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#### Question 17–11
Which of the following statements are correct? Explain your answers.
- A. Kinesin moves endoplasmic reticulum membranes along microtubules so that the network of ER tubules becomes stretched throughout the cell.
- B. Without actin, cells can form a functional mitotic spindle and pull their chrom... | {
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"Header 3": "Questions",
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There are no known motor proteins that move on intermediate filaments. Suggest an explanation for this.
#### Question 17–16
When cells enter mitosis, their existing array of cytoplasmic microtubules has to be rapidly broken down and replaced with the mitotic spindle that forms to pull the chromosomes into the daugh... | {
"Header 1": "Different Types of Muscle Cells Perform Different Functions",
"Header 3": "Question 17–15",
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The electron micrographs shown in Figure Q17–20A were obtained from a population of microtubules that were growing rapidly. Figure Q17–20B was obtained from microtubules undergoing "catastrophic" shrinking. Comment on any differences between A and B, and suggest likely explanations for the differences that you observe.... | {
"Header 1": "Different Types of Muscle Cells Perform Different Functions",
"Header 3": "Question 17–20",
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"Where a cell arises, there must be a previous cell, just as animals can only arise from animals and plants from plants." This statement, which appears in a book written by German pathologist Rudolf Virchow in 1858, carries with it a profound message for the continuity of life. If every cell comes from a previous cell,... | {
"Header 1": "The Cell-Division Cycle",
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Consider the following statement: "All present-day cells have arisen by an uninterrupted series of cell divisions extending back in time to the first cell division." Is this strictly true?
eukaryotic cell manufactures other components, such as proteins, membranes, organelles, and cytoskeletal filaments. In this chapt... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "Question 18–1",
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The most basic function of the cell cycle is to duplicate accurately the vast amount of DNA in the chromosomes and then to segregate the DNA into genetically identical daughter cells such that each cell receives a complete copy of the entire genome (Figure 18–1). In most cases, a cell also duplicates its other macromol... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "Overview of the Cell Cycle",
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Seen in a microscope, the two most dramatic events in the cell cycle are when the nucleus divides, a process called *mitosis*, and when the cell later splits in two, a process called *cytokinesis*. These two processes together constitute the **M phase** of the cycle. In a typical mammalian cell, the whole of M phase ta... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "The Eukaryotic Cell Cycle Usually Includes Four Phases",
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A population of proliferating cells is stained with a dye that becomes fluorescent when it binds to DNA, so that the amount of fluorescence is directly proportional to the amount of DNA in each cell. To measure the amount of DNA in each cell, the cells are then passed through a flow cytometer, an instrument that measur... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "Question 18–2",
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To ensure that they replicate all their DNA and organelles, and divide in an orderly manner, eukaryotic cells possess a complex network of regulatory proteins known as the *cell-cycle control system*. This system guarantees that the events of the cell cycle—DNA replication, mitosis, and so on—occur in a set sequence an... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "A Cell-Cycle Control System Triggers the Major Processes of the Cell Cycle",
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Some features of the cell cycle, including the time required to complete certain events, vary greatly from one cell type to another, even within the same organism. The basic organization of the cycle, however, is essentially the same in all eukaryotic cells, and all eukaryotes appear to use similar machinery and contro... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "Cell-Cycle Control is Similar in All Eukaryotes",
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The cell-cycle control system governs the cell-cycle machinery by cyclically activating and then inactivating the key proteins and protein
ECB4 e18.04/18.04 Figure 18–4 Progression through the cell cycle depends on cyclin-dependent protein kinases (Cdks). A Cdk must bind a regulatory p... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "The Cell-Cycle Control System Depends on Cyclically Activated Protein Kinases called Cdks",
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The fertilized eggs of many animals are especially suitable for biochemical studies of the cell cycle because they are exceptionally large and divide rapidly. An egg of the frog *Xenopus*, for example, is just over 1 mm in diameter (Figure 18–6). After fertilization, it divides rapidly to partition the egg into many sm... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "Back to the egg",
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In such experiments, Kazuo Matsui and colleagues found that an extract from an M-phase egg instantly drives the oocyte into M phase, whereas cytoplasm from a cleaving egg at other phases of the cycle does not. When they first made this discovery, they did not know the molecules or the mechanism responsible, so they ref... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "Give us an M",
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While biochemists were identifying the proteins that regulate the cell cycles of frog and clam embryos, yeast geneticists—led by Lee Hartwell, studying baker's yeast (*S. cerevisiae*), and Paul Nurse, studying fission yeast
(S. pombe)—were taking a genetic approach to dissecting the cell-cycle control system. By stud... | {
"Header 1": "The Cell-Division Cycle",
"Header 3": "All in the family",
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As discussed in Chapter 7, the concentration of a given protein in the cell is determined by the rate at which the protein is synthesized and the rate at which it is degraded. Over the course of the cell cycle, the concentration of each type of cyclin rises gradually and then falls abruptly (see Figure 18–8). The gradu... | {
"Header 1": "Cyclin Concentrations are Regulated by Transcription and by Proteolysis",
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As mentioned earlier, the cell-cycle control system can transiently delay progress through the cycle at various transition points to ensure that the major events of the cycle occur in a specific order. At these transitions,
Figure 18–10 For M-Cdk to be active, inhibitory phosphates must be removed.
As soon as the M... | {
"Header 1": "The Cell-Cycle Control System Can Pause the Cycle in Various Ways",
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In addition to being a bustling period of metabolic activity, cell growth, and repair, $G_1$ is an important point of decision-making for the cell. Based on intracellular signals that provide information about the size of the cell and extracellular signals reflecting the environment, the cell-cycle control machinery ... | {
"Header 1": "The Cell-Cycle Control System Can Pause the Cycle in Various Ways",
"Header 3": "**G<sub>1</sub> PHASE**",
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During M phase, when cells are actively dividing, the cell is awash with active cyclin–Cdk complexes. If those S-Cdks and M-Cdks are not disabled by the end of M phase, the cell will immediately replicate its DNA and initiate another round of division, without spending any significant time in the G1 or G2 phases. Such ... | {
"Header 1": "The Cell-Cycle Control System Can Pause the Cycle in Various Ways",
"Header 3": "Cdks are Stably Inactivated in G1",
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As a general rule, mammalian cells will multiply only if they are stimulated to do so by extracellular signals, called *mitogens*, produced by other cells. If deprived of such signals, the cell cycle arrests in G1; if the cell is deprived of mitogens for long enough, it will withdraw from the cell cycle and enter a non... | {
"Header 1": "The Cell-Cycle Control System Can Pause the Cycle in Various Ways",
"Header 3": "Mitogens Promote the Production of the Cyclins that Stimulate Cell Division",
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The cell-cycle control system uses several distinct mechanisms to halt progress through the cell cycle if DNA is damaged. It can do so at various transition points from one phase of the cell cycle to the next. The mechanism that operates at the G<sub>1</sub>-to-S transition, which prevents the cell from replicating dam... | {
"Header 1": "DNA Damage Can Temporarily Halt Progression Through G<sub>1</sub>",
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As mentioned earlier, cells can delay progress through the cell cycle at specific transition points, to wait for suitable conditions or to repair damaged DNA. They can also withdraw from the cell cycle for prolonged periods—either temporarily or permanently.
The most radical decision that the cell-cycle control syste... | {
"Header 1": "Cells Can Delay Division for Prolonged Periods by Entering Specialized Nondividing States",
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Like any monumental task, configuring chromosomes for replication requires a certain amount of preparation. For eukaryotic cells, this preparation begins early in G1, when DNA is made replication-ready by the recruitment of proteins to the sites along each chromosome where replication will begin. These nucleotide seque... | {
"Header 1": "Cells Can Delay Division for Prolonged Periods by Entering Specialized Nondividing States",
"Header 3": "S-Cdk Initiates DNA Replication and Blocks Re-Replication",
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Earlier, we described how DNA damage can signal the cell-cycle control system to delay progress through the G1-to-S transition, preventing the cell from replicating damaged DNA. But what if errors occur during DNA replication—or if replication is delayed? How does the cell keep from dividing with DNA that is incorrectl... | {
"Header 1": "Cells Can Delay Division for Prolonged Periods by Entering Specialized Nondividing States",
"Header 3": "Incomplete Replication Can Arrest the Cell Cycle in G2",
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Although M phase (mitosis plus cytokinesis) occurs over a relatively short amount of time—about one hour in a mammalian cell that divides once a day, or even once a year—it is by far the most dramatic phase of the cell cycle. During this brief period, the cell reorganizes virtually all of its components and distributes... | {
"Header 1": "Cells Can Delay Division for Prolonged Periods by Entering Specialized Nondividing States",
"Header 3": "M Phase",
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One of the most remarkable features of the cell-cycle control system is that a single protein complex, M-Cdk, brings about all the diverse and intricate rearrangements that occur in the early stages of mitosis. Among its many duties, M-Cdk helps prepare the duplicated chromosomes for segregation and induces the assembl... | {
"Header 1": "Cells Can Delay Division for Prolonged Periods by Entering Specialized Nondividing States",
"Header 3": "M-Cdk Drives Entry Into M Phase and Mitosis",
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When the cell enters M phase, the duplicated chromosomes condense, becoming visible under the microscope as threadlike structures. Protein complexes, called **condensins**, help carry out this **chromosome condensation**, which reduces mitotic chromosomes to compact bodies that can be more easily segregated within the ... | {
"Header 1": "Cohesins and Condensins Help Configure Duplicated Chromosomes for Separation",
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After the duplicated chromosomes have condensed, two complex cytoskeletal machines assemble in sequence to carry out the two mechanical processes that occur in M phase. The mitotic spindle carries out nuclear division (mitosis), and, in animal cells and many unicellular eukaryotes, the *contractile ring* carries out cy... | {
"Header 1": "Different Cytoskeletal Assemblies Carry Out Mitosis and Cytokinesis",
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Although M phase proceeds as a continuous sequence of events, it is traditionally divided into a series of stages. The first five stages of M phase—prophase, prometaphase, metaphase, anaphase, and telophase constitute mitosis, which was originally defined as the period in which the chromosomes are visible (because they... | {
"Header 1": "Different Cytoskeletal Assemblies Carry Out Mitosis and Cytokinesis",
"Header 3": "M Phase Occurs in Stages",
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Before M phase begins, two critical events must be completed: DNA must be fully replicated, and, in animal cells, the centrosome must be duplicated. The **centrosome** is the principal *microtubule-organizing center* in animal cells. It duplicates so that it can help form the two poles of the mitotic spindle and so tha... | {
"Header 1": "Centrosomes Duplicate To Help Form the Two Poles of the Mitotic Spindle",
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In the light micrographs of dividing animal cells shown in this panel, chromosomes are stained *orange* and microtubules are *green*.
(Micrographs courtesy of Julie Canman and Ted Salmon; "Metaphase" from cover of *J. Cell. Sci.* 115(9), 2002. With permission from The Company of Biologists Ltd; "Telophase" from J.C. ... | {
"Header 1": "Centrosomes Duplicate To Help Form the Two Poles of the Mitotic Spindle",
"Header 3": "microtubules cytosol plasma membrane duplicated centrosome nuclear envelope decondensed chromosomes in nucleus During interphase, the cell increases in size. The DNA of the chromosomes is replicated, and the centro... |
MITOSIS
Prometaphase starts abruptly with the breakdown of the nuclear envelope. Chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement.
time = 79 min
At metaphase, the chromosomes are aligned at the equ... | {
"Header 1": "Centrosomes Duplicate To Help Form the Two Poles of the Mitotic Spindle",
"Header 2": "2 PROMETAPHASE spindle pole kinetochore microtubule chromosome in motion fragments of nuclear envelope",
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CYTOKINESIS completed nuclear envelope surrounds decondensing chromosomes contractile ring creating cleavage furrow
During cytokinesis of an animal cell, the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches the cell into two daughters, each with one nucleus.
re-formatio... | {
"Header 1": "CYTOKINESIS",
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The mitotic spindle begins to form in prophase. This assembly of the highly dynamic spindle depends on the remarkable properties of microtubules. As discussed in Chapter 17, microtubules continuously polymerize and depolymerize by the addition and loss of their tubulin subunits, and individual filaments alternate betwe... | {
"Header 1": "CYTOKINESIS",
"Header 3": "The Mitotic Spindle Starts to Assemble in Prophase",
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Prometaphase starts abruptly with the disassembly of the nuclear envelope, which breaks up into small membrane vesicles. This process is triggered by the phosphorylation and consequent disassembly of nuclear pore proteins and the intermediate filament proteins of the nuclear lamina, the network of fibrous proteins that... | {
"Header 1": "CYTOKINESIS",
"Header 3": "Chromosomes Attach to the Mitotic Spindle at Prometaphase",
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(A) A fluorescence micrograph of a duplicated mitotic chromosome. The DNA is stained with a fluorescent dye, and the kinetochores are stained *red* with fluorescent antibodies that recognize kinetochore proteins. These antibodies come from patients with scle... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
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During prometaphase, the duplicated chromosomes, now attached to the mitotic spindle, begin to move about, as if jerked first this way and then that. Eventually, they align at the equator of the spindle, halfway between the two spindle poles, thereby forming the *metaphase plate*. This event defines the beginning of me... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "Chromosomes Line Up at the Spindle Equator at Metaphase",
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Anaphase begins abruptly with the breakage of the cohesin linkages that hold sister chromatids together (see Figure 18–18A). This release allows each chromatid—now considered a chromosome—to be pulled to the spindle pole to which it is attached (Figure 18–27). This movement segregates the two identical sets of chromoso... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "Proteolysis Triggers Sister-Chromatid Separation at Anaphase",
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If fine glass needles are used to manipulate a chromosome inside a living cell during early M phase, it is possible to trick the kinetochores on the two sister chromatids into attaching to the same spindle pole. This arrangement is normally unstable, but the attachments can be stabilized if the needle is used to gently... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "Question 18–6",
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If a dividing cell were to begin to segregate its chromosomes before all the chromosomes were properly attached to the spindle, one daughter cell would receive an incomplete set of chromosomes, while the other would receive a surplus. Both situations could be lethal. Thus, a dividing cell must ensure that every last ch... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "An Unattached Chromosome Will Prevent Sister-Chromatid Separation",
"token_count": 212,
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By the end of anaphase, the daughter chromosomes have separated into two equal groups, one at each pole of the spindle. During telophase, the final stage of mitosis, the mitotic spindle disassembles, and a nuclear envelope reassembles around each group of chromosomes to form the two daughter nuclei (Movie 18.8). Vesicl... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "The Nuclear Envelope Re-forms at Telophase",
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The first visible sign of cytokinesis in animal cells is a puckering and furrowing of the plasma membrane that occurs during anaphase (Figure 18–31). The furrowing invariably occurs in a plane that runs perpendicular to the long axis of the mitotic spindle. This positioning ensures that the *cleavage furrow* cuts betwe... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "The Mitotic Spindle Determines the Plane of Cytoplasmic Cleavage",
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The contractile ring is composed mainly of an overlapping array of actin filaments and myosin filaments (Figure 18–32). It assembles at anaphase and is attached to membrane-associated proteins on the cytoplasmic face of the plasma membrane. Once assembled, the contractile ring is capable of exerting a force strong enou... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "The Contractile Ring of Animal Cells Is Made of Actin and Myosin Filaments",
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The mechanism of cytokinesis in higher plants is entirely different from that in animal cells, presumably because plant cells are surrounded by a tough cell wall (discussed in Chapter 20). The two daughter cells are separated not by the action of a contractile ring at the cell surface but instead by the construction of... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "Cytokinesis in Plant Cells Involves the Formation of a New Cell Wall",
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Organelles such as mitochondria and chloroplasts cannot assemble spontaneously from their individual components; they arise only from the growth and division of the preexisting organelles. Likewise, endoplasmic reticulum (ER) and Golgi apparatus also derive from preexisting organelle fragments. How, then, are these var... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "Membrane-Enclosed Organelles Must Be Distributed to Daughter Cells When a Cell Divides",
"token_count": 327,
... |
A fertilized mouse egg and a fertilized human egg are similar in size—about 100 µm in diameter. Yet an adult mouse is much smaller than an adult human. What are the differences between the control of cell behavior in humans and mice that generate such big differences in size? The same fundamental question can be asked ... | {
"Header 1": "CYTOKINESIS",
"Header 2": "bules that form the mitotic spindle are differently colored in Figure 18–24. spindle pole duplicated chromosome (sister chromatids) kinetochore",
"Header 3": "CONTROL OF CELL NUMBERS AND CELL SIZE",
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At the beginning of telophase, after the chromosomes have segregated, a new cell wall starts to assemble inside the cell at the equator of the old spindle (A). The interpolar microtubules of the mitotic spindle remaining at telophase form the phragmoplast and guide vesicles, derived from the Golgi apparatus, toward the... | {
"Header 1": "Figure 18–34 Cytokinesis in a plant cell is guided by a specialized microtubule-based structure called the phragmoplast.",
"token_count": 275,
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The cells of a multicellular organism are members of a highly organized community. The number of cells in this community is tightly regulated not simply by controlling the rate of cell division, but also by controlling the rate of cell death. If cells are no longer needed, they can commit suicide by activating an intra... | {
"Header 1": "Figure 18–34 Cytokinesis in a plant cell is guided by a specialized microtubule-based structure called the phragmoplast.",
"Header 3": "Apoptosis Helps Regulate Animal Cell Numbers",
"token_count": 465,
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Cells that die as a result of acute injury typically swell and burst, spilling their contents all over their neighbors, a process called *cell necrosis* (Figure 18–37A). This eruption triggers a potentially damaging inflammatory response. By contrast, a cell that undergoes apoptosis dies neatly, without damaging its n... | {
"Header 1": "Figure 18–34 Cytokinesis in a plant cell is guided by a specialized microtubule-based structure called the phragmoplast.",
"Header 3": "Apoptosis Is Mediated by an Intracellular Proteolytic Cascade",
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All nucleated animal cells contain the seeds of their own destruction: in these cells, inactive procaspases lie waiting for a signal to destroy the cell. It is therefore not surprising that caspase activity is tightly regulated inside the cell to ensure that the death program is held in check until it is needed—for exa... | {
"Header 1": "Figure 18–34 Cytokinesis in a plant cell is guided by a specialized microtubule-based structure called the phragmoplast.",
"Header 3": "The Intrinsic Apoptotic Death Program Is Regulated by the Bcl2 Family of Intracellular Proteins",
"token_count": 356,
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Sometimes the signal to commit suicide is not generated internally, but instead comes from a neighboring cell. Some of these extracellular signals activate the cell death program by affecting the activity of members of the Bcl2 family of proteins. Others stimulate apoptosis more directly by activating a set of cell-sur... | {
"Header 1": "Figure 18–34 Cytokinesis in a plant cell is guided by a specialized microtubule-based structure called the phragmoplast.",
"Header 3": "Extracellular Signals Can Also Induce Apoptosis",
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... |
In a multicellular organism, the fate of individual cells is controlled by signals from other cells. For either tissue growth or cell replacement, cells must grow before they divide. Nutrients are not enough for an animal cell to survive, grow, or divide. It must also receive chemical signals from other cells, usually ... | {
"Header 1": "Animal Cells Require Extracellular Signals to Survive, Grow, and Divide",
"token_count": 709,
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Animal cells need signals from other cells just to survive. If deprived of such survival factors, cells activate a caspase-dependent intracellular suicide program and die by apoptosis. This requirement for signals from other cells helps ensure that cells survive only when and where they are needed. Many types of nerve ... | {
"Header 1": "Animal Cells Require Extracellular Signals to Survive, Grow, and Divide",
"Header 3": "Survival Factors Suppress Apoptosis",
"token_count": 366,
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} |
Most mitogens are secreted signal proteins that bind to cell-surface receptors. When activated by mitogen binding, these receptors initiate various intracellular signaling pathways (discussed in Chapter 16) that stimulate cell division. As we saw earlier, these signaling pathways act mainly by releasing the molecular b... | {
"Header 1": "Mitogens Stimulate Cell Division by Promoting Entry into S Phase",
"token_count": 708,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The extracellular signal proteins that we have discussed so far—survival factors, mitogens, and growth factors—act positively to increase the size of organs and organisms. Some extracellular signal proteins, however, act to oppose these positive regulators and thereby inhibit tissue growth. *Myostatin*, for example, is... | {
"Header 1": "Mitogens Stimulate Cell Division by Promoting Entry into S Phase",
"Header 3": "Some Extracellular Signal Proteins Inhibit Cell Survival, Division, or Growth",
"token_count": 702,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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- • The eukaryotic cell cycle consists of several distinct phases. In interphase, the cell grows and the nuclear DNA is replicated; in M phase, the nucleus divides (mitosis) followed by the cytoplasm (cytokinesis).
- • In most cells, interphase consists of an S phase when DNA is duplicated, plus two gap phases—G1 and G... | {
"Header 1": "Mitogens Stimulate Cell Division by Promoting Entry into S Phase",
"Header 3": "Essential Concepts",
"token_count": 811,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
#### Question 18–11
Roughly, how long would it take a single fertilized human egg to make a cluster of cells weighing 70 kg by repeated divisions, if each cell weighs 1 nanogram just after cell division and each cell cycle takes 24 hours? Why does it take very much longer than this to make a 70-kg adult human?
####... | {
"Header 1": "Mitogens Stimulate Cell Division by Promoting Entry into S Phase",
"Header 3": "Questions",
"token_count": 2014,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Individual cells reproduce by replicating their DNA and dividing in two. This basic process of cell proliferation occurs in all living species—in the cells of multicellular organisms and in free-living cells such as bacteria and yeasts—and it allows each cell to pass on its genetic information to future generations.
... | {
"Header 1": "Sexual Reproduction and the Power of Genetics",
"token_count": 535,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Most of the creatures we see around us reproduce sexually. However, many organisms, especially those invisible to the naked eye, can produce offspring without resorting to sex. Most bacteria and other single-celled organisms multiply by simple cell division (Figure 19–1). Many plants also reproduce asexually, forming m... | {
"Header 1": "Sexual Reproduction and the Power of Genetics",
"Header 3": "The Benefits of Sex",
"token_count": 262,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Organisms that reproduce sexually are generally *diploid*: each cell contains two sets of chromosomes—one inherited from each parent. Because the two parents are members of the same species, the *maternal* chromosome set and the *paternal* chromosome set are very similar. The most notable difference between them is the... | {
"Header 1": "Sexual Reproduction and the Power of Genetics",
"Header 3": "Sexual Reproduction Involves Both Diploid and Haploid Cells",
"token_count": 557,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Sexual reproduction produces novel chromosome combinations. During meiosis, the maternal and paternal chromosome sets present in diploid germ-line cells are partitioned out into the single chromosome sets of the gametes. Each gamete will receive a mixture of maternal homologs and paternal homologs; when the genomes of ... | {
"Header 1": "Sexual Reproduction and the Power of Genetics",
"Header 3": "Sexual Reproduction Generates Genetic Diversity",
"token_count": 670,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The processes that generate genetic diversity during meiosis operate at random, as we will shortly discuss. That means that the alleles an individual receives from its parents are as likely to represent a change for the worse as they are a change for the better. Why, then, should the ability to try out new genetic comb... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"token_count": 968,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Before a diploid cell divides by mitosis, it duplicates its two sets of chromosomes. This duplication allows a full set of chromosomes—including a complete maternal set plus a complete paternal set—to be transmitted to each daughter cell (discussed in Chapter 18). Although meiosis ultimately halves this diploid chromos... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Meiosis Involves One Round of DNA Replication Followed by Two Rounds of Cell Division",
"token_count": 991,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pd... |
As mentioned earlier, before a eukaryotic cell divides—by either meiosis or mitosis—it first duplicates all of its chromosomes. The twin copies of each duplicated chromosome, called sister chromatids, at first remain tightly linked along their length. The way these duplicated chromosomes are handled, however, differs b... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Meiosis Requires the Pairing of Duplicated Homologous Chromosomes",
"token_count": 651,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The picture of meiotic division I we have just painted is greatly simplified, in that it leaves out a crucial feature. In sexually reproducing organisms, the pairing of the maternal and paternal chromosomes is accompanied by homologous recombination, a process in which two identical or very similar nucleotide sequences... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Crossing-Over Occurs Between the Duplicated Maternal and Paternal Chromosomes in Each Bivalent",
"token_count": 772,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4t... |
In most organisms, crossing-over during meiosis is required for the correct segregation of the two duplicated homologs into separate daughter nuclei. The chiasmata created by crossover events keep the maternal and paternal homologs bundled together until the spindle separates them during meiotic anaphase I. Before anap... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Chromosome Pairing and Crossing-Over Ensure the Proper Segregation of Homologs",
"token_count": 673,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Even though they share the same parents, no two siblings are genetically the same (unless they are identical twins). These genetic differences are initiated long before sperm meets egg, when meiosis I produces two kinds of randomizing genetic reassortment.
Figure 19–14 In meiosis II, a... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Haploid Gametes Contain Reassorted Genetic Information",
"token_count": 622,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Why do you think that organisms do not use the first steps of meiosis (up to and including meiotic cell division I) for the ordinary mitotic division of somatic cells?
Figure 19–15 Two kinds of genetic reassortment generate new chromosome combinations during meiosis. (A) The independent assortment of the maternal and... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Question 19–1",
"token_count": 348,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The sorting of chromosomes that takes place during meiosis is a remarkable feat of molecular bookkeeping: in humans, each meiosis requires that the starting cell keep track of 92 chromosomes (23 pairs, each of which has duplicated), handing out one complete set to each gamete. Not surprisingly, mistakes can occur in th... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Meiosis Is Not Flawless",
"token_count": 625,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Having seen how chromosomes are parceled out during meiosis to form haploid germ cells, we now briefly consider how they are reunited in the process of fertilization to form a new zygote with a diploid set of chromosomes.
Of the 300 million human sperm ejaculated during coitus, only about 200 reach the site of fertil... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Fertilization Reconstitutes a Complete Diploid Genome",
"token_count": 528,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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In organisms that reproduce without sex, the genetic material of the parent is transmitted faithfully to its progeny. The resulting offspring are thus genetically identical to a single parent. Before Mendel started working with peas, some biologists suspected that inheritance might work that way in humans (Figure 19–18... | {
"Header 1": "Sexual Reproduction Gives Organisms a Competitive Advantage in a Changing Environment",
"Header 3": "Mendel and the Laws of Inheritance",
"token_count": 776,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The breeding experiments that Mendel performed were straightforward. He started with stocks of genetically pure, "true-breeding" plants—those that produce offspring of the same variety when allowed to self-fertilize. If he followed pea color, for example, he used plants with yellow peas that always produced offspring w... | {
"Header 1": "Mendel Studied Traits That Are Inherited in a Discrete Fashion",
"Header 3": "Mendel Disproved the Alternative Theories of Inheritance",
"token_count": 385,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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One look at the offspring of Mendel's initial cross-fertilization experiments, such as those shown in Figure 19–21, raises an obvious question: what happened to the traits that disappeared in the $F_1$ generation? Did the plants bearing green peas, for example, fail to make a genetic contribution to their offspring? ... | {
"Header 1": "Mendel Studied Traits That Are Inherited in a Discrete Fashion",
"Header 3": "Mendel's Experiments Revealed the Existence of Dominant and Recessive Alleles",
"token_count": 883,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Mendel's theory—that for every gene, an individual inherits one copy from its mother and one copy from its father—raised some logistical issues. If an organism has two copies of every gene, how does it pass only one copy of each to its progeny? And how do these gene sets come together again in the resulting offspring? ... | {
"Header 1": "Mendel Studied Traits That Are Inherited in a Discrete Fashion",
"Header 3": "Each Gamete Carries a Single Allele for Each Character",
"token_count": 722,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Mendel's law of segregation explained the data for every trait he examined in pea plants, and he replicated his basic findings with corn and beans. But his rules governing inheritance are not limited to plants: they apply to all sexually reproducing organisms (Figure 19–24).
Consider a phenotype in humans that reflec... | {
"Header 1": "Mendel Studied Traits That Are Inherited in a Discrete Fashion",
"Header 3": "Mendel's Law of Segregation Applies to All Sexually Reproducing Organisms",
"token_count": 829,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Mendel deliberately simplified the problem of heredity by starting with breeding experiments that focused on the inheritance of one trait at a time, called *monohybrid crosses*. He then turned his attention to multihybrid crosses, examining the simultaneous inheritance of two or more apparently unrelated traits.
In t... | {
"Header 1": "Mendel Studied Traits That Are Inherited in a Discrete Fashion",
"Header 3": "Alleles for Different Traits Segregate Independently",
"token_count": 897,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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So far we have discussed alleles and genes as if they are disembodied entities. We now know that Mendel's "factors"—the things we call genes—are carried on chromosomes that are parceled out during the formation of gametes and then brought together in novel combinations in the zygote at fertilization. Chromosomes theref... | {
"Header 1": "Mendel Studied Traits That Are Inherited in a Discrete Fashion",
"Header 3": "The Behavior of Chromosomes During Meiosis Underlies Mendel's Laws of Inheritance",
"token_count": 550,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Mendel studied seven traits, each of which is controlled by a separate gene. It turns out that most of these genes are located on different chromosomes, which readily explains the independent segregation he observed. But the independent segregation of different traits does not necessarily require that the responsible g... | {
"Header 1": "Mendel Studied Traits That Are Inherited in a Discrete Fashion",
"Header 3": "Even Genes on the Same Chromosome Can Segregate Independently by Crossing-Over",
"token_count": 475,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Mutations produce heritable changes in DNA sequence. They can arise in various ways (discussed in Chapter 6) and can be classified by the effect
ECB4 e19.25/19.29 Figure 19–29 Genes that lie far enough apart on the same chromosome will segregate independently. (A) Because several crosso... | {
"Header 1": "Mendel Studied Traits That Are Inherited in a Discrete Fashion",
"Header 3": "Mutations in Genes Can Cause a Loss of Function or a Gain of Function",
"token_count": 860,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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As we saw in Chapter 9, mutations provide the fodder for evolution. They can alter the fitness of an organism, making it either less or more likely for the individual to survive and leave progeny. Natural selection determines whether these mutations are preserved: those that confer a selective advantage on an organism ... | {
"Header 1": "Each of Us Carries Many Potentially Harmful Recessive Mutations",
"token_count": 454,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Our understanding of how chromosomes shuttle genetic information from one generation to the next did more than demystify the basis of inheritance: it united the science of genetics with other life sciences, from cell biology and biochemistry to physiology and medicine. Genetics provides a powerful way to discover what ... | {
"Header 1": "Each of Us Carries Many Potentially Harmful Recessive Mutations",
"Header 3": "Genetics as an Experimental Tool",
"token_count": 285,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Imagine that each chromosome undergoes one and only one crossover event on each chromatid during each meiosis. How would the co-inheritance of traits that are determined by genes at opposite ends of the same chromosome compare with the co-inheritance observed for genes on two different chromosomes? How does this compar... | {
"Header 1": "Each of Us Carries Many Potentially Harmful Recessive Mutations",
"Header 3": "Question 19–3",
"token_count": 355,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
A genetic screen typically involves examining many thousands of mutagenized individuals to find the few that show a specific altered phenotype of interest. To search for genes involved in cell metabolism, for example, one might screen mutagenized bacterial or yeast cells to pick out those that have lost the ability to ... | {
"Header 1": "Each of Us Carries Many Potentially Harmful Recessive Mutations",
"Header 3": "Genetic Screens Identify Mutants Deficient in Specific Cell Processes",
"token_count": 387,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Given two mutations that produce the same phenotype, how can we tell whether they are mutations in the same gene? If the mutations are recessive (as they most often are), the answer can be found by a complementation test.
In the simplest type of complementation test, an individual who is homozygous for one mutation i... | {
"Header 1": "Each of Us Carries Many Potentially Harmful Recessive Mutations",
"Header 3": "TWO GENES OR ONE?",
"token_count": 508,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Genetic screens are a powerful approach for isolating and characterizing mutations that are compatible with life—those that change the appearance or behavior of an organism without killing it. A problem arises, however, if we wish to study essential genes—those that are absolutely required for fundamental cell processe... | {
"Header 1": "Each of Us Carries Many Potentially Harmful Recessive Mutations",
"Header 3": "Conditional Mutants Permit the Study of Lethal Mutations",
"token_count": 499,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
A large-scale genetic screen can turn up many mutant organisms with the same phenotype. These mutations might affect the same gene or they might affect different genes that function in the same process. How can we distinguish between the two? If the mutations are recessive and cause a loss of function, a **complementat... | {
"Header 1": "Each of Us Carries Many Potentially Harmful Recessive Mutations",
"Header 3": "A Complementation Test Reveals Whether Two Mutations Are in the Same Gene",
"token_count": 587,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Genetic screens in model experimental organisms have been spectacularly successful in identifying genes and relating them to various phenotypes, including many that are conserved between these organisms and humans. But the same approach cannot be used in humans. Unlike flies, worms, yeast, and bacteria, humans do not r... | {
"Header 1": "Rapid and Cheap DNA Sequencing Has Revolutionized Human Genetic Studies",
"token_count": 274,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
When we compare the sequences of multiple human genomes, we find that any two individuals will differ in about 1 nucleotide pair in 1000. Most of these variations are common and relatively harmless. When two sequence variants coexist in the population and are both common, the variants are called **polymorphisms**. The ... | {
"Header 1": "Linked Blocks of Polymorphisms Have Been Passed Down from Our Ancestors",
"token_count": 201,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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When two individuals from different isolated, inbred subpopulations of a species come together and mate, their offspring often show "hybrid vigor": that is, they appear more robust, healthy, and fertile than either parent. Can you suggest a possible explanation for this phenomenon?
Figure 19–36 Single-nucleotide poly... | {
"Header 1": "QUESTION 19–4",
"token_count": 392,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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A detailed examination of haplotype blocks has provided intriguing insights into the history of human populations. New alleles of genes are continually being generated by mutation; many of these variants will be neutral, in that they will not affect the reproductive success of the individual. These have a chance of bec... | {
"Header 1": "QUESTION 19–4",
"Header 3": "Our Genome Sequences Provide Clues to our Evolutionary History",
"token_count": 740,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The study of polymorphisms may also have more practical relevance to human health. CNVs, indels, and SNPs can be used as markers for building human genetic maps or for conducting searches for mutations that predispose individuals to a specific disease. Mutations that give rise, in a reproducible way, to rare but clearl... | {
"Header 1": "QUESTION 19–4",
"Header 3": "Polymorphisms Can Aid the Search for Mutations Associated with Disease",
"token_count": 1416,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
One way that SNPs have facilitated the search for alleles that predispose to disease is by providing the physical markers needed to construct detailed genetic linkage maps. A genetic linkage map displays the relative locations of a set of genes. Such maps are based on the frequency with which two alleles are co-inherit... | {
"Header 1": "QUESTION 19–4",
"Header 3": "Making a map",
"token_count": 678,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Genome-wide association studies allow us to discover common genetic variants that affect the risk for a common disease, even if each variant alters susceptibility only slightly. Because mutations that destroy the activity of a key gene are likely to have a disastrous effect on the fitness of the mutant individual, they... | {
"Header 1": "QUESTION 19–4",
"Header 3": "Making associations",
"token_count": 1031,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
- • Sexual reproduction involves the cyclic alternation of diploid and haploid states: diploid germ-line cells divide by meiosis to form haploid gametes, and the haploid gametes from two individuals fuse at fertilization to form a new diploid cell—the zygote.
- • During meiosis, the maternal and paternal homologs are p... | {
"Header 1": "QUESTION 19–4",
"Header 3": "Essential Concepts",
"token_count": 755,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
#### Question 19–6
It is easy to see how deleterious mutations in bacteria, which have a single copy of each gene, are eliminated by natural selection: the affected bacteria die and the mutation is thereby lost from the population. Eukaryotes, however, have two copies of most genes—that is, they are diploid. Often an... | {
"Header 1": "QUESTION 19–4",
"Header 3": "Questions",
"token_count": 373,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Distinguish between the following genetic terms:
- A. Gene and allele.
- B. Homozygous and heterozygous.
- C. Genotype and phenotype.
- D. Dominant and recessive.
#### Question 19–11
You have been given three wrinkled peas, which we shall call A, B, and C, each of which you plant to produce a mature pea plant. ... | {
"Header 1": "QUESTION 19–4",
"Header 3": "Question 19–10",
"token_count": 1614,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Cells are the building blocks of multicellular organisms. This seems a simple statement, but it raises deep problems. Cells are not like bricks: they are small and squishy. How can they be used to construct a giraffe or a giant redwood tree? Each cell is enclosed in a flimsy membrane less than a hundred-thousandth of a... | {
"Header 1": "Cell Communities: Tissues, Stem Cells, and Cancer",
"token_count": 834,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Plants and animals have evolved their multicellular organization independently, and their tissues are constructed on different principles. Animals prey on other living things—and often are preyed on by other animals—and for this they must be strong and agile: they must possess tissues capable of rapid movement, and the... | {
"Header 1": "Cell Communities: Tissues, Stem Cells, and Cancer",
"Header 3": "Extracellular Matrix and Connective Tissues",
"token_count": 728,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
A naked plant cell, artificially stripped of its wall, is a delicate and vulnerable thing. With care, it can be kept alive in culture; but it is easily ruptured, and even a small decrease in the osmotic strength of the culture medium can cause the cell to swell and burst. Its cytoskeleton lacks the tension-bearing inte... | {
"Header 1": "Cell Communities: Tissues, Stem Cells, and Cancer",
"Header 3": "Plant Cells Have Tough External Walls",
"token_count": 1295,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
#### Question 20–1
Cells in the stem of a seedling that is grown in the dark orient their microtubules horizontally. How would you expect this to affect the growth of the plant?
Figure 20–7 Microtubules help direct the deposition of cellulose in the plant cell wall. Electron micrographs show (A) oriented cellulose ... | {
"Header 1": "1 µm (A) (B) 200 nm 0.1 µm cellulose microfibril being added to preexisting wall plasma membrane connector protein microtubule attached to plasma membrane cellulose synthase complex CYTOSOL",
"token_count": 417,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.p... |
It is traditional to distinguish four major types of tissues in animals: connective, epithelial, nervous, and muscular. But the basic architectural distinction is between connective tissues and the rest. In connective tissues, extracellular matrix is plentiful and carries the mechanical load. In other tissues, such as ... | {
"Header 1": "1 µm (A) (B) 200 nm 0.1 µm cellulose microfibril being added to preexisting wall plasma membrane connector protein microtubule attached to plasma membrane cellulose synthase complex CYTOSOL",
"Header 3": "Animal Connective Tissues Consist Largely of Extracellular Matrix",
"token_count": 332,
"sou... |
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