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BIOLOGY 1A03 (168)
Xudong Zhu (12)
Chapter 11

Textbook and Class Notes Collaborated - Week 4 - Unit 2 - Chapter 11 Bio 1A03

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Xudong Zhu

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Bio 1A03 Unit Two: Cell Structure and Function Chapter 11: The Cell Cycle  All cells arise from preexisting cells by the process of cell division  Chromosomes – the carriers of hereditary material – the instructions for building and operating the cell  Gametes – sperm and eggs; male and female reproductive cells  Somatic Cells – all other cells of the body that are not gametes Types of Nuclear Division Mitosis Meiosis  Occurs in somatic cells  Occurs only in reproductive cells  Produces gametes  Daughter cells are genetically identical to  Daughter cells (ova, sperm) are genetically parent cell different from the parent cells Functions of Mitotic Cell Division o Reproduction in asexually reproducing species o Growth and Repair in multicellular species  Chromosome o A single, long DNA double helix that is wrapped around proteins in a highly organized manner  DNA encodes the cell’s hereditary information, or genetic material  Gene – a length of DNA that codes for a particular protein or ribonucleic acid (RNA) found in the cell 11.1 Mitosis and the Cell Cycle  4 Cell Cycle Phases o Figure 11.5; The Cell Cycle Has Four Phases 1. G 1hase (Interphase) 2. S Phase (Interphase) 3. G 2hase (Interphase) 4. Mitotic (or M) Phase Overview of Mitosis Prior to mitosis Each chromosome is copied As mitosis starts The chromosomes also condense from long, thin, fibrous forms into compact structures that can be moved around the cell efficiently In the remainder of mitosis One of the chromosomes copies is distributed to each of two daughter cells  Figure 11.3, page 136; Chromosome Morphology Changed Before an during Mitosis M Phase and Interphase  Growing cells cycle between Mitotic Phase and Interphase o Mitotic (M) Phase – dividing phase  Cells can be stained and observed with a light microscope during M phase, when they are condensed into compact structures o Interphase – non-dividing phase  Cells spend most of their time in interphase  When stains are used, individual interphase chromosomes are not normally visible in the light microscope Bio 1A03  Autoradiography o Radioactive Phosphorous and Thymidine (components of deoxyribonucleotides, which are components of DNA) o Label DNA as chromosomes were being copied, then wash away any radioactive isotope that hadn’t been incorporated o The labeled DNA could then be visualized by exposing the treated cells to X-Ray film o Emissions from the radioactive phosphorus or thymidine create a black dot in the film o Alma Howard and Stephen Pelc  Found that black dots only appeared in interphase cells, not M-phase cells  Evidence that DNA replication occurs during Interphase  identifies Synthesis (S) Phase  Synthesis (S) Phase – DNA replication o Duplication of genetic matieral occurs, independent of mitosis o Distributes chromosome copies to daughter cells  Cell Cycle – the orderly sequence of events that occurs starting from the formation of a eukaryotic cell through the duplication of its chromosomes to the time it undergoes division itself o Replication or copying of the hereditary material in chromosomes o The partitioning of copied chromosomes to the two daughter cells during M phase  The hereditary material is duplicated, with one copy going to each daughter cell during mitosis  Daughter cells contain genetic information identical to that of the parent cell The Discovery of the Gap Phases  Howard and Pelc  Figure 11.4, page 238  Experimental Design o Exposed growing cells to radioactive phosphorous or thymidine and then waited various lengths of time before looking at the cells o In one experiment, researchers worked with cells that were growing in culture and stopped the labeling of DNA after 30 minutes of exposure to radioactive thymidine o To stop DNA from being labeled with radioactive thymidine. They flooded the solution surrounding the cultured cells with nonradioactive thymidine – used the pulse chase approach o Analyzed the labeled cells 2 hours, 4 hours and 6 hours after labeling them and so on o Then they graphed the percentage of marked cells that were undergoing mitosis versus the time after exposure  Observations o Cells that the researchers were studying take about 24 hours to complete one cell cycle o The cells do not divide synchronously – different cells in a sample are in all different parts of interphase and M phase at any given time o When the pulse ended, some cells should have been just completing S phase o If they went into M phase immediately – some labeled M-phase cells should have been present just after the pulse ended  Results o No labeled dividing cells appeared for about 4 hours – corresponds with the time lag between the end of S phase and the beginning of M phase  There is a gap in the cycle where chromosome replication has ended, but mitosis has not yet begun  G Phase 2 o Labeled nuclei undergoing mitosis are observed over a period of about 6 to 8 hours – S phase o When S phase, M phase and G2 phase are added up then subtracted from 24 hours, there is a discrepancy of 7 to 9 hours 1 G phase  Occurs after M phase but before S phase  G and G phase 1 2 o Provide the time for the parent cells to grow large enough and synthesize enough organelles that its daughter cells will be normal in size and function before mitosis o G1Phase – Cell growth and duplication of organelles Bio 1A03 o G Phase – Cell growth and duplication of organelles continues to build the protein “machinery” 2  Bacterial Division o Box 11.2, page 239 o Binary Fission – produced two genetically identical daughter cells o Most bacterial contain a single, circular chromosome, coiled upon itself and composed of DNA o Lacking a nucleus – bacterial chromosome is located in the nucleoid (distinctive region, no membrane) o After the chromosome is replicated, the two daughter chromosomes stat attached to each other for a period of time and them separate to opposite sides of the cell o As the chromosomes part, a contractile ring composed of FtsZ fibers forms between the two chromosomes  FtsZ fibers are a major component of the bacterial cytoskeleton and are similar in structure to microtubules o As the FtsZ ring closes, the bacterial cytoplasm is divided in two, completing cell division 11.2 How Does Mitosis Take Place  Mitosis represents only a small portion of the cell cycle  Mitosis results in the division of replicated chromosomes and the formation of two daughter nuclei with identical chromosomes and genes  Mitosis is usually accompanied by cytokinesis – cytoplasmic division and the formation of two daughter cells  The number of chromosomes in each cell varies widely among species o Eg/ Humans have 46 chromosomes o Eg/ Fruit Flies have 8 chromosomes  Figure 11.7, page 240 o G1Phase  Chromatin – extremely long, threadlike strands consisting of DNA associated with histones (globular proteins) o S Phase and 2 Phase  Chromosomes that have been copied prior to mitosis  Chromatid – each of the DNA copies in a replicated chromosome  Each pair of chromatids are joined along their entire length as well as at the centromere  Sister Chromatids – chromatids from the same chromosome o Represent exact copies of the same genetic information  Each chromatid contains one long DNA double helix  At the start of M phase, each chromosome consists of two sister chromatids that are attached to one another at the centromere o Mitosis  As mitosis begins, chromatin condenses to form a much more compact structure  The two sister chromatids separate to form independent chromosomes  One copy of each chromosome goes to each of the two daughter cells  Each daughter cell receives a copy of the genetic information that is contained in each chromosome  Each daughter cell ends up with exactly the same complement of chromosomes as the parent cell had prior to replication  Every daughter cell receives the same genetic information Mitotic Phases  Figure 11.8, page 240; Mitosis and Cytokinesis  Figure 11.10, page 243; Mitosis and Cytokinesis in Action  Mitosis (Nuclear Division) consists of: o Prophase o Prometaphase o Metaphase Bio 1A03 o Anaphase o Telophase  Prophase – “before phase” or Preparation Phase o In the Nucleus  Nucleoli begin to disappear  Chromatin fibers contract (DNA condenses) by tightly coiling  Chromosomes are visible and consist of two identical sister chromatids joined together at the centromere  Chromosomes first become visible in the light microscope o In the Cytoplasm  Mitotic spindle forms  Structure that produces mechanical forces that pull chromosomes into the daughter cells during mitosis  Consists of an array of microtubules (components of the cytoskeleton) that have polymerized  Spindle Fibers – groups of microtubules that attach to the chromosomes o Originate from the microtubule organizing center  Assembly of microtubules begins in the centrosome (animals) or microtubule organizing center (plants)  Centrosome – microtubule organizing center; contains a pair of centrioles  In animals, the centrioles begin to move apart to opposite sides of nucleus (2 poles)  Prometaphase – “before middle phase” o Chromosomes do not appear completely aligned or organized o Nucleolus disappears once chromosomes have condensed and nuclear envelope breaks down o After the nuclear envelope has disintegrates, spindle fibers from each mitotic spindle attach to one of the two sister chromatids of each chromosome o Kinetochore  Located at the centromere region of the chromosome  Each chromosome has two kinetochores where spindle fibers attach – one on each side  Spindle fibers attach to sister chromatids at each kinetochore region o Kinetochore microtubules are polymerized, randomly oriented at first, then they become aligned parallel with microtubules  Kinetochore Microtubules  Extend from the poles to the kinetochore  Attach to the kinetochore  Functions of the Kinetochore Microtubules  Kinetochore microtubules assist in movement of chromosomes  Nonkinetochore Microtubules  Radiate from each centrosome towards the metaphase plate without attaching to chromosomes  May overlap with those from the opposite pole  Functions of the Nonkinetochore Microtubules  Assist in elongating the entire cell during anaphase  Form a cage-like network which facilitates the activities of the cell cycle components o Centrosomes continue their movement to opposite poles of the cell (animal) o The microtubules that are attached to the kinetochores begin moving the chromosomes to the middle of the cell  Metaphase – “middle phase” or Organizing Phase Bio 1A03 o Centrosomes complete their migration to the opposite poles of the cell (animas) o Chromosomes line up along the metaphase plate o Centromeres are aligned on the metaphase plate, which is located equidistant from the two poles o Formation of mitotic spindle is complete o Each chromatid is attached to spindle fibers that run from its kinetochore to one of the two poles of the cell o Each chromosome is held by kinetochore spindle fibers reaching to opposite poles and exerting the same amount of tension  Anaphase – “against phase” or Separating DNA Copies o Binding proteins between the sister chromatids break down o Disjunctional Segregation – Centromeres of sister chromatids disjoin and segregate o Chromosomes move centromere first (they appear V-shaped) o The centromeres that are holding sister chromatids together split o Sister chromatids are pulled apart equally to create independent chromosomes o The kinetochore spindle fibers begin to shorten o Motor proteins pull the chromosomes to opposite poles of the cell o The two poles of the cell are pushed away from each other by motor proteins associated with microtubules that are not attached to chromosomes o Towards the end of anaphase, the two poles have identical numbers of chromosomes o The cell elongates  Telophase – “end phase” o Nuclear envelope beings to form around each set of chromosomes o Mitotic spindle disintegrates o Chromosomes begin to de-condense o Nonkinetochore microtubules further elongates the cell o Two daughter nuclei form o Formation of the nuclear envelopes around each set of chromosomes o Once two independent nuclei have formed, mitosis is complete  Cytokinesis  Telophase is often, but not always followed by cytokinesis  The cytoplasm divides to form two daughter cells, each with its own nucleus and complete set of organelles o Organelles such as mitochondria, lysosomes, chloroplasts etc were replicated prior to mitosis o Animals, slime molds and fungi – form a cleavage furrow  Furrow appears because a ring of actin filament forms just inside the plasma membrane, in a plane that bisects the cell  Myosin (motor protein) binds to these actin filaments  When myosin binds to ATP or ADP, part of the protein moves in a way that causes actin filaments to slide  As myosin moves the rind of actin filaments on the inside of the plasma membrane, the ring shrinks in size and tightens  Because the ring is attached to the plasma membrane, the shrinking ring pulls the membrane with it – the plasma membrane pinches inward  The actin and myosin filaments continue to slide past each other, tightening the ring further, until the original membrane is pinched in two and cell division is complete  Figure 11.9, page 242 a) o Plants – form a cell plate  A series of microtubules and other proteins define and organize the region where the new plasma membranes and cell walls will form  Vesicles from the Golgi apparatus are then transported to the middle o the dividing cell, where they form a cell plate  The vesicles carry components of the cell wall and plasma membrane that gradually build up, completing the cell plate and dividing the two daughter cells  Figure 11.9, page 242 b) Bio 1A03 Chromosome Movement During Mitosis  Mitotic Spindle Forces o Spindle Fibers are composed of microtubules  Microtubules  Composed of α-tubulin and β-tubulin dimers  The length is determined by the number of tubulin dimers it contains  Asymmetric – have a plus and minus end  Grow at plus end  Spindle Fibers grow from the microtubule organizing center until their plus ends attach to the kinetochore o The tubulin subunits of the kinetochore microtubules are depolymerized (lost) from the kinetochore plus ends o Experiment  Figure 11.11, page 244  Fluorescently labeled tubulin subunits into prophase or metaphase cells – made entire mitotic spindle visible  Once anaphase had begun, the researchers marked a region of the spindle with a bar-shaped beam of laser light – laser quenched the fluorescence in the exposed region, making that section of the spindle dark (“photo bleached”), although still functional  As Anaphase progressed  The photo bleached region remained stationary  The spindle fires got shorter between the photo bleached region and the kinetochore  A Kinetochore Motor o Motor proteins, such as dynein attach and detach along the kinetochore microtubules length  This results in chromosome movement o The kinetochore is thought to have a base that attaches to the centromere region of the chromosome and a “crown” of fibrous proteins projecting outward o Dynein’s and other motor proteins in kinetochore  Attached to the kinetochores fibrous crown  Are capable of “walking” down microtubules from their plus ends near the kinetochore toward their minus ends at the spindle  Reminiscent of the way that kinesin walks down microtubules during vesicle transport or tat dynein walks down the microtubule doublets in flagella o The combination of shape changes and attachment-detachment cycle in motor proteins causes them to walk down the length of a microtubule or other fiber o As Anaphase gets under way, proteins in the kinetochore catalyze the loss of tubulin subunits at the plus end of the spindle fiber, while dynein’s and other kinetochore motor proteins walk toward the minus ends o As the microtubules shortens and the detach-move-reattach cycle of the motor proteins repeats, the chromosome is pulled to one end of the mitotic spindle o Figure 11.12, page 245 11.3 Control of the Cell Cycle  Intestinal cells divide more than twice a day to renew tissue that is lost during digestion, mature human nerve and muscle cells do not divide at all  differences due to variation in len1th of G Phase o In rapidly dividing cells – G1 Phase is essentially eliminated o Most non dividing cells – Stuck in the G1 Phase  referred to 0s G Phase  G0Phase – The nondividing state o Mature, fully formed nerve cells and muscle cells are0in G o Note that in other cells, such as liver cells, they can go from G back into the cell 0 o This is triggered by external cues (such as growth factors that are released upon injury) o Cells in 0 have effectively exited the cell cycle; sometimes referred to as post-mitotic  Cell division rate can vary in response to changes in conditions Bio 1A03 o Human liver cells normally divide about once per year – but if a part of the liver is damaged or lost, the remaining cells divide every one or two days until repair is accomplished o Cells of unicellular organisms such as yeasts, bacteria or Archae divide rapidly only if the environment is rich in nutrients – otherwise they enter a quiescent (inactive) state  The cell cycle must be regulated in some way  Regulation varies among cells and organisms  Cell Cycle Control Molecules o Heterokaryon – “different nuclei” The Discovery of Cell-cycle Regulatory Molecules  Cell Fusion Experiments (Johnson and Rao Experiments) o Experiments on fusing pairs of mammalian cells that were growing in cell culture was the first solid evidence for cell-cycle control molecules o In the presence of certain chemicals, viruses or an electric shock – the membranes f two cells that are growing in culture can be made to fuse, creating a single cell with two nuclei o Investigators fused cells that were in different stages of the cell cycle, causing certain nuclei changed phases  Figure 11.13a, page 246; Cell Fusion Experiments  M Phase (mitotic) cell is fused to 1 G phase cell  Nucleus of G 1hase cell enters M phase  S Phase cell is fused to 1 G phase cell  G 1hase cell beings replicating its DNA (enters S phase)  S phase cell is fused to 2 G phase cell  Chromosomes in g ph2se cell do not replicate (do not enter S phase)  Microinjection Experiments (Markert and Masui Experiments) o Experiments on the South African claw-toed frog, Xenopus laevis  As an egg of these frogs matures it changes from an oocyte (arrested in a phase similar t2 G ), to a mature egg that has entered M phase o When biologists purified cytoplasm from frog eggs in one phase and injected it into the cytoplasm of frog oocytes in another phase  Figure 11.13b, page 246; Microinjection Experiments  M Phase cytoplasm is injected into a 2 phase cell  G 2hase cell begins M phase  Interphase cytoplasm is injected into G phase cell 2  G 2hase cell remains in G 2hase  Observations suggest  Regulatory molecules control entry into M and S phases  Cytoplasm of M-phase cells (but not cytoplasm of interphase cells) contains a factor/molecular signal that drives immature oocytes into M phase to complete their maturation and initiate mitosis o Mitosis-Promoting Factor (MPF) – factor that induces mitosis in all eukaryotes  Mitosis-Promoting Factor (MPF) o Made up of two distinctive polypeptide subunits
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