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Chapter 8 Cell Division and Reproduction
Chapter Outline
I. Cancer Is a Genetic Disorder
A. Cancer is present when abnormal cells form a tumor (not organ dependant).
B. Abnormal cells are the hallmark…no control on cell division.
C. All cells of the tumor share a common cell ancestor.
D. Therefore cancer is a cellular disease.
E. Cancer is uncontrolled cell division found in eukaryotic organisms.
1. Stem cells usually are the type of cells that divide in a controlled manner.
a) Skin stem cells replenish the skin cells as they die.
b) All embryonic cells can divide.
2. Specialization of cells occurs as they mature and form organs/tissues.
a) These cells typically do not undergo cell division.
b) These cells communicate with other cells.
3. Cancerous cells lose their specialization.
a) These cells divide continuously.
b) This cell division forms the tumor.
c) The normal function of the cells is lost.
4. The cells of multicellular organisms contain the genetic controls of cell
division in their DNA.
a) The DNA encodes for the process of cell division.
b) The DNA encodes for the controls of cell division.
c) Certain genes promote and inhibit cell division.
5. Mutations of genes controlling cell division result in cancers.
6. Cancer is a genetic disorder as it is the result of genetic disruptions.
7. Cancer-causing mutations can be induced by multiple mechanisms:
a) Chemicals or radiation.
b) Incorporation of certain viruses carrying mutated genes.
c) Random errors during DNA synthesis (replication).
8. A series of mutations typically precedes a cancer.
II. Cell Division Ensures the Passage of Genetic Information
-Critical concepts include: roles of cell division and division in prokaryotes.
8.1 Cell division is involved in both asexual and sexual reproduction
A. Cell division occurs when a parent cell divides to produce two new cells.
B. Cell division occurs during both sexual and asexual reproduction.
C. Division is essential to both unicellular and multicellular organisms.
D. Somatic cells are body cells.
E. Roles for cell division
1. Growth (from zygote to maturation through death)
2. Cell replacement (skin and red blood cells) and repair (heal cuts)
3. Reproduction
a) Asexual reproduction – Production of identical genetic copies of
original cell/organism.
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1) Includes single cell organism reproduction.
2) Includes cuttings of plants or offshoot structures.
3) Includes any increase in somatic cell production.
b) Sexual reproduction – The fusion of sperm and egg from two sources.
1) Germ cells are those that make sperm or eggs.
2) Found exclusively in testes (male) and ovaries (female).
3) Fusion results in genetically distinct offspring (zygote).
4) Produces new independent organisms.
F. All cells come from preexisting cells.
G. Cell division is required for the production of new cells and organisms.
H. The nucleus (with the inherited DNA) has to be copied and distributed.
1. This is accomplished by cell division.
2. This allows for the passage of genetic material to the next generation.
8.2 Prokaryotes reproduce asexually
A. Prokaryotes undergo cell division.
B. They reproduce via asexual reproduction producing clones.
C. Prokaryotes lack a nucleus and other membranous organelles.
D. They have a single chromosome that is circular (plasmid).
1. The prokaryotic chromosome is simpler with fewer proteins produced.
E. Bacteria have a nucleoid region.
1. This region contains the chromosomal DNA but no membrane.
F. Prokaryotes divide by binary fission.
1. The division produces two (binary) new cells that are genetically identical.
2. Binary fission is accomplished via the following steps:
a) The single chromosome is attached to the plasma membrane.
b) The cell enlarges.
c) The chromosome is replicated.
d) The cell wall and plasma membrane begin to indent.
e) The chromosome copies are separated.
f) The cell continues to elongate.
g) The plasma membrane and cell wall continue to develop until they
grow together dividing the cell and separating the chromosome copies.
G. Bacteria like E. coli (in our intestines) can divide every 20 minutes.
1. This time is called the generation time.
2. Generation time in other bacteria varies.
III. Somatic Cells Have a Cell Cycle and Undergo Mitosis and Cytokinesis
-Critical concepts include: the cell cycle phases, the phases of mitosis, chromosome
doubling, characteristics/roles of each phase, and cytokinesis in animals vs. plants.
8.3 The eukaryotic cell cycle is a set series of events
A. The cell cycle is an ordered sequence of stages that take place between the
time a cell arises through the completion of cell division.
1. The cell cycle encompasses the all phases of the cell’s life.
2. The cell cycle accomplishes a duplication of the cell contents.
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B. The cell cycle can be separated into two major phases.
1. Interphase.
a) Most of the cell cycle is spent in this phase.
b) Usual functions of cell are performed in this phase.
c) Length of time in interphase is variable between different cells.
1) Differentiated cells (nerve and muscle cells) remain in interphase.
2) These cells are considered to have entered the G0 stage.
d) Adult stem cells are body cells that can divide.
1) Interphase may last approximately 20 hours = 90% of cell cycle.
e) Interphase has three sub-phases:
1) G1 (growth) phase = protein synthesis and double organelles.
2) S phase = DNA replication.
3) G2 (growth) phase = protein synthesis and cell growth pre mitosis.
4) Chromosomes are copied into sister chromatids during S phase.
2. M (Mitotic) Phase.
a) Cell division occurs during this phase (both nucleus and cytoplasm).
b) Mitosis is associated with this type of division.
c) Daughter nuclei are identical to the parent cell and each other.
1. Have the same number and kinds of chromosomes.
d) Cytokinesis is the division of the cytoplasm and occurs in this phase.
8.4 Eukaryotic chromosomes are visible during cell division
A. Both the nucleus and cytoplasm must divide in eukaryotes.
B. Chromosomes are associated with histone proteins that organize them.
C. Prior to division, the chromosomes are in a tangled mass of nucleic acid
threads called the chromatin.
D. Chromatin has to be tightly coiled and condensed prior to division.
1. Results in easily seen chromosomes (condensed DNA version).
E. Somatic cells are diploid (2n).
1. The condensed chromosomes are visible and countable.
2. Every organism has a distinct number of chromosomes present.
a) Zea mays, corn (plant) = 20 chromosomes.
b) Saccharomyces cerevisiae, yeast (fungi) = 32 chromosomes.
c) Felis catus, cat (animal) = 38 chromosomes.
d) Homo sapiens, human (animal) = 46 chromosomes.
e) Ophioglossum vulgatum (plant) = 1320 chromosomes.
3. The total number of chromosomes in a somatic cell is the diploid number.
4. This is the number of chromosomes in all of the somatic cells of that
particular organism.
5. The diploid number includes two chromosomes of each kind.
F. Mitosis of somatic parent cells produces diploid somatic daughter cells.
1. Mitosis results in diploid cells from diploid cells.
2. Prior to division, chromosomes are replicated = copies.
a) A cell with 46 chromosomes would have 92 copies.
b) The identical copies are called sister chromatids.
c) Sister chromatids are attached at the centromere.
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G. During mitosis sister chromatids separate at the centromere.
1. This results in two daughter cells with the same amount and kind of
chromosomes.
H. Gametes are haploid (n).
1. These cells have half of the amount of DNA found in a diploid cell.
a) If a diploid cell has 46 chromosomes, a haploid cell has 23.
2. These cells contain only one chromosome of each kind.
3. Gametes include sperm and eggs.
4. Typically only these cells are haploid.
8.5 Mitosis maintains the chromosome number
A. Mitosis is a continuous process.
B. DNA replication has to occur prior to mitosis initiation.
C. The sister chromatids joined by a centromere must be separated.
D. Sexual reproduction results in the donation of chromosomes from both the
mother (egg) and father (sperm).
1. These chromosomes are treated individually, doubled, and separated.
E. Centrosomes are the organization centers for microtubules in animals only.
1. They contain pairs of centrioles that synthesize the microtubules.
2. The mitotic spindle is produced from these structures.
3. Spindle fibers consist of microtubules and separate sister chromatids.
F. Separated sister chromatids are called daughter chromosomes.
G. Daughter nuclei, therefore, are identical containing the same
chromosomes/genes.
H. The phases of mitosis and their primary roles are listed below.
1. Early Prophase:
a) Centrosomes divide.
b) Chromatin is condensing into chromosomes.
c) Nuclear envelope begins to fragment.
2. Prophase:
a) Nucleolus disappears.
b) Duplicated chromosomes are visible.
c) Centrosomes move apart.
d) The mitotic spindle begins to form.
3. Early Metaphase:
a) Chromatids are attached to spindle fibers.
b) Some spindle fibers overlap themselves (do not attach chromatids).
4. Metaphase:
a) Centromeres align on the equator (center of formed spindle).
b) Spindle fibers from each pole are attached to doubled chromosomes.
5. Anaphase:
a) Sister chromatids divide and become daughter chromosomes.
b) Daughter chromosomes move toward spindle poles.
6. Telophase:
a) Daughter cells are forming.
b) Nuclear envelopes and nucleoli reappear.
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c) Chromosomes revert to dispersed chromatin organization.
8.6 Cytokinesis divides the cytoplasm
A. Cytokinesis follows mitosis (for most cells).
1. If cytokinesis does not occur, the cell has two nuclei = multinucleated.
B. Begins during telophase.
C. Continues after nuclei have formed.
D. Following cytokinesis, the cell enters interphase immediately.
E. Rapidly dividing mammalian cells undergo the cell cycle in about 24 hours.
F. Animal Cell Cytokinesis:
1. Cleavage furrow forms around dividing cell.
2. This is an indentation of the membrane between two daughter nuclei.
3. Begins to form during anaphase.
4. Band of actin filaments (contractile ring) forms along cleavage furrow.
5. The contractile ring contracts, tightening the pinching of the membrane.
6. Continues to contract until a physical separation between cells.
G. Plant Cell Cytokinesis:
1. No cleavage furrow forms in plant cells due to rigid cell wall.
2. Plant cells build new plasma membranes and cell walls.
3. Flattened disks composed of vesicles, appear between two daughter nuclei.
4. Golgi apparatus produces the vesicles which contain cell wall components.
5. A cell plate is formed by the fusion of vesicles.
6. The plasma membrane of each vesicle fuses with the next vesicle.
7. Eventually the cell plate fuses with the membrane/walls of the parent cell.
8. The addition of cellulose microfibrils later strengthens the cell wall.
IV. Cancer is Uncontrolled Cell Division
-Critical concepts include: cell cycle checkpoints and control, characteristics of cancer,
and relationship between cancer and behaviors.
8.7 Cell cycle control occurs at checkpoints
A. Control of the cell cycle allows correct cell reproduction.
B. Cell cycle controls ensure that the phases occur in the correct order.
1. Much like a washing machine, only moves to the next phase as possible.
2. Controls the transition from G1 S G2 M.
C. Cell cycle checkpoints control transition into/from stages.
D. The three main checkpoints exist at G1, G2 and M.
1. The G1 checkpoint.
a) Most significant.
b) Passage of this checkpoint commits the cell to divide.
c) If cell does not pass G1 checkpoint, it enters G0 (no division).
d) p53 is the internal checkpoint protein that determines if DNA is OK.
1) If the DNA is damaged, the p53 protein detects it.
2) p53 initiates DNA repair.
3) If DNA is not repairable, p53 initiates apoptosis (cell death).
2. The G2 checkpoint.
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a) The cell cycle hesitates at this checkpoint.
b) This allows DNA to complete replication.
c) Prevents transition into M unless the chromosomes are duplicated.
d) Also allows time to repair damage that may have occurred to the DNA.
e) Again, apoptosis occurs if damage is not repairable.
3. The M checkpoint.
a) Occurs during the mitotic stage.
b) Hesitates the cell cycle to ensure chromosomes are distributed correctly
to daughter cells.
E. Apoptosis.
1. Refers to programmed cell death.
2. Cell-to-cell communication is lost first.
3. Nucleus fragments and plasma membrane blisters.
4. Cell breaks into fragments, which are taken up by blood cells.
5. Caspases are the enzymes that initiate apoptosis (usually present in cells).
a) Typically inhibited and work in response to signals.
6. Works with cell division to keep cell numbers in the appropriate balance.
7. Is a normal cellular process.
a) The webbing between our fingers is lost during our development due to
apoptosis.
8.8 Signals affect the cell cycle control system
A. Internal and external signals control checkpoints.
B. Signaling molecules stimulate or inhibit events.
C. Internal signaling molecules occur within a cell.
D. External signaling molecules come from outside a cell.
E. Kinases are enzymes that add phosphate (from ATP) to another molecule.
1. S-cyclin is phosphorylated by S-kinase prior to the S phase.
2. M-cyclin is phosphorylated by M-kinase prior to the M phase.
3. Cyclins are proteins with vary in their concentrations in the cell.
a) They increase concentration until come into contact with a kinase.
b) They are activated by a kinase.
F. Growth factors (hormones) are external signaling molecules.
1. Stimulate repair of tissues.
2. Stimulate the cell cycle (even for cells in G0 phase).
3. Epidermal growth factor (hormone) stimulates skin growth.
a) Repairs damage of cuts.
4. Hormones act on tissues long distances from their origin.
G. Cell-signaling pathways activate the controls of the cell cycle.
1. External signal is perceived (received) by a receptor protein.
2. The receptor relays the signal to proteins inside the cell.
3. A signal transduction pathway is followed (signal passed on).
4. Signal reaches the nucleus of the cell.
5. The control system in the nucleus is regulated.
H. Controls of cell division include:
1. Contact Inhibition.
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a) Cells stop dividing when they touch each other.
2. Age of the cell.
a) Senescence (aging) is recorded by the cell (max 70 x divisions).
b) Telomeres are repeating DNA base sequences at the ends of
chromosomes.
c) Telomeres shorten as cells divide stimulating senescence.
d) Telomeres have the sequence TTAGGG.
e) They act like protective caps on the ends of chromosomes.
f) Also function to prevent chromosomes from fusing together.
g) As telomeres shorten, chromosomes can fuse and the cell dies.
8.9 Cancer cells have abnormal characteristics
A. Improperly controlled cell cycles result in cancer.
B. Cancer is characterized by uncontrolled cell reproduction.
C. Cancers are classified according to location.
1. Carcinomas = epithelial tissue that lines organs.
2. Sarcomas = muscle or connective tissue (bone or cartilage).
3. Leukemias = blood.
D. Mutations that cause cancer can arise from repeated cell division.
1. Occur in response to the repeated copying of the DNA.
E. Carcinogenesis is the development of cancer.
F. Cancer cells have the following characteristics:
1. Cancer cells lack differentiation.
a) Nonspecialized and do not contribute to the functioning region.
b) Appears abnormal compared to differentiated cells.
c) Cancer cells are immortal (not limited in number of divisions).
2. Cancer cells have abnormal nuclei.
a) Nuclei are enlarged potentially with the wrong number of
chromosomes.
b) Chromosomes may have duplicated regions or lost regions.
c) Gene amplification (multiple copies) can be present.
d) Fail to undergo apoptosis.
3. Cancer cells form tumors.
a) Do not have contact inhibition – grow in layers.
b) Form tumors from these overlapping layers.
c) Have a reduced requirement for growth factors.
d) Do not respond to inhibitory signals.
4. Cancer cells undergo metastasis and promote angiogenesis.
a) Benign tumors – Grow but do not invade neighboring tissues.
b) Cancerous tumors – Invade other tissues as they grow.
c) Cancer in situ – A tumor in the place of origin, not encapsulated.
d) Invasive tumors – Produce cancer cells that travel through the body.
1) Invade lymphs and start cancers elsewhere.
e) Metastasis refers to the spread of cancer cells.
f) Metastatic tumors – Those found at new sites (not origin).
g) Angiogenesis – Formation of new blood vessels.
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1) Required to bring nutrients/O2 to the cancerous tumor.
G. Some cancer treatments inhibit angiogenesis from occurring.
H. Prognosis depends on:
1. If the cancer has invaded surrounding tissue.
2. The presence of metastatic tumors in distant parts of the body.
How Biology Impacts Our Lives:
8.10 Protective behaviors and diet help prevent cancer
A. The risk of cancer can be reduced based on protective measures.
B. Protective Behaviors include:
1. Don’t smoke.
a) Responsible for 90% of lung cancer in men, 79% in women.
b) Responsible for 30% of all cancers.
c) Smokeless tobacco (chewing tobacco) increases the risk of mouth,
larynx, throat, and esophagus cancer.
2. Use sunscreen.
a) Nearly all skin cancers are related to sun exposure.
b) Use SPF 15 or higher sunscreen.
c) Don’t sunbathe on the beach or a tanning salon.
3. Avoid radiation.
a) Avoid exposure to ionizing radiation such as unnecessary X-rays.
b) Radon gas in homes should be monitored and remedied.
4. Be tested for cancer.
a) Shower check for breast or testicular cancer.
b) Have a physician check you out.
5. Be aware of occupational hazards.
a) Hazards include: nickel, chromate, asbestos, and vinyl chloride.
6. Carefully consider home therapy.
a) Estrogen-progestin therapy to treat menopause may increase occurrence
of breast cancer.
C. The right diet may aid in decreasing the onset of cancer.
1. Increase consumption of foods rich in vitamins A and C.
a) Beta-carotene (which makes vitamin A) is found in carrots, fruits, and
dark green leafy vegetables.
b) Vitamin C is present in citrus fruits.
c) These vitamins are antioxidants and prevent free radical formation.
1) Free radicals are organic ions having an unpaired electron.
2) Can damage DNA.
d) Vitamin C prevents formation of carcinogenic nitrosamines.
2. Limit consumption of salt-cured, smoked or nitrate-cured foods.
a) Can increase rate of stomach or esophageal cancer.
b) Chemical carcinogens can be present in smoked foods.
c) Nitrites added to protect from spoilage can be converted to nitrosamine.
3. Include vegetables from the cabbage family in the diet.
a) Includes cabbage, broccoli, Brussels sprouts, kohlrabi, and cauliflower.
b) May reduce the risk of gastrointestinal and respiratory cancers.
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4. Be moderate in the consumption of alcohol.
a) Cancer risk rises with alcohol consumption.
b) Strongest association with oral, pharyngeal, esophageal, and laryngeal
cancers.
5. Maintain a healthy weight.
a) Risk of cancer is 55% greater in obese women.
b) Risk of cancer is 33% greater in obese men.
V. Meiosis Produces Cells that Become the Gametes in Animals and Spores in Other
Organisms
- Critical concepts include: homologous chromosomes, stages of meiosis I and meiosis II,
genetic variability, sexual reproduction, life cycles, and differences between mitosis and
meiosis.
8.11 Homologous chromosomes separate during meiosis
A. Sexual reproduction transfers chromosomes from each parent to offspring.
B. Results in offspring having a unique combination.
C. Offspring also has two versions of every type of chromosome.
D. Chromosomes have identifying characteristics.
1. Can be visualized when condensed and stained.
2. Pictures can be taken of the chromosomes in a cell.
3. Karyotype – Picture of the paired representative chromosomes from a cell.
4. Homologous Chromosomes – Members of a matched pair.
a) Are homologues because have the same shape, size, and constriction.
5. One homologue comes from the male parent, and one from the female.
6. Homologues have the same characteristic banding pattern upon staining.
7. Homologues contain the genes for the same traits.
8. Alleles – Alternative forms of a gene.
9. Homologues can have different alleles but they are at the same position.
E. Humans have 23 pairs of chromosomes.
1. Sex chromosomes – Contain genes that determine gender.
a) One pair with two sex chromosome members = X and Y.
b) Males have one of each (so the pair is not uniform).
c) Females have two X chromosomes.
d) X is a larger chromosome than Y.
2. Autosomes – All other chromosomes.
a) 22 pairs of autosomes.
3. Sperm and Egg (gametes) are unique.
a) Only contain 23 chromosomes total in each cell type.
b) These cells are haploid (n).
c) These cells form via the form of cell division called meiosis.
F. Meiosis requires two cell divisions and produces haploid gamete cells.
1. Meiosis I is the first cell division that separates homologous chromosomes.
2. Meiosis II is the second cell division that separates sister chromatids.
3. Daughter cells receive one copy of each chromosome pair.
4. Four daughter cells are derived from one parent cell via meiosis.
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8.12 Synapsis and crossing-over occur during meiosis I
A. Chromosomes duplicate forming two sister chromatids before meiosis I.
B. Sister chromatids are joined at the centromere.
C. Synapsis and crossing over distinguish meiosis I from mitosis.
D. Synapsis.
1. Homologous chromosomes come together and line up side by side.
2. These chromosomes are in synapsis (joined together).
3. Held in place by proteins.
4. Tetrad – The resulting four sister chromatids joined together.
E. Crossing Over.
1. The formation of a tetrad encourages the exchange of genetic material.
2. Nonsister chromatids are the chromatids of opposite homology
chromosomes.
3. Crossing-over is the exchange of genetic material.
4. Homologues carry similar genetic info but can vary in alleles.
5. Exchange of regions of nonsister chromatids can change alleles present.
6. Results in different combinations of alleles.
7. This results in the production of variable gametes.
8. Crossing over increases genetic variability of gametes and offspring.
8.13 Sexual reproduction increases genetic variation
A. Diploid parent cells contain pairs of homologous chromosomes.
B. Synapsis occurs at the beginning of meiosis I.
C. No control is put on which homologous chromosome goes to which pole.
D. For each pair of doubled homologous chromosomes.
1. The paternal doubled chromosome can go to pole A or B.
2. The maternal doubled chromosome can go to pole B or A.
3. The only restriction is that only one doubled chromosome from a
homologous pair can move to a single pole (can’t double up).
E. These chromosomes align independently at the cell equator.
F. Daughter cells only get one member of each pair so they are haploid.
G. Gametes can be any of the possible combinations of chromosomes.
H. Independent Assortment – The unregulated dispersal of homologous
chromosomes and contributes to genetic diversity.
I. Fertilization
1. The union of male and female gametes.
2. Contributes to genetic variation.
3. Fertilization combines the chromosomes from the male gamete (paternal
set) with the chromosomes from the female gamete (maternal set).
4. A single gamete has the potential to be one of 223 possibilities.
5. Fertilization combines two gametes resulting in (223)2 possibilities.
6. With crossing over occurring, this results in (423)2 possibilities.
7. With each possibility being genetically distinct.
J. Significance of Genetic Variation.
1. Asexual reproduction passes exact copies of chromosomes and genes.
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a) This is not favorable in a changing environment.
2. Sexual reproduction introduces genetic recombination in a population.
a) This is advantageous in a changing environment.
b) Some offspring may have a better chance to survive.
8.14 Meiosis requires two division cycles
A. Meiosis uses the same stages of mitosis.
1. Prophase, Metaphase, Anaphase, and Telophase.
B. Meiosis is divided into two stages of division: Meiosis I and Meiosis II.
C. During Meiosis I:
1. Prophase I.
a) Nuclear envelope fragments, nucleolus disappears, and spindle appears.
b) Synapsis organizes the doubled homologous chromosomes.
c) Tetrads form and crossing over occurs.
2. Metaphase I.
a) Tetrads are attached to the spindle fibers.
b) Homologous chromosomes align independently at the equator.
3. Anaphase I.
a) Homologous chromosomes separate and move toward poles.
4. Telophase I.
a) Daughter cells have one double chromosome from each pair (haploid).
b) Nuclear envelope reforms.
D. Interkinesis – The time between meiosis I and II.
E. During Meiosis II:
1. The events are the same as for mitosis except that cells are haploid.
2. DNA is not replicated since it is still present as sister chromatids.
3. Duplicated chromosomes are aligned (metaphase II) and separated
(anaphase II) and haploid daughter nuclei form (telophase II).
8.15 The life cycle of most multicellular organisms includes both mitosis and
meiosis
A. Life Cycle – All reproductive events occurring from generation to generation.
B. The human life cycle includes mitosis and meiosis.
C. Mitosis builds the body (somatic cells).
D. Meiosis generates the gametes.
1. Spermatogenesis – Development of sperm in testes.
2. Oogenesis – Development of eggs in ovaries.
E. Zygote – The diploid cell that is the result of sperm and egg fusion.
F. The zygote then undergoes mitosis, which results in the diploid body.
G. Meiosis keeps the number of chromosomes constant between generations.
H. Humans use the life cycle described above.
I. Alternative life cycles exist in other organisms.
1. Plants have an adult haploid phase that alternates with an adult diploid
phase.
a) Gametophyte – The adult haploid phase.
b) Sporophyte – The adult diploid phase.
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c) Haploid generation typically is short lived.
2. Fungi and algae have a diploid phase that consists only of the zygote.
a) The zygote undergoes meiosis directly to produce haploid gametes.
b) The majority of the body structure is haploid (opposite of most plants).
c) The haploid phase produces gametes without the need for meiosis.
1) This is because it happened earlier.
8.16 Meiosis can be compared to mitosis
A. Meiosis and mitosis can be directly compared.
B. Mitosis:
1. One nuclear division.
2. Two daughter cells formed.
3. Daughter cells are diploid.
4. Genetically identical daughter cells.
C. Meiosis:
1. Two nuclear divisions.
2. Four daughter cells formed.
3. Daughter cells are haploid.
4. Genetically dissimilar daughter cells.
5. Meiosis specific differences include:
a) Tetrads form and crossing over occurs during prophase I.
b) Tetrads align at the equator in metaphase I.
c) Homologous chromosomes are separated during anaphase I.
d) Haploid cells begin meiosis II (not diploid).
VI. Chromosomal Abnormalities Can Be Inherited
-Critical concepts include: polyploidy, aneuploidy, nondisjunction, syndromes,
relationship between genetic diseases, and changes in chromosome structure/number.
8.17 An abnormal chromosome number is sometimes traceable to nondisjunction
A. Changes in chromosome number increases genetic variation.
B. Polyploidy – Having three or more complete sets of chromosomes.
1. Polyploids – Eukaryotic organisms with multiple sets of chromosomes.
2. Triploids = 3n, tetraploids = 4n, pentaploids = 5n.
3. Not typical in animals but important in plants.
4. Polyploidy is an evolutionary mechanism in plants including wheat, corn,
cotton, sugarcane, watermelons, bananas, strawberries, and apples.
C. Aneuploidy – Having more or less than the normal number of chromosomes.
1. Anaploid – An organism with the change in chromosomes.
2. Monosomy – Individual with only one of a particular pair of chromosomes
a) 2n-1.
3. Trisomy – Individual with three of a particular type of chromosome.
a) 2n + 1
4. Nondisjunction – The process by which numbers of chromosomes change.
a) Occurs when homologues fail to separate during meiosis I.
b) Occurs when sister chromatids fail to separate during meiosis II.
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5. Monosomy and trisomy occur in both plants and animals.
a) Animal autosomal monosomies and trisomies typically are lethal.
b) Trisomic individuals are more likely to survive than monosomic ones.
c) Sex chromosome aneuploids can exist.
8.18 Abnormal chromosome numbers cause syndromes
A. Syndrome – Group of symptoms that occur together and comprise a disorder.
B. Trisomy 21 = Down Syndrome.
1. Most common autosomal trisomy in humans.
2. Characteristics include:
a) Short stature.
b) Eyelid fold.
c) Flat face.
d) Stubby fingers.
e) Wide gap between first and second toes.
f) Large, fissured tongue.
g) Round head.
h) Distinctive palm crease.
i) Heart problems.
j) Mental retardation.
k) Increased risk to develop Alzheimer disease later in life.
3. Typically characterized by having three copies of chromosome 21.
a) Typically two copies are donated by the egg.
b) 23% of the time, the sperm donates two copies.
4. Age increases the risk of having child with Down syndrome.
a) 20-30 = 1 in 1400
b) 30-35 = 1 in 750
5. Karyotypes can detect a Down syndrome child.
C. Abnormal Sex Chromosome Inheritance.
1. Offspring with abnormal sex chromosome numbers have better survival
than those with abnormal autosomal chromosome numbers.
2. Only one X chromosome functions.
a) Extra X chromosomes are Barr bodies.
3. Turner Syndrome Females.
a) Born with only a single X chromosome.
b) Characteristics: short, broad chest, widely spaced nipples, neck
webbing, low posterior hairline, underdeveloped reproductive
structures, do not menstruate, normal intelligence, and require
hormone supplements.
4. Klinefelter Syndrome in Males.
a) Born with two or more X chromosomes in addition to Y chromosome.
b) Extra X chromosome is inactivated.
c) Characteristics: underdeveloped testes and prostrate, no facial hair,
some breast development, large hands and feet, and long arms and
legs.
d) Regardless of the number of X chromosomes, an individual with
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a Y chromosome is a male.
8.19 Abnormal chromosome structure also causes syndromes
A. Changes in chromosome structure can lead to various syndromes.
B. Chromosome breakage can produce syndromes.
1. Agents include: radiation, organic chemicals, or viruses.
C. If broken ends do not reattach, mistakes in sequence can occur.
D. Changes in chromosome structure include:
1. Deletions – Loss of end or internal segment.
2. Duplications – Presence of a segment more than once in a chromosome.
3. Inversions – Segment of chromosome turned around 180 degrees
(reversed).
4. Translocations – Movement of segments across chromosomes.
a) Some Down syndrome patients have this (makes another copy of 21).
E. Changes in chromosome may be detectable with karyotypes.
F. Inheritance patterns in families can also provide information.
G. Williams syndrome – Chromosome 7 loses a tiny end piece.
1. Children look like pixies.
2. The gene that encodes the protein elastin is missing.
3. The skin ages prematurely.
H. Cri du chat (cat’s cry) syndrome – Chromosome 5 is missing an end piece.
1. Characteristics: small head, mentally retarded, abnormal glottis and larynx
result in an infant’s cry that resembles that of a cat.
I. Translocations do not always cause problems, if they occur on both
homologous chromosomes and result in just recombination.
J. If movement breaks alleles or disrupts them, then problems occur.
K. Translocations can be involved in certain cancers.
1. Part of chromosome 22 moved to chromosome 9 can be responsible for
chronic myelogenous leukemia.
2. Burkitt lymphoma develops from translocation of part of chromosome 8 to
chromosome 14.
VII. Connecting The Concepts
A. All cells receive DNA from preexisting cells.
B. Cell division ensures correct DNA distribution to subsequent generations.
C. Mitosis (ordinary cell division) produces diploid cells.
D. Diploid cells have the same number of chromosomes as their parent cells.
E. The cell cycle is ordered and must be regulated to ensure proper division.
F. Meiosis produces gametes (haploid sex cells).
G. Haploid cells have half the number of chromosome as their parent cell.
H. Sexually reproducing organisms have greater genetic variation.
I. Asexually reproducing organisms make genetic clones of themselves.
J. Genetic variation is essential to survival of a species (evolution).
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