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CHAPTER 12THE CELL CYCLE
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Section B2: The Mitotic Cell Cycle (continued)3. Cytokinesis divides the cytoplasm: a closer look4. Mitosis in eukaryotes may have evolved from binary fission in bacteria
• Cytokinesis, division ofthe cytoplasm, typicallyfollows mitosis.
• In animals, the first sign ofcytokinesis (cleavage)is the appearance of acleavage furrow in thecell surface near the oldmetaphase plate.
3. Cytokinesis divides the cytoplasm:a closer look
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.8a
• On the cytoplasmic sideof the cleavage furrow acontractile ring of actinmicrofilaments and themotor protein myosinform.
• Contraction of the ringpinches the cell in two.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.8a
• Cytokinesis in plants, which have cell walls,involves a completely different mechanism.
• During telophase, vesiclesfrom the Golgi coalesce atthe metaphase plate,forming a cell plate.• The plate enlarges until its
membranes fuse with theplasma membrane at theperimeter, with the contentsof the vesicles forming newwall material in between.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.8b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.9
• Prokaryotes reproduce by binary fission, notmitosis.
• Most bacterial genes are located on a single bacterialchromosome which consists of a circular DNAmolecule and associated proteins.
• While bacteria do not have as many genes or DNAmolecules as long as those in eukaryotes, theircircular chromosome is still highly folded and coiledin the cell.
4. Mitosis in eukaryotes may have evolvedfrom binary fission in bacteria
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In binary fission, chromosome replication begins atone point in the circular chromosome, the origin ofreplication site.
• These copied regions begin to move to oppositeends of the cell.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.10
• The mechanism behind the movement of thebacterial chromosome is still an open question.• A previous hypothesis proposed that this movement
was driven by the growth of new plasma membranebetween the two origin regions.
• Recent observations have shown more directedmovement, reminiscent of the poleward movement ofeukaryotic chromosomes.
• However, mitotic spindles or even microtubules areunknown in bacteria.
• As the bacterial chromosome is replicating and thecopied regions are moving to opposite ends of thecell, the bacterium continues to grow until itreaches twice its original size.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Cell division involvesinward growth of theplasma membrane,dividing the parentcell into two daughtercells, each with acomplete genome.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.10
• It is quite a jump from binary fission to mitosis.
• Possible intermediate evolutionary steps are seenin the division of two types of unicellular algae.• In dinoflagellates, replicated chromosomes are attached
to the nuclear envelope.
• In diatoms, the spindle develops within the nucleus.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 12.11
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