Category: Cellular Processes

Chapter 11    Cell Communication    Lecture Outline

Overview

·         Cell-to-cell communication is absolutely essential for multicellular organisms.

°         Cells must communicate to coordinate their activities.

·         Communication between cells is also important for many unicellular organisms.

·         Biologists have discovered universal mechanisms of cellular regulation involving the same small set of cell-signaling mechanisms.

°         The ubiquity of these mechanisms provides additional evidence for the evolutionary relatedness of all life.

·         Cells most often communicate by chemical signals, although signals may take other forms.

A. An Overview of Cell Signaling

·         What messages are passed from cell to cell? How do cells respond to these messages?

·         We will first consider communication in microbes, to gain insight into the evolution of cell signaling.

1. Cell signaling evolved early in the history of life.

·         One topic of cell “conversation” is sex.

·         Saccharomyces cerevisiae, the yeast of bread, wine, and beer, identifies potential mates by chemical signaling.

°         There are two sexes, a and a, each of which secretes a specific signaling molecule, a factor and a factor, respectively.

°         These factors each bind to receptor proteins on the other mating type.

·         Once the mating factors have bound to the receptors, the two cells grow toward each other and undergo other cellular changes.

·         The two cells fuse, or mate, to form an a/a cell containing the genes of both cells.

·         The process by which a signal on a cell’s surface is converted into a specific cellular response is a series of steps called a signal-transduction pathway.

°         The molecular details of these pathways are strikingly similar in yeast and animal cells, even though their last common ancestor lived more than a billion years ago.

°         Signaling systems of bacteria and plants also share similarities.

·         These similarities suggest that ancestral signaling molecules evolved long ago in prokaryotes and have since been adopted for new uses by single-celled eukaryotes and multicellular descendents.

2. Communicating cells may be close together or far apart.

·         Multicellular organisms release signaling molecules that target other cells.

·         Cells may communicate by direct contact.

°         Both animals and plants have cell junctions that connect to the cytoplasm of adjacent cells.

°         Signaling substances dissolved in the cytosol can pass freely between adjacent cells.

°         Animal cells can communicate by direct contact between membrane-bound cell surface molecules.

°         Such cell-cell recognition is important to such processes as embryonic development and the immune response.

·         In other cases, messenger molecules are secreted by the signaling cell.

°         Some transmitting cells release local regulators that influence cells in the local vicinity.

°         One class of local regulators in animals, growth factors, includes compounds that stimulate nearby target cells to grow and multiply.

°         This is an example of paracrine signaling, which occurs when numerous cells simultaneously receive and respond to growth factors produced by a single cell in their vicinity.

·         In synaptic signaling, a nerve cell produces a neurotransmitter that diffuses across a synapse to a single cell that is almost touching the sender.

°         The neurotransmitter stimulates the target cell.

°         The transmission of a signal through the nervous system can also be considered an example of long-distance signaling.

·         Local signaling in plants is not well understood. Because of their cell walls, plants must have different mechanisms from animals.

·         Plants and animals use hormones for long-distance signaling.

°         In animals, specialized endocrine cells release hormones into the circulatory system, by which they travel to target cells in other parts of the body.

°         Plant hormones, called growth regulators, may travel in vessels but more often travel from cell to cell or move through air by diffusion.

·         Hormones and local regulators range widely in size and type.

°         The plant hormone ethylene (C2H4), which promotes fruit ripening and regulates growth, is a hydrocarbon of only six atoms, capable of passing through cell walls.

°         Insulin, which regulates blood sugar levels in mammals, is a protein with thousands of atoms.

·         What happens when a cell encounters a signal?

°         The signal must be recognized by a specific receptor molecule, and the information it carries must be changed into another form, or transduced, inside the cell before the cell can respond.

3. The three stages of cell signaling are reception, transduction, and response.

·         E. W. Sutherland and his colleagues pioneered our understanding of cell signaling.

°         Their work investigated how the animal hormone epinephrine stimulates breakdown of the storage polysaccharide glycogen in liver and skeletal muscle.

°         Breakdown of glycogen releases glucose derivatives that can be used for fuel in glycolysis or released as glucose in the blood for fuel elsewhere.

°         Thus one effect of epinephrine, which is released from the adrenal gland during times of physical or mental stress, is mobilization of fuel reserves.

·         Sutherland’s research team discovered that epinephrine activated a cytosolic enzyme, glycogen phosphorylase.

°         However, epinephrine did not activate the phosphorylase directly in vitro but could only act via intact cells.

°         Therefore, there must be an intermediate step or steps occurring inside the cell.

°         The plasma membrane must be involved in transmitting the epinephrine signal.

·         The process involves three stages: reception, transduction, and response.

°         In reception, a chemical signal binds to a cellular protein, typically at the cell’s surface or inside the cell.

°         In transduction, binding leads to a change in the receptor that triggers a series of changes in a series of different molecules along a signal-transduction pathway. The molecules in the pathway are called relay molecules.

°         In response, the transduced signal triggers a specific cellular activity.

B. Signal Reception and the Initiation of Transduction

1. A signal molecule binds to a receptor protein, causing the protein to change shape.

·         The cell targeted by a particular chemical signal has a receptor protein on or in the target cell that recognizes the signal molecule.

°         Recognition occurs when the signal binds to a specific site on the receptor that is complementary in shape to the signal.

·         The signal molecule behaves as a ligand, a small molecule that binds with specificity to a larger molecule.

·         Ligand binding causes the receptor protein to undergo a change in shape.

·         This may activate the receptor so that it can interact with other molecules.

°         For other receptors, this causes aggregation of receptor molecules, leading to further molecular events inside the cell.

·         Most signal receptors are plasma membrane proteins, whose ligands are large water-soluble molecules that are too large to cross the plasma membrane.

2. Some receptor proteins are intracellular.

·         Some signal receptors are dissolved in the cytosol or nucleus of target cells.

°         To reach these receptors, the signals pass through the target cell’s plasma membrane.

°         Such chemical messengers are either hydrophobic enough or small enough to cross the phospholipid interior of the plasma membrane.

·         Hydrophobic messengers include the steroid and thyroid hormones of animals.

·         Nitric oxide (NO) is a gas whose small size allows it to pass between membrane phospholipids.

·         Testosterone is secreted by the testis and travels through the blood to enter cells throughout the body.

°         The cytosol of target cells contains receptor molecules that bind testosterone, activating the receptor.

°         These activated proteins enter the nucleus and turn on specific genes that control male sex characteristics.

·         How does the activated hormone-receptor complex turn on genes?

·         These activated proteins act as transcription factors.

·         Transcription factors control which genes are turned on—that is, which genes are transcribed into messenger RNA.

·         mRNA molecules leave the nucleus and carry information that directs the synthesis (translation) of specific proteins at the ribosome.

·         Other intracellular receptors (such as thyroid hormone receptors) are found in the nucleus and bind to the signal molecules there.

3. Most signal receptors are plasma membrane proteins.

·         Most signal molecules are water-soluble and too large to pass through the plasma membrane.

·         They influence cell activities by binding to receptor proteins on the plasma membrane.

°         Binding leads to changes in the shape of the receptor or to the aggregation of receptors.

°         These cause changes in the intracellular environment.

·         There are three major types of membrane receptors: G-protein-linked receptors, receptor tyrosine kinases, and ion-channel receptors.

·         A G-protein-linked receptor consists of a receptor protein associated with a G protein on the cytoplasmic side.

°         Seven alpha helices span the membrane.

°         G-protein-linked receptors bind many different signal molecules, including yeast mating factors, epinephrine and many other hormones, and neurotransmitters.

·         The G protein acts as an on/off switch.

°         If GDP is bound to the G protein, the G protein is inactive.

°         When the appropriate signal molecule binds to the extracellular side of the receptor, the G protein binds GTP (instead of GDP) and becomes active.

°         The activated G protein dissociates from the receptor and diffuses along the membrane, where it binds to an enzyme, altering its activity.

°         The activated enzyme triggers the next step in a pathway leading to a cellular response.

·         The G protein can also act as a GTPase enzyme to hydrolyze GTP to GDP.

°         This change turns the G protein off.

·         Now inactive, the G protein leaves the enzyme, which returns to its original state.

·         The whole system can be shut down quickly when the extracellular signal molecule is no longer present.

·         G-protein receptor systems are extremely widespread and diverse in their functions.

°         They play important roles during embryonic development.

°         Vision and smell in humans depend on these proteins.

·         Similarities among G proteins and G-protein-linked receptors of modern organisms suggest that this signaling system evolved very early.

·         Several human diseases involve G-protein systems.

°         Bacterial infections causing cholera and botulism interfere with G-protein function.

·         The tyrosine-kinase receptor system is especially effective when the cell needs to trigger several signal transduction pathways and cellular responses at once.

°         This system helps the cell regulate and coordinate many aspects of cell growth and reproduction.

·         The tyrosine-kinase receptor belongs to a major class of plasma membrane receptors that have enzymatic activity.

°         A kinase is an enzyme that catalyzes the transfer of phosphate groups.

°         The cytoplasmic side of these receptors functions as a tyrosine kinase, transferring a phosphate group from ATP to tyrosine on a substrate protein.

·         An individual tyrosine-kinase receptor consists of several parts:

°         An extracellular signal-binding site.

°         A single alpha helix spanning the membrane.

°         An intracellular tail with several tyrosines.

·         The signal molecule binds to an individual receptor.

°         Ligands bind to two receptors, causing the two receptors to aggregate and form a dimer.

·         This dimerization activates the tyrosine-kinase section of the receptors, each of which then adds phosphate from ATP to the tyrosine tail of the other polypeptide.

·         The fully activated receptor proteins activate a variety of specific relay proteins that bind to specific phosphorylated tyrosine molecules.

°         One tyrosine-kinase receptor dimer may activate ten or more different intracellular proteins simultaneously.

°         These activated relay proteins trigger many different transduction pathways and responses.

·         A ligand-gated ion channel is a type of membrane receptor that can act as a gate when the receptor changes shape.

·         When a signal molecule binds as a ligand to the receptor protein, the gate opens to allow the flow of specific ions, such as Na+ or Ca2+, through a channel in the receptor.

°         Binding by a ligand to the extracellular side changes the protein’s shape and opens the channel.

°         When the ligand dissociates from the receptor protein, the channel closes.

·         The change in ion concentration within the cell may directly affect the activity of the cell.

·         Ligand-gated ion channels are very important in the nervous system.

°         For example, neurotransmitter molecules released at a synapse between two neurons bind as ligands to ion channels on the receiving cell, causing the channels to open.

°         Ions flow in and trigger an electrical signal that propagates down the length of the receiving cell.

·         Some gated ion channels respond to electrical signals, instead of ligands.

C. Signal-Transduction Pathways

·         The transduction stage of signaling is usually a multistep pathway.

·         These pathways often greatly amplify the signal.

°         If some molecules in a pathway transmit a signal to multiple molecules of the next component in the series, the result can be large numbers of activated molecules at the end of the pathway.

·         A small number of signal molecules can produce a large cellular response.

·         Also, multistep pathways provide more opportunities for coordination and regulation than do simpler systems.

1. Pathways relay signals from receptors to cellular responses.

·         Signal-transduction pathways act like falling dominoes.

°         The signal-activated receptor activates another protein, which activates another, and so on, until the protein that produces the final cellular response is activated.

·         The relay molecules that relay a signal from receptor to response are mostly proteins.

°         The interaction of proteins is a major theme of cell signaling.

°         Protein interaction is a unifying theme of all cellular regulation.

·         The original signal molecule is not passed along the pathway and may not even enter the cell.

°         It passes on information.

°         At each step, the signal is transduced into a different form, often by a conformational change in a protein.

°         The conformational change is often brought about by phosphorylation.

2. Protein phosphorylation, a common mode of regulation in cells, is a major mechanism of signal transduction.

·         The phosphorylation of proteins by a specific enzyme (a protein kinase) is a widespread cellular mechanism for regulating protein activity.

°         Most protein kinases act on other substrate proteins, unlike tyrosine kinases that act on themselves.

·         Most phosphorylation occurs at either serine or threonine amino acids of the substrate protein (unlike tyrosine phosphorylation in tyrosine kinases).

·         Many of the relay molecules in a signal-transduction pathway are protein kinases that act on other protein kinases to create a “phosphorylation cascade.”

·         Each protein phosphorylation leads to a conformational change because of the interaction between the newly added phosphate group and charged or polar amino acids on the protein.

·         Phosphorylation of a protein typically converts it from an inactive form to an active form.

°         Rarely, phosphorylation inactivates protein activity.

·         A single cell may have hundreds of different protein kinases, each specific for a different substrate protein.

°         Fully 2% of our genes are thought to code for protein kinases.

°         Together, they regulate a large proportion of the thousands of cell proteins.

·         Abnormal activity of protein kinases can cause abnormal cell growth and may contribute to the development of cancer.

·         The responsibility for turning off a signal-transduction pathway belongs to protein phosphatases.

°         These enzymes rapidly remove phosphate groups from proteins, a process called dephosphorylation.

°         Phosphatases also make the protein kinases available for reuse, enabling the cell to respond again to a signal.

·         At any given moment, the activity of a protein regulated by phosphorylation depends on the balance of active kinase molecules and active phosphatase molecules.

·         When the extracellular signal molecule is absent, active phosphatase molecules predominate, and the signaling pathway and cellular response are shut down.

·         The phosphorylation/dephosphorylation system acts as a molecular switch in the cell, turning activities on and off as required.

3. Certain signal molecules and ions are key components of signaling pathways (second messengers).

·         Many signaling pathways involve small, water-soluble, nonprotein molecules or ions called second messengers.

°         These molecules rapidly diffuse throughout the cell.

·         Second messengers participate in pathways initiated by both G-protein-linked receptors and tyrosine-kinase receptors.

°         Two of the most widely used second messengers are cyclic AMP and Ca2+.

·         Once Sutherland knew that epinephrine caused glycogen breakdown without entering the cell, he looked for a second messenger inside the cell.

·         Binding by epinephrine leads to increases in the cytosolic concentration of cyclic AMP, or cAMP.

°         This occurs because the activated receptor activates adenylyl cyclase, which converts ATP to cAMP.

°         The normal cellular concentration of cAMP can be boosted twentyfold within seconds.

°         cAMP is short-lived, as phosphodiesterase converts it to AMP.

°         Another surge of epinephrine is needed to reboost the cytosolic concentration of cAMP.

·         Caffeine-containing beverages such as coffee provide an artificial way to keep the body alert.

°         Caffeine blocks the conversion of cAMP to AMP, maintaining the system in a state of activation in the absence of epinephrine.

·         Many hormones and other signal molecules trigger the formation of cAMP.

°         G-protein-linked receptors, G proteins, and protein kinases are other components of cAMP pathways.

°         cAMP diffuses through the cell and activates a serine/threonine kinase called protein kinase A.

°         The activated kinase phosphorylates various other proteins.

·         Regulation of cell metabolism is also provided by G-protein systems that inhibit adenylyl cyclase.

°         These use a different signal molecule to activate a different receptor that activates an inhibitory G protein.

·         Certain microbes cause disease by disrupting G-protein signaling pathways.

°         The cholera bacterium, Vibrio cholerae, may be present in water contaminated with human feces.

°         This bacterium colonizes the small intestine and produces a toxin that modifies a G protein that regulates salt and water secretion.

°         The modified G protein is unable to hydrolyze GTP to GDP and remains stuck in its active form, continuously stimulating adenylyl cyclase to make cAMP.

°         The resulting high concentration of cAMP causes the intestinal cells to secrete large amounts of water and salts into the intestines, leading to profuse diarrhea and death from loss of water and salts.

·         Treatments for certain human conditions involve signaling pathways.

°         One pathway uses cyclic GMP, or cGMP, as a signaling molecule. Its effects include the relaxation of smooth muscle cells in artery walls.

°         A compound was developed to treat chest pains. This compound inhibits the hydrolysis of cGMP to GMP, prolonging the signal and increasing blood flow to the heart muscle.

°         Under the trade name Viagra, this compound is now widely used as a treatment for erectile dysfunction. Viagra causes dilation of blood vessels, allowing increased blood flow to the penis.

·         Many signal molecules in animals induce responses in their target cells via signal-transduction pathways that increase the cytosolic concentration of Ca2+.

°         In animal cells, increases in Ca2+ may cause contraction of muscle cells, secretion of certain substances, and cell division.

°         In plant cells, increases in Ca2+ trigger responses such as the pathway for greening in response to light.

·         Cells use Ca2+ as a second messenger in both G-protein pathways and tyrosine-kinase pathways.

·         The Ca2+ concentration in the cytosol is typically much lower than that outside the cell, often by a factor of 10,000 or more.

°         Various protein pumps transport Ca2+ outside the cell or into the endoplasmic reticulum or other organelles.

°         As a result, the concentration of Ca2+ in the ER is usually much higher than the concentration in the cytosol.

·         Because cytosolic Ca2+ is so low, small changes in the absolute numbers of ions causes a relatively large percentage change in Ca2+ concentration.

·         Signal-transduction pathways trigger the release of Ca2+ from the cell’s ER.

·         The pathways leading to release involve still other second messengers, diacylglycerol (DAG) and inositol trisphosphate (IP3).

°         DAG and IP3 are created when a phospholipase cleaves membrane phospholipid PIP2.

°         The phospholipase may be activated by a G protein or by a tyrosine-kinase receptor.

°         IP3 activates a gated-calcium channel, releasing Ca2+ from the ER.

·         Calcium ions activate the next protein in a signal-transduction pathway.

D. Cellular Responses to Signals

1. In response to a signal, a cell may regulate activities in the cytoplasm or transcription in the nucleus.

·         Ultimately, a signal-transduction pathway leads to the regulation of one or more cellular activities.

°         This may be the opening or closing of an ion channel or a change in cell metabolism.

°         For example, epinephrine helps regulate cellular energy metabolism by activating enzymes that catalyze the breakdown of glycogen.

·         The stimulation of glycogen breakdown by epinephrine involves a G-protein-linked receptor, a G protein, adenylyl cyclase, cAMP, and several protein kinases before glycogen phosphorylase is activated.

·         Other signaling pathways do not regulate the activity of enzymes but the synthesis of enzymes or other proteins.

·         Activated receptors may act as transcription factors that turn specific genes on or off in the nucleus.

2. Elaborate pathways amplify and specify the cell’s response to signals.

·         Signaling pathways with multiple steps have two benefits.

They amplify the response to a signal.

They contribute to the specificity of the response.

·         At each catalytic step in a cascade, the number of activated products is much greater than in the preceding step.

°         In the epinephrine-triggered pathway, binding by a small number of epinephrine molecules can lead to the release of hundreds of millions of glucose molecules.

·         Various types of cells may receive the same signal but produce very different responses.

°         For example, epinephrine triggers liver or striated muscle cells to break down glycogen, but stimulates cardiac muscle cells to contract, leading to a rapid heartbeat.

·         The explanation for this specificity is that different kinds of cells have different collections of proteins.

°         The response of a particular cell to a signal depends on its particular collection of receptor proteins, relay proteins, and proteins needed to carry out the response.

°         Two cells that respond differently to the same signal differ in one or more of the proteins that handle and respond to the signal.

·         A signal may trigger a single pathway in one cell but trigger a branched pathway in another.

·         Two pathways may converge to modulate a single response.

·         Branching of pathways and interactions between pathways are important for regulating and coordinating a cell’s response to incoming information.

·         Rather than relying on diffusion of large relay molecules such as proteins, many signal pathways are linked together physically by scaffolding proteins.

°         Scaffolding proteins may themselves be relay proteins to which several other relay proteins attach.

°         This hardwiring enhances the speed, accuracy, and efficiency of signal transfer between cells.

·         The importance of relay proteins that serve as branch or intersection points in signaling pathways is underscored when these proteins are defective or missing.

°         The inherited disorder Wiskott-Aldrich syndrome (WAS) is caused by the absence of a single relay protein.

°         Symptoms include abnormal bleeding, eczema, and a predisposition to infections and leukemia, due largely to the absence of the protein in the cells of the immune system.

°         The WAS protein is located just beneath the cell surface, where it interacts with the microfilaments of the cytoskeleton and with several signaling pathways, including those that regulate immune cell proliferation.

°         When the WAS protein is absent, the cytoskeleton is not properly organized and signaling pathways are disrupted.

·         As important as activating mechanisms are inactivation mechanisms.

°         For a cell to remain alert and capable of responding to incoming signals, each molecular change in its signaling pathways must last only a short time.

°         If signaling pathway components become locked into one state, whether active or inactive, the proper function of the cell can be disrupted.

°         Binding of signal molecules to receptors must be reversible, allowing the receptors to return to their inactive state when the signal is released.

°         Similarly, activated signals (cAMP and phosphorylated proteins) must be inactivated by appropriate enzymes to prepare the cell for a fresh signal.

Chapter 12    The Cell Cycle    Lecture Outline

Overview

·         The ability of organisms to reproduce their kind is the one characteristic that best distinguishes living things from nonliving matter.

·         The continuity of life is based on the reproduction of cells, or cell division.

A. The Key Roles of Cell Division

1. Cell division functions in reproduction, growth, and repair.

·         The division of a unicellular organism reproduces an entire organism, increasing the population.

·         Cell division on a larger scale can produce progeny for some multicellular organisms.

°         This includes organisms that can grow by cuttings.

·         Cell division enables a multicellular organism to develop from a single fertilized egg or zygote.

·         In a multicellular organism, cell division functions to repair and renew cells that die from normal wear and tear or accidents.

·         Cell division is part of the cell cycle, the life of a cell from its origin in the division of a parent cell until its own division into two.

2. Cell division results in genetically identical daughter cells.

·         Cell division requires the distribution of identical genetic material—DNA—to two daughter cells.

·         What is remarkable is the fidelity with which DNA is passed along, without dilution, from one generation to the next.

·         A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and then splits into two daughter cells.

·         A cell’s genetic information, packaged as DNA, is called its genome.

°         In prokaryotes, the genome is often a single long DNA molecule.

°         In eukaryotes, the genome consists of several DNA molecules.

·         A human cell must duplicate about 2 m of DNA and separate the two copies such that each daughter cell ends up with a complete genome.

·         DNA molecules are packaged into chromosomes.

°         Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.

§            Human somatic cells (body cells) have 46 chromosomes, made up of two sets of 23 (one from each parent).

§            Human gametes (sperm or eggs) have one set of 23 chromosomes, half the number in a somatic cell.

·         Eukaryotic chromosomes are made of chromatin, a complex of DNA and associated protein.

°         Each single chromosome contains one long, linear DNA molecule carrying hundreds or thousands of genes, the units that specify an organism’s inherited traits.

·         The associated proteins maintain the structure of the chromosome and help control gene activity.

·         When a cell is not dividing, each chromosome is in the form of a long, thin chromatin fiber.

·         Before cell division, chromatin condenses, coiling and folding to make a smaller package.

·         Each duplicated chromosome consists of two sister chromatids, which contain identical copies of the chromosome’s DNA.

°         The chromatids are initially attached by adhesive proteins along their lengths.

°         As the chromosomes condense, the region where the chromatids connect shrinks to a narrow area, the centromere.

·         Later in cell division, the sister chromatids are pulled apart and repackaged into two new nuclei at opposite ends of the parent cell.

°         Once the sister chromatids separate, they are considered individual chromosomes.

·         Mitosis, the formation of the two daughter nuclei, is usually followed by division of the cytoplasm, cytokinesis.

·         These processes start with one cell and produce two cells that are genetically identical to the original parent cell.

°         Each of us inherited 23 chromosomes from each parent: one set in an egg and one set in sperm.

°         The fertilized egg, or zygote, underwent cycles of mitosis and cytokinesis to produce a fully developed multicellular human made up of 200 trillion somatic cells.

°         These processes continue every day to replace dead and damaged cells.

°         Essentially, these processes produce clones—cells with identical genetic information.

·         In contrast, gametes (eggs or sperm) are produced only in gonads (ovaries or testes) by a variation of cell division called meiosis.

°         Meiosis yields four nonidentical daughter cells, each with half the chromosomes of the parent.

°         In humans, meiosis reduces the number of chromosomes from 46 to 23.

°         Fertilization fuses two gametes together and doubles the number of chromosomes to 46 again.

B. The Mitotic Cell Cycle

1. The mitotic phase alternates with interphase in the cell cycle.

·         The mitotic (M) phase of the cell cycle alternates with the much longer interphase.

°         The M phase includes mitosis and cytokinesis.

°         Interphase accounts for 90% of the cell cycle.

·         During interphase, the cell grows by producing proteins and cytoplasmic organelles, copies its chromosomes, and prepares for cell division.

·         Interphase has three subphases: the G1 phase (“first gap”), the S phase (“synthesis”), and the G2 phase (“second gap”).

°         During all three subphases, the cell grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum.

°         However, chromosomes are duplicated only during the S phase.

·         The daughter cells may then repeat the cycle.

·         A typical human cell might divide once every 24 hours.

°         Of this time, the M phase would last less than an hour, while the S phase might take 10–12 hours, or half the cycle.

°         The rest of the time would be divided between the G1 and G2 phases.

°         The G1 phase varies most in length from cell to cell.

·         Mitosis is a continuum of changes.

·         For convenience, mitosis is usually broken into five subphases: prophase, prometaphase, metaphase, anaphase, and telophase.

·         In late interphase, the chromosomes have been duplicated but are not condensed.

°         A nuclear membrane bounds the nucleus, which contains one or more nucleoli.

°         The centrosome has replicated to form two centrosomes.

°         In animal cells, each centrosome features two centrioles.

·         In prophase, the chromosomes are tightly coiled, with sister chromatids joined together.

°         The nucleoli disappear.

°         The mitotic spindle begins to form.

§            It is composed of centrosomes and the microtubules that extend from them.

°         The radial arrays of shorter microtubules that extend from the centrosomes are called asters.

°         The centrosomes move away from each other, apparently propelled by lengthening microtubules.

·         During prometaphase, the nuclear envelope fragments, and microtubules from the spindle interact with the condensed chromosomes.

°         Each of the two chromatids of a chromosome has a kinetochore, a specialized protein structure located at the centromere.

°         Kinetochore microtubules from each pole attach to one of two kinetochores.

°         Nonkinetochore microtubules interact with those from opposite ends of the spindle.

·         The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equidistant from the poles, defining metaphase.

·         At anaphase, the centromeres divide, separating the sister chromatids.

°         Each is now pulled toward the pole to which it is attached by spindle fibers.

°         By the end, the two poles have equivalent collections of chromosomes.

·         At telophase, daughter nuclei begin to form at the two poles.

°         Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system.

°         The chromosomes become less tightly coiled.

·         Cytokinesis, division of the cytoplasm, is usually well underway by late telophase.

°         In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two.

°         In plant cells, vesicles derived from the Golgi apparatus produce a cell plate at the middle of the cell.

2. The mitotic spindle distributes chromosomes to daughter cells: a closer look.

·         The mitotic spindle, fibers composed of microtubules and associated proteins, is a major driving force in mitosis.

·         As the spindle assembles during prophase, the elements come from partial disassembly of the cytoskeleton.

·         The spindle fibers elongate by incorporating more subunits of the protein tubulin.

·         Assembly of the spindle microtubules starts in the centrosome.

°         The centrosome (microtubule-organizing center) is a nonmembranous organelle that organizes the cell’s microtubules.

°         In animal cells, the centrosome has a pair of centrioles at the center, but the centrioles are not essential for cell division.

·         During interphase, the single centrosome replicates to form two centrosomes.

·         As mitosis starts, the two centrosomes are located near the nucleus.

°         As the spindle microtubules grow from them, the centrioles are pushed apart.

°         By the end of prometaphase, they are at opposite ends of the cell.

·         An aster, a radial array of short microtubules, extends from each centrosome.

·         The spindle includes the centrosomes, the spindle microtubules, and the asters.

·         Each sister chromatid has a kinetochore of proteins and chromosomal DNA at the centromere.

°         The kinetochores of the joined sister chromatids face in opposite directions.

·         During prometaphase, some spindle microtubules (called kinetochore microtubules) attach to the kinetochores.

·         When a chromosome’s kinetochore is “captured” by microtubules, the chromosome moves toward the pole from which those microtubules come.

·         When microtubules attach to the other pole, this movement stops and a tug-of-war ensues.

·         Eventually, the chromosome settles midway between the two poles of the cell, on the metaphase plate.

·         Nonkinetochore microtubules from opposite poles overlap and interact with each other.

·         By metaphase, the microtubules of the asters have grown and are in contact with the plasma membrane.

·         The spindle is now complete.

·         Anaphase commences when the proteins holding the sister chromatids together are inactivated.

°         Once the chromosomes are separate, full-fledged chromosomes, they move toward opposite poles of the cell.

·         How do the kinetochore microtubules function into the poleward movement of chromosomes?

·         One hypothesis is that the chromosomes are “reeled in” by the shortening of microtubules at the spindle poles.

·         Experimental evidence supports the hypothesis that motor proteins on the kinetochore “walk” the attached chromosome along the microtubule toward the nearest pole.

°         Meanwhile, the excess microtubule sections depolymerize at their kinetochore ends.

·         What is the function of the nonkinetochore microtubules?

·         Nonkinetochore microtubules are responsible for lengthening the cell along the axis defined by the poles.

°         These microtubules interdigitate and overlap across the metaphase plate.

°         During anaphase, the area of overlap is reduced as motor proteins attached to the microtubules walk them away from one another, using energy from ATP.

°         As microtubules push apart, the microtubules lengthen by the addition of new tubulin monomers to their overlapping ends, allowing continued overlap.

3. Cytokinesis divides the cytoplasm: a closer look.

·         Cytokinesis, division of the cytoplasm, typically follows mitosis.

·         In animal cells, cytokinesis occurs by a process called cleavage.

·         The first sign of cleavage is the appearance of a cleavage furrow in the cell surface near the old metaphase plate.

·         On the cytoplasmic side of the cleavage furrow is a contractile ring of actin microfilaments associated with molecules of the motor protein myosin.

°         Contraction of the ring pinches the cell in two.

·         Cytokinesis in plants, which have cell walls, involves a completely different mechanism.

·         During telophase, vesicles from the Golgi coalesce at the metaphase plate, forming a cell plate.

°         The plate enlarges until its membranes fuse with the plasma membrane at the perimeter.

°         The contents of the vesicles form new cell wall material between the daughter cells.

4. Mitosis in eukaryotes may have evolved from binary fission in bacteria.

·         Prokaryotes reproduce by binary fission, not mitosis.

·         Most bacterial genes are located on a single bacterial chromosome that consists of a circular DNA molecule and associated proteins.

·         While bacteria are smaller and simpler than eukaryotic cells, they still have large amounts of DNA that must be copied and distributed equally to two daughter cells.

·         The circular bacterial chromosome is highly folded and coiled in the cell.

·         In binary fission, chromosome replication begins at one point in the circular chromosome, the origin of replication site, producing two origins.

°         As the chromosome continues to replicate, one origin moves toward each end of the cell.

°         While the chromosome is replicating, the cell elongates.

°         When replication is complete, its plasma membrane grows inward to divide the parent cell into two daughter cells, each with a complete genome.

·         Researchers have developed methods to allow them to observe the movement of bacterial chromosomes.

°         The movement is similar to the poleward movements of the centromere regions of eukaryotic chromosomes.

°         However, bacterial chromosomes lack visible mitotic spindles or even microtubules.

·         The mechanism behind the movement of the bacterial chromosome is becoming clearer but is still not fully understood.

°         Several proteins have been identified and play important roles.

·         How did mitosis evolve?

°         There is evidence that mitosis had its origins in bacterial binary fission.

°         Some of the proteins involved in binary fission are related to eukaryotic proteins.

°         Two of these are related to eukaryotic tubulin and actin proteins.

·         As eukaryotes evolved, the ancestral process of binary fission gave rise to mitosis.

·         Possible intermediate evolutionary steps are seen in the division of two types of unicellular algae.

°         In dinoflagellates, replicated chromosomes are attached to the nuclear envelope.

°         In diatoms, the spindle develops within the nucleus.

·         In most eukaryotic cells, the nuclear envelope breaks down and a spindle separates the chromosomes.

C. Regulation of the Cell Cycle

·         The timing and rates of cell division in different parts of an animal or plant are crucial for normal growth, development, and maintenance.

·         The frequency of cell division varies with cell type.

°         Some human cells divide frequently throughout life (skin cells).

°         Others have the ability to divide, but keep it in reserve (liver cells).

°         Mature nerve and muscle cells do not appear to divide at all after maturity.

·         Investigation of the molecular mechanisms regulating these differences provide important insights into the operation of normal cells, and may also explain cancer cells escape controls.

1. Cytoplasmic signals drive the cell cycle.

·         The cell cycle appears to be driven by specific chemical signals present in the cytoplasm.

·         Some of the initial evidence for this hypothesis came from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei.

°         Fusion of an S phase cell and a G1 phase cell induces the G1 nucleus to start S phase.

§            This suggests that chemicals present in the S phase nucleus stimulated the fused cell.

°         Fusion of a cell in mitosis (M phase) with one in interphase (even G1 phase) induces the second cell to enter mitosis.

·         The sequential events of the cell cycle are directed by a distinct cell cycle control system.

°         Cyclically operating molecules trigger and coordinate key events in the cell cycle.

°         The control cycle has a built-in clock, but it is also regulated by external adjustments and internal controls.

·         A checkpoint in the cell cycle is a critical control point where stop and go-ahead signals regulate the cycle.

°         The signals are transmitted within the cell by signal transduction pathways.

°         Animal cells generally have built-in stop signals that halt the cell cycle at checkpoints until overridden by go-ahead signals.

°         Many signals registered at checkpoints come from cellular surveillance mechanisms.

°         These indicate whether key cellular processes have been completed correctly.

°         Checkpoints also register signals from outside the cell.

·         Three major checkpoints are found in the G1, G2, and M phases.

·         For many cells, the G1 checkpoint, the “restriction point” in mammalian cells, is the most important.

°         If the cell receives a go-ahead signal at the G1 checkpoint, it usually completes the cell cycle and divides.

°         If it does not receive a go-ahead signal, the cell exits the cycle and switches to a nondividing state, the G0 phase.

§            Most cells in the human body are in this phase.

§            Liver cells can be “called back” to the cell cycle by external cues, such as growth factors released during injury.

§            Highly specialized nerve and muscle cells never divide.

·         Rhythmic fluctuations in the abundance and activity of cell cycle control molecules pace the events of the cell cycle.

°         These regulatory molecules include protein kinases that activate or deactivate other proteins by phosphorylating them.

·         These kinases are present in constant amounts but require attachment of a second protein, a cyclin, to become activated.

°         Levels of cyclin proteins fluctuate cyclically.

°         Because of the requirement for binding of a cyclin, the kinases are called cyclin-dependent kinases, or Cdks.

·         Cyclin levels rise sharply throughout interphase, and then fall abruptly during mitosis.

·         Peaks in the activity of one cyclin-Cdk complex, MPF, correspond to peaks in cyclin concentration.

·         MPF (“maturation-promoting factor” or “M-phase-promoting-factor”) triggers the cell’s passage past the G2 checkpoint to the M phase.

°         MPF promotes mitosis by phosphorylating a variety of other protein kinases.

°         MPF stimulates fragmentation of the nuclear envelope by phosphorylation of various proteins of the nuclear lamina.

°         It also triggers the breakdown of cyclin, dropping cyclin and MPF levels during mitosis and inactivating MPF.

§            The noncyclin part of MPF, the Cdk, persists in the cell in inactive form until it associates with new cyclin molecules synthesized during the S and G2 phases of the next round of the cycle.

·         At least three Cdk proteins and several cyclins regulate the key G1 checkpoint.

·         Similar mechanisms are also involved in driving the cell cycle past the M phase checkpoint.

2. Internal and external cues help regulate the cell cycle.

·         While research scientists know that active Cdks function by phosphorylating proteins, the identity of all these proteins is still under investigation.

·         Scientists do not yet know what Cdks actually do in most cases.

·         Some steps in the signaling pathways that regulate the cell cycle are clear.

°         Some signals originate inside the cell, others outside.

·         The M phase checkpoint ensures that all the chromosomes are properly attached to the spindle at the metaphase plate before anaphase.

°         This ensures that daughter cells do not end up with missing or extra chromosomes.

·         A signal to delay anaphase originates at kinetochores that have not yet attached to spindle microtubules.

°         This keeps the anaphase-promoting complex (APC) in an inactive state.

°         When all kinetochores are attached, the APC activates, triggering breakdown of cyclin and inactivation of proteins holding sister chromatids together.

·         A variety of external chemical and physical factors can influence cell division.

°         For example, cells fail to divide if an essential nutrient is left out of the culture medium.

·         Particularly important for mammalian cells are growth factors, proteins released by one group of cells that stimulate other cells to divide.

°         For example, platelet-derived growth factors (PDGF), produced by platelet blood cells, bind to tyrosine-kinase receptors of fibroblasts, a type of connective tissue cell.

°         This triggers a signal-transduction pathway that allows cells to pass the G1 checkpoint and divide.

·         Each cell type probably responds specifically to a certain growth factor or combination of factors.

·         The role of PDGF is easily seen in cell culture.

°         Fibroblasts in culture will only divide in the presence of a medium that also contains PDGF.

·         In a living organism, platelets release PDGF in the vicinity of an injury.

°         The resulting proliferation of fibroblasts helps heal the wound.

·         At least 50 different growth factors can trigger specific cells to divide.

·         The effect of an external physical factor on cell division can be seen in density-dependent inhibition of cell division.

°         Cultured cells normally divide until they form a single layer on the inner surface of the culture container.

°         If a gap is created, the cells will grow to fill the gap.

°         At high densities, the amount of growth factors and nutrients is insufficient to allow continued cell growth.

·         Most animal cells also exhibit anchorage dependence for cell division.

°         To divide, they must be anchored to a substratum, typically the extracellular matrix of a tissue.

°         Control appears to be mediated by pathways involving plasma membrane proteins and elements of the cytoskeleton linked to them.

·         Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence.

3. Cancer cells have escaped from cell cycle controls.

·         Cancer cells divide excessively and invade other tissues because they are free of the body’s control mechanisms.

°         Cancer cells do not stop dividing when growth factors are depleted.

°         This is either because a cancer cell manufactures its own growth factors, has an abnormality in the signaling pathway, or has an abnormal cell cycle control system.

·         If and when cancer cells stop dividing, they do so at random points, not at the normal checkpoints in the cell cycle.

·         Cancer cells may divide indefinitely if they have a continual supply of nutrients.

°         In contrast, nearly all mammalian cells divide 20 to 50 times under culture conditions before they stop, age, and die.

·         Cancer cells may be “immortal.”

°         HeLa cells from a tumor removed from a woman (Henrietta Lacks) in 1951 are still reproducing in culture.

·         The abnormal behavior of cancer cells begins when a single cell in a tissue undergoes a transformation that converts it from a normal cell to a cancer cell.

°         Normally, the immune system recognizes and destroys transformed cells.

°         However, cells that evade destruction proliferate to form a tumor, a mass of abnormal cells.

·         If the abnormal cells remain at the originating site, the lump is called a benign tumor.

°         Most do not cause serious problems and can be fully removed by surgery.

·         In a malignant tumor, the cells become invasive enough to impair the functions of one or more organs.

·         In addition to chromosomal and metabolic abnormalities, cancer cells often lose attachment to nearby cells, are carried by the blood and lymph system to other tissues, and start more tumors in an event called metastasis.

°         Cancer cells are abnormal in many ways.

°         They may have an unusual number of chromosomes, their metabolism may be disabled, and they may cease to function in any constructive way.

°         Cancer cells may secrete signal molecules that cause blood vessels to grow toward the tumor.

·         Treatments for metastasizing cancers include high-energy radiation and chemotherapy with toxic drugs.

°         These treatments target actively dividing cells.

°         Chemotherapeutic drugs interfere with specific steps in the cell cycle.

°         For example, Taxol prevents mitotic depolymerization, preventing cells from proceeding past metaphase.

°         The side effects of chemotherapy are due to the drug’s effects on normal cells.

·         Researchers are beginning to understand how a normal cell is transformed into a cancer cell.

°         The causes are diverse, but cellular transformation always involves the alteration of genes that influence the cell cycle control system.

Cell Reproduction Lecture Guide

SECTION 8-1    CHROMOSOMES

DNA stores?                        Estimated length?

Coiled DNA in eukaryote nucleus called?

Chromosome shape?                              Made of?

Can be seen inside nucleus by?

Histones?

Function of histones?

Function of nonhistone proteins?

Sister chromatids?

When form?

Centromere?

Function of centromere?

Sketch sister chromatids & label centromere.

Prokaryotic chromosomes?

Shape?                         Number?                     Location & attachment?

Number of chromosomes in human body cells?               Called what?

How abbreviated?

Are all diploid numbers in organisms the same?   Explain and give examples.

Human body cells called what?               Examples?

Reproductive cells are called?                        Name them.

Chromosome number of gametes?                              Abbreviation?

Haploid number also called?                                      Haploid number for humans?

Fertilization?

Chromosome number that fertilization restores?

Fertilized egg called?                          Sets of chromosomes in zygote?

Chromosomes in egg and sperm called?             Name them.

Sex chromosomes of female?                                 Male?

Other 22 pairs or 44  chromosomes called?

Karyotype?

Homologous pairs of chromosomes?

Example of information contained in homologs.

SECTION 8-2    CELL DIVISION

All cells come from?                          Process called?                     

Same in prokaryotes & eukaryotes?

Binary fission?

Used by?                               Number of steps or stages.

Stage 1 of binary fission?

Stage 2 of binary fisssion?

Stage 3 of binary fission?

Is binary fission sexual or asexual reproduction?

Original cell that forms eukaryotes is called?

How new cells compare to each other & the original cell after cell division? Why?

Phases cell goes through in its life cycle called?

Number of phases?                  Name them.

Two parts of cell division?

Mitosis?

Interphase?

Also called?                          Length in cell cycle?

What’s occurring to cells in interphase?

Number of phases in interphase?                   Name them.

G1 phase?

S phase?

G2 phase?

Replication?

Results in forming?                                            Occurs when?

Why all new cells must have exact copy of DNA?

Daughter cells?

How form?                                       Compare to each other?

Two steps of cell division called?

Another name for mitosis?                                    What’s dividing?

Division of the cytoplasm called?                          When occurs?

Parent cell?

How multicellular organisms grow?

Number of steps or phases in mitosis?            Name them in order.

What’s made during mitosis?

When did the chromosomes replicate (make copies of the DNA)?

Prophase?

Chromatin condenses into what?                                 Held together by?

Two structures that disappear in prophase?

Centrosomes located near?                                        Number of centrosomes?

Contain what cylindrical bodies?                                 Found in plant &/or animal cells?
Made of bundles of?                                                   Where centrosomes move?

Help form?

Function of mitotic spindle?

Two types of spindle fibers?

Attach to centromere of sister chromatids?                                          Function?

Metaphase?

             Where are chromosomes moved?

What moves the chromosomes?

Center of cell called?                                        Ends of cell called?

Anaphase?

What happens to sister chromatids?

Once chromatids separate, they’re now individual what?

Telophase?

What happens to spindle fibers?

Chromosomes again tightly coil becoming what?

What two structures reform?

Cytokinesis?

How occurs in animal cells?

How occurs in plant cells?

How many new cells formed?                                    Cells called?

Size of new cells to each other?                   Size of new cells & parent cell?

Daughter cells & parent cell genetically identical or different?

Is mitosis sexual or asexual reproduction?

SECTION 8-3    MEIOSIS

Meiosis?

What happens to chromosome number?

Cells produced by meiosis are called?              Their chromosome number?

Fusion of gametes?                                                      Effect on chromosome number?

Number of chromosomes in human egg?               Sperm?            Zygote?

Sexual reproduction?

Combines what 2 cells?                                                Forms what cell?

Eggs?

Sperm?

How sperm reaches nonmotile egg?

Gametes produced by what process?

Where occurs in females?                                            In males?

What called in females?                                               In males?

Diploid egg or sperm after meiosis have what chromosome number?

How do daughter cells made in meiosis compare to the original cell?

How many divisions do cells undergo during meiosis?

How many new cells are produced?

How many main stages are there in meiosis?                                   Name them.

What occurs in Meiosis I?

What occurs in Meiosis II?

Chromosome number at the beginning of Meiosis I?

Homolog?

Synapsis?

Pair of homologs after synapsis called?                                    Sketch a tetrad.

First step in Meiosis I called?

            Are chromosomes visible?

Chromosome number in meiosis I?

Genes?

Crossing over?

Genetic recombination?

What 2 structures disappear?

What structure appears & attaches to homologs?

Where are tetrads moved during Metaphase I?

What happens to homologs in Anaphase I?

            What separates the homologs?

Random separation of homologous chromosomes called?

What happens to cytoplasm during Telophase I?

Chromosome number of new cells?                             How many new cells formed?

Do chromosomes replicate before Meiosis II?

            Name the 4 steps in Meiosis II.

How many new cells form in males?                          In females?

Polar bodies?

What usually happens to polar bodies? Why?

New cells in females called?                                      Cells after maturing called?

New cells in males called?                                         Cells after maturing called?

Chromosome number of new cells?

Evolution?

Which type of reproduction causes change in organisms?

Reproduction involving one parent?                                               Give 3 examples.

Chromosome number of parent & new cells?

Reproduction involving two parents?

Chromosome number of parent cell?                           Chromosome number of new cells?

Are organisms in a sexually reproducing population genetically identical?

Variations?

“Survival of the fittest”?

How environmental changes affect asexually reproducing organisms?        Sexually reproducing organisms?