Chapter 9 Outline and Terms

9.1. How Prokaryotic Cells Divide (p. 148)

A. Prokaryotic cells lack a nucleus and other membranous organelles; include only the bacteria.

B. The Chromosome Is Singular

1. Prokaryotic chromosome contains mostly DNA and some associated proteins, but much less protein than eukaryotic chromosomes.

2. Single prokaryotic chromosome is tightly coiled inside the cell.

3. Chromosome consists of nucleoid---an irregularly-shaped region, electron dense, and not enclosed by a membrane.

4. Chromosome, when stretched out, is a circular loop attached to the inside of the plasma membrane; about 1,000 times the length of the cell.

C. Division is by Fission (Fig. 9.1) [transp. 57 and micro. slide 13]

1. Asexual reproduction requires a single parent; offspring are genetically identical to parent.

2. Binary fission of prokaryotic cells produces two genetically identical daughter cells by division (fission).

3. Before cell division takes place, DNA is replicated so two chromosomes are attached inside the plasma membrane.

4. Following DNA replication, the two chromosomes separate when cell lengthens and pulls them apart.

5. 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.

6. Generation times of Escherichia coli is 20 minutes; most bacteria need one hour to a day.

9.2. Eukaryotic Chromosomes and the Cell Cycle (p. 149)

A. Chromosomes (Fig. 9.2) [micro. slide 14]

1. DNA in the chromosomes of eukaryotic cells is associated with proteins; histone proteins organize chromosomes.

2. When a eukaryotic cell is not undergoing division, DNA within a nucleus is a tangled mass of threads called chromatin.

3. At cell division, chromatin becomes highly coiled and condensed and visible as chromosomes.

4. Each species has a characteristic number of chromosomes. (Table 9.1)

a. Diploid (2n) number includes two sets of chromosomes of each type.

1) Found in all the nonsex 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 include humans (23), crayfish (100), etc.

5. Cell division in eukaryotes involves nuclear division and cytokinesis (division of the cytoplasm).

a. Somatic (body) cells undergo mitosis involved in the development, growth, and repair of multicellular organisms.

1) This nuclear division leaves the chromosome number constant.

2) A 2n nucleus divides to provide daughter nuclei that are also 2n.

b. A chromosome begins cell division with two sister chromatids. (Fig. 9.2) [micro. slide 14]

1) Sister chromatids are two parts of a chromosome, at the beginning of cell division, that are attached at a centromere; each consists of a DNA molecule identical to the DNA molecule of the other chromatid.

2) Centromere---a region of constriction on a chromosome, where sister chromatids are attached.

B. How Eukaryotic Cells Cycle

1. Interphase was considered a "resting state" until DNA replication was detected in the 1950s.

2. Cell cycle involves 4-stage sequence of events. (Fig. 9.3) [transp. 58]

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.

C. How the Cycle is Controlled

1. Some cells (e.g., skin cells) divide continuously throughout the life of the multicellular organism.

2. Other cells (e.g., 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. Still other cells, such as cardiac muscle cells, are arrested in the G2 stage; if this type of cell is 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 cyclin-dependent kinases (Cdks) regulate passage of cells through these checkpoints.

a. Kinases are enzymes that remove a phosphate group from ATP and add it to another protein.

1) As a result of phosphorylation, the recipient protein (which may be another kinase) becomes activated.

2) Phosphorylated molecules are a common way for the cell to turn on metabolic pathways.

3) Phosphorylation of a protein precedes the S and M stages of the cell cycle.

b. Enzymes of cell cycle are called cyclin dependent because they activate when they combine with cyclin.

c. Cyclin proteins activate cyclin-dependent kinases, which in turn activate enzymes; one destroys cyclin.

d. Kinases that combine with cyclins (i.e., cyclin-dependent kinases) become activated and regulate the passage of cells through the various stages of the cell cycle. (Fig. 9.4) [transp. 59]

1) Oncogenes (cancer-causing genes) may code for cyclins that no longer function as they should.

2) Growth factors are molecules that attach to plasma membrane receptors and bring about cell growth.

3) Ordinarily a cyclin combines with its kinase only when a growth factor is present.

4) Cyclin that has gone awry combines with its kinase when growth factor is absent, resulting in a tumor.

5) Tumor-suppressor genes usually function to prevent cancer; e.g., tumor-suppressor gene (p53) causes production of protein that combines with a cyclin-kinase complex to stop that kinase from being active.

9.3. How Eukaryotic Cells Divide (p. 152)

A. In mitosis, a spindle assists orderly distribution of chromosomes to daughter cell nuclei; chromosome number stays constant.

1. The spindle contains many fibers, each composed of a bundle of microtubules.

2. Microtubules are made of the protein tubulin, assemble when tubulin subunits join, disassemble when tubulin subunits become free, and form interconnected filaments of cytoskeleton; disassemble as spindle fibers form.

3. Centrosomes are believed responsible for organizing the spindle.

a. The centrosome is the main microtubule organizing center of the cell.

b. The centrosome has divided before mitosis begins.

c. Each centrosome contains a pair of barrel-shaped organelles called centrioles; plant cells lack centrioles.

B. How Animal Cells Divide

1. Mitosis (karyokinesis) is divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase. (Fig. 9.5) [transp. 60 and micro. slides 14-19]

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 centrioles located in each centrosome; form starlike asters.

3. Prometaphase [not depicted]

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. The chromosomes move back and forth until they are aligned at the metaphase plate.

4. Metaphase [micro. slide 18]

a. Chromosomes, attached to kinetochore fibers, are aligned at the metaphase plate.

b. Nonattached spindle fibers, called polar spindle fibers, can reach beyond the metaphase plate and overlap.

5. Anaphase [micro. slide 19]

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 [micro. slide 20]

a. Spindle disappears.

b. Chromosomes decondense and return to chromatin; the nuclear envelope reforms and nucleoli reappear.

c. Cytokinesis is nearly complete.

C. How Plant Cells Divide (Figure 9.6, 9.7) [micro. slides 21-24]

1. Plant meristematic tissue in tips of roots and shoots of stems retains ability to divide throughout life.

2. Stages are same as in animal cells.

3. Although plant cells have a centrosome, there are no centrioles, and asters do not form.

D. Cytokinesis (cytoplasmic cleavage) Accompanies Mitosis

1. In Plant Cells (Figure 9.7) [micro. slides 25]

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 cell nuclei.

c. Vesicles fuse, forming a cell plate; their membranes complete membranes of developing daughter cells.

d. The vesicles also release molecules that signal the formation of plant cell walls.

e. Later, these walls are strengthened by the addition of cellulose fibrils.

2. In Animal Cells (Figure 9.8) [micro. slides 26-27]

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 slowly constricts between the two daughter cells.

d. Narrow bridge exists between daughter cells during telophase; then constriction separates the cytoplasm.

3. Cell Division and Cytokinesis in Other 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.

9.4. Comparing Prokaryotes and Eukaryotes (Table 9.2) (p. 156)

A. Functions

1. Binary fission and mitosis both ensure each daughter cell is genetically identical to the parent cell.

2. Cell division in unicellular organisms accomplishes asexual reproduction.

3. Cell division in multicellular organisms accomplishes development, growth, and repair.

B. Prokaryotes differ because of the following:

1. The single chromosome consists largely of DNA with some associated proteins.

2. Cell elongation separates the anchored DNA strands and a new cell wall forms between daughter cells.

3. No spindle apparatus is involved in binary fission.

C. Eukaryotes differ because of the following:

1. protein histone organizes a chromosome, allowing extension to chromatin and coiling for mitosis.

2. species vary with a characteristic number of chromosomes.

3. a spindle appears and helps distribute daughter chromosomes to daughter nuclei.

4. cytokinesis involves formation of a cell plate in plants or furrowing in animal cells.


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