Chapter 9 – Cellular Reproduction and the Cell Cycle

I. Section 9.1: How Prokaryotic Cells Divide

A. Prokaryotic cells (archaea and bacteria) lack a nucleus and other membrane

bound organelles

B. The Prokaryotic Chromosome

1. prokaryotic chromosome contains DNA and associated proteins, but

much less protein than eukaryotic chromosomes

2. chromosome consists of a nucleoid (irregularly-shaped region) not

enclosed by a membrane

3. chromosome, when stretched, is a circular loop attached to the inside of

the PM; about 1,000X 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 replicated so two chromosomes are attached

to inside of PM

3. following DNA replication, two chromosomes separate as cell

lengthens and pulls them apart

4. when cell about twice its normal length, the PM grows inward, a new

cell wall forms dividing the cell into two approximately equal daughter

cells

5. generation time can be anywhere from minutes (20 for Escherichia coli)

to an hour to an entire day

II. Section 9.2: How Eukaryotic Cells Divide

A. Eukaryotic Chromosomes

1. DNA in chromosomes of eukaryotic cells associated with proteins;

histone proteins organize chromosomes

2. DNA in nucleus normally found in threads called chromatin

3. at cell division, chromatin condenses into chromosomes

4. each species has a characteristic number of chromosomes

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

type

1) found in all non-sex cells of organism’s body (few

exceptions)

2) e.g. humans have 46, crayfish have 200

b. haploid (n) number contains one of each kind of chromosome

1) in life cycle of many animals, only sperm and egg cells

(gametes) have haploid number

2) e.g. humans have 23, crayfish have 100

5. cell division involves nuclear division and cytokinesis (division of

cytoplasm)

a. somatic (body) cells undergo mitosis for development, growth,

and repair

1) number of chromosomes constant

2) 2n nucleus replicates and divides to provide daughter

nuclei that are also 2n

b. chromosome begins cell division with two sister chromatids

1) sister chromatids are two strands of genetically

identical chromosomes

2) attached at centromere at beginning of cell division

3) centromere is region of constriction on a chromosome

where sister chromatids are attached

B. Mitotic Spindle

1. centrosomes responsible for organizing spindle

2. centrosome is main microtubule organizing center of cell

3. centrosome has divided before mitosis begins

4. each centrosome contains a pair of barrel-shaped organelles called

centrioles; plant cells lack centrioles

5. spindle contains many fibers, each composed of a bundle of

microtubules

6. microtubules made of protein called 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 Animal Cells

1. mitosis is divided into five phases: prophase, metaphase, anaphase, and

telophase

2. prophase

a. nuclear division is about to occur because chromatin condenses

and chromosomes become visible

b. nucleolus disappears and nuclear envelope fragments

c. already duplicated chromosomes are composed of two sister

chromatids held together by a centromere

1) chromosomes have no particular orientation at this time

2) specialized protein complex (kinetochore fibers) develop

on each side of centromere for future chromosome orientation

d. spindle fibers begin to assemble as pairs of centrosomes migrate

away from each other

e. short microtubules radiate out from the pair of centrioles located

in each chromosome to form star-like asters

3. prometaphase

a. spindle consists of poles, asters, and fibers that are bundles of

microtubules

b. attachment of chromosomes to spindle fibers and chromosomes

are aligned at metaphase plate (equator) of spindle

c. kinetochores of sister chromatids capture kinetochore spindle

fibers

d. chromosomes move back and forth until they are aligned at

metaphase plate

4. metaphase

a. chromosomes, attached to kinetochore fibers, are aligned at

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 split at

centromere

b. daughter chromosomes, each with 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 kinetochores;

this pulls daughter chromosomes to each pole

6. telophase

a. spindle disappears

b. chromosomes de-condense and return to chromatin; nuclear

envelope reforms and nucleoli reappear

c. cytokinesis nearly complete

D. Mitosis in Plant 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 centrosome and spindle, there are no

centrioles and asters do not form

E. Cytokinesis in Plant and Animal Cells

1. plant cells

a. rigid cell wall does not permit cytokinesis by furrowing

b. Golgi apparatus produces vesicles that move to the midpoint

between daughter nuclei

c. vesicles fuse forming cell plate; their membranes complete PM

of daughter cells

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

walls

e. walls strengthened by addition of cellulose fibers

2. animal cells

a. cleavage furrow indents the PM 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 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, nuclear envelope does not fragment

but divides and one nucleus goes to each daughter cell

III. Section 9.3: How Eukaryotic Cells Cycle

1. interphase was considered a “resting state” until DNA replication was

detected in 1950s

2. cell cycle involves 4-stage sequence of events

3. M stage (M = mitosis) is entire cell division state, including both

mitosis and cytokinesis

4. cell growth and increase in organelles occurs in G₁ stage, just prior to

DNA replication

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

division

7. interphase consists of G₁, S, G₂ stages

A. Cell Cycle Clock

1. some cells (e.g. skin cells) divide continuously throughout life of

organisms

2. skeletal muscle cells and nerve cells are arrested in G₁ stage; if nucleus

from an arrested cell is placed in cytoplasm of an S-stage cell, it finishes cell cycle

3. cardiac muscle cells are arrested in G₂ stage; if fused with a cell

undergoing mitosis, will undergo mitosis too

4. appear to be stimulatory substances causing cell to proceed through two

critical checkpoints

a. G₁ stage à S stage

b. G₂ stage à M stage

5. enzymes known as cyclins and kinases regulate passage of cells thru

two checkpoints

a. kinases are enzymes that remove a -P group from ATP and add

it to another protein (common switch for metabolic activation)

b. cyclin proteins activate kinases, which in turn activate enzymes;

one destroys cyclins, therefore cyclin levels vary

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) chromosomes to condense

2) nuclear envelope to disassemble and

3) spindle to form

4) then M-cyclin is destroyed

d. growth factors

1) molecules that attach to PM receptors and bring about

cell growth

2) ordinarily 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 tumor formation

4) tumor-suppression genes usually function to prevent

cancer; e.g. tumor-suppressor gene p53 causes production of protein that combines with cyclin kinase complex to stop kinase from being active; this stops cell cycle

IV. Section 9.4: 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 faulty

p53 gene

3. normal p53 gene halts the cell cycle when DNA matures and is in need

of repair

4. carcinogens are agents that cause cancer and include:

a. radiation (e.g. UV light, X rays, radon gas, etc.)

b. organic chemicals (e.g. tobacco smoke, some foods, pesticides,

etc.)

c. certain viruses

5. a p53 protein mobilizes repair enzymes and stops the cell cycle; only

when the cell is repaired does the cell cycle begin again

6. if DNA repair is not possible, then p53 protein promotes cell death

(apoptosis)

B. Apoptosis is Programmed Cell Death

1. apoptosis is a sequence of cellular changes involving:

a. shattering of nucleus

b. chopping up of chromosomes

c. packaging cellular remains into vesicles to be 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

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 surviving protein which blocks

apoptosis; if we can inactivate surviving, 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 nerve 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. 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 are 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 destroy normal tissue; new growth is

called neoplasia

c. a benign tumor is disorganized but encapsulated and does not

invade adjacent tissue

4. cancer cells under angiogenesis and metastasis

a. cancer cells release a growth factor that causes 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