9 The Cell Cycle

The Mitotic Spindle: A Closer Look The mitotic spindle is a structure made of microtubules and associated proteins It controls chromosome movement during mitosis In animal cells, assembly of spindle microtubules begins in the centrosome, a type of microtubule organizing center © 2016 Pearson Education, Inc. 2

The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase An aster (radial array of short microtubules) extends from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters © 2016 Pearson Education, Inc. 3

During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes Kinetochores are protein complexes that assemble on sections of DNA at centromeres At metaphase, the centromeres of all the chromosomes are at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles © 2016 Pearson Education, Inc. 4

Video: Mitosis Spindle © 2016 Pearson Education, Inc.

Aster Centrosome Sister chromatids Metaphase plate (imaginary) Kineto- Figure 9.8 Aster Centrosome Sister chromatids Metaphase plate (imaginary) Kineto- chores Microtubules Chromosomes Overlapping nonkinetochore microtubules Figure 9.8 The mitotic spindle at metaphase Kinetochore microtubules 1 m 0.5 m Centrosome © 2016 Pearson Education, Inc.

Microtubules Chromosomes 1 m Centrosome Figure 9.8-1 Figure 9.8-1 The mitotic spindle at metaphase (part 1: TEM) 1 m Centrosome © 2016 Pearson Education, Inc.

Kinetochores Kinetochore microtubules 0.5 m Figure 9.8-2 Figure 9.8-2 The mitotic spindle at metaphase (part 2: kinetochore TEM) 0.5 m © 2016 Pearson Education, Inc.

The microtubules shorten by depolymerizing at their kinetochore ends In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell The microtubules shorten by depolymerizing at their kinetochore ends Chromosomes are also “reeled in” by motor proteins at spindle poles, and microtubules depolymerize after they pass by the motor proteins © 2016 Pearson Education, Inc. 9

Experiment Results Conclusion Kinetochore Spindle pole Mark Chromosome Figure 9.9 Experiment Results Kinetochore Spindle pole Conclusion Mark Chromosome movement Kinetochore Figure 9.9 Inquiry: At which end do kinetochore microtubules shorten during anaphase? Motor protein Tubulin subunits Microtubule Chromosome © 2016 Pearson Education, Inc.

Experiment Kinetochore Spindle pole Mark Figure 9.9-1 Figure 9.9-1 Inquiry: At which end do kinetochore microtubules shorten during anaphase? (part 1: experiment) © 2016 Pearson Education, Inc.

Results Conclusion Chromosome movement Kinetochore Motor Tubulin Figure 9.9-2 Results Conclusion Chromosome movement Figure 9.9-2 Inquiry: At which end do kinetochore microtubules shorten during anaphase? (part 2: results and conclusion) Kinetochore Motor protein Tubulin subunits Microtubule Chromosome © 2016 Pearson Education, Inc.

Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell Cytokinesis begins during anaphase or telophase, and the spindle eventually disassembles © 2016 Pearson Education, Inc. 13

Cytokinesis: A Closer Look In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow In plant cells, a cell plate forms during cytokinesis © 2016 Pearson Education, Inc. 14

Animation: Cytokinesis © 2016 Pearson Education, Inc.

Video: Cytokinesis and Myosin © 2016 Pearson Education, Inc.

(a) Cleavage of an animal cell (SEM) Figure 9.10 (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM) 100 m Vesicles forming cell plate Wall of parent cell 1 m Cleavage furrow Cell plate New cell wall Figure 9.10 Cytokinesis in animal and plant cells Contractile ring of microfilaments Daughter cells Daughter cells © 2016 Pearson Education, Inc.

100 m Cleavage furrow Figure 9.10-1 Figure 9.10-1 Cytokinesis in animal and plant cells (part 1: cleavage of animal cell, SEM) 100 m Cleavage furrow © 2016 Pearson Education, Inc.

Vesicles Wall of forming parent cell 1 m cell plate Figure 9.10-2 Figure 9.10-2 Cytokinesis in animal and plant cells (part 2: cell plate formation in plant cell, TEM) Vesicles forming cell plate Wall of parent cell 1 m © 2016 Pearson Education, Inc.

Chromosomes condensing Nucleus Nucleolus Chromosomes 10 m Prophase Figure 9.11 Chromosomes condensing Nucleus Nucleolus Chromosomes 10 m Prophase Prometaphase Cell plate Figure 9.11 Mitosis in a plant cell Metaphase Anaphase Telophase © 2016 Pearson Education, Inc.

Nucleus Chromosomes condensing Nucleolus 10 m Prophase Figure 9.11-1 Figure 9.11-1 Mitosis in a plant cell (part 1: prophase) 10 m Prophase © 2016 Pearson Education, Inc.

Chromosomes 10 m Prometaphase Figure 9.11-2 Figure 9.11-2 Mitosis in a plant cell (part 2: prometaphase) 10 m Prometaphase © 2016 Pearson Education, Inc.

Figure 9.11-3 Figure 9.11-3 Mitosis in a plant cell (part 3: metaphase) 10 m Metaphase © 2016 Pearson Education, Inc.

Figure 9.11-4 Figure 9.11-4 Mitosis in a plant cell (part 4: anaphase) 10 m Anaphase © 2016 Pearson Education, Inc.

Cell plate 10 m Telophase Figure 9.11-5 Figure 9.11-5 Mitosis in a plant cell (part 5: telophase) 10 m Telophase © 2016 Pearson Education, Inc.

Binary Fission in Bacteria Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission In E. coli, the single chromosome replicates, beginning at the origin of replication The two daughter chromosomes actively move apart while the cell elongates The plasma membrane pinches inward, dividing the cell into two © 2016 Pearson Education, Inc. 26

Origin of replication Cell wall Plasma membrane E. coli cell Figure 9.12-s1 Origin of replication Cell wall Plasma membrane E. coli cell Chromosome replication begins. Bacterial chromosome Two copies of origin Figure 9.12-s1 Bacterial cell division by binary fission (step 1) © 2016 Pearson Education, Inc.

Origin of replication Cell wall Plasma membrane E. coli cell Figure 9.12-s2 Origin of replication Cell wall Plasma membrane E. coli cell Chromosome replication begins. Bacterial chromosome Two copies of origin Origin Origin One copy of the origin is now at each end of the cell. Figure 9.12-s2 Bacterial cell division by binary fission (step 2) © 2016 Pearson Education, Inc.

Origin of replication Cell wall Plasma membrane E. coli cell Figure 9.12-s3 Origin of replication Cell wall Plasma membrane E. coli cell Chromosome replication begins. Bacterial chromosome Two copies of origin Origin Origin One copy of the origin is now at each end of the cell. Figure 9.12-s3 Bacterial cell division by binary fission (step 3) Replication finishes. © 2016 Pearson Education, Inc.

Origin of replication Cell wall Plasma membrane E. coli cell Figure 9.12-s4 Origin of replication Cell wall Plasma membrane E. coli cell Chromosome replication begins. Bacterial chromosome Two copies of origin Origin Origin One copy of the origin is now at each end of the cell. Figure 9.12-s4 Bacterial cell division by binary fission (step 4) Replication finishes. Two daughter cells result. © 2016 Pearson Education, Inc.

The Evolution of Mitosis Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission Certain protists (dinoflagellates, diatoms, and some yeasts) exhibit types of cell division that seem intermediate between binary fission and mitosis © 2016 Pearson Education, Inc. 31

(b) Diatoms and some yeasts Figure 9.13 Chromosomes Microtubules Intact nuclear envelope (a) Dinoflagellates Kinetochore microtubule Figure 9.13 Mechanisms of cell division Intact nuclear envelope (b) Diatoms and some yeasts © 2016 Pearson Education, Inc.