Chapter 12: The Cell Cycle

1. Chapter 12: The Cell Cycle • Why do cells need to divide? • As they grow larger, their surface area-to-volume ratio decreases. It is therefore more difficult to transport all needed materials in/out of the cell! A large cell is an inefficient cell.

2. 100 µm 200 µm 20 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM). Chapter 12: The Cell Cycle • When is cell division important? • Reproduction – unicellular organisms – binary fission in bacteria • Growth & development from fertilized egg • Repair (& replacement) of damaged cells

3. Chapter 12: The Cell Cycle • When is cell division important? • What is an organism’s genome? • Total hereditary endowment (all genes) in the cell of a species • Nuclear & extra-nuclear (mito & chloro) • How many chromosomes do we have? • 46 – somatic cell (cells of the body) • 2n • diploid • 23 – gamete (sex cells – sperm & egg) • n • haploid • What are chromosomes made of? • Chromatin • DNA & proteins

4. 0.5 µm A eukaryotic cell has multiplechromosomes, one of which is represented here. Before duplication, each chromosomehas a single DNA molecule. Chromosomeduplication(including DNA synthesis) Once duplicated, a chromosomeconsists of two sister chromatidsconnected at the centromere. Eachchromatid contains a copy of the DNA molecule. Centromere Sisterchromatids Separation of sister chromatids Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Centromeres Sister chromatids Fig 12.4 Chromosome duplication & distribution during cell division

5. INTERPHASE S(DNA synthesis) G1 CytokinesisMitosis G2 MITOTIC(M) PHASE Chapter 12: The Cell Cycle • When is cell division important? • What is an organism’s genome? • How many chromosomes do we have? • What are chromosomes made of? • What are the 2 major phases of the cell cycle? • Interphase – 90% • Mitotic phase – 10%

6. Chapter 12: The Cell Cycle • When is cell division important? • What is an organism’s genome? • How many chromosomes do we have? • What are chromosomes made of? • What are the 2 major phases of the cell cycle? • Interphase • Mitotic phase • What are the steps of the cell cycle? • IPMATC • Interphase • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis

7. Combine pro- & prometaphase PROMETAPHASE G2 OF INTERPHASE PROPHASE Centrosomes(with centriole pairs) Aster Fragmentsof nuclearenvelope Early mitoticspindle Kinetochore Chromatin(duplicated) Centromere Nonkinetochoremicrotubules Kinetochore microtubule Chromosome, consistingof two sister chromatids Nuclearenvelope Plasmamembrane Nucleolus Chromosomes appear as Pairs

8. METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Metaphaseplate Cleavagefurrow Nucleolusforming Nuclear envelopeforming Daughter chromosomes Centrosome at one spindle pole Spindle Middle Apart Two

9. Chapter 12: The Cell Cycle • Kinetochores: • Located near centromeres of attached chromosomes; microtubules attach here • Microtubules: • Make up “spindle fibers”; framework on which chromosome movement occurs • Motor Proteins: • Located within kinetochores; move chromosomes during Anaphase • Actin Filaments: • Interact with myosin to create cleavage furrow/pinching during cytokinesis in animal cells

10. Chapter 12: The Cell Cycle • When is cell division important? • What is an organism’s genome? • How many chromosomes do we have? • What are chromosomes made of? • What are the 2 major phases of the cell cycle? • What are the steps of the cell cycle? • What is the difference between animal and plant cytokinesis? • Animal – cleavage furrow – cell forms from outside in • Plants – cell plate – cell forms from inside out

11. 100 µm Cleavage furrow Vesiclesforming cell plate 1 µm Wall of patent cell Cell plate New cell wall Contractile ring of microfilaments Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM) Figure 12.9 Cytokinesis in animal and plant cells

12. Chapter 12: The Cell Cycle • When is cell division important? • What is an organism’s genome? • How many chromosomes do we have? • What are chromosomes made of? • What are the 2 major phases of the cell cycle? • What are the steps of the cell cycle? • What is the difference between animal and plant cytokinesis? • How is the cell cycle regulated? • Checkpoints • Make sure cell has enough “ingredients” to move to next stage

13. G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint Figure 12.14 Mechanical analogy for the cell cycle control system

14. G0 G1 checkpoint G1 G1 (b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a non-dividing state. (a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues      on in the cell cycle. Figure 12.15 The G1 checkpoint Most functioning cells are in G0

15. Chapter 12: The Cell Cycle • When is cell division important? • What is an organism’s genome? • How many chromosomes do we have? • What are chromosomes made of? • What are the 2 major phases of the cell cycle? • What are the steps of the cell cycle? • What is the difference between animal and plant cytokinesis? • How is the cell cycle regulated? • Checkpoints • Make sure cell has enough “ingredients” to move to next stage • Cyclins • Cyclin-dependent kinase (CDK) • MPF • maturation promoting factor • Cyclin + CDK = MPF

16. 3 G1 G1 M G2 G2 (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle S S M MPF activity Cyclin Relative Concentration Time (b) Molecular mechanisms that help regulate the cell cycle 1 Synthesis of cyclin begins in late S phase and continues through G2. Because cyclin is protected from degradation during this stage, it accumulates. 5 During G1, conditions in the cell favor degradation of cyclin, and the Cdk component of MPF is recycled. G1 S Cdk M G2 DegradedCyclin G2checkpoint 2 Cdk Cyclin is degraded Cyclin MPF MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase. 4 During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G1 phase. Fig. 12.16 Molecular control of the cell cycle at the G2 checkpoint Accumulated cyclin molecules combine with recycled Cdk mol- ecules, producing enough molecules of MPF to pass the G2 checkpoint and initiate the events of mitosis.