and spindle, there are no centrioles and asters do not form.
E. Cytokinesis in Plant and Animal Cells
1. In Plant Cells
a. The
rigid cell wall that surrounds plant cells does not permit cytokinesis by
furrowing.
b. Golgi
apparatus produces vesicles that move to the midpoint between the daughter
nuclei.
c. Vesicles
fuse forming cell plate; their membranes complete plasma
memberanes of daughter cells.
d. Vesicles
also release molecules that signal the formation of plant cell walls.
e. Walls are
strengthened by the addition of cellulose fibrils.
2. In Animal Cells
a. Cleavage
furrow indents the plasma membrane between the two daughter nuclei at a
midpoint;
progressively divides cytoplasm during cell division.
b.
Cytoplasmic cleavage begins as anaphase draws to a close.
c. Cleavage
furrow deepens as band of actin filaments constricts between the two daughter
cells.
d. Narrow
bridge exists between daughter cells during telophase; constriction separates
cytoplasm.
3. Cell Division in Other Eukaryotic
Organisms
a. Protists
and fungi also undergo mitosis and cytokinesis.
b. In fungi
and some protists, the nuclear envelope does not fragment but divides and one
nucleus
goes to each daughter cell.
11.3 How
Eukaryotic Cells Cycle
1. Interphase was considered a "resting state" until
DNA replication was detected in the 1950s.
2. Cell cycle involved
4-stage sequence of events.
3. M stage (M=mitosis)
is the entire cell division state, including both mitosis and cytokinesis.
4. G1 stage just prior
to DNA replication is when cell grows in size and organelles increase in
number.
5. S Stage is DNA
synthesis period where replication occurs; proteins associated with DNA are
also synthesized.
6. G2 stage occurs
just prior to cell division; preparation for mitotic cell division.
7. Interphase consists of G1, S,
and G2 stages.
A. The Cell-Cycle Clock
1. Some cells (e.g., skin cells) divide continuously throughout the life of an
organism.
2. Skeletal muscle cells and nerve
cells are arrested in the G1 stage; if the nucleus from an arrested cell
is placed in
cytoplasm of an S-stage cell, it starts to finish the cell cycle.
3. Cardiac muscle cells, are
arrested in the G2 stage; if fused with a cell undergoing mitosis, it too
starts
to undergo
mitosis.
4. There appear to be stimulatory
substances causing a cell to proceed through two critical checkpoints:
a. G1
stage 
S stage
b. G2
stage M stage
5. Enzymes known as cyclins and
kinases regulate passage of cells through these two checkpoints.
a. Kinases
are enzymes that remove a phosphate group from ATP and add it to another
protein; this is
a common way for the cell to turn on metabolic pathways.
b. Cyclin
proteins activate kinases, which in turn activate enzymes; one destroys cyclin;
therefore cyclin
levels vary, which gives them their name.
c. When
M-kinase combines with M-cyclin, the kinase phosphorylates a protein causing
the cell to move
from G2 to the M stage. This causes:
1) the chromosomes to condense,
2) the nuclear envelope to disassemble, and
3) the spindle to form.
4) Then M-cyclin is destroyed.
d. Growth
Factors
1) Growth factors are molecules that attach to plasma membrane receptors and
bring about cell growth.
2) Ordinarily a cyclin combines with its kinase only when a growth factor is
present.
3) Cyclin that has gone awry combines with its kinase when growth factor is
absent, resulting in a tumor.
4) Tumor suppressor genes usually function to prevent cancer; e.g., tumor
suppressor gene causes production
of protein that combines with a cyclin kinase complex to stop that kinase from
being active; this stops the
cell cycle.
11.4 How
Cancer Develops
A. Carcinogenesis is development of cancer.
1. Cancer is a genetic disease requiring a series of mutations toward
developing a tumor.
2. A tumor indicates a
failure in controlling cell division, usually due to a faulty gene.
3. Normal gene protein halts the
gene cycle when DNA mutates and is in need of repair.
4. Carcinogens are agents that cause
cancer and include
a. radiation
(e.g., U-V light, X-rays, radon gas, etc.)
b. organic
chemicals (e.g., tobacco smoke, some foods, pesticides, etc.), and
c. certain
viruses.
5. A protein mobilizes repair
enzymes and stops the cell cycle; only when repaired does a cell cycle begin
again.
6. If DNA repairs is not possible,
the protein promotes cell death—a good thing.
B. Apoptosis is programmed cell death.
1. Apoptosis is a sequence of cellular changes involving:
a.
shattering the nucleus,
b. chopping
up the chromosomes, and
c. packaging
the cellular remains into vesicles to the engulfed by macrophages.
2. Apoptosis is caused by cells
harboring enzymes called caspases.
3. Cells normally contain caspases
by using inhibitors.
4. Caspases can be released in two
ways:
a. during
development, external signals trigger cells to die, as in the webbing between
fingers.
b. in
adults, cells with severe DNA damage kill themselves.
5. Caspases are activated in two
ways.
a.
Initiators receive a message to activate the executioners which activate the
enzymes.
b.
Initiators, executioners and dismantling enzymes become active when they are
clipped and shortened.
6. Apoptosis research may lead to
new therapy.
a. Tumor
cells contain high levels of survivin protein which blocks apoptosis; if we can
inactivate survivin,
cancer cells would be more vulnerable to radiation and chemicals.
b. Excess
apoptosis kills off brain cells in Parkinson’s disease and stroke; inhibitors
of apoptosis could
keep the brain cells alive.
C. Characteristics of Cancer Cells
1. Cancer cells lack differentiation.
a. Unlike
normal cells that differentiate into muscle or nerves cells, cancer cells have
a general abnormal form.
b. Normal
cells enter the cell cycle only about 50 times; cancer cells are immortal.
2. Cancer cells have abnormal
nuclei.
a. The nuclei
may have an abnormal number of chromosomes, and be enlarged.
b. Some
chromosomes may be duplicated or deleted.
c. Extra
copies of specific genes is more frequent.
3. Cancer cells form tumors.
a. Normal cells
are anchored and stop dividing when in contact with other cells.
b. Cancer
cells invade and destroys normal tissue; new growth is called neoplasia.
c. A benign
tumor is disorganized but encapsulated and does not invade adjacent tissue.
4. Cancer cells undergo angiogenesis
and metstasis.
a. Cancer cells
release a growth factor that caused nearby blood vessels to grow and bring more
nutrients
and oxygen to the tumor.
b. Cancer in
situ is still in its place of origin and has not spread to other tissues.
c.
Malignancy occurs when metastasis spreads new tumors distant from the primary
tumor.