Chapter 11: The Cell Cycle
All cells arise from pre-existing cells by the process of cell division.
Fig. 11.1 Two Types of Nuclear Division Occur in Many Species
In animals, meiosis leads to the production of eggs and sperm. Mitosis is
responsible for producing somatic cells.
TYPES OF NUCLEAR DIVISION
Mitosis Meiosis (covered later in Ch. 12)
Occurs in somatic cells Occurs only in reproductive cells
Daughter cells are genetically Daughter cells (ova, sperm) are
identical to parent cells genetically different from the parent
FUNCTIONS OF MITOTIC CELL DIVISION
Reproduction in asexually reproducing species (unicellular organisms)
Growth and Tissue Repair in multicellular species
MITOSIS AND THE CELL CYCLE
Bacteria (Box 11.2)
Undergo the process of binary fission
Binary fission produces two genetically identical daughter cells
Animals, Fungi and Plants complete The Cell Cycle
Fig. 11.5 The Cell Cycle has Four Phases
The time required for the G and G phases varies dramatically among cells and
4 CELL CYCLE PHASES
1. G P1ase (Interphase)
2. S Phase (Interphase)
3. G Phase (Interphase)
4. Mitotic (or M) Phase
Interphase- represents a very large portion of the Cell Cycle
What occurs during the G P1ase?
Cell growth and the duplication of organelles.
What occurs during the S Phase?
DNA Synthesis (chromosomes replicate)
What occurs during the G Phase?
2 Cell growth and duplication of organelles continues to build the protein
Continuation of G phase
Why do Gap Phases exist?
Before mitosis= the parent cell must be large enough in size and must have
made the required organelles, so that daughter cells will operate normally.
Cytokinesis= separation of cytoplasm (slime moulds don’t undergo cytokinesis;
only eukaryotes do).
What is G 0hase (Gap 0 Phase)?
Not a dividing state (the non-dividing state)
No synthesis, no cell replication
Example: mature nerve cells and muscle cells (fully formed)
Exception: alcoholics that abuse alcohol damage their liver and scarring of the
liver occurs. If they get help to stop, then the liver cells can go from th0 G stage
back to the cell cycle allowing it to heal. This is triggered by external cues
(growth release factors; released upon injury).
Is a small portion of the cell cycle
NUCLEAR DIVISION- events in the nucleus and cytoplasm
Table 11.1 Structures involved in Mitosis
Prophase- distinct chromosomes, spindle formation of microtubules
Prometaphase- chromosomes aren’t aligned, breakdown of nuclear envelope
G 2f Interphase- microtubule arrangement (3 bundles of 9)= chromatin
Prophase (The Preparation Phase)
In the Nucleus:
Nucleoli begin to disappear
Chromatin fibers contract (DNA condenses) by tightly coiling
Chromosomes are visible and consist of two identical sister chromatids joined
together at the centromere.
In the Cytoplasm:
Mitotic spindle forms (microtubules polymerize)
Assembly of microtubules begins in the centrosome (animals) or microtubule
organizing centre (plants)
In animals, the centrioles begin to move apart to opposite sides of nucleus (2
Chromosomes do not appear completely aligned or organized
Nucleoli disappear and nuclear envelope breaks down and is recycled
A specialized structure called a kinetochore is formed near the centromere
Spindle fibers attach to sister chromatids at the kinetochore regions
Kinetochore microtubules are polymerized, randomly oriented at first, then they
become aligned parallel with microtubules.
Kinetochore microtubules begin moving the chromosomes towards the middle of
Extend from the poles to the kinetochores
Attach to the kinetochores
What is the function of the kinetochore microtubules?
They are attached to the microtubules
Radiate from each centrosome towards the metaphase plate without attaching to
May overlap with those from the opposite pole
What are the functions of the nonkinetochore microtubules?
Don’t attach to kinetochores
Assist in elongating the entire cell during anaphase
Interdigitate= cage-like network
They form a cage-like network, which facilitates the activities of the cell cycle
Metaphase (The Organizing Phase)
Chromosomes line up along the metaphase plate (zone)
Centromeres are aligned on the metaphase plate, which is located equidistant
from the two poles.
Anaphase (Separating DNA Copies)
Binding proteins between the sister chromatids break down
Centromeres of sister chromatids disjoin and segregate, this process is called
Chromosomes move centromere first (they appear “V-shaped”)
Towards the end of anaphase, the two poles have identical numbers of
The cell elongates
8 chromosomes- single stranded (not replicated); replication’s during the S phase Telophase
Nonkinetochore microtubules further elongate the cell
Two daughter nuclei form
Formation of the nuclear envelopes around each set of chromosomes
Cleavage furrow= pinching off- separation of daughter cells
Polysaccharides= how they divide
Cytokinesis (Cytoplasm Division)
Telophase is often followed by cytokinesis
Fig. 11.9 The Mechanism of Cytokinesis Varies among Eukaryote Groups
a) In animals, the cytoplasm is divided by a cleavage furrow that pinches the
parent cell in two.
b) In plants, the cytoplasm is divided by a cell plate that forms in the middle of
the parent cell.
Animals, Slime Molds and Fungi- form a Cleavage Furrow
Plants- form a Cell Plate
Fig. 11.10 Mitosis and Cytokinesis in Action
Micrographs showing newt cells in interphase and undergoing mitosis and
cytokinesis. Chromosomes are stained blue, microtubules are green and actin filaments
Fig. 11.11 During Anaphase, Microtubules Shorten at the Kinetochore
A Laser beam experiment is done to determine how microtubules shorten to pull
sister chromatids apart at anaphase. The conclusion is that microtubules shorten at one
end- at the kinetochore.
How do the chromosomes move during the process of Mitosis?
Tubulin subunits of the kinetochore microtubules are depolarized from the
Motor proteins (dynein) attach and detach along the kinetochore microtubule’s
length= chromosomes move.
Fig. 11.12 How do Microtubules Move Chromosomes during Mitosis
The kinetochore consists of an inner plate, an outer plate, and associated fibers
that contain motor proteins. The fibers are thought to act as “legs” that walk the
chromosomes down the length of the kinetochore microtubules as the microtubules
shorten, due to loss of tubulin dimers at the kinetochore.
Polarity- chromosomes move in the proper direction
Down syndrome- lack of chromosomes in the nucleus CONTROL OF THE CELL CYCLE
Cell Cycle Control Molecules
Heterokaryon- “different nuclei”—cell knows about different phases
Fig. 11.13 Experimental Evidence for Cell Cycle Control Molecules
Not about the sequence of the cell cycle
Are there signal factors