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CHAPTER 12. THE CELL CYCLE. I. OVERVIEW. 1 . Cell division is defined as the reproduction of cells and is the characteristic that best distinguishes living things from nonliving matter. 2. In unicellular organisms, cell division reproduces an entire organism (Ex: bacteria)

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chapter 12



i overview

1. Celldivisionis defined as the reproduction of cells and is the characteristic that best distinguishes living things from nonliving matter.

2. In unicellular organisms, cell division reproduces an entire organism (Ex: bacteria)

3. In multicellular organisms, cell division functions in:

  • Development from a fertilized egg
  • Growth
  • Repair

4. Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division.


5. Cell division involves:

a. Precise replication of DNA

b. Movement of this DNA to opposite ends of the cell

c. Separation into two daughter cells with identical genetic information, DNA

ii concept 12 1 cell division results in genetically identical daughter cells
II. Concept 12.1: Cell division results in genetically identical daughter cells

A. Cellular Organization of the Genetic Material

  • All the DNA in a cell constitutes the cell’s genome
  • A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells)
  • DNA molecules in a cell are packaged into chromosomes
  • Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus (humans-46)

Somatic cells (nonreproductive cells) have two sets of chromosomes; produced by mitosis; genetically identical

  • Gametes(reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells; produced by meiosis; genetically unique
  • Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division, present in this state when cell is not dividing

B. Distribution of Chromosomes During Eukaryotic Cell Division

  • In preparation for cell division, DNA is replicated and the chromosomes condense
  • Each duplicated chromosome has two sister chromatids, which separate during cell division
  • The centromereis the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached
      • Chromatin
      • Active form of DNA found in a non-dividing cell
      • DNA—genetic material
      • Proteins—help maintain chromosomal structure and help control gene activity
doubled chromosome
Doubled Chromosome





Eukaryotic cell division consists of:

  • Mitosis—the division of the nucleus
  • Cytokinesis—the division of the cytoplasm
  • Gametes are produced by a variation of cell division called meiosis
  • Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell
iii concept 12 2 the cell cycle
III. Concept 12.2: The Cell Cycle
  • In 1882, the German anatomist Walther Flemming developed dyes to observe chromosomes during mitosis and cytokinesis

A. Phases of the Cell Cycle

  • The cell cycle consists of:
    • Mitotic (M) phase (mitosis and cytokinesis)
    • Interphase(cell growth and copying of chromosomes in preparation for cell division)

Interphase (about 90% of the cell cycle) can be divided into subphases:

a. G1 Phase (first gap)—cell grows; first growth phase when the centrioles replicate

b. S Phase (synthesis)--DNA replicates

c. G2 Phase (second gap)—cell grows; second growth phase during which the cell is preparing for mitosis by producing proteins, ATP, etc.

  • The cell grows during all three phases, but chromosomes are duplicated only during the S phase

B. G2 of Interphase

  • Nucleus well defined and bound by nuclear envelope
  • One or more nucleoli present
  • Two pairs of centrioles adjacent to nucleus
  • Around each pair of centrioles are microtubules that form a radial array called an aster (animal cells only)
  • Chromosomes have duplicated and appear as chromatin

C. Actual Stages of Mitosis Are:






D. Prophase

1. Changes in nucleus:

  • Nucleoli disappear
  • Chromatin fibers become tightly coiled and folded to form observable chromosomes

2. Changes in cytoplasm

  • Mitotic spindle forms

--Composed of microtubules and associated proteins which are arranged between the two centrosomes (microtubule-organizing center)

  • Centrosomes move apart

--Apparently they are propelled along the surface of the nucleus by lengthening of microtubule bundles between them


B. Prometaphase

1. Nuclear envelope fragments

2. Microtubules interact with chromosomes

3. Spindle fibers (bundles of microtubules) extend from each pole toward the equator (midpoint between the poles) (aka metaphase plate) of the cell

4. Each chromatid has akinetochore (specialized structure located at the centromere region)

5. Bundles of microtubules (kinetochore microtubules) attach to kinetochores and put chromosomes into agitated motion

6. Microtubules (nonkinetochore microtubules) radiated from each centrosome toward the metaphase plate (equator) without attaching to chromosome


C. Metaphase

1. Centrosomes positioned at poles (opposite

ends ) of the cell

2. Chromosomes move to metaphase plate (plane equidistant between the spindle poles)

3. Centromeres of all chromosomes aligned on metaphase plate

4. Kinetochore fibers of sister chromatids face opposite poles so identical chromatids are attached to kinetochore fibers radiating from opposite ends of the parent cell

5. Entire structure formed by nonkinetochore microtubules plus kinetochore microtubules called the spindle


D. Anaphase

1. Begins when paired centromeres of each chromosome move apart

2. Sister chromatids separate and are considered chromosomes

3. Spindle apparatus starts moving the separate chromosomes toward opposite poles

4. Chromosomes move in a V-shape because of attachment of kinetochore fibers to centromere

5. Kinetochore microtubules shorten at end attached to kinetochores

6. Poles of cell move farther apart slightly elongating the cell

7. At end of phase two poles have complete and equivalent collections of chromosomes


E. Telophase

1. Nonkinetochore microtubules further elongate the cell

2. Daughter nuclei begin to form at the two poles

3. Nuclear envelop reforms from fragments of parent cell nuclear envelope and other portions of endomembrane system

4. Nucleoli reappear

5. Chromosomes uncoil and appear as chromatin

6. Cytokinesis occurs

7. At the end of the phase:

  • Mitosis is complete
  • Cytokinesis has begun and the appearance of two separate daughters cells occurs shortly after mitosis is complete
f mitotic spindle
F. Mitotic Spindle

1. Important to events occurring in mitosis

2. Forms in cytoplasm during prophase

3. Composed of microtubules and associated proteins

4. Elements come from partial disassembly of the cytoskeleton

5. Elongation occurs by the addition of tubulin (protein) subunits at one end

6. Assembly begins in the centrosome(microtubule organizing center).

7. In animal cells, a pair of centrioles is in the center of the centrosome.


8. F(x) of centrioles is undefined.

9. Parallel microtubules form bundles called spindle fibers.

10. Fibers elongate or shorten by adding and removing tubulin subunits from end away from centrosome.

11. 2 Types of Spindle Fibers:

a. kinetochore microtubules

  • Attached to kinetochore
  • F(x): -arrange chromosomes so kinetochores face the poles

-align chromosomes at cell’s midline

-to move chromosomes to poles


b. nonkinetochore microtubules

  • Are not attached to kinetochore
  • Overlap at center of cell
  • F(x): elongate whole cell during anaphase
  • Movement requires ATP
g cytokinesis
G. Cytokinesis

1. Process of cytoplasmic division that follows mitosis

2. Begins in telophase

3. Differs in plant and animal cells

4. Animal Cells

  • Occurs as cleavage
  • Cleavage furrow forms as a shallow groove on cell surface near metaphase plate (in animal cells only)
  • Contractile ring of actin and myosin microfilaments forms on cytoplasmic side of furrow
  • Microfilaments contract until parent cell is pinched in two

5. Plant Cells

  • Cell plate forms across midline of parent cell (equator)
  • Cell plate forms from fusing vesicles derived from Golgi apparatus that have moved along microtubules to cell’s center
  • Cell plate enlarges until its surrounding membrane fuses with the existing plasma membrane
  • New cell wall forms between two membranes from contents of cell plate
g binary fission
G. Binary Fission

1. Cells grow to double their size and divide to form two cells

2. Prokaryotes (bacteria and cyanobacteria) reproduce by binary fission (division in half).

3. Have a single circular DNA molecule that carries most of the genes

4. Soon after chromosome replication begins, one copy of the origin of replication (first replicated region) begins to move toward the other end of the cell.

5. Replication continues and eventually one copy of the origin is at each end of the cell.

5. Replication finishes.

6. Plasma membrane grows inward, and new cell wall is deposited.

7. Two daughter cells

8. Single celled eukaryotes must still go through mitosis prior to division.

h the evolution of mitosis
H. The Evolution of Mitosis
  • Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission
  • Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis
vi concept 12 3 regulation of the cell cycle
VI. Concept 12.3: Regulation of the Cell Cycle
  • Normal growth, development and maintenance of a cell depends on the timing and rate of mitosis.
  • The frequency of cell division varies with the type of cell

-human skin cells divide frequently

-liver cells divide to repair damaged cells

-nerve, muscle, and other specialized cells do not divide in mature humans

  • These cell cycle differences result from regulation at the molecular level

A. Cell Cycle Control System

1. Cell cycle appears to be driven by specific chemical signals in the cytoplasm.

2. The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock

  • The cell cycle control system is regulated by both internal and external controls
  • The clock has specific checkpointswhere the cell cycle stops until a go-ahead signal is received
  • When the control system malfunctions, cancer may result.

6. Checkpoints:

      • Stop and go signals that regulate the cell cycle
      • Report status of cellular conditions
      • Integrate internal (intracellular) and external (extracellular) information

7.For many cells, the G1 checkpoint seems to be the most important one

  • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide
  • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

-Ex: a. muscle and nerve cells until death

b. liver cells until recruited back to cell cycle by such cues as growth factors

b cyclins and cyclin dependent kinases
B. Cyclins and Cyclin-Dependent Kinases
  • Two types of regulatory proteins are involved in cell cycle control: cyclinsand cyclin-dependent kinases (Cdks)
      • During interphase levels of cyclins rise, then fall abruptly during mitosis
  • Kinases catalyze the transfer of Pi group from ATP to a target protein and synchronize the ordered sequence of cell cycle events
  • Protein kinases - provide go ahead signals of G1 and G2 checkpoints
b cyclins and cyclin dependent kinases1
B. Cyclins and Cyclin-Dependent Kinases

2. The activity of cyclins and Cdks fluctuates during the cell cycle

* cyclins – proteins that bind to and activate protein kinase

3. MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

  • Cyclins + Cdk = MPF (maturation promoting factor)
    • MPF activity is directly proportional to the concentration of cyclins. Concentration/Activity rises during S and G2 phases and falls during M phase.
    • MPF phosphorylates many proteins, initiating mitosis.
c stop and go signs internal and external signals at the checkpoints
C. Stop and Go Signs: Internal and External Signals at the Checkpoints
  • An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase
  • Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide
  • For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture
  • Another example of external signals is density-dependent inhibition, in which crowded cells stop dividing
  • Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide

D. Cancer Cells (abnormal cells)

1. Cancer cells do not respond normally to the body’s control mechanisms

2. Cancer cells may not need growth factors to grow and divide:

  • They may make their own growth factor
  • They may convey a growth factor’s signal without the presence of the growth factor
  • They may have an abnormal cell cycle control system

3. A normal cell is converted to a cancerous cell by a process called transformation

4. Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue


5. If abnormal cells remain at the original site, the lump is called a benign tumor

6. Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

you should now be able to
You should now be able to:
  • Describe the structural organization of the prokaryotic genome and the eukaryotic genome
  • List the phases of the cell cycle; describe the sequence of events during each phase
  • List the phases of mitosis and describe the events characteristic of each phase
  • Draw or describe the mitotic spindle, including centrosomes, kinetochore microtubules, nonkinetochore microtubules, and asters
  • Compare cytokinesis in animals and plants

Describe the process of binary fission in bacteria and explain how eukaryotic mitosis may have evolved from binary fission

  • Explain how the abnormal cell division of cancerous cells escapes normal cell cycle controls
  • Distinguish between benign, malignant, and metastatic tumors
exit slip
Exit Slip
  • How many chromatids are in a duplicated chromosome?
  • A chicken has 78 chromosomes in its somatic cells.
    • How many chromosomes did the chicken inherit from each parent?
    • How many chromosomes are in each of the chicken’s gametes?
    • How many chromosomes will be in each somatic cell of the chicken’s offspring?