Chapter 10 Cell Division and Mitosis Cengage Learning. All Rights Reserved.

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1 Chapter 10 Cell Division and Mitosis

2 A Cell Undergoing Mitosis

3 Why It Matters The zebrafish is a model organism for study of the stages of regeneration at the molecular level If a predator takes a bite out of a zebrafish s fins, the entire fin will regenerate within a week skin, nerves, muscles, bones, and related tissues Regeneration occurs because cells that had stopped growing and dividing are suddenly stimulated to grow and divide in a highly regulated way

4 Zebrafish

5 10.1 The Cycle of Cell Growth and Division: An Overview Single-celled prokaryotic or eukaryotic organisms grow and divide as long as environmental conditions allow In multicellular eukaryotes, cell division is under strict control to develop and maintain different subpopulations of cells The life cycle of a cell (cell cycle) is divided into three parts: Cell growth and activity, including replication of DNA Nuclear division (mitosis) Division of the cytoplasm (cytokinesis)

6 Two Types of Nuclear Division Mitosis (a growth process) divides the replicated DNA equally and precisely, generating daughter cells, which are exact genetic copies of the parent cell Meiosis (a process of sexual reproduction) produces daughter nuclei with half the number of chromosomes of the parental nucleus The arrangements of genes on chromosomes are different from those in the parent cell

7 The Products of Mitosis Mitosis partitions replicated DNA equally and precisely: Master program of molecular checks and balances DNA synthesis replicates each DNA chromosome into two copies with almost perfect fidelity Mitotic cytoskeleton separates replicated DNA molecules precisely into the daughter cells Chromosomes, the nuclear units of genetic information divided and distributed by mitotic cell division

8 Chromosomes In eukaryotes, the hereditary information within the nucleus is distributed among individual, linear DNA molecules DNA molecules combine with proteins that stabilize the DNA molecules, assist in packaging DNA during cell division, and influence the expression of individual genes In a cell, each chromosome is composed of one DNA molecule and its associated proteins

9 Ploidy The number of chromosome sets in a cell or species is called its ploidy Some microorganisms have only one copy of each type of chromosome in their nuclei they are haploid, or n Many plant species, have three, four, or even more complete sets of chromosomes in each cell They are polyploid

10 Ploidy (cont'd.) Most eukaryotes have two copies of each type of chromosome in their nuclei they are diploid, or 2n The two chromosomes of each pair in a diploid cell are called homologous chromosomes one is from the mother, the other from the father Homologous chromosomes have the same genes in the same order in the DNA of the chromosomes

11 Packing DNA DNA fits into a nucleus because it is packed into a shorter length by histone proteins Other nonhistone proteins also associate with DNA; the complex of DNA and all its associated proteins is chromatin In a nucleosome, an eight-protein nucleosome core particle forms when DNA winds around the histones H2A, H2B, H3, and H4 A short linker segment of DNA connects nucleosomes

12 Packing DNA (cont'd.) Nucleosomes and linkers appear as beads on a string under electron microscopes The 10-nm chromatin fiber is named from the diameter of the beads and compacts DNA by a factor of about 7 Further packing occurs in the 30-nm chromatin fiber when the nucleosome and linker are bound by the fifth histone protein H1 The solenoid model predicts the nucleosomes spiral helically with about six nucleosomes per turn Chromatin packing continues at higher levels, with euchromatin being loosely packed and heterochromatin showing dense packing

13 O.L. Miller, Jr., Steve McKnight B. Hamkalo The Solenoid Model Histone Histone tail Histone Hl binds to nucleosomes and linker DNA, causing nucleosomes to form coiled structure. Solenoid 2 nm DNA Nucleosome: DNA wound around a nucleosome core particle consisting of 2 molecules each of H2A, H2B, H3, H4 10-nm chromatin fiber Linke r Chromosome in metaphase stage of cell in mitosis, the most compact form of a 30-nm chromatin fiber chromosome Nucleosome s Linker s Chromatin fiber

14 STUDY BREAK What are the three interrelated systems that contribute to the eukaryotic cell cycle? 2. What is the general composition of a eukaryotic chromosome? 3. What is the structure of the nucleosome? 4. What is the role of histone H1 in eukaryotic chromosome structure?

15 10.2 The Mitotic Cell Cycle There are three main events in a cell cycle: interphase, mitosis (M phase), and cytokinesis Interphase is divided into three phases: G 1 phase, in which the cell grows S phase, in which DNA replicates and chromosomal proteins are duplicated G 2 phase, in which cell growth continues and the cell prepares for mitosis

16 (Interphase ends in parent cell) The Cell Cycle A. Cell cycle events G 2 refers to the second gap in which there is no DNA synthesis. During G 2, the cell continues to synthesize RNAs and proteins, including those for mitosis, and it continues to grow. The end of G 2 marks the end of interphase; mitosis then begins. Cytokinesi s G 2 G l S If the cell is going to divide, DNA G l phase is a period of growth replication begins. During S before the DNA replicates. The phase, the cell duplicates each cell makes various RNAs, chromo- some, including both the proteins, and other types of DNA cellular molecules but not DNA and the chromosomal proteins, (the G in G l stands for gap, and it also continues synthesis referring to the absence of other cellular molecules. of DNA synthesis).

17 Sister Chromatids Before a cell divides in mitosis, duplication of each chromosome (and its proteins) produces two identical copies called sister chromatids Sister chromatids are held together by sister chromatid cohesion until mitosis separates them, placing one in each of two daughter nuclei Cohesins hold the sister chromatids together until they are removed The equal distribution of chromosomes into each of two daughter nuclei is called chromosome segregation

18 Clones Chromosome replication and segregation in the mitotic cell cycle creates a group of genetically identical cells clones of the original cell All cells of a multicellular organism arise by mitosis from a single zygote, and contain the same genetic information In the laboratory, clones may be grown in cell cultures

19 Interphase Usually, G 1 is the only phase of the cell cycle that varies in length other phases are typically uniform in length G 1 is also the stage in which many cell types stop dividing and are shunted into a G 0 phase some cells in G 0 reenter G 1, others never resume the cell cycle Internal regulatory controls trigger each phase of the cell cycle and regulate the overall number of cycles that a cell goes through

20 Photograph by Dr. Conly L. Rieder, Wadsworth Center, Albany, New York The Stages of Mitosis: Interphase Nucleolu s Centrosom e Pair of centrioles Microtubules ofcentrosome Plasma Pairof membrane chromosomes G l of interphase The chromosomes are unreplicated and extend throughout the nucleus. For simplicity we show only two pairs of chromosomes. One of each pair was inherited from one parent, and the other was inherited from the otherparent. Nuclear envelope G 2 of interphase After replication during the S phase of interphase, each chromosome is double at all points and now consists of two sister chromatids. Cohesins encircle each pair of sister chromatids along their lengths, aligning them tightly. The centrioles within the centrosome have also doubled intopairs.

21 Mitosis Proceeds in Five Stages Following interphase, mitosis can be divided into five sequential stages: Prophase Prometaphase Metaphase Anaphase Telophase Cytoplasmic division (cytokinesis) coincides with telophase

22 Prophase Prophase Chromosomes condense into chromatin Nucleolus becomes smaller and disappears The mitotic spindle begins to form between the two centrosomes as they migrate toward the opposite ends of the cell, where they will form spindle poles

23 Photograph by Dr. Conly L. Rieder, Wadsworth Center, Albany, New York The Stages of Mitosis: Prophase Microtubules of developing spindle Sister chromatids Chromosome Prophase The chromosomes condense into threads that become visible under the light microscope. The tight alignment of the pairs of sister chromatids can now be seen. The centrosome has divided into two parts, which are generating the spindle as they separate.

24 Prometaphase Prometaphase Begins when the nuclear envelope breaks down Spindle microtubules grow from centrosomes at opposite spindle poles toward the center of the cell A kinetochore forms on each sister chromatid at the centromere (the point where chromatids are joined in sister chromatid adhesion) Kinetochore microtubules bind to the kinetochores Nonkinetochore microtubules overlap those from the opposite spindle pole

25 Photograph by Dr. Conly L. Rieder, Wadsworth Center, Albany, New York The Stages of Mitosis: Prometaphase Centrosomeat a spindlepole Kinetochore microtubule Kinetochor e Nonkinetochore microtubule Centrosome at opposite spindlepole Prometaphase The nuclear envelope has disappeared and the spindle enters the former nuclear area. Microtubules from opposite spindle poles attach to the two kinetochores of each chromosome.

26 Spindle Connections at Prometaphase Prometaphase Spindle pole Kinetochore microtubules Sister chromatid I Kinetochore I Prometaphase chromosome Sister chromatid II Kinetochore II Spindle pole

27 Metaphase Metaphase Spindle microtubules move chromosomes into alignment at the spindle midpoint (metaphase plate) Condensation gives each chromosome a characteristic shape, determined by length and centromere location An image of a complete set of metaphase chromosomes, arranged according to size and shape, forms a karyotype

28 Photograph by Dr. Conly L. Rieder, Wadsworth Center, Albany, New York The Stages of Mitosis: Metaphase Kinetochore microtubule Nonkinetochore microtubule Metaphase The chromosomes become aligned at the spindle midpoint. Each sister chromatid pair is held in position by opposing forces: the kinetochore microtubules pulling to the poles and the cohesins binding the sister chromatids together.

29 Anaphase Anaphase The spindle separates sister chromatids and pulls them toward opposite spindle poles Movement continues until the separated chromatids (daughter chromosomes) have reached the two poles At this point, chromosome segregation is complete

30 Photograph by Dr. Conly L. Rieder, Wadsworth Center, Albany, New York The Stages of Mitosis: Anaphase Kinetochore microtubule Nonkinetochore microtubule Anaphase Separase cleaves the cohesin rings responsible for sister chromatid cohesion. The spindle separates the two sister chromatids of each chromosome and moves them to opposite spindle poles.

31 Telophase Telophase The spindle disassembles and chromosomes at each spindle pole decondense, returning to the extended state typical of interphase The nucleolus reappears, RNA transcription resumes A new nuclear envelope forms around the chromosomes at each pole producing the two daughter nuclei At this point, nuclear division is complete the cell has two nuclei

32 Cytokinesis Cytokinesis produces two daughter cells, each with one of the two daughter nuclei In animals, protists, and many fungi, a furrow girdles the cell and deepens until it cuts the cytoplasm into two parts In plants, a cell plate forms between the daughter nuclei and grows laterally until it divides the cytoplasm in two The plane of cytoplasmic division is determined by the layer of microtubules that persists at the former spindle midpoint

33 The Stages of Mitosis: Telophase and Cytokinesis Photograph by Dr. Conly L. Rieder, Wadsworth Center, Albany, New York Telophase and Cytokinesis The chromosomes unfold and return to the interphase state, and new nuclear envelopes form around the daughter nuclei. Cytokinesis now commences as seen by the cytoplasm beginning to divide by furrowing at the points marked by arrows. G l of the following interphase The two daughter cells are genetic duplicates of the parental cell that entered mitotic division.

34 Dr. David Phillips/Visuals Unlimited, Inc. Cytokinesis by Furrowing Contractile ring of microfilaments (see Section 5.5) just inside the plasma membrane 1 The microfilaments slide together, tightening the ring and constricting the cell. The constriction forms a groove the furrow in the plasma membrane. 2 Continued constriction causes the furrow to deepen gradually, much like the tightening of a drawstring. 3 Furrowing continues until the daughter nuclei are enclosed in separate cells. At the same time, the cytoplasmic division distributes organelles and other structures (which also have increased in number) approximately equally between the cells.

35 Dr. Robert Calentine/Visuals Unlimited, Inc. Cytokinesis by Cell Plate Formation Vesicl e Cell wall l A layer of vesicles containing wall material collects in the plane of the former spindle midpoint (arrow). The vesicles are produced by the endoplasmic reticulum and Golgi complex. 2 More vesicles are added to the layer until it extends across the cell. The vesicles begin to fuse together. 3 The vesicles fuse 4 Vesicle fusion continues dumping together, their contents into a gradually expanding wall between the daughter nuclei. until the daughter nuclei are separated into two cells by a continuous new wall, the cell plate. The plasma membranes that line the two surfaces of the cell plate are derived from vesicle membranes.

36 A. S. Bajer, University of Oregon Mitosis in a Plant Cell A cell at interphase Prophase Metaphase Anaphase Spindle pole Telophase Prometaphase Microtubules Cytokinesis Spindle midpoint Cytoplasm Nucleus Chromosomes Spindle pole

37 Research Method: Preparing a Human Karyotype Protocol: 1. Add sample to culture medium that has stimulator for growth and division of cells (white blood cells in the case of blood). Incubate at 37 o C. 2. Stain the cells so that the chromosomes are distinguished. Pair of homologous chromosomes Pair of sister chromatids closely aligned sideby-side by sister chromatid cohesion 3. View the stained cells under a microscope equipped with a digital imaging system and take a digital photograph. Interpreting the Results: The karyotype is evaluated with respect to the scientific question being asked.

38 The Mitotic Cell Cycle in Development and Reproduction The mitotic cell cycle accounts for the growth of multicellular eukaryotes from a fertilized egg to fully developed adults Mitosis is used in vegetative or asexual reproduction, which occurs in many plants and protists and some animals In asexual reproduction, daughter cells produced by mitotic cell division are released from the parent and grow separately by further mitosis into complete individuals

39 STUDY BREAK Compare the chromosome content of daughter cells following mitosis with that of the parent cell before its chromosomes were duplicated. 2. In what order do the stages of mitosis occur? 3. What is the importance of centromeres to mitosis?

40 10.4 Cell Cycle Regulation Robert T. Johnson and Potu N. Rao fused human HeLa cells (cultured cancer cells) in different stages of the cell cycle and determined whether one nucleus could influence the other Their results suggested that specific molecules in the cytoplasm cause the progression of cells from G 1 to S, and from G 2 into M

41 Experimental Research: Molecules Controlling the Cell Cycle Demonstrating the Existence of Molecules Controlling the Cell Cycle by Cell Fusion Question: Do molecules in the cytoplasm direct the progression through the cell cycle? Experiment: Johnson and Rao fused human HeLa cells at different stages of the cell cycle. Cell fusion produces a single cell with two separate nuclei. The researchers allowed the fused cells to grow and determined whether one nucleus influenced the other in terms of progression through the cell cycle. 1.Fusion of cell in S phase with cell in G1 phase. 2.Fusion of cell in M (mitosis) with cell in any other stage. S G 1 M G 1 S S M M Result: DNA synthesis quickly began in the original G1 nucleus. Normally, the G1 nucleus would not have initiated DNA synthesis until it reached S phase itself, which could have been several hours later. The result suggested that one or more molecules that activate S phase are present in the cytoplasm of S phase cells. Result: Regardless of the phase of the cell, the nucleus of the cell with which the M phase cell was fused immediately began the early stages of mitosis. This included condensation of the chromosomes, spindle formation, and breaking down of the nuclear envelope. For a cell in G1 (shown in the diagram), the condensed chromosomes that appear have not replicated.

42 Cell Cycle Regulation (cont'd.) Leland Hartwell investigated yeast mutants that become stuck at some point in the cell cycle he identified many genes involved in the cell cycle and hypothesized where in the cycle their protein products operated Paul Nurse identified a gene in yeast (cdc2), which encodes a protein kinase needed for the cell to progress from G 2 to M all eukaryotic cells have counterparts of the cdc2 gene

43 Checkpoints The cell cycle has three key checkpoints to prevent critical phases from beginning until the previous phases are completed correctly: G 1 /S checkpoint G 2 /M checkpoint Mitotic spindle checkpoint Checkpoints are signals to stop inactivation of a checkpoint allows the cell cycle to proceed

44 G 1 /S Checkpoint The G 1 /S checkpoint is the main point in the cell cycle at which a cell decides whether to divide or not The cell cycle arrests at the G 1 /S checkpoint if DNA is damaged by radiation or chemicals if the DNA damage is repaired, the cycle starts again Cell cycle arrest also occurs at this checkpoint if the cell is nutritionally deficient or growth factors are absent

45 G 2 /M Checkpoint The G 2 /M checkpoint commits a cell to mitosis Cells arrest at the G 2 /M checkpoint if DNA was not replicated accurately in S or if the DNA has been damaged by radiation or chemicals Accurate DNA replication is essential for producing genetically identical daughter cells

46 Mitotic Spindle Checkpoint The mitotic spindle checkpoint is before metaphase in the M phase This checkpoint assesses whether chromosomes are attached properly to the mitotic spindle so that they align correctly at the metaphase plate Once the cell begins anaphase, it is irreversibly committed to completing M

47 Cyclins and Cyclin-Dependent Kinases Direct regulation of the cell cycle itself involves an internal control system consisting of proteins called cyclins and enzymes called cyclindependent kinases (Cdks) A Cdk is a protein kinase, which phosphorylates and regulates the activity of target proteins Cdk enzymes are active only when bound to cyclin Concentrations of cyclins change as the cell cycle progresses

48 Cyclins and Cdks (cont'd.) A specific Cdk becomes active when the cell synthesizes the cyclin that binds to it and remains active until the cyclin is degraded Phosphorylation regulates proteins that initiate or regulate key events in the cell cycle (DNA replication, mitosis, and cytokinesis) Regulation of the activity of cyclin-cdk complexes is integrated with the regulatory events at the key cell cycle checkpoints

49 Three Classes of Cyclins G 1 /S cyclin binds to Cdk2 near the end of G 1 required for transition from G 1 to S, and to commit to DNA replication S cyclin binds to Cdk2 in the S phase required for initiation of DNA replication and progression of the cell through S M cyclin binds to Cdk1 in G 2 required for the transition from G 2 to M, and the progression of the cell through mitosis

50 G 1 Cyclin In most cells, a fourth class of cyclins, G 1 cyclin, binds to Cdk4 and Cdk6 before the G 1 /S transition to form two cyclin Cdk complexes These complexes are needed to move the cell through the G 1 checkpoint and proceed from G 1 to S

51 M Cyclin-Cdk1 Complex The M cyclin-cdk1 complex is also called M phase-promoting factor (MPF) When chromosomes are correctly attached to the mitotic spindle, MPF activates another enzyme complex, the anaphase-promoting complex (APC) Activated APC degrades an inhibitor of anaphase, leading to the separation of sister chromatids later APC directs the degradation of M cyclin, causing Cdk1 to lose its activity

52 Cyclin is degraded Cyclin is degraded M cyclin binds to CDK1 M cyclin CDK1 G 2 /M checkpoint G 2 S M G 1 Cyclin is degraded Mitotic spindle checkpoint G 1 /S checkpoint S cyclin CDK2 G 1 /S cyclin binds to CDK2 G 1 /S cyclin CDK2 S cyclin binds to CDK2 Stepped Art

53 Molecular Insights: The Cell Cycle Research Question: What human proteins are cell cycle-dependent in level or localization? Discussion: About 40% of the proteins studied showed cell cycle-dependent behavior Most of the cell cycle-dependent proteins identified in the study showed changes in cellular localization

54 External Controls Internal controls are modified by external controls (signal molecules originating outside the dividing cells), including peptide hormones and growth factors Hormones and growth factors act on the cell by the reception-transduction-response pattern Reactions triggered by the activated receptor may speed, slow, or stop the progress of cell division

55 External Controls (cont'd.) Cell-surface receptors in animals also recognize contact with other cells or with molecules of the extracellular matrix Contact inhibition triggers internal reaction pathways that inhibit division by arresting the cell cycle, stabilizing cell growth in fully developed organs and tissues Cells in contact with one another are shunted into the G 0 phase and prevented from dividing if contacts are broken, the freed cells often enter rounds of division

56 Cell Cycle Controls Are Lost in Cancer Cancer occurs when cells lose normal controls over division cancer cells divide continuously and uncontrollably, producing a rapidly growing mass called a tumor Cancer cells typically lose adhesions to other cells and spread throughout the body (metastasis) producing new tumors in other body regions Metastasis is promoted by changes that block contact inhibition and alter cell-surface molecules that link cells together or to the extracellular matrix

57 Cancer Cancer cells typically have a number of mutated genes that promote uncontrolled cell division or metastasis Many of these genes code for components of the cyclin/cdk system that regulates cell division Others encode proteins that regulate gene expression, form cell surface receptors, or elements of receptor systems The mutated form of the genes, called oncogenes encode altered versions of these products

58 STUDY BREAK Why is a Cdk not active throughout the entire cell cycle? 2. How do cyclin Cdk complexes typically trigger transitions in the cell cycle? 3. What is an oncogene? How might an oncogene affect the cell cycle? 4. What is metastasis?

59 10.5 Cell Division in Bacteria Prokaryotic cells divide and reproduce by binary fission splitting or dividing into two parts Although binary fission is regulated, the small size of prokaryotic cells makes observation of chromosome movements difficult Although actin-like proteins are present in bacteria, their role in chromosome segregation remains unclear

60 The Prokaryotic Cell Cycle Most prokaryotes have a single, circular DNA molecule known as the prokaryotic chromosome (either a bacterial chromosome or an archaeal chromosome) DNA replication occupies most of the period between cytoplasmic divisions In E. coli cells, which are capable of dividing every 20 minutes, DNA replication occupies 19 minutes of the 20-minute division cycle

61 Bacterial chromosome Origin of replication (ori) Replication origins Unreplicated region of chromosome Replicated chromosomes Cell wall Plasma membrane Stepped Art

62 Mitosis Evolved from Binary Fission Most prokaryotic cells inherit a single chromosome in eukaryotes, each cell must inherit one each of several different chromosomes Mitosis allows each daughter cell to receive a complete complement of the chromosomes that the parent cell possessed Scientists believe that the ancestral division process was binary fission and that mitosis evolved from that process

63 STUDY BREAK How do bacteria divide? 2. What processes involved in eukaryotic cell division are absent from bacterial cell division?