Notes on Cellular Reproduction
Page 9 of 11
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:
ü Diffusion slows down dramatically;
ü Oxygen, nutrients arrive at mitochondria too slowly;
ü RNA (genetic messages) cannot be made by nucleus fast enough for effective protein synthesis to occur
(“DNA overload”);
ü Carbon dioxide / other waste cannot diffuse out of cell fast enough — cell “drowns in its own poisons.”
« Therefore most cells are about 2- 200 m (microns) in size (small bacteria vs. large plant cells)
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.
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)
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)
ü Football-shaped; cage-like; made of microtubules
ü Eventually attaches to centromeres, pulling
sister chromatids apart
« In plant cells, chromosomes are simply moved by microtubules
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”)
Anaphase
« Centromeres split, separating chromatids;
« Each Spindle fiber shortens, pulling chromatids apart
(Enzymes digest tubulin strands; return
their amino acids to cytoplasm)
Telophase
« Uncoiling of chromatin, making it invisible
« Formation of new nuclei / nucleoli:
ü Protein synthesis resumes;
ü 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)
ü Solidifies into a shared cell wall
ü Eventually separates cytoplasm, forms a cell wall for each daughter cell;
ü Is lined by a new cell membrane
« Complete digestion of spindle
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
ü Are rarely helpful to the organism
ü Often are harmful – result in changed proteins
ü May result in uncontrolled cellular reproduction (Cancer) if used to produce proteins for contact inhibition
¨ Mutagenic chemicals
¨ Ultra-violet (UV) radiation
¨ Nuclear radiation
¨ ?