The Cell Cycle and Cell Division

The Cell Cycle and Cell Division
7 The Cell Cycle and Cell Division

Chapter 7 The Cell Cycle and Cell Division
Key Concepts 7.1 Different Life Cycles Use Different Modes of Cell Reproduction 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells 7.3 Cell Reproduction Is Under Precise Control

Chapter 7 The Cell Cycle and Cell Division
Key Concepts 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

Chapter 7 Opening Question
How does infection with HPV result in uncontrolled cell reproduction?

Organisms have two basic strategies for reproducing themselves:
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The lifespan of an organism is linked to cell reproduction—usually called cell division. Organisms have two basic strategies for reproducing themselves: Asexual reproduction Sexual reproduction Cell division is also important in growth and repair of tissues. See Chapter 4

Figure 7.1 The Importance of Cell Division (Part 1)

Any genetic variations are due to mutations.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction In asexual reproduction the offspring are clones—genetically identical to the parent. Any genetic variations are due to mutations. A unicellular prokaryote may reproduce itself by binary fission. Single-cell eukaryotes can reproduce by mitosis. Other eukaryotes are also able to reproduce through asexual or sexual means.

Figure 7.2 Asexual Reproduction on a Large Scale

Gametes form by meiosis—a process of cell division.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Sexual reproduction requires gametes—two parents each contribute one gamete to an offspring. Gametes form by meiosis—a process of cell division. Gametes—and offspring—differ genetically from each other and from the parents.

DNA in eukaryotic cells is organized into chromosomes.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction DNA in eukaryotic cells is organized into chromosomes. A chromosome consists of a single molecule of DNA and proteins. Somatic cells—body cells not specialized for reproduction Each somatic cell contains two sets of chromosomes (homologs) that occur in homologous pairs. VIDEO 7.1 Cell Visualization: From DNA to chromosomes LINK The inheritance of characteristics such as seed shape is discussed in Chapter 8 See Chapter 4

Haploid cell—Number of chromosomes = n
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Gametes contain only one set of chromosomes—one homolog from each pair. Haploid cell—Number of chromosomes = n Fertilization—Two haploid gametes (female egg and male sperm) fuse to form a zygote. Chromosome number in zygote = 2n and cells are diploid.

All kinds of sexual life cycles involve meiosis:
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction All kinds of sexual life cycles involve meiosis: Haplontic life cycle—in protists, fungi, and some algae—zygote is only diploid stage After zygote forms it undergoes meiosis to form haploid spores, which germinate to form a new organism. Organism is haploid, and produces gametes by mitosis—cells fuse to form diploid zygote.

Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)

Spores divide by mitosis to form the haploid generation (gametophyte).
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Alternation of generations—most plants, some protists; meiosis gives rise to haploid spores Spores divide by mitosis to form the haploid generation (gametophyte). Gametophyte forms gametes by mitosis. Gametes then fuse to form diploid zygote (sporophyte), which in turn produces haploid spores by meiosis.

A mature organism is diploid and produces gametes by meiosis.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Diplontic life cycle—animals and some plants; gametes are the only haploid stage A mature organism is diploid and produces gametes by meiosis. Gametes fuse to form diploid zygote; zygote divides by mitosis to form mature organism.

Thus, no two individuals have exactly the same genetic makeup.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The essence of sexual reproduction is that it allows the random selection of half the diploid chromosome set. This forms a haploid gamete that fuses with another to make a diploid cell. Thus, no two individuals have exactly the same genetic makeup. See Part Four

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction
What life cycle is this? You are studying two, tiny plant-like organisms that you have discovered in your yard. One is a small, green fuzzy structure about half an inch high. It is multicellular, and each cell has 6 chromosomes. You notice that it sometimes produces single cells that can move away, independently, in rainwater or dew. Each of these cells also has 6 chromosomes. You observe two of these independent cells fusing together. The second organism is an upright, brown spike about an inch tall. Each of its cells has 12 chromosomes. You observe that cells in its upper end are undergoing cell division. Each of the parent cells produces four daughter cells that each have 6 chromosomes. The daughter cells are blown away by the wind. You perform DNA analysis on both organisms. To your surprise, they turn out to share the same genome! They are two different life stages of the same species. 21

What life cycle is this? (continued)
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction What life cycle is this? (continued) Working in small groups, develop a hypothesis about this organism’s life cycle. How are the green fuzzy stage and the brown spike stage related to each other? How could you test your ideas? Next, diagram your organism’s life cycle, using the figures on p.126 of your textbook as a guide. Decide whether this life cycle is most likely haplontic, diplontic, or alternation of generations. On your diagram, label the following structures: sporophyte, gametophyte, spore, and gamete. Specify whether each life stage is haploid (1n) or diploid (2n). Finally, label where meiosis and fertilization occur. Answers: This is a moss. The green fuzzy stage is the haploid gametophyte, which gives rise to haploid gametes. Two haploid gametes fuse together to produce a diploid zygote, which develops (via mitosis) into the “brown spike,” which is the diploid sporophyte. Cells at the top of the diploid sporophyte undergo meiosis to produce haploid spores. The spores are airborne; each eventually can give rise (via mitosis) to a new haploid gametophyte. You can test your ideas by observing what happens to the spores and gametes. The gametes should fuse together to form a zygote, and the zygote should grow to become the “brown spike” (i.e., the diploid sporophyte). Similarly, the spores from the brown spike, if planted in an appropriate environment, should each develop into a new “green fuzzy stage” - the haploid gametophyte. INSTRUCTOR NOTES: Students should be able to realize this is alternation of generations, because both the haploid stage and the diploid stage are multicellular. Students should produce a diagram similar to the middle part of Figure 7.3 (the fern). 22

The small, green fuzzy organism is a. diploid. b. haploid.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Recall that the small, green fuzzy organism consists of many cells that each have 6 chromosomes, while the brown spike consists of many cells that each have 12 chromosomes. The small, green fuzzy organism is a. diploid. b. haploid. c. I don’t know. Answer: b [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don't know” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] 23

The small, green fuzzy organism is producing a. diploid spores.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The small, green fuzzy organism consists of many cells that each have 6 chromosomes. It sometimes produces independent cells that also have 6 chromosomes. Two of these cells can fuse together. The small, green fuzzy organism is producing a. diploid spores. b. haploid spores. c. diploid gametes. d. haploid gametes. e. I don’t know. Answer: d INSTRUCTOR NOTES: Most students will realize that if these cells have 6 chromosomes, they must be haploid; and if two of these cells can fuse together, they must be gametes. Some students may stumble on the fact that the gametes here are being produced by mitosis rather than by meiosis; it may help to remind these students that though meiosis must occur at some point in the life cycle, it need not occur immediately before gamete formation. In some life cycles there is a long gap between meiosis and gamete formation (i.e., meiosis produces spores, which then grow to become gametophytes, which eventually produce gametes via mitosis.) [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don't know” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] 24

The small, green fuzzy organism is a. a sporophyte. b. a gametophyte.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The small, green fuzzy organism is a. a sporophyte. b. a gametophyte. c. I don’t know. Answer: b INSTRUCTOR NOTES: This is often a helpful time to point out that the sporophyte and gametophyte are named for what they produce. The small, green fuzzy stage produces gametes, and therefore it is a gametophyte. [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don't know” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] 25

The brown spike organism is producing a. diploid spores.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The brown spike stage consists of many cells that each have 12 chromosomes. Cells at the top are undergoing cell division; each of these parent cells produces four daughter cells that each have 6 chromosomes. The daughter cells are blown away by the wind. The brown spike organism is producing a. diploid spores. b. haploid spores. c. diploid gametes. d. haploid gametes. e. I don’t know. Answer: b INSTRUCTOR NOTES: This is often a good time to point out that both spores and gametes are haploid. (Students may have a tendency to think that if one is haploid, the other should be diploid.) [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don't know” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] 26

a. diploid gametophyte; haploid sporophyte
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The green fuzzy stage is a _______; the brown spike stage is a _______. a. diploid gametophyte; haploid sporophyte b. haploid gametophyte; diploid sporophyte c. diploid sporophyte; haploid gametophyte d. haploid sporophyte; diploid gametophyte e. I don’t know. Answer: b [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don't know” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] 27

c. alternation of generations. d. I don’t know.
Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The life cycle of this green fuzzy / brown spike species is best described as a. haplontic. b. diplontic. c. alternation of generations. d. I don’t know. Answer: c [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don't know” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] 28

Four events must occur for cell division:
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Four events must occur for cell division: Reproductive signal—to initiate cell division Replication of DNA Segregation—distribution of the DNA into the two new cells Cytokinesis—division of the cytoplasm and separation of the two new cells

Separates the DNA and cytoplasm into two cells through binary fission
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells In prokaryotes, cell division results in reproduction of the entire organism. The cell: Grows in size Replicates its DNA Separates the DNA and cytoplasm into two cells through binary fission

Two important regions in reproduction: ori - where replication starts
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Most prokaryotes have one chromosome, a single molecule of DNA—usually circular. Two important regions in reproduction: ori - where replication starts ter - where replication ends

Replication begins at the ori site and moves towards the ter site.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Replication occurs as the DNA is threaded through a “replication complex” of proteins in the center of the cell. Replication begins at the ori site and moves towards the ter site.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells
As replication proceeds, the ori complexes move to opposite ends of the cell. DNA sequences adjacent to the ori region actively bind proteins for the segregation, hydrolyzing ATP for energy. An actin-like protein provides a filament along which ori and other proteins move. LINK Review the description of the cytoskeleton and its components in Concept 4.4

Figure 7.4 Prokaryotic Cell Division

Cytokinesis begins after chromosome segregation by a pinching in of the plasma membrane—protein fibers form a ring. As the membrane pinches in, new cell wall materials are synthesized resulting in separation of the two cells. VIDEO 7.2 Cytokinesis in the euglenoid Phacus VIDEO 7.3 Cytokinesis in a green alga, Micrasterias

Eukaryotic cells divide by mitosis followed by cytokinesis.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Eukaryotic cells divide by mitosis followed by cytokinesis. Replication of DNA occurs as long strands are threaded through replication complexes. DNA replication only occurs during a specific stage of the cell cycle. See Chapter 9

In eukaryotes, the chromosomes become highly condensed.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells In segregation of DNA after cell division, one copy of each chromosome ends up in each of the two new cells. In eukaryotes, the chromosomes become highly condensed. Mitosis segregates them into two new nuclei— the cytoskeleton is involved in the process.

Cytokinesis follows mitosis.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Cytokinesis follows mitosis. The process in plant cells (which have cell walls) is different than in animal cells (which do not have cell walls).

The cell cycle—the period between cell divisions
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells The cell cycle—the period between cell divisions In eukaryotes it is divided into mitosis and cytokinesis—called the M phase—and a long interphase. During interphase, the cell nucleus is visible and cell functions including replication occur Interphase begins after cytokinesis and ends when mitosis starts.

Interphase has three subphases: G1, S, and G2.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Interphase has three subphases: G1, S, and G2. G1 (Gap 1)—variable, a cell may spend a long time in this phase carrying out its functions S phase (Synthesis)—DNA is replicated G2 (Gap 2)—the cell prepares for mitosis, synthesizes microtubules for segregating chromosomes ANIMATED TUTORIAL 7.1 Mitosis

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 1)

In mitosis, one nucleus produces two daughter nuclei each containing the same number of chromosomes as the parent nucleus. Mitosis is continuous, but can be can be divided into phases—prophase, prometaphase, metaphase, anaphase, and telophase. VIDEO 7.4 Division of bacteria, Salmonella enteritidis

The chromatin (DNA) is not yet condensed.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells During interphase, only the nuclear envelope and and the nucleolus are visible. The chromatin (DNA) is not yet condensed. Three structures appear in prophase: The condensed chromosomes Centrosome Spindle See Concept 4.3

Condensed chromosomes appear during prophase.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Condensed chromosomes appear during prophase. Sister chromatids—two DNA molecules on each chromosome after replication Centromere—region where chromatids are joined Kinetochores are protein structures on the centromeres, and are important for chromosome movement. See Chapter 9

The karyotype of an organism reflects the number and sizes of its condensed chromosomes. Karyotype analysis can be used to identify organisms, but DNA sequence is more commonly used.

Segregation is aided by other structures:
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Segregation is aided by other structures: The centrosome determines the orientation of the spindle apparatus. Each centrosome can consist of two centrioles—hollow tubes formed by microtubules. Centrosome is duplicated during S phase and each moves towards opposite sides of the nucleus.

Polar microtubules form a spindle and overlap in the center.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Centrosomes serve as mitotic centers or poles; the spindle forms between the poles from two types of microtubules: Polar microtubules form a spindle and overlap in the center. Kinetochore microtubules—attach to kinetochores on the chromatids. Sister chromatids attach to opposite halves of the spindle. VIDEO 7.5 Formation of mitotic spindles

Chromosome separation and movement is highly organized.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Chromosome separation and movement is highly organized. During prometaphase, the nuclear envelope breaks down. Chromosomes consisting of two chromatids attach to the kinetochore mictotubules. VIDEO 7.6 Cell Visualization: Mitosis and cell division

Figure 7.6 The Phases of Mitosis (1)

Durin metaphase, chromosomes line up at the midline of the cell.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Durin metaphase, chromosomes line up at the midline of the cell. During anaphase, the separation of sister chromatids is controlled by M phase cyclin- Cdk; cohesin is hydrolyzed by separase. After separation, they move to opposite ends of the spindle and are referred to as daughter chromosomes.

Figure 7.6 The Phases of Mitosis (2)

Microtubules also shorten, drawing chromosomes toward poles.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells A protein at the kinetochores—cytoplasmic dynein—hydrolyzes ATP for energy to move chromosomes along the microtubules towards the poles. Microtubules also shorten, drawing chromosomes toward poles.

Telophase occurs after chromosomes have separated: Spindle breaks down
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Telophase occurs after chromosomes have separated: Spindle breaks down Chromosomes uncoil Nuclear envelope and nucleoli appear Two daughter nuclei are formed with identical genetic information

Division of the cytoplasm differs in plant and animals
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Cytokinesis: Division of the cytoplasm differs in plant and animals In animal cells, plasma membrane pinches between the nuclei because of a contractile ring of microfilaments of actin and myosin. VIDEO 7.7: Mitosis in a newt lung epithelial cell

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 1)

These fuse to form a new plasma membrane.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Plant cells: Vesicles from the Golgi apparatus appear along the plane of cell division These fuse to form a new plasma membrane. Contents of vesicles form the cell plate—the beginning of the new cell wall. VIDEO 7.8 Mitosis in a plant cell

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 2)

Each daughter cell contains all of the components of a complete cell.
Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells After cytokinesis: Each daughter cell contains all of the components of a complete cell. Chromosomes are precisely distributed. The orientation of cell division is important to development, but organelles are not always evenly distributed.

Concept 7.2 Both Binary Fission And Mitosis Produce Genetically Identical Cells
Reviewing the stages of mitosis Working in pairs and not looking at your notes, briefly review the stages of mitosis. First, put the following five stages in order: Metaphase Telophase Propase Anaphase Prometaphase Then take turns naming a feature of each of the five stages. (Person A names one feature of the first stage; the person B names another feature of the first stage. Then proceed to the second stage and name two more features, etc. Any feature counts.) 61

During which of these stages is DNA replicated? a. Telophase
Concept 7.2 Both Binary Fission And Mitosis Produce Genetically Identical Cells During which of these stages is DNA replicated? a. Telophase b. Prometaphase c. Prophase d. Anaphase e. None of the above Answer: e (DNA is replicated long before mitosis begins - in the S phase of the cell cycle.) 62

Concept 7.3 Cell Reproduction Is Under Precise Control
The reproductive rates of most prokaryotes respond to environmental conditions. In eukaryotes, cell division is related to the needs of the entire organism. Cells divide in response to extracellular signals, like growth factors.

The eukaryotic cell cycle has four stages: G1, S, G2, and M. Progression is tightly regulated—the G1-S transition is called R, the restriction point. Passing this point usually means the cell will proceed with the cell cycle and divide.

Figure 7.8 The Eukaryotic Cell Cycle

Specific signals trigger the transition from one phase to another. Evidence for substances as triggers came from cell fusion experiments. Nuclei in cells at different stages, fused by polyethylene glycol, both entered the phase of DNA replication (S).

Figure 7.9 Regulation of the Cell Cycle (Part 1)

Transitions also depend on activation of cyclin- dependent kinases (Cdk’s). A protein kinase is an enzyme that catalyzes phosphorylation from ATP to a protein. Phosphorylation changes the shape and function of a protein by changing its charges. See Chapter 5

Cdk is activated by binding to cyclin (by allosteric regulation); this alters its shape and exposes its active site. The G1-S cyclin-Cdk complex acts as a protein kinase and triggers transition from G1 to S. Other cyclin-Cdk’s act at different stages of the cell cycle, called cell cycle checkpoints. See Chapter 3

Figure 7.10 Cyclins Are Transient in the Cell Cycle

Example of G1-S cyclin-Cdk regulation: Progress past the restriction point in G1 depends on retinoblastoma protein (RB). RB normally inhibits the cell cycle, but when phosphorylated by G1-S cyclin-Cdk, RB becomes inactive and no longer blocks the cell cycle. See Chapters 3 and 5

The cell cycle game The entire classroom is a cell in the G1 phase. Will we enter the S phase and copy our DNA? You will be assigned one of the following roles: DNA copiers: You are the cell’s DNA replication machinery. There is a DNA sequence on the board. Your job is to copy the sequence, exactly, onto a piece of paper. If interrupted or if you make a mistake, you must start over. Call out “DNA copied!” when you finally succeed. Retinoblastoma protein: Your job is to prevent the DNA from being copied; that is, you are keeping the cell in the G1 phase. You may try any techniques as long as you don’t touch the DNA-copying people directly. You can block their line of sight, cover their eyes with jackets, block the board with your own body, hide their pencils, etc.—anything that will keep them from copying the DNA. When the game starts, you are not phosphorylated, and that means you are active. Once you are phosphorylated (by being given a phosphate group), you’ll move out of the way and become inactive. INSTRUCTOR NOTES: This exercise give students an interactive and memorable representation of how Cdk and cyclin interact at cell cycle checkpoints. The entire exercise takes about 20 minutes (10 min to explain and position students, about 3 min to actually do it, and 5 min to discuss). [Note: As written, the exercise takes the class all the way through S phase (i.e., fully copying the DNA). You may wish point out to students that the key checkpoint is actually before the S phase begins.] The exercise can be done in small groups, in a single group up front that everybody else watches, or with the entire class. For the full-class version, bring the following: Many copies of the cell cycle roles above (1 role per sheet of paper). Most of these should be for DNA copiers and retinoblastoma proteins, with a few more retinoblastoma proteins than DNA copiers. Very few people are needed to play the other three roles; you will only need just enough Cdk’s to be able to run around and tag all of the people playing retinoblastoma proteins (between 2-8, depending on the size of the class); 1-3 cyclin makers (depending on the size of the class); and just 1 growth factor. Identifiers for students to be labeled as Cdk, cyclin, etc. If multiple sections will do this exercise, you may wish to make re-usable nametags, colored hats with labels, etc. For a simple quick label, print a very large-font title on the cell cycle role pages (e.g., “CYCLIN” in huge letters and then a smaller-font explanation of the cyclin role); then bring safety pins and ask students to pin their pages to their shirts after they have read them. Colored pieces of paper and pens, for cyclin-making people to make cyclin molecules. A visual prop to represent phosphorylation - colorful foam balls, neckties, round colored pieces of paper with a “P” on them, etc. - that Cdk people can put on, or hand to, retinoblastoma-protein people. (There should be a few more phosphate groups than there are retinoblastoma-protein people.) Proceed as follows: Before class, write a DNA sequence on the board - long enough that DNA copiers won't be able to copy it quickly (at least 20 bp). Set out the role-playing pages for students to pick up when they enter class. Put the colored paper and pens near the door, and scatter the phosphate groups throughout the room. When you are ready to start the exercise, first ask all students to read their roles; then answer any questions. Point out that this exercise dramatically oversimplifies the process, particularly what exactly retinoblastoma protein is doing and how long the cyclin lasts; and that we will take the cell well past checkpoint and all the way through the S phase. The point is to illustrate the sequence of which protein activates which, and to illustrate how the arrival of a growth factor molecule can ultimately cause a cell to divide. Position students as follows. Ask for a show of hands about where the retinoblastoma-proteins and DNA copiers are, and quickly check that DNA copiers are not clumped together (retinoblastoma proteins may need to get to them quickly to block them); also check that there are some retinoblastoma proteins in the front row who will be able to get to the board quickly (to block the view of the DNA). Then, ask the growth factor person to stand just outside in the hallway (looking in); get the cyclin-making people lined up near the door with their colored pieces of paper and pens; and tell the Cdk people to wander around aimlessly until they receive a cyclin. Ask all students if they will know what to do if someone hands them something, or if they know who exactly they will need to hand something to. Encourage students to verbalize what they do, saying out loud phrases like “Here's a cyclin,” “I'm phosphorylating you,” “I'm inactive,” and so on. When you say “Start”, DNA-copiers should start trying to copy the DNA, and retinoblastoma proteins should try to stop them. The cyclin-makers and Cdk’s are inactive and the growth factor has not arrived. Allow a minute or two to pass for students to get used to the idea that the cell is stuck in G1 because retinoblastoma protein is preventing the cell cycle from proceeding. If time permits, pause here to ask the Cdk people to explain to the class why they weren’t doing anything. Gesture to the growth factor to come in - he/she should make a dramatic announcement here, something like “The growth factor has arrived!” and should tap the cyclin makers on the head (representing activation). Cyclin makers should start making cyclin (writing “CYCLIN” on the colored pieces of paper) and handing the cyclins to Cdk’s. Cdk’s should then start picking up phosphate groups from around the room, and handing them to all the retinoblastoma proteins they can find. Soon, enough retinoblastoma proteins will be inactivated that a large number of DNA copiers will start successfully copying the DNA. Once all the DNA copiers have finished copying, end the exercise. Discuss. Have students review each molecule’s name and its function. [Note: If class size permits, you can add some protease people to break down the cyclins.] 72

The cell cycle game (continued) Cyclin-dependent kinase (Cdk): [Note: You are only active if you are holding a cyclin molecule.] Once you have a cyclin molecule, you will become active and you will phosphorylate all the retinoblastoma proteins you can find. You will do this by giving them phosphate groups. Cyclin maker: You are an enzyme that will make many copies of the molecule cyclin (by writing “CYCLIN” on a piece of paper) and handing them to Cdk people. You are not active yet; you’ll only become active when a growth factor arrives and taps you on the head. Growth factor: When the game starts, you have not arrived at the cell yet. When cued, make a dramatic entrance, and announce your arrival clearly by saying “The growth factor has arrived!” Then tap all the cyclin makers on the head to activate them. INSTRUCTOR NOTES: This exercise give students an interactive and memorable representation of how Cdk and cyclin interact at cell cycle checkpoints. The entire exercise takes about 20 minutes (10 min to explain and position students, about 3 min to actually do it, and 5 min to discuss). [Note: As written, the exercise takes the class all the way through S phase (i.e., fully copying the DNA). You may wish point out to students that the key checkpoint is actually before the S phase begins.] The exercise can be done in small groups, in a single group up front that everybody else watches, or with the entire class. For the full-class version, bring the following: Many copies of the cell cycle roles above (1 role per sheet of paper). Most of these should be for DNA copiers and retinoblastoma proteins, with a few more retinoblastoma proteins than DNA copiers. Very few people are needed to play the other three roles; you will only need just enough Cdk’s to be able to run around and tag all of the people playing retinoblastoma proteins (between 2-8, depending on the size of the class); 1-3 cyclin makers (depending on the size of the class); and just 1 growth factor. Identifiers for students to be labeled as Cdk, cyclin, etc. If multiple sections will do this exercise, you may wish to make re-usable nametags, colored hats with labels, etc. For a simple quick label, print a very large-font title on the cell cycle role pages (e.g., “CYCLIN” in huge letters and then a smaller-font explanation of the cyclin role); then bring safety pins and ask students to pin their pages to their shirts after they have read them. Colored pieces of paper and pens, for cyclin-making people to make cyclin molecules. A visual prop to represent phosphorylation - colorful foam balls, neckties, round colored pieces of paper with a “P” on them, etc. - that Cdk people can put on, or hand to, retinoblastoma-protein people. (There should be a few more phosphate groups than there are retinoblastoma-protein people.) Proceed as follows: Before class, write a DNA sequence on the board - long enough that DNA copiers won't be able to copy it quickly (at least 20 bp). Set out the role-playing pages for students to pick up when they enter class. Put the colored paper and pens near the door, and scatter the phosphate groups throughout the room. When you are ready to start the exercise, first ask all students to read their roles; then answer any questions. Point out that this exercise dramatically oversimplifies the process, particularly what exactly retinoblastoma protein is doing and how long the cyclin lasts; and that we will take the cell well past checkpoint and all the way through the S phase. The point is to illustrate the sequence of which protein activates which, and to illustrate how the arrival of a growth factor molecule can ultimately cause a cell to divide. Position students as follows. Ask for a show of hands about where the retinoblastoma-proteins and DNA copiers are, and quickly check that DNA copiers are not clumped together (retinoblastoma proteins may need to get to them quickly to block them); also check that there are some retinoblastoma proteins in the front row who will be able to get to the board quickly (to block the view of the DNA). Then, ask the growth factor person to stand just outside in the hallway (looking in); get the cyclin-making people lined up near the door with their colored pieces of paper and pens; and tell the Cdk people to wander around aimlessly until they receive a cyclin. Ask all students if they will know what to do if someone hands them something, or if they know who exactly they will need to hand something to. Encourage students to verbalize what they do, saying out loud phrases like “Here's a cyclin,” “I'm phosphorylating you,” “I'm inactive,” and so on. When you say “Start”, DNA-copiers should start trying to copy the DNA, and retinoblastoma proteins should try to stop them. The cyclin-makers and Cdk’s are inactive and the growth factor has not arrived. Allow a minute or two to pass for students to get used to the idea that the cell is stuck in G1 because retinoblastoma protein is preventing the cell cycle from proceeding. If time permits, pause here to ask the Cdk people to explain to the class why they weren’t doing anything. Gesture to the growth factor to come in - he/she should make a dramatic announcement here, something like “The growth factor has arrived!” and should tap the cyclin makers on the head (representing activation). Cyclin makers should start making cyclin (writing “CYCLIN” on the colored pieces of paper) and handing the cyclins to Cdk’s. Cdk’s should then start picking up phosphate groups from around the room, and handing them to all the retinoblastoma proteins they can find. Soon, enough retinoblastoma proteins will be inactivated that a large number of DNA copiers will start successfully copying the DNA. Once all the DNA copiers have finished copying, end the exercise. Discuss. Have students review each molecule’s name and its function. [Note: If class size permits, you can add some protease people to break down the cyclins.] 73

Which of the following is a true statement about Cdk’s and cyclins? a. Both are always present in the cell. b. Cyclins are always present; Cdk’s are only made at certain times. c. Cdk’s are always present; cyclins are only made at certain times. d. Cdk’s are active when not bound to a cyclin. e. I don’t know. Answer: c [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don't know” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] 74

Reduce the chromosome number from diploid to haploid
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiosis consists of two nuclear divisions but DNA is replicated only once. The function of meiosis is to: Reduce the chromosome number from diploid to haploid Ensure that each haploid has a complete set of chromosomes Generate diversity among the products ANIMATED TUTORIAL 7.2 Meiosis

Figure 7.11 Mitosis and Meiosis: A Comparison

Figure 7.12 Meiosis: Generating Haploid Cells (Part 1)

Meiotic division reduces the chromosome number. Two unique features:
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiotic division reduces the chromosome number. Two unique features: In meiosis I, homologous pairs of chromosomes come together and line up along their entire lengths. After metaphase I, the homologous chromosome pairs separate, but individual chromosomes made up of two sister chromatids remain together.

Meiosis I is preceded by an S phase during which DNA is replicated.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiosis I is preceded by an S phase during which DNA is replicated. Each chromosome then consists of two sister chromatids, held together by cohesin proteins. At the end of meiosis I, two nuclei form, each with half the original chromosomes—still composed of sister chromatids.

These four cells are not genetically identical.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Sister chromatids separate during meiosis II, which is not proceeded by DNA replication. The products of meiosis I and II are four cells with a haploid number of chromosomes. These four cells are not genetically identical. Two processes may occur: Crossing over and independent assortment VIDEO 7.9 Meiosis in a cranefly spermatocyte

In prophase of meiosis I homologous chromosomes pair by synapsis.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity In prophase of meiosis I homologous chromosomes pair by synapsis. The four chromatids of each pair of chromosomes form a tetrad,or bivalent. The homologs seem to repel each other but are held together at chiasmata.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity
Crossing over is an exchange of genetic material that occurs at the chiasma. Crossing over results in recombinant chromatids and increases genetic variability of the products.

In-Text Art, Ch. 7, p. 138

Figure 7.13 Crossing Over Forms Genetically Diverse Chromosomes

Prophase I may last a long time.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Prophase I may last a long time. Human males: Prophase I lasts about 1 week, and 1 month for entire meiotic cycle Human females: Prophase I begins before birth, and ends up to decades later during the monthly ovarian cycle

The more chromosomes involved, the more combinations possible.
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Independent assortment during anaphase I also allows for chance combinations and genetic diversity. After homologous pairs of chromosomes line up at metaphase I, it is a matter of chance which member of a pair goes to which daughter cell. The more chromosomes involved, the more combinations possible. APPLY THE CONCEPT Meiosis halves the nuclear chromosome content and generates diversity

Either results in aneuploidy—chromosomes lacking or present in excess
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiotic errors: Nondisjunction—homologous pairs fail to separate at anaphase I—sister chromatids fail to separate, or homologous chromosomes may not remain together Either results in aneuploidy—chromosomes lacking or present in excess

Organisms with triploid (3n), tetraploid (4n), and even higher levels are called polyploid. This can occur through an extra round of DNA duplication before meiosis, or the lack of spindle formation in meiosis II. Polyploidy occurs naturally in some species, and can be desirable in plants.

If crossing over happens between non- homologous chromosomes, the result is a translocation. A piece of chromosome may rejoin another chromosome, and its location can have profound effects on the expression of other genes. Example: Leukemia See Chapters 10 and 11

In-Text Art, Ch. 7, p. 140

Concept 7.4 Meiosis Halves The Nuclear Chromosome Content And Generates Diversity
Meiosis footrace Divide the class into two halves. Each half will race the other to produce a complete diagram of the stages of meiosis. Start with two volunteer “runners,” one from each half of the classroom, lined up at the back of the room. When the clock is started, these two runners will run up to the board from the back of the room and draw a single, diploid, eukaryotic cell that has 2 pairs of chromosomes (a long pair and a short pair) that is in Prophase I. Once a runner’s diagram is completed, he or she will run back to the back of the room and tag the next runner, who will run to the board and draw the next stage of meiosis. INSTRUCTOR NOTES: This game takes about 10 minutes and requires a large amount of whiteboard space (or poster paper, etc.). If space is insufficient, the exercise can also be done in small groups of 4 (one team of two races another team of two, each team drawing on pieces of paper). If desired, a calmer and non-competitive version of this exercise can be done simply by asking pairs of students to draw a full diagram of meiosis and turn it in at the end of class. 95

Meiosis footrace (continued) Every diagram should include:
Concept 7.4 Meiosis Halves The Nuclear Chromosome Content And Generates Diversity Meiosis footrace (continued) Every diagram should include: the name of the phase all the chromosomes (drawn clearly enough to see if chromatids are present) spindle (if present) nuclear membrane (if present) Non-runners can coach runners from the sidelines. Each team should also be compiling a list of differences between mitosis and meiosis. (Designate one person per team to write down the list.) At the end, check all diagrams for any errors. Each team gets 2 points per correct diagram, 1 point for every correct item on their list of mitosis/meiosis differences, and a 5 point bonus for finishing first! Note: This game takes about 10 minutes, and requires a large amount of whiteboard space (or poster paper, etc.). If space is insufficient, the exercise can also be done in small groups of 4 (one team of two races another team of two, each team drawing on pieces of paper). If desired, a calmer and non-competitive version of this exercise can be done simply by asking pairs of students to draw a full diagram of meiosis and to turn it in at the end of class. 96

b. Tetrads form during meiosis but not during mitosis.
Concept 7.4 Meiosis Halves The Nuclear Chromosome Content And Generates Diversity Which of the following is not a difference between mitosis and meiosis? a. Meiosis produces four daughter cells from one parent cell; mitosis produces two. b. Tetrads form during meiosis but not during mitosis. c. Meiosis halves the 2n number; mitosis does not. d. Crossing over occurs only during mitosis. e. Mitosis has only 1 cell division; meiosis has 2. Answer: d 97

Meiosis halves the nuclear chromosome content and generates diversity
Apply the concept Meiosis halves the nuclear chromosome content and generates diversity In the anther (male sex organ) of the lily plant, cells undergo mitosis in synchrony. These cells can be removed and studied in the laboratory. An antibody was developed that would bind specifically to a chromosomal protein called B1. A procedure involving antibody binding and staining of the antibody was used to detect the protein in anther cells. The protein was detected at the centromeres of mitotic chromosomes, and its presence or absence was monitored at different stages of the cell cycle. The results were G1, absent; early S, absent; late S through metaphase, present; anaphase, absent; telophase, absent. Propose a role for this protein in sister chromatid function. What rsults would you expect if meiotic cells were examined for this protein? At what stages of meiosis would the protein be present or absent?

Cell death occurs in two ways:
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Cell death occurs in two ways: In necrosis, the cell is damaged or starved for oxygen or nutrients. The cell swells and bursts. Cell contents are released to the extracellular environment and can cause inflammation. See Concept 31.1 APPLY THE CONCEPT Programmed cell death is a necessary process in living organisms

Apoptosis is genetically programmed cell death. Two possible reasons:
7.5 ProConcept 7.ammed Cell Death Is a Necessary Process in Living Organisms Apoptosis is genetically programmed cell death. Two possible reasons: The cell is no longer needed, e.g., the connective tissue between the fingers of a fetus. Old cells may be prone to genetic damage that can lead to cancer—blood cells and epithelial cells die after days or weeks.

Cell detaches from its neighbors
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Events of apoptosis: Cell detaches from its neighbors Cuts up its chromatin into nucleosome-sized pieces Forms membranous lobes called “blebs” that break into fragments Surrounding living cells ingest the remains of the dead cell VIDEO 7.10 Apoptosis

Figure 7.14 Apoptosis: Programmed Cell Death (Part 1)

Cell death cycle is controlled by signals:
Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Cell death cycle is controlled by signals: Lack of a mitotic signal (growth factor) Recognition of damaged DNA External signals cause membrane proteins to change shape and activate enzymes called caspases—hydrolyze proteins of membranes.

Figure 7.14 Apoptosis: Programmed Cell Death (Part 2)

Answer to Opening Question
Human papilloma virus (HPV) stimulates the cell cycle when it infects the cervix. Two proteins regulate the cell cycle: Oncogene proteins are positive regulators of the cell cycle—in cancer cells they are overactive or present in excess Tumor suppressors are negative regulators of the cell cycle, but in cancer cells they are inactive—can be blocked by a virus such as HPV VIDEO 7.11 Human melanoma cells dividing in culture

Figure 7.15 Molecular Changes Regulate the Cell Cycle in Cancer Cells

Cancer represents a failure of apoptosis.
Concept 7.5 Programmed Cell Death Is A Necessary Process In Living Organisms Cancer and apoptosis Working in pairs and not looking at your notes, first review what apoptosis is. See if you can name two major reasons for apoptosis (i.e., two reasons that a multicellular body might need a certain cell to perform apoptosis). Then discuss the following statement: Cancer represents a failure of apoptosis. Do you agree with this statement? Why or why not? Be prepared to explain your answer to the class. 107

The Cell Cycle and Cell Division

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