Download Cellular Growth and Reproductive Cycles

Notes on Cellular Reproduction
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Cellular Growth and Reproductive Cycles
Cell Size is Limited by Geometry and Physics:
 Surface area of objects is a “2nd order function”
(squares if the diameter doubles; increases faster than height)
 Volume of objects is a “3rd order function”
(cubes if the diameter doubles; increases incredibly fast)
 Doubling the diameter of a cell:
— Produces a 4X increase in surface area,
— Produces an 8X increase in volume
 When diameter gets too large, cells have too much
volume (in comparison to area) for them to survive:

Diffusion slows down dramatically;

Oxygen, nutrients arrive at mitochondria too slowly;

RNA (genetic messages) cannot be made by nucleus
fast enough for effective protein synthesis to occur
(“DNA overload”);

Carbon dioxide / other waste cannot diffuse out of cell
fast enough — cell “drowns in its own poisons.”
 Therefore most cells are about 2- 200  (microns) in
size (small bacteria vs. large plant cells)
Notes on Cellular Reproduction
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Cells Must Reproduce
Unicellular organisms play a “game of numbers”:

Cannot grow indefinitely (forever);

Survive & compete by multiplying
Multicellular organisms “win by getting bigger.”

Growth of single cells is limited by geometry and
physics – cells are doomed if they exceed 200 microns.

Growth above this size, or development
(a change in size and shape) REQUIRES a
CONSTANT SUPPLY of HEALTHY NEW CELLS.

Cell reproduction is needed for replacement of
dead/dying tissues
Mitosis (orderly separation and distribution of genetic
materials in new somatic cells)
 Occurs only in eukaryotic cells
(flowering plants, Protista, animals, and fungi)
 Does NOT occur in prokaryotes — “binary fission”
is the term used to describe copying of bacterial cells.
 Is NOT used to produce sperm/egg cells —
process is different and is called “meiosis.”
 Involves a cyclical, orderly sequence of well-defined steps
 Must begin AFTER copying of chromatin –
the strands of DNA & proteins (histones) that contain
genetic information.
Notes on Cellular Reproduction
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Phases of the Cell Cycle:
Interphase (once thought to be a “resting phase”)
 About 75% – 90% of a cell’s life
 Growth and synthesis of needed molecules and
organelles;
 Most metabolic activities;
 Careful duplication of chromatin;
 Divided into 3 sub-phases:
 Interphase 1 (G1): rapid growth, protein
synthesis, copying of paired centrioles to make 4
(Each centriole is composed of 9 triads
of microtubules – centrioles can be
lengthened or digested by enzymes.)
 Interphase 2 (S): (chromatin copying
only occurs during this period)
 Interphase 3 (G2):
 production of organelles for division
(especially mitochondria & ribosomes)
 beginnings of mitotic cycle (Chromosomes
condense and become visible)
Notes on Cellular Reproduction
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Notes on Cellular Reproduction
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Prophase
 Copied chromatin “super-coils” around histones—
visible pairs in enlarged nucleus are chromosomes;
 Each pair of identical chromatin strands
(sister chromatids) is joined at a centromere;
 Nuclear envelope & nucleolus is destroyed by
digestive enzymes;
 1 pair of centrioles move to each end of cell, begin
formation of a spindle (animal cells only)

Football-shaped; cage-like; made of microtubules

Eventually attaches to centromeres, pulling
sister chromatids apart
 In plant cells, chromosomes are simply moved by
microtubules
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Notes on Cellular Reproduction
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Metaphase
 Enzymes build & lengthen spindle fibers;
 2 Spindle fibers attach to each centromere
(1 from each polar end of cell – assures
identical DNA distribution to “daughter cells”)
Notes on Cellular Reproduction
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Anaphase
 Centromeres split, separating chromatids;
 Each Spindle fiber shortens, pulling chromatids apart
(Enzymes digest tubulin strands; return
their amino acids to cytoplasm)
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Telophase
 Uncoiling of chromatin, making it invisible
 Formation of new nuclei / nucleoli:

Protein synthesis resumes;

DNA resumes regulation of metabolic activity
 Cytokinesis (NOT IN PLANT CELLS)
(plasma membrane “pinches off” cytoplasm,
then closes – separating cell into 2 daughter cells)
Or:
 Formation of a “cell plate” from Golgi Apparatus
(PLANT CELLS ONLY)

Solidifies into a shared cell wall

Eventually separates cytoplasm, forms a cell wall for
each daughter cell;

Is lined by a new cell membrane
 Complete digestion of spindle
Notes on Cellular Reproduction
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Notes on Cellular Reproduction
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Interphase (and entire cycle) repeats.
Additional Information on Mitosis:
 Death of a cell without faithful reproduction –
“Cell Apoptosis”
 Cause unknown – programmed genetically
 Without apoptosis, human life expectancy
might reach 150 years
 The reproductive process is regulated by enzymes
(proteins that speed/control chemical reactions)
 “Gene:” a section of the DNA coding for proteins
 Genes that slow or stop mitosis are activated by
contact with other cells (“contact inhibition”)
 Incorrectly-Copied (mutated) genes

Are rarely helpful to the organism

Often are harmful – result in changed proteins

May result in uncontrolled cellular reproduction
(Cancer) if used to produce proteins for contact
inhibition

Mutagenic chemicals

Ultra-violet (UV) radiation

Nuclear radiation

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