Cell Cycle Review by steepslope9876

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									Ch 12 The Cell Cycle
Review Questions
1.   During which stages of the cell cycle would chromosomes consist of two identical
     chromatids?
       Since chromosomes are duplicated only during the S phase of interphase, it is
       during this subphase that the two sister chromatids will form.
       “Each duplicated chromosome has two sister chromatids. The two chromatids,
       each containing an identical DNA molecule, are initially attached by adhesive
       proteins all along their lengths. In its condensed form, the duplicated
       chromosome has a narrow “waist” at a specialized region called the centromere
       where the two chromatids are most closely attached (Figure 12.4).” (Text quoted
       from page 219 of the textbook)
2.   Which type of cell division (sexual or asexual) is primarily concerned with
     reproducing cells for growth and repair?
       With the process of growth and repair an asexual reproductive process will be
       the main process for most multicellular plants and animals. Each cell whether it
       be haploid or diploid will form a new cell by mitosis so each resulting cell has the
       same number of chromosomes as the parent cell. Mitosis is the process that is
       normally associated with an asexual division process since there is not any
       mechanism for diversity as new cells form unless there is a mutation from the
       original parental cell that divides.( See Activity 1 Roles of Cell Division on the CD
       in the book)
3.   Contrast differences between a human somatic cell and gamete.
       “Every eukaryotic species has a characteristic number of chromosomes in each
       cell nucleus. For example, the nuclei of human somatic cells (all body cells
       except the reproductive cells) each contain 46 chromosomes made up of two
       sets of 23, one set inherited from each parent. Reproductive cells, or gametes—
       sperm cells and egg cells—have half as many chromosomes as somatic cells, or
       one set of 23 chromosomes in humans.” (Text quoted from page 219 of the
       textbook)
       In this chapter we will study how somatic cells divide by a process called mitosis
       where a parent cell divides producing two daughter cells containing the same
       number of chromosomes. Normally this process takes place in diploid cells (2n)
       but it can also occur in the formation of some haploid cells (n) in multicellular
       haploid plants. Gametes normally form by the process of meiosis which differs
       from mitosis by having two series of divisions and producing haploid cells (n)
       from the diploid parent cell (2n). We will study the process of meiosis in detain in
       chapter 13.
4.   What are some examples of locations where somatic cells could be found in
     human beings?
       Mitosis can occur in most cells once the zygote forms. Some cells most notably
       those in the nervous system and muscles cells loose their ability to divide.
       Mitosis can be readily seen in cells of the epidermis in skin where viable cells are
       capable of replacing the cells that may be lost in the normal day to day
       functioning of the skin. Also the sites where red blood cells form is another good
       example of cells that may be dividing by mitosis to replace dead or damaged
       erythrocytes.
5.   What does the symbol 2N → 2N represent? What does the symbol N → N
     represent? Which cell division process (Mitosis, or Meiosis) would normally be
     associated with 2N → 2N?
        Since the symbol 2n represents a diploid cell, the symbol 2N → 2N refers to a
        diploid cell dividing to make more diploid cells. The process that is normally
        associated with this symbol is mitosis. In plants and a few other organisms,
        there is another symbol that may also be used. This second symbol is the
        symbol N → N, which represents a haploid cell forming a haploid organism. This
        second symbol is common in plants during the formation of a multicellular haploid
        plant called a gametophyte.
6.    How many chromosomes are typically found in a human somatic cell? How many
      chromosomes are typically found in a human gamete?
        A human somatic cell normally has 46 chromosomes. When a normal human
        somatic cell divided each daughter cell would typically have 46 chromosomes.

        Since gametes are haploid cells a normal human gamete would typically contain
        half the number of chromosomes found in a somatic cell. Human gametes would
        have 23 chromosomes.
7.    In which ways are prokaryotic and eukaryotic cells similar when they divide
      asexually?
        Both prokaryotic and eukaryotic cells duplicate their DNA and separate the
        parent cell into two smaller daughter cells.
8.    What is the process by which bacteria divide? Explain what happens during this
      process of cell division in bacteria.
        “Prokaryotes (bacteria) reproduce by a type of cell division called binary fission,
        meaning literally “division in half.” Most bacterial genes are carried on a single
        bacterial chromosome that consists of a circular DNA molecule and associated
        proteins. Although bacteria are smaller and simpler than eukaryotic cells, the
        problem of replicating their genomes in an orderly fashion and distributing the
        copies equally to two daughter cells is still formidable.” (Text quoted from page
        226 of the textbook See Figure 12.11 on page 227 of the textbook)
        “In E. coli, the process of cell division begins when the DNA of the bacterial
        chromosome begins to replicate at a specific place on the chromosome called
        the origin of replication, producing two origins. As the chromosome continues to
        replicate, one origin moves rapidly toward the opposite end of the cell (Figure
        12.11). While the chromosome is replicating, the cell elongates. When replication
        is complete and the bacterium has reached about twice its initial size, its plasma
        membrane grows inward, dividing the parent E. coli cell into two daughter cells.
        Each cell inherits a complete genome.” (Text quoted from page 226 of the
        textbook. Refer to Figure 12.11 on page 227 of the textbook)
9.    The cell cycle consists of two major subdivisions, the Mitotic phase and interphase.
      Which of these divisions is the longest subdivision?
        “Mitosis is just one part of the cell cycle (Figure 12.5). In fact, the mitotic (M)
        phase, which includes both mitosis and cytokinesis, is usually the shortest part of
        the cell cycle. Mitotic cell division alternates with a much longer stage called
        interphase, which often accounts for about 90% of the cycle.” Text quoted from
        page 221 of the textbook. See Figure 12.5 on page 221 of the textbook) .( See
        Activity 2 The Cell Cycle on the CD in the book)
10.   What happens during interphase of the cell cycle?
        “It is during interphase that the cell grows and copies its chromosomes in
        preparation for cell division.” (Text quoted from page 221 of the textbook) .( See
        Activity 2 The Cell Cycle on the CD in the book)
11.   What three subphases are parts of interphase? Identify what happens during each
      of the three subphases of interphase.
        “Interphase can be divided into 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 (we discuss synthesis of DNA in Chapter 16).
        Thus, a cell grows (G1), continues to grow as it copies its chromosomes (S),
        grows more as it completes preparations for cell division (G2), and divides (M).
        The daughter cells may then repeat the cycle.” (Text quoted from page 221 of
        the textbook) .( See Activity 2 The Cell Cycle on the CD in the book)
12.   What are five (5) subphases of the mitotic phase (M phase)? Besides Mitosis, what
      else occurs in the mitotic or M phase?
        “For purposes of description, however, mitosis is conventionally broken down into
        five stages: prophase, prometaphase, metaphase, anaphase, and telophase.”
        (Text quoted from page 221 of the textbook)
        Each of these subphases involve changes in the nucleus where the cell divides
        the DNA content in the nucleus to form two nuclei. It is during the last subphase
        that the process of cytokinesis also occurs to divide the cytoplasm and
        organelles to form two complete cells. .( See Activity 2 The Cell Cycle, Activity 3
        Mitosis and Cytokinesis and the Video Mitosis and Cytokinesis on the CD in the
        book)
13.   Explain what happens in the M phase of the cell cycle to include prophase,
      metaphase, anaphase, telophase and cytokinesis.
        The M phase includes Mitosis and cytokinesis.
        “During interphase, the single centrosome replicates, forming two centrosomes,
        which remain together near the nucleus (see Figure 12.6). The two centrosomes
        move apart from each other during prophase and prometaphase of mitosis, as
        spindle microtubules grow out from them. By the end of prometaphase, the two
        centrosomes, one at each pole of the spindle, 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 of the two sister chromatids of a chromosome has a kinetochore, a
       structure of proteins associated with specific sections of chromosomal DNA at
       the centromere. The chromosome′s two kinetochores face in opposite directions.
       During prometaphase, some of the spindle microtubules attach to the
       kinetochores; these are called kinetochore microtubules. (The number of
       microtubules attached to a kinetochore varies among species, from one
       microtubule in yeast cells to 40 or so in some mammalian cells.) When one of a
       chromosome′s kinetochores is “captured” by microtubules, the chromosome
       begins to move toward the pole from which those microtubules extend. However,
       this movement is checked as soon as microtubules from the opposite pole attach
       to the other kinetochore. What happens next is like a tug-of-war that ends in a
       draw. The chromosome moves first in one direction, then the other, back and
       forth, finally settling midway between the two ends of the cell. At metaphase, the
       centromeres of all the duplicated chromosomes are on a plane midway between
       the spindle′s two poles. This imaginary plane is called the metaphase plate of the
       cell (Figure 12.7). Meanwhile, microtubules that do not attach to kinetochores
       have been growing, and by metaphase they overlap and interact with other
       nonkinetochore microtubules from the opposite pole of the spindle. (These are
       sometimes called “polar” microtubules.) By metaphase, the microtubules of the
       asters have also grown and are in contact with the plasma membrane. The
       spindle is now complete.

       Anaphase commences suddenly when proteins holding together the sister
       chromatids of each chromosome are inactivated. Once the chromatids become
       separate, full-fledged chromosomes, they move toward opposite ends of the cell.
       How do the kinetochore microtubules function in this poleward movement of
       chromosomes? One possibility is that the chromosomes are “reeled in” by
       microtubules that are shortening at the spindle poles.

        At the end of anaphase, duplicate groups of chromosomes have arrived at
        opposite ends of the elongated parent cell. Nuclei re-form during telophase.
        Cytokinesis generally begins during these later stages of mitosis, and the spindle
        eventually disassembles.” (Text quoted from pages 221 and 224 of the textbook
        See Figure 12.6 on pages 222 and 223 and Figure 12.7 on page 224 of the
        textbook)
14.   What happens in cytokinesis? How does cytokinesis in a plant cell differ from
      cytokinesis in an animal cell?
        During cytokinesis, the cell divides the cytoplasm and organelles into two new
        cells For a plant cell, the division occurs along the metaphase plane where a
        cell plate begins to form toward the outer regions of the cell where the cell wall is
        located. In an animal cell a cleavage furrow forms from the plasma membrane
        on both sides of the metaphase plane and grows toward each other pinching the
        animal cell into two cells. .( See Activity 3 Mitosis and Cytokinesis on the CD in
        the book)
15.   How is cell division in a plant cell different from that of an animal cell?
        “Cytokinesis in plant cells, which have cell walls, is markedly different. There is
        no cleavage furrow. Instead, during telophase, vesicles derived from the Golgi
        apparatus move along microtubules to the middle of the cell, where they
        coalesce, producing a cell plate (Figure 12.9b). Cell wall materials carried in the
        vesicles collect in the cell plate as it grows. The cell plate enlarges until its
        surrounding membrane fuses with the plasma membrane along the perimeter of
        the cell. Two daughter cells result, each with its own plasma membrane.
        Meanwhile, a new cell wall arising from the contents of the cell plate has formed
        between the daughter cells (Text quoted from page 226 of the textbook)
        “In animal cells, cytokinesis occurs by a process known as cleavage. The first
        sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the
        cell surface near the old metaphase plate (Figure 12.9a). On the cytoplasmic
        side of the furrow is a contractile ring of actin microfilaments associated with
        molecules of the protein myosin. (Actin and myosin are the same proteins that
        are responsible for muscle contraction as well as many other kinds of cell
        movement.) The actin microfilaments interact with the myosin molecules, causing
        the ring to contract. The contraction of the dividing cell′s ring of microfilaments is
        like the pulling of drawstrings. The cleavage furrow deepens until the parent cell
        is pinched in two, producing two completely separated cells, each with its own
        nucleus and share of cytosol and organelles. (text quoted from pages 224 and
        226 of the textbook)
16.   Explain the role of checkpoints in the cell cycle.
        “The timing and rate of cell division in different parts of a plant or animal are
        crucial to normal growth, development, and maintenance. The frequency of cell
        division varies with the type of cell. For example, human skin cells divide
       frequently throughout life, whereas liver cells maintain the ability to divide but
       keep it in reserve until an appropriate need arises—say, to repair a wound. Some
       of the most specialized cells, such as mature, fully formed nerve cells and
       muscle cells, do not divide at all in a mature human. These cell cycle differences
       result from regulation at the molecular level. The mechanisms of this regulation
       are of intense interest, not only for understanding the life cycles of normal cells
       but also for understanding how cancer cells manage to escape the usual
       controls.” (Text quoted from page 228 of the textbook)
       “A checkpoint in the cell cycle is a critical control point where stop and go-ahead
       signals can regulate the cycle. (The signals are transmitted within the cell by the
       kinds of signal transduction pathways discussed in Chapter 11.) 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 inside the cell; the signals report whether
       crucial cellular processes up to that point have been completed correctly and
       thus whether or not the cell cycle should proceed. Checkpoints also register
       signals from outside the cell, as we will discuss later. Three major checkpoints
       are found in the G1, G2, and M phases (see Figure 12.14). (Text quoted from
       page 229 of the textbook)
       “For many cells, the G1 checkpoint—dubbed the “restriction point” in mammalian
       cells—seems to be the most important. If a cell receives a go-ahead signal at the
       G1 checkpoint, it will usually complete the S, G2, and M phases and divide.
       Alternatively, if it does not receive a go-ahead signal at that point, it will exit the
       cycle, switching into a nondividing state called the G0 phase (Figure 12.15). Most
       cells of the human body are actually in the G0 phase. As mentioned earlier, fully
       formed, mature nerve cells and muscle cells never divide. Other cells, such as
       liver cells, can be “called back” from the G0 phase to the cell cycle by certain
       external cues, such as growth factors released during injury.” (Text quoted from
       page 229 of the textbook. See Figure 12.15 on page 229 of the textbook)

KEY TERMS
    Anaphase                           Diploid chromosome                  Interphase
    Aster                              number                              Malignant tumor
    Benign tumor                       G0 Phase                            Metaphase
    Binary fission                     G1 phase                            Metaphase plate
    Cell plate                         G2 phase                            Metastasis
    Centromere                         Gamete                              Mitosis
    Chromosome                         Genome                              Prophase
    Cleavage furrow                    Growth factor                       Sister chromatids
    Cytokinesis                        Haploid cell                        Somatic cell
    Density-dependent                  Haploid                             S phase
    inhibition                         chromosome                          Telophase
    Diploid cell                       number

Concept check 12.1 page 220
Concept check 12.2 page 228
Concept check 12.3 page 233
Questions 2, 3, 4, 5, 7, 8, 9, 10 and 11 on pages 234 and 235 of textbook

								
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