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Chapter 12

Chapter 12. The Cell Cycle. Cell Division. You began life as a single cell, but there are now more cells in your body than stars in the Milky Way Just in the past second, millions of your cells have divided in two. 200 µm. 20 µm.

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Chapter 12

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  1. Chapter 12 The Cell Cycle

  2. Cell Division • You began life as a single cell, but there are now more cells in your body than stars in the Milky Way • Just in the past second, millions of your cells have divided in two

  3. 200 µm 20 µm (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). Figure 12.2 B, C The Key Roles of Mitosis • Mitosis is a type of cell division that results in genetically identical daughter cells • Multicellular organisms depend on mitosis for • Development from a fertilized cell • Growth • Repair

  4. 100 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). Figure 12.2 A The Key Roles of Mitosis • Unicellular organisms • Reproduce by mitosis • Asexual reproduction

  5. Binary Fission • Prokaryotes (bacteria) • Reproduce by a type of cell division called binary fission • In binary fission • The bacterial chromosome replicates • The two daughter chromosomes actively move apart

  6. Origin of replication 4 3 1 2 Cell wall Plasma Membrane E. coli cell Bacterial Chromosome Two copies of origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy ofthe origin is now at each end of the cell. Origin Origin Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result. Figure 12.11 Bacterial cell division (binary fission)

  7. Distribution of Chromosomes During Cell Division • Eukaryotic chromosomes • Consist of chromatin, a complex of DNA and protein that condenses during cell division • Before a cell divides, it must make a complete copy of the genetic material for each daughter cell • All of its chromosomes are duplicated, resulting in 2 copies called sister chromatids • Sister chromatids are attached at the centromere

  8. 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. Centrometers Sister chromatids Figure 12.4 Chromosome duplication and distribution during cell division

  9. The Cell Cycle • Eukaryotic cells that divide undergo an orderly sequence of events called the cell cycle • The cell cycle consists of • Interphase • The time between cell divisions • A cell spends 90% of the time in interphase • Mitotic (M) Phase INTERPHASE S(DNA synthesis) G1 CytokinesisMitosis G2 MITOTIC(M) PHASE Figure 12.5

  10. Interphase • Interphase can be divided into three subphases • G1 phase (Gap 1) • Cell increases in size, performs vital cell functions • An important cell cycle control mechanism during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis • S phase (Synthesis) • DNA replication occurs during the S (synthesis) phase • G2 phase (Gap 2) • Cell continues to grow and produces new proteins in preparation for cell division

  11. G0 • G0 phase (Gap 0) • Is the designation for a cell that has quit dividing. • This may be a temporary resting period, or • A permanent condition for certain cells that reach an end stage of development and will no longer divide (e.g. nerve cells).

  12. Mitotic Phase • The mitotic phase consists of • Mitosis, the division of the nucleus and its chromosomes • Cytokinesis, the division of the cytoplasm

  13. Mitosis • Mitosis, the division of the nucleus, can be subdivided into four distinct phases • Prophase • Prometaphase • Metaphase • Anaphase • Telophase • Chromosomes move around the cell with the help of a spindle apparatus

  14. Stages of Mitosis:Prophase (including Prometaphase) • Chromosomes condense and shorten • Nuclear membrane fragments • Nucleoli disappear • Centrosomes migrate to the poles while producing the fibers of the spindle apparatus Pro means first

  15. The Mitotic Spindle: A Closer Look • The mitotic spindle (= spindle apparatus) • Is an apparatus of microtubules that controls chromosome movement during mitosis • The spindle arises from the centrosomes (centrioles) • And includes the spindle microtubules and asters

  16. Stages of Mitosis:Metaphase Meta means middle • Centrosomes are at the poles and the nuclear membrane is absent • Spindle fibers from each pole attach to the chromosomes • Chromosomes line up at the spindle equator (metaphase plate)

  17. Aster Centrosome MetaphasePlate Sisterchromatids Kinetochores Overlappingnonkinetochoremicrotubules Kinetochores microtubules 0.5 µm Microtubules Chromosomes Figure 12.7 Centrosome 1 µm The Mitotic Spindle at Metaphase • Some spindle microtubules • Attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate

  18. Structure and Location of the Centromere and Kinetochore • The kinetochore is the protein structure which assembles on the centromere and links the chromosome to the spindle apparatus during mitosis. Figure 23-38, p. 1094, Molecular Cell Biology, 3rd ed., Lodish, et al., copyright 1995, W.H. Freeman and Company

  19. Stages of Mitosis:Anaphase Ana means split • Kinetochore microtubules shorten and pull sister chromatids apart • The 2 daughter chromosomes move along the microtubules to opposite poles of the cell • Non-kinetochore microtubules overlap and push against each other, lengthening the cell and pushing the poles apart

  20. Stages of Mitosis:Telophase Telo means end • Daughter chromosomes have arrived at opposite poles of the cell • Chromosomes decondense and detach from spindle fibers • Nucleoli reappear • Nuclear membranes form around each region of chromosomes • Cytokinesis begins during telophase

  21. Cleavage furrow 100 µm Contractile ring of microfilaments Daughter cells Figure 12.9 A (a) Cleavage of an animal cell (SEM) Cytokinesis: A Closer Look • In animal cells • Cytokinesis occurs by a process known as cleavage, forming a cleavage furrow • Actin filaments contract and pinch the cell in two at the equator

  22. Cytokinesis: A Closer Look • In plant cells, during cytokinesis • A cell plate forms • Vesicles containing cell wall material migrate to the equator and coalesce to form a new cell wall between the daughter cells Vesiclesforming cell plate Wall of patent cell 1 µm Cell plate New cell wall Figure 12.9 B Daughter cells (b) Cell plate formation in a plant cell (SEM)

  23. The Cell Cycle Control • The cell cycle is regulated by a molecular control system • The frequency of cell division • Varies with the type of cell • These cell cycle differences • Result from regulation at the molecular level

  24. G1 checkpoint Control system S G1 G2 M M checkpoint Figure 12.14 G2 checkpoint The Cell Cycle Control System • The sequential events of the cell cycle are directed by a distinct control system • The most important checkpoints are • G1 checkpoint • G2 checkpoint • M checkpoint

  25. G0 G1 checkpoint G1 G1 (a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues      on in the cell cycle. (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 nondividing state. Figure 12.15 A, B Cell Cycle Checkpoints • At each checkpoint • the cell cycle stops until a go-ahead signal is received

  26. The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases • Two types of regulatory proteins are involved in cell cycle control • Cyclins • Cyclins are cytoplasmic proteins that fluctuate in concentration • Cyclin-dependent kinases (Cdks) • activate other proteins in the cell by phosphorylation (protein kinases)

  27. Stop and Go Signs: Internal and External Signals at the Checkpoints • Both internal and external signals • Control the cell cycle checkpoints • Growth factors • Stimulate other cells to divide • In density-dependent inhibition • Crowded cells stop dividing • Most animal cells exhibit anchorage dependence • In which they must be attached to a substratum to divide

  28. Cancer cells do not exhibitanchorage dependence or density-dependent inhibition. Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells. (b) Figure 12.18 B 25 µm Cancer • Cancer cells • Do not respond normally to the body’s control mechanisms • Exhibit neither density-dependent inhibition nor anchorage dependence

  29. 2 4 3 1 Lymphvessel Tumor Bloodvessel Glandular tissue Cancer cell MetastaticTumor A small percentage of cancer cells may survive and establish a new tumor in another part of the body. A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body. Figure 12.19 The growth and metastasis of a malignant breast tumor

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