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Lecture

BIOLOGY 1A03 Lecture Notes - Cleavage Furrow, Sister Chromatids, Spindle Apparatus


Department
Biology
Course Code
BIOLOGY 1A03
Professor
Lovaye Kajiura

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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
Produces gametes
Daughter cells are genetically
identical
to parent cells
Daughter cells (ova, sperm) are
genetically
different
from the parent
cells
FUNCTIONS OF MITOTIC CELL DIVISION
Reproduction in asexually reproducing species (unicellular organisms)
Growth and Tissue Repair in multicellular species
Development
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 G1 and G2 phases varies dramatically among cells and
organisms.
4 CELL CYCLE PHASES
1. G1 Phase (Interphase)
2. S Phase (Interphase)
3. G2 Phase (Interphase)
4. Mitotic (or M) Phase
Interphase- represents a very large portion of the Cell Cycle
What occurs during the G1 Phase?
Cell growth and the duplication of organelles.
What occurs during the S Phase?
DNA Synthesis (chromosomes replicate)
What occurs during the G2 Phase?

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Cell growth and duplication of organelles continues to build the protein
“machinery”
Continuation of G1 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 G0 Phase (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 the G0 stage
back to the cell cycle allowing it to heal. This is triggered by external cues
(growth release factors; released upon injury).
Mitosis
Is a small portion of the cell cycle
NUCLEAR DIVISION- events in the nucleus and cytoplasm
Mitotic Phases
Table 11.1 Structures involved in Mitosis
Prophase- distinct chromosomes, spindle formation of microtubules
Prometaphase- chromosomes aren’t aligned, breakdown of nuclear envelope
Metaphase
Anaphase
Telophase
G2 of Interphase- microtubule arrangement (3 bundles of 9)= chromatin
duplicated
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
poles)
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Prometaphase
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
the cell.
Kinetochore Microtubules
Extend from the poles to the kinetochores
Attach to the kinetochores
What is the function of the kinetochore microtubules?
Chromosome migration
They are attached to the microtubules
Nonkinetochore Microtubules
Radiate from each centrosome towards the metaphase plate without attaching to
chromosomes
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
components.
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
Disjunctional Segregation.
Chromosomes move centromere first (they appear “V-shaped”)
Towards the end of anaphase, the two poles have identical numbers of
chromosomes
The cell elongates
Daughter chromosomes
8 chromosomes- single stranded (not replicated); replication’s during the S phase
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