Chapter Outline
9.1 The Cell Cycle
1.
B.
C.
D.
E.
The cell cycle is an orderly set of stages from the first division to the time the daughter cells
divide.
2. When a cell is preparing for division, it grows larger, the number of organelles doubles, and the
DNA replicates.
Interphase
1. Most of a cell's life is spent in interphase, in which the cell performs its usual functions.
2. Time spent in interphase varies by cell type: nerve and muscle cells do not complete the cell
cycle and remain in the G0 stage while embryonic cells complete the cycle every few hours.
3. The G1 stage is just prior to DNA replication; a cell grows in size, organelles increase in number,
and material accumulates for DNA synthesis.
4. The S stage is the DNA synthesis (replication) period; proteins associated with DNA are also
synthesized; at the end of the S stage, each chromosome has two identical DNA double helix
molecules, called sister chromatids.
5. The G2 stage occurs just prior to cell division; the cell synthesizes proteins needed for cell
division, such as proteins in microtubules.
6. Interphase therefore consists of G1, S, and G2.
M (Mitotic) Stage
1. M stage (M = mitosis) is the entire cell division stage, including both mitosis and cytokinesis.
2. Mitosis is nuclear division, cytokinesis is division of the cytoplasm.
3. When division of the cytoplasm is complete, two daughter cells are produced.
Control of the Cell Cycle
1. The cell cycle is controlled by both internal and external signals.
2. A signal is a molecule that either stimulates or inhibits a metabolic event.
3. Growth factors are external signals received at the plasma membrane.
4. Cell Cycle Checkpoints
a. There appear to be three checkpoints where the cell cycle either stops or continues
onward, depending on the internal signals it receives.
b. Researchers have identified a family of proteins called cyclins, internal signals that
increase or decrease during the cell cycle.
c. Cyclin must be present for the cell to move from the G1 stage to the S stage, and from
the G2 stage to the M stage.
d. The cell cycle stops at the G2 stage if DNA has not finished replicating; stopping the cell
cycle at this stage allows time for repair of possible damaged DNA.
e. Also, the cycle stops if chromosomes are not distributed accurately to daughter cells.
f. DNA damage also stops the cycle at the G1 checkpoint by the protein p53; if the DNA is
not repaired, p53 triggers apoptosis.
Apoptosis
1. Apoptosis is programmed cell death and involves a sequence of cellular events involving:
a. fragmenting of the nucleus,
b. blistering of the plasma membrane, and
c. engulfing of cell fragments by macrophages and/or neighboring cells.
2. Apoptosis is caused by enzymes called caspases.
3. Cells normally hold caspases in check with inhibitors.
4. Caspases are released by internal or external signals.
5. Apoptosis and cell division are balancing processes that maintain the normal level of somatic
(body) cells.
6. Cell death is a normal and necessary part of development: frogs, for example, must destroy tail
tissue they used as tadpoles, and the human embryo must eliminate webbing found between
fingers and toes.
7. Death by apoptosis prevents a tumor from developing.
9.2 Mitosis and Cytokinesis
A.
Eukaryotic Chromosomes
1. DNA in chromosomes of eukaryotic cells is associated with proteins; histone proteins organize
chromosomes.
2.
B.
When a cell is not undergoing division, DNA in the nucleus is a tangled mass of threads called
chromatin.
3. At cell division, chromatin becomes highly coiled and condensed and is now visible as individual
chromosomes.
4. Each species has a characteristic number of chromosomes.
a. The diploid (2n) number includes two sets of chromosomes of each type.
i.
The diploid number is found in all the non-sex cells of an organism's body (with
a few exceptions).
ii.
Examples include humans (46), crayfish (200), etc.
b. The haploid (n) number contains one of each kind of chromosome.
i.
In the life cycle of many animals, only sperm and egg cells have the haploid
number.
ii.
Examples include humans (23), crayfish (100), etc.
5. Cell division in eukaryotes involves nuclear division and cytokinesis.
a. Somatic cells undergo mitosis for development, growth, and repair.
i.
This nuclear division leaves the chromosome number constant.
ii.
A 2n nucleus replicates and divides to provide daughter nuclei that are also 2n.
b. A chromosome begins cell division with two sister chromatids.
i.
Sister chromatids are two strands of genetically identical chromosomes.
ii.
At the beginning of cell division, they are attached at a centromere, a region of
constriction on a chromosome.
Stages of Mitosis
1. The centrosome, the main microtubule organizing center of the cell, divides before mitosis
begins.
2. Each centrosome contains a pair of barrel-shaped organelles called centrioles.
3. The mitotic spindle contains many fibers, each composed of a bundle of microtubules.
4. 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.
5. Mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase, and
telophase.
6. Prophase
a. Nuclear division is about to occur: chromatin condenses and chromosomes become
visible.
b. The nucleolus disappears and the nuclear envelope fragments.
c. Duplicated chromosomes are composed of two sister chromatids held together by a
centromere; chromosomes have no particular orientation in the cell at this time.
d. The spindle begins to assemble as pairs of centrosomes migrate away from each other.
e. An array of microtubules called asters radiates toward the plasma membrane from the
centrosomes.
7. Prometaphase (Late Prophase)
a. Specialized protein complexes (kinetochores) develop on each side of the centromere
for future chromosome orientation.
b. An important event during prometaphase is attachment of the 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 toward alignment at the metaphase plate.
8. Metaphase
a. Chromosomes, attached to kinetochore fibers, are now aligned at the metaphase plate.
b. Non-attached spindle fibers, called polar spindle fibers, can reach beyond the
metaphase plate and overlap.
9. Anaphase
a. The two sister chromatids of each duplicated chromosome separate at the centromere.
b. Daughter chromosomes, each with a centromere and single chromatid, move to
opposite poles.
i.
Polar spindle fibers lengthen as they slide past each other.
ii.
Kinetochore spindle fibers disassemble at the kinetochores; this pulls daughter
chromosomes to poles.
iii.
The motor molecules kinesin and dynein are involved in this sliding process.
iv.
Anaphase is the shortest stage of mitosis.
C.
D.
E.
10. Telophase
a. Spindle disappears in this stage.
b. The nuclear envelope reforms around the daughter chromosomes.
c. The daughter chromosomes diffuse, again forming chromatin.
d. The nucleolus reappears in each daughter nucleus.
Cytokinesis in Animal and Plant Cells
1. Cytokinesis in Animal Cells
a. A cleavage furrow indents the plasma membrane between the two daughter nuclei at a
midpoint; this deepens to divide the cytoplasm during cell division.
b. Cytoplasmic cleavage begins as anaphase draws to a close and organelles are
distributed.
c. The cleavage furrow deepens as a band of actin filaments, called the contractile ring,
constricts between the two daughter cells.
d. A narrow bridge exists between daughter cells during telophase until constriction
completely separates the cytoplasm.
2. Cytokinesis in Plant Cells
a. The rigid cell wall that surrounds plant cells does not permit cytokinesis by furrowing.
b. The Golgi apparatus produces vesicles, which move along the microtubules to a small
flattened disc that has formed.
c. Vesicles fuse forming a cell plate; their membranes complete the plasma membranes of
the daughter cells.
d. The new membrane also releases molecules from the new plant cell walls; the cell walls
are strengthened by the addition of cellulose fibrils.
The Functions of Mitosis
1. Mitosis permits growth and repair.
2. In flowering plants, the meristematic tissue retains the ability to divide throughout the life of the
plant; this accounts for the continued growth, both in height and laterally, of a plant.
3. In mammals, mitosis is necessary as a fertilized egg becomes an embryo and as the embryo
becomes a fetus; throughout life, mitosis allows a cut to heal or a broken bone to mend.
Stem Cells
1. Many mammalian organs contain stem cells (or adult stem cells), which retain the ability to divide.
2. Red bone marrow stem cells repeatedly divide to produce the various types of blood cells.
3. Therapeutic cloning to produce human tissues can begin with either adult stem cells or
embryonic stem cells.
4. Embryonic stem cells can be used for reproductive cloning, the production of a new individual.
9.3 The Cell Cycle and Cancer
1.
2.
3.
B.
A neoplasm is an abnormal growth of cells.
A benign neoplasm is not cancerous; a malignant neoplasm is cancerous.
Cancer is a cellular growth disorder that results from the mutation of genes that regulate the cell
cycle; i.e., cancer results from the loss of control and a disruption of the cell cycle.
4. Carcinogenesis, the development of cancer is gradual—it may take decades before a cell has
the characteristics of a cancer cell.
Characteristics of Cancer Cells
1. Cancer cells lack differentiation.
a. Unlike normal cells that differentiate into muscle or nerves cells, cancer cells have an
abnormal form and are nonspecialized.
b. Normal cells enter the cell cycle only about 50 times; cancer cells are immortal in that
they can enter the cell cycle repeatedly.
2. Cancer cells have abnormal nuclei.
a. The nuclei may be enlarged and may have an abnormal number of chromosomes.
b. The chromosomes have mutated; some chromosomes may be duplicated or deleted.
c. Gene amplification, extra copies of genes, is more frequent in cancerous cells.
d. Whereas ordinary cells with DNA damage undergo apoptosis, cancer cells do not.
3. Cancer cells form tumors.
a. Normal cells are anchored and stop dividing when in contact with other cells; i.e., they
exhibit contact inhibition.
b. Cancer cells invade and destroy normal tissue and their growth is not inhibited.
c. Cancer cells pile on top of each other to form a tumor.
4. Cancer cells undergo metastasis and angiogenesis.
a.
b.
c.
d.
e.
C.
D.
E.
F.
A benign tumor is encapsulated and does not invade adjacent tissue.
Cancer in situ is a tumor in its place of origin but is not encapsulated—it will invade
surrounding tissues.
Many types of cancer can undergo metastasis, in which new tumors form which are
distant from the primary tumor.
Angiogenesis, the formation of new blood vessels, is required to bring nutrients and
oxygen to the tumor.
A cancer patient's prognosis depends on whether the tumor has invaded surrounding
tissue, whether there is lymph node involvement, and whether there are metastatic
tumors elsewhere in the body.
Origin of Cancer
1. A DNA repair system corrects mutations during replication; mutations in genes encoding the
various repair enzymes can cause cancer.
2. Proto-oncogenes specify proteins that stimulate the cell cycle while tumor-suppressor genes
specify proteins that inhibit the cell cycle; mutations of either of these genes can cause cancer.
3. DNA segments called telomeres form the ends of chromosomes and shorten with each
replication, eventually signaling the cell to end division; cancer cells produce telomerase that
keeps telomeres at a constant length and thus the cells to continue dividing.
Regulation of the Cell Cycle
1. Proto-oncogenes are at the end of a stimulatory pathway from the plasma membrane to the
nucleus; a growth factor binding at the plasma membrane can result in turning on an oncogene.
2. Tumor-suppressor genes are at the end of an inhibitory pathway; a growth-inhibitory factor can
result in turning on a tumor suppressor gene that inhibits the cell cycle.
3. The balance between stimulatory and inhibitory signals determines whether proto-oncogenes or
tumor-suppressor genes are active, and therefore whether or not cell division occurs.
Oncogenes
1. Proto-oncogenes can undergo mutation to become oncogenes (cancer-causing genes).
2. An oncogene may code for a faulty receptor in the stimulatory pathway, or,
3. An oncogene can specify an abnormal protein product or abnormally high levels of a normal
product that stimulates the cell cycle.
4. About 100 oncogenes have been described; the ras gene family includes variants associated with
lung, colon, pancreatic cancers as well as leukemias, lymphomas, and thyroid cancers; the
BRCA1gene is associated with certain forms of breast and ovarian cancer.
Tumor-suppressor Genes
1. Mutation of a tumor-suppressor gene results in unregulated cell growth.
2. Researchers have identified about a half dozen tumor-suppressor genes.
3. The RB tumor-suppressor gene prevents retinoblastoma, a cancer of the retina, and has been
found to malfunction in cancers of the breast, prostate, bladder, and small-cell lung carcinoma.
4. The p53 tumor-suppressor gene is more frequently mutated in human cancers than any other
known gene; it normally functions to trigger cell cycle inhibitors and stimulate apoptosis.
9.4 Prokaryotic Cell Division
1.
B.
C.
Unicellular organisms reproduce via asexual reproduction, in which the offspring are genetically
identical to the parent.
The Prokaryotic Chromosome
1. Prokaryotic cells (bacteria and archaea) lack a nucleus and other membranous organelles.
2. The prokaryotic chromosome is composed of DNA and associated proteins, but much less
protein than eukaryotic chromosomes.
3. The chromosome appears as a nucleoid, an irregular-shaped region that is not enclosed by a
membrane.
4. The chromosome is a circular loop attached to the inside of the plasma membrane; it is about
1,000 times the length of the cell.
Binary Fission
1. Binary fission of prokaryotic cells produces two genetically identical daughter cells.
2. Before cell division, DNA is replicated--both chromosomes are attached to a special site inside
the plasma membrane.
3. The two chromosomes separate as a cell lengthens and pulls them apart.
4. When the cell is approximately twice its original length, the plasma membrane grows inward, a
septum (consisting of new cell wall and plasma membrane) forms, dividing the cell into two
daughter cells.
5.
D.
The generation time of bacteria depends on the species and environmental conditions;
Escherichia coli's generation time is about 20 minutes.
Comparing Prokaryotes and Eukaryotes
1. Both binary fission and mitosis ensure that each daughter cell is genetically identical to the
parent.
2. Bacteria and protists use asexual reproduction to produce identical offspring.
3. In multicellular fungi, plants, and animals, cell division is part of the growth process that produces
and repairs the organism.
4. Prokaryotes have a single chromosome with mostly DNA and some associated protein; there is
no spindle apparatus.
5. Eukaryotic cells have chromosomes with DNA and many associated proteins; histone proteins
organize the chromosome.
6. The spindle is involved in distributing the daughter chromosomes to the daughter nuclei.
10.1 Halving the Chromosome Number
1.
B.
C.
Meiosis is nuclear division, reducing the chromosome number from the diploid (2n) to the haploid
(n) number.
2. The haploid (n) number is half of the diploid number of chromosomes.
3. Sexual reproduction requires gamete (reproductive cell, often sperm and egg) formation and then
fusion of gametes to form a zygote.
4. A zygote always has the full or diploid (2n) number of chromosomes.
5. If gametes contained same number of chromosomes as body cells, doubling would soon fill cells.
Homologous Pairs of Chromosomes
1. In diploid body cells, chromosomes occur as pairs.
a. Each set of chromosomes is a homologous pair; each member is a homologous
chromosome or homologue.
b. Homologues look alike, have the same length and centromere position, and have a similar
banding pattern when stained.
c. A location on one homologue contains gene for the same trait that occurs at this locus on
the other homologue, although the genes may code for different variations of that trait;
alternate forms of a gene are called alleles.
2. Chromosomes duplicate immediately prior to nuclear division.
a. Duplication produces two identical parts called sister chromatids; they are held together at
the centromere.
3. One member of each homologous pair is inherited from the male parent, the other member from
the female parent.
4. One member of each homologous pair will be placed in each sperm or egg.
Overview of Meiosis
1. Meiosis involves two nuclear divisions and produces four haploid daughter cells.
2. Each daughter cell has half the number of chromosomes found in the diploid parent nucleus.
3. Meiosis I is the nuclear division at the first meiotic division.
a. Prior to meiosis I, DNA replication occurs, each chromosome thus has two sister
chromatids.
b. During meiosis I, homologous chromosomes pair; this is called synapsis.
c. During synapsis, the two sets of paired chromosomes lay alongside each other as a
bivalent (sometimes called a tetrad).
4. In meiosis II, the centromeres divide and daughter chromosomes (derived as sister chromatids)
separate.
a. No replication of DNA is needed between meiosis I and II because chromosomes are
already doubled (DNA replication occurred prior to meiosis I).
b. Chromosomes in the four daughter cells have only one chromatid.
c. Counting the number of centromeres verifies that parent cells were diploid; each daughter
cell is haploid.
d. In the animal life cycle, daughter cells become gametes that fuse during fertilization.
e. Fertilization restores the diploid number in cells.
10.2 Genetic Variation
A.
Genetic Recombination
1. Due to genetic recombination, offspring have a different combination of genes than their parents.
2.
B.
C.
D.
Without recombination, asexual organisms must rely on mutations to generate variation among
offspring; this is sufficient because they have great numbers of offspring.
3. Meiosis brings about genetic recombination in two ways: crossing-over and independent
assortment.
4. Crossing-over of non-sister chromatids results in exchange of genetic material between non-sister
chromatids of a bivalent; this introduces variation.
5. At synapsis, homologous chromosomes are held in position by a nucleoprotein lattice (the
synaptonemal complex).
6. As the lattice of the synaptonemal complex breaks down at the beginning of anaphase I,
homologues are temporarily held together by chiasmata, regions where the non-sister chromatids
are attached due to crossing-over.
7. The homologues separate and are distributed to daughter cells.
8. Due to this genetic recombination, daughter chromosomes derived from sister chromatids are no
longer identical.
Independent Assortment of Homologous Chromosomes
1. During independent assortment, the homologous chromosomes separate independently or in a
random manner.
2. Independent assortment in a cell with only three pairs of chromosomes is 23 or eight combinations
of maternal and paternal chromosomes.
3. In humans with 23 pairs of chromosomes, the combinations possible are 2 23 or 8,388,608, and this
does not consider the variation from crossing-over.
Fertilization
1. When gametes fuse at fertilization, chromosomes donated by parents combine.
2. The chromosomally different zygotes from same parents have (2 23)2 or 70,368,744,000,000
combinations possible without crossing-over.
3. If crossing-over occurs once, then (423)2 or 4,951,760,200,000,000,000,000,000,000 genetically
different zygotes are possible for one couple.
Significance of Genetic Recombination
1. A successful parent in a particular environment can reproduce asexually and produce offspring
adapted to that environment.
2. If the environment changes, differences among offspring provide the sexual parents with much
improved chances of survival.
10.3 The Phases of Meiosis
B.
C.
D.
E.
F.
1. Both meiosis I and meiosis II have four phases: prophase, metaphase anaphase, and telophase.
Prophase I
1. Nuclear division is about to occur: nucleolus disappears; nuclear envelope fragments; centrosomes
migrate away from each other; and spindle fibers assemble.
2. Homologous chromosomes undergo synapsis to form bivalents; crossing-over may occur at this
time in which case sister chromatids are no longer identical.
3. Chromatin condenses and chromosomes become microscopically visible.
Metaphase I
1. Bivalents held together by chiasmata have moved toward the metaphase plate at the equator of the
spindle.
2. In metaphase I, there is a fully formed spindle and alignment of the bivalents at the metaphase
plate.
3. Kinetochores, protein complexes just outside the centromeres attach to spindle fibers called
kinetochore spindle fibers.
4. Bivalents independently align themselves at the metaphase plate of the spindle.
5. Maternal and paternal homologues of each bivalent may be oriented toward either pole.
Anaphase I
1. The homologues of each bivalent separate and move toward opposite poles.
2. Each chromosome still has two chromatids.
Telophase I
1. In animals, this stage occurs at the end of meiosis I.
2. When it occurs, the nuclear envelope reforms and nucleoli reappear.
3. This phase may or may not be accompanied by cytokinesis.
Interkinesis
1. This period between meiosis I and II is similar to the interphase between mitotic divisions; however,
no DNA replication occurs (the chromosomes are already duplicated).
G. Meiosis II
1. During metaphase II, the haploid number of chromosomes align at the metaphase plate.
2. During anaphase II, the sister chromatids separate at the centromeres; the two daughter
chromosomes move toward the poles.
3. Due to crossing-over, each gamete can contain chromosomes with different types of genes.
4. At the end of telophase II and cytokinesis, there are four haploid cells.
5. In animals, the haploid cells mature and develop into gametes.
6. In plants, the daughter cells become spores and divide to produce a haploid generation; these
haploid cells fuse to become a zygote that develops into a diploid generation.
7. The type of life cycle of alternating haploid and diploid generations is called alternation of
generations.
10.4 Meiosis Compared to Mitosis
1.
2.
3.
B.
C.
D.
Meiosis requires two nuclear divisions; mitosis requires only one nuclear division.
Meiosis produces four daughter nuclei and four daughter cells; mitosis produces only two.
The daughter cells produced by meiosis are haploid; the daughter cells produced by mitosis are
diploid.
4. The daughter cells produced by meiosis are not genetically identical; the daughter cells produced
by mitosis are genetically identical to each other and to the parental cell.
A Occurrence
1. In humans, meiosis occurs only in reproductive organs to produce gametes.
2. Mitosis occurs in all tissues for growth and repair.
Meiosis I Compared to Mitosis
1. DNA is replicated only once before both mitosis and meiosis; in mitosis there is only one nuclear
division; in meiosis there are two nuclear divisions.
2. During prophase I of meiosis, homologous chromosomes pair and undergo crossing-over; this does
not occur during mitosis.
3. During metaphase I of meiosis, bivalents align at the metaphase plate; in mitosis individual
chromosomes align.
4. During anaphase I in meiosis, homologous chromosomes (with centromeres intact) separate and
move to opposite poles; in mitosis at this stage, sister chromatids separate and move to opposite
poles.
Meiosis II Compared to Mitosis
1. Events of meiosis II are the same stages as in mitosis.
2. However, in meiosis II, the nuclei contain the haploid number of chromosomes.
10.5 The Human Life Cycle
1.
2.
3.
B.
Life cycle refers to all reproductive events between one generation and next.
In animals, the adult is always diploid [Instructors note: some bees, etc. have haploid male adults].
In plants, there are two adult stages: one is diploid (called the sporophyte) and one is haploid
(called the gametophyte).
4. Mosses are haploid most of their cycle; the majority of higher plants are diploid most of their cycle.
5. In fungi and some algae, only the zygote is diploid, and it undergoes meiosis.
6. In human males, meiosis is part of spermatogenesis (the production of sperm), and occurs in the
testes.
7. In human females, meiosis is part of oogenesis (the production of eggs), and occurs in the
ovaries.
8. After birth, mitotic cell division is involved in growth and tissue regeneration of somatic tissue.
Spermatogenesis and Oogenesis in Humans
1. Spermatogenesis
a. In the testes of males, primary spermatocytes with 46 chromosomes undergo meiosis I
to form two secondary spermatocytes, each with 23 duplicated chromosomes.
b. Secondary spermatocytes divide (meiosis II) to produce four spermatids, also with 23
daughter chromosomes.
c. Spermatids then differentiate into sperm (spermatozoa).
d. Meiotic cell division in males always results in four cells that become sperm.
2. Oogenesis
a. In the ovaries of human females, primary oocytes with 46 chromosomes undergo
meiosis I to form two cells, each with 23 duplicated chromosomes.
b.
c.
d.
e.
f.
g.
One of the cells, a secondary oocyte, receives almost all the cytoplasm; the other cell, a
polar body, disintegrates or divides again.
The secondary oocyte begins meiosis II and then stops at metaphase II.
At ovulation, the secondary oocyte leaves the ovary and enters an oviduct where it may
meet a sperm.
If a sperm enters secondary oocyte, the oocyte is activated to continue meiosis II to
completion; the result is a mature egg and another polar body, each with 23 daughter
chromosomes.
Meiosis produces one egg and three polar bodies; polar bodies serve to discard
unnecessary chromosomes and retain most of the cytoplasm in the egg.
The cytoplasm serves as a source of nutrients for the developing embryo.