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BIOL 130 Study Notes Unit IX Cell Cycle

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University of Waterloo
BIOL 130
Richard Ennis

Unit 9 Cell Cycle, Mitosis, and Meiosis Part A: Mitosis  Key Concepts to Consider: • Cell division involves an orderly sequence of events in which a cell duplicates its contents and divides • Details of cell cycle vary depending on the organism and on different times in an organism’s life • Unicellular organisms generate a completely new organism (bacteria, yeast, etc.) • Metazoans (animals) o Many cell divisions required to generate new organisms from fertilized egg The Features of Cell Division that are Universal: • Cell must replicate DNA faithfully • Replicated DNA must be accurately distributed to daughter cells • Before the cell divides, it needs to be large enough and have duplicated its macromolecules and organelles Eukaryotes: Mitosis vs. Meiosis • Mitosis o Division produces daughter cells that are genetically identical to parent cell o Two daughter cells are formed at the end when the cytoplasm splits (cytokinesis) o Mitosis is a process used generally by all cells except for ones associated with sexual reproduction (i.e. gametes) • Meiosis o Division produces daughter cells that have only half the chromosomes found in parent o Meiosis is a process that develops gametes (eggs and sperm) only based on sexual reproduction and genetic inheritance Cell Division is Essential for Key Events in Eukaryotes: • Cell division is essential for: o Growth and development (e.g. a human embryo) o Asexual reproduction (e.g. yeast) o Repairing damage (e.g. epithelial cells lining in intestine) • The cell cycle is not the same in all cell types! o Some cells like epithelial cells divide continuously throughout life o Other cells like neurons will never divide once they are mature Eukaryotic Chromosomes: • Multiple linear chromosomes in the nucleus • Two copies of each are essential to survive • Highly variable in number; anywhere between 1 – 500 pairs o Most eukaryotes have 10-50 o Chromosome number is, however, not correlated with the complexity of the organism  E.g. humans have 46, silkworms have 56, chickens have 78 • DNA of eukaryotes is complexly packaged  associated with proteins o Typical human chromosome is 140 million nucleotides long When do Chromosomes Become Visible? • Only when the cell is about to divide  specifically during late prophase • Chromosomes condense when preparing for cell division Homologous Chromosomes vs. Sister Chromatids: • Homologous chromosomes are two different chromosomes, one paternal and one maternal, that pair up with each other during meiosis o They have the same genes, but different alleles (different variations of the genes) o When the maternal one is paired up with the paternal one, they are referred to as non-sister chromatids relative to one another o Allowing them to “cross-over” and potentially swap segments of genetic information with the other homolog • A single replicated homolog is considered to be a single chromosome that consists of 2 sister chromatids • Sister chromatids of the mitotic process are genetically identical since they were replicated from the same gene, whereas the homolog pairings in the meiosis process are different from one another A Quick Overview of the Cell Cycle: • Consists of interphase (G1, S-phase, and G2): o G1:  Period of cell growth and normal metabolism  Organelles duplicate  Chromosomes directing cell activity  Chromosomes are unipartite (1 chromosome; 1 chromatid; 1 DNA molecule) o S-phase:  DNA synthesis via semi-conservative replication  Chromosomes duplicate but their numbers remains equal  By the end, all chromosomes are bipartite (1 chromosome; 2 sister chromatids; 2 DNA molecule) o G2:  Cell grows and prepares for mitosis  Chromosomes are bipartite • The cell spends most of its time (90%) in interphase • M-phase: o Chromosomes become highly condensed and eventually visible o Series of stages for cell division o Produces 2 genetically identical daughter cells when chromatids separate and cytoplasm cleaves Two Transient Cytoskeletal Structures Required for Mitosis in Animal Cells: • These two structures are mitotic spindle fibers and the contractile ring • Mitotic spindle fiber: o Lots of tubulin synthesized during G2 phase because it is used to make the spindle microtubules used in mitosis o These fibers are required to attach onto chromosomes to pull the chromatids apart during mitosis • Contractile ring: o Made up of non-muscle actin and myosin filaments o The ring is formed equatorially in the middle of the cell and constricts the cell membrane to form a cleavage furrow o Furrow continually moves inward and allows the cell cytoplasm to eventually be cleaved into two Stages of Mitosis: 1. Prophase: o From the onset of G2 phase of interphase, all chromosomes now are bipartite, where each chromosome consists of 2 sister chromatids joined at the center by a centromere o Chromosomes condense and become visible o Early mitotic spindle begins to form and move to the poles 2. Prometaphase: o By the end of prophase the nuclear membrane and the nucleoli will have disappeared o Definitive mitotic spindle is formed and moves to the poles o Chromosomes begins to move to the center of the cell 3. Metaphase: o Metaphase chromosomes consist of 2 sister chromatids joined at the center by a centromere  Each centromere has a kinetochore that allows for the spindle fibers to bind onto it o All chromosomes line up along the metaphase plate o Spindle fibers will attach onto each sister chromatid via the kinetochore 4. Anaphase: o Spindle fibers pull the sister chromatids apart to either sides of the poles o Centromeres separate creating 2 unipartite daughter chromosomes from the bipartite chromosomes 5. Telophase & Cytokinesis: o Unipartite chromosomes reach the end of their respective poles o By late telophase chromosomes uncoil from their previously condensed forms and are not visible o Nuclear envelop reforms; spindle apparatus disintegrates o Cleavage furrow by contractile ring is present, eventually allowing the two new cells to be cleaved apart Cytokinesis: Animals vs. Plants  Animals: o Involves ring of non-muscle actin filaments under plasma membrane associated with motor proteins called myosin o Creates the cleavage furrow in the cytoplasm to allow the cell to be cleaved  Plants: o A new cell wall must be constructed between dividing plant cells o Microtubules and proteins define and organize the regions where new cell membrane and wall will form o Vesicles from the Golgi arrive carrying polysaccharides and glycoproteins to lay down the matrix for the new cell wall by fusing together o Later on cellulose fibers are added to complete the wall Do All Cells Go Through the Cell Cycle Continuously and Is the Length of the Cycle Constant?  No, not all cells go through the cycle continuously – some do, others do not o Neurons will not divide once they mature  With regards to the length of the cell cycle, that largely depends on the cell type, its developmental stage, and external signals o Examples:  Early frog embryos divide in 30 minutes  Yeast cells divide in 1.5-3 hours  Mammalian intestinal epithelial cells divide in 12 hours  Human liver cells divide in 1 year Control of the Cell Cycle: • There was a mitosis-promoting factor (MPF) found in mammalian cells that induced mitosis o Shown to do so in all eukaryotic cells • MPF is a hetero dimeric protein composed of: o Cyclin-dependent kinase (CdK)  A catalytic subunit (enzyme) that transfers phosphate from ATP to certain amino acids on target proteins  Not active unless bound to cyclin  Levels are CONSTANT o Cyclin  The regulatory subunit  Levels change/oscillate throughout the cell cycle  Has a variety of roles: • Can phosphorylate chromosomal proteins to initiate mitosis • Phosphorylate nuclear lamins to break down nuclear envelop • Phosphorylate microtubules to activate mitotic spindle fibers • Phosphorylate an enzyme that degrades cyclin causing it to decline • Not surprisingly… o M-CdK activity is most high during mitosis and correlates with the rise and fall of M-cyclin o It begins to rise during interphase from G1 to S to G2 phases Regulation of Mammalian Cell Cycle by Cyclins: • During interphase: o G1 Phase:  G1-CdK is present to help with passage through restriction point o G1/S-phase:  Near the end of G1 and the beginning of S, G1/S-CdK commits cells to DNA replication o S-phase:  S-CdK is present to actually carry out DNA replication • During Mitosis: o M-CdK is present to promotes events of mitosis Some Cells Opt Out of the Cell Cycle: • Some cells divide very slowly, but can be induced to re-enter the cell cycle o Example: liver cells • However, some cells becoming so highly specialized that they can no longer divide o We use the term terminally differentiated to describe them o Examples: neurons, muscle cells, red blood cells How Does the Cell Decide Whether to Proceed Through the Cell Cycle? • During G1 or S: Are there external signals received? Are there enough resources? o If not, then stay in G0 phase and do not proceed o This is especially pertaining to animal cells, since they require a signal to tell them to divide; dividing is NOT a default mechanism for animal cells o Also needs to be enough resources available for the cell to support mitosis, and the cell also needs to be large enough to do so • During G2: Is everything correctly in place for mitosis? o Mitosis will only occur if DNA is replicated properly and if there is no DNA damage! o If these things are not 100%, then the cell must either attempt to fix it o And if it is unable to fix it, then it will undergo programmed cell death (apoptosis) • During M: Can mitosis be completed? o Are the chromosomes properly attached to spindles? o If not, then mitosis is halted and the cell attempts to fix it  “metaphase arrest” • At the given checkpoints (indicated by red T) molecular “breaks” (usually CdK inhibitors) can pause or halt progression Fun fact: If radioactive thymine was introduced to a growth medium of the cell, it would be incorporated into the DNA during S-phase because that’s when the DNA replicates via semi- conservative replication. The Checkpoints of the Cell Cycle: • The G1/S Checkpoint & Restriction Point o Fun fact: Discovered in yeast o Involves the integration of external and internal signals o Internal signals:  Are there enough nutrients/resources to support cell division?  Is the DNA damaged?  Is the cell large enough to split into 2? o External signals:  Normal animal cells depend on an external signal to tell them to divide  If there is no signal  stay in G0 phase and do not proceed  Inappropriate “start” signals or the progression through the cell cycle without the presence of a signal is associated with cancer • G2/M Checkpoint: DNA Check! o Is the DNA properly replicated? o Is the DNA undamaged? o Is M-CdK present? • Spindle Checkpoint (Metaphase  Anaphase) o Have all the chromosomes properly moved and aligned along the metaphase plate? o Are all of them attached by a spindle fiber? Cancer: A Failure to Respect Checkpoints! • Cell division in the absence of external signals • Or inappropriate start signals: o Mutations with Ras • Failure to induce programmed cell death of a damaged cell o i.e. dividing despite damaged DNA and become genetically unstable • Each cancer is caused by a unique combination of errors Cell Cycle “Breaks”: p53 the Guardian of the Genome • Tumour suppressors:
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