Download Binary Fission-Bacterial Cell Division

Binary Fission-Bacterial Cell Division
-Asexual reproduction of prokaryotes
-No mitosis
-Circular DNA and organelles replicate, the
copies migrate to opposite sides of the
elongating cell, and the cell splits in two forming
two identical cells
Roles of Cell Division
-Reproduction
-Growth
-Repair
-Remember: In all organisms, cell division must
distribute identical genetic material (an exact
copy of the genome) into two daughter cells
Genome
-The cell’s hereditary endowment of DNA
-Usually packaged into chromosomes for
manageability
Chromosome
-Made of a DNA and histone protein complex
called chromatin
-During cell division, the chromatin becomes
highly condensed into the chromosomes
Structure:
-At cell division, each chromosome has been
duplicated during interphase
-The duplicated chromosome consist of two
sister chromatids
-Centromere: The point where two sister
chromatids are connected
Goal of Cell Division
-To create two identical daughter cells by
splitting all the sister chromatids and giving one
copy of each to each new cell
Cell Cycle
-Interphase: (90% of cycle) when the cell grows
and duplicates the chromosomes
-Mitotic Phase: When the chromosome are split
into separate cells. This phase includes
cytokinesis
Interphase
-G1: First Gap
--The cell grows and carries out regular
biochemical functions
-S: Synthesis
--When the DNA is replicated or synthesis
(chromosomes are replicated)
-G2: Second Gap
--Cell completes preparations for division
Mitotic Phase
-Mitosis: Division of replicated chromosomes
-Cytokinesis: Division of the cell’s cytoplasm
Purpose
-To divide the two copies of the DNA equally
-To separate the sister chromatids into separate
cells
Mitosis Steps
-Prophase
-Prometaphase
-Metaphase
-Anaphase
-Telophase
Prophase
-Nucleoli disappear
-Chromatin condenses into chromosomes (wrap
around histones)
-Centrioles separate to opposite ends of the cell
(grows microtubules)
-Mitotic spindle begins to form
Prometaphase
-Nuclear envelope dissolves
-Spindle fibers join with the kinetochore of the
centromeres (middle of chromosomes)
Kinetochore
-Specialized regions of the centromeres where
spindle microtubules attach
Metaphase
-Centrioles now at opposite ends of the cell
-Chromosomes line up on the metaphase plate
-Spindle apparatus fully developed
Anaphase
-Centromeres break and the duplicated
chromosomes are pulled away from each other
toward opposite ends of the cell
-Cell elongates: poles move slightly further apart
Telophase
-Chromosomes uncoil back to chromatin
-Nuclear envelope reforms
-Nucleoli reappear
-Spindle fibers disappear
-Cytokinesis usually starts
(Reverse of prophase)
Cytokinesis-Animal
-Cleavage furrows form
-Microfilaments contrast and divides the
cytoplasm into two parts
Cytokinesis-Plants
-Cell plate develops from Golgi vesicles
-New cell wall developed around the cell plate
(middle)
Regulation of Cell Division
-The cell cycle is regulated by a molecular control
mechanism to tell cells when to divide
-This system moves the cell through its stages
through a series of checkpointsenzymes
(Kinase)
-At each checkpoint, signals tell the cell to either
keep dividing or stop
Checkpoints
-A critical control point in the cell cycle
-The major checkpoints in the cell cycle are the
G1 checkpoint (growth), G2 checkpoint (DNA),
and M phase checkpoint (spindles)
-Cells must receive a “go-ahead” signal at each
checkpoint before proceeding to the next
G1 Checkpoint
Most important checkpoint
-If a cell gets the “go-ahead” signal at this
checkpoint, it usually completes the whole cell
cycle and divides
-If it does not receive this signal, it enters a nondividing phase called the G o phase. (Most
specialized cells are in this state)
-Later, if a cell needs to divide, a “go-ahead”
signal is given and the cell reactivates into the Mphase
Control of the Cell Cycle
-Kinases are enzymes that control the cell cycle
(adds activate phosphate)
-They are always in a cell but only active when
they are connected to a cyclin protein so they
are called cyclin-dependent Kinases (Cdk)
-When cyclin is connected to a Cdk, it forms an
activated protein complex called MPF (M-phase
Promoting Factor)
-Specific Kinases give the “go-ahead” signals at
the G1 and G2 checkpoints
How Mitosis is Initiated?
-As a specific example, cyclin molecules combine
with Cdk molecules producing enough molecules
of MPF to pass the G2 checkpoint and initiate the
events of mitosis
-MPF is the protein complex required for a cell to
progress from G2 to Mitosis
--Role of MPF: to trigger a chain of protein kinase
activations and starts mitosis
How does a Cell Stop Mitosis?
-During anaphase, MPF switches itself off by
starting a process that leads to the destruction of
cyclin molecules
-It activates a cyclin-degrading enzyme, which
lowers the amount of cyclin in the cell
-Without cyclin molecules Cdk molecules
become inactive bringing mitosis to a close
Growth Factors
-External signals that affect mitosis
Ex:
-PDGF
-Density-dependent inhibition
-Anchorage dependence
PDGF
-Platelet-Derived Growth Factor
-Stimulates cell division to heal injuries
Normal Cell Division has 2 Key Characteristics
-Density dependent inhibition
-Anchorage dependency
Density Dependent Inhibition
-The process in which crowded cells stop dividing
-The number of cells in an area force
competition for nutrients, space, and growth
factors, so when cells are crowded, they get
signals to stop dividing
-When density is high- no cell division
-When density is low- cells divide
Anchorage Dependence
-Normal cells must be attached to a substratum
like the extracellular matrix of a tissue to divide
-Prevents cells from dividing and floating off in
the body
Cancer Cells
-Do not exhibit either density dependent
inhibition or anchorage dependency
-The control mechanisms for cell division have
failed
-The process that changes a normal cell to a
cancerous one id transformation
-A tumor is a mass of abnormal cells within
otherwise normal tissue
-If the abnormal cells remain at the original site,
it is a benign tumor
Malignant
-A tumor becomes invasive enough to impair the
functions of one or more organs
-Individuals who have malignant tumors have
cancer
Metastasis
-A condition where cells from a malignant tumor
separate and enter the blood or lymph vessels
spreading the cancer to other organs
Comment
-Regulation of cell division is a balance between:
--Mitosis: making new cells
--Apoptosis: cell suicide or death
-Cancer can result if either process doesn’t work