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Biology 101 Chapter 9 The Cell Cycle Lecture Outline 9.1 Cell Division Most cell division results in the distribution of identical genetic material—DNA—to two daughter cells DNA is passed from one generation of cells to the next with remarkable fidelity All the DNA in a cell constitutes the cell’s genome A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) DNA molecules in a cell are packaged into chromosomes Eukaryotic chromosomes consist of chromatin a complex of DNA and protein Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Stomatic cells (nonreproductive cells) have two sets of chromosomes Gametes (reproductive cells: sperm and eggs) have one set of chromosomes In preparation for cell division, DNA is replicated and the chromosomes condense Each duplicated chromosome has two sister chromatids, joined identical copies of the original chromosome The centromere is where the two chromatids are most closely attached During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei Once separate, the chromatids are called chromosomes Eukaryotic cell division consists of Mitosis the division of the genetic material in the nucleus Cytokinesis the division of the cytoplasm Gametes are produced by a variation of cell division called meiosis Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell 9.2 Interphase and Mitotic Phases The cell cycle consists of Mitotic (M) phase, including mitosis and cytokinesis Interphase, including cell growth and copying of chromosomes in preparation for cell division Interphase (about 90% of the cell cycle) can be divided into subphases G1 phase (“first gap”) S phase (“synthesis”) G2 phase (“second gap”) The cell grows during all three phases, but chromosomes are duplicated only during the S phase Mitotic (M) Phase, including mitosis and cytokinesis Mitosis is conventionally divided into five phases (PMAT) Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis overlaps the latter stages of mitosis The mitotic spindle is a structure made of microtubules and associated proteins It controls chromosome movement during mitosis In animal cells, assembly of spindle microtubules begins in the centrosome, a type of microtubule organizing center The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase An aster (radial array of short microtubules) extends from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes Kinetochores are protein complexes that assemble on sections of DNA at centromeres At metaphase, the centromeres of all the chromosomes are at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles In anaphase, sister chromatids separated and move along the kinetochore microtubules toward opposite ends of the cell The microtubules shorten by depolymerizing at their kinetochore ends Chromosomes are also “reeled in” by motor proteins at spindle poles, and microtubules depolymerize after they pass by the motor proteins Non-kinetochore microtubules from opposite poles overlap and push against each other, elongating the cell At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell Cytokinesis begins during anaphase or telophase, and the spindle eventually disassembles In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow In plant cells, a cell plate forms during cytokinesis Binary Fission in Bacteria Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission In E. coli, the single chromosome replicates, beginning at the origin of replication The two daughter chromosomes actively move apart while the cell elongates The plasma membrane pinches inward, dividing the cell into two Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission Certain protists (dinoflagellates, diatoms, and some yeasts) exhibit types of cell division that seem intermediate between binary fission and mitosis 9.3 Regulation of Eukaryotic Cell Cycle The frequency of cell division varies with the type of cell These differences result from regulation at the molecular level Cancer cells manage to escape the usual controls on the cell cycle The cell cycle is driven by specific signaling molecules present in the cytoplasm Some evidence for this hypothesis comes from experiments with cultured mammalian cells Cells at different phases of the cell cycle were fused to form a single cell with two nuclei at different stages Cytoplasmic signals from one of the cells could cause the nucleus from the second cell to enter the “wrong” stage of the cell cycle The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a timing device of a washing machine The cell cycle control system is regulated by both internal and external controls The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received For many cells, the G1 checkpoint 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 If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase Internal Control of Cell Cycle The cell cycle is regulated by a set of regulatory proteins and protein complexes including kinases and proteins called cyclins. An example of an internal signal occurs at the M phase checkpoint In this case, anaphase does not begin if any kinetochores of the chromosomes remain unattached to spindle microtubules Attachment of all of the kinetochores activates a regulatory complex, which then activates the enzyme separase Separase allows sister chromatids to separate, triggering the onset of anaphase External Control of Cell Cycle Some external signals are growth factorproteins released by certain cells that stimulate other cells to divide For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture Another example of external signals is density- dependent inhibition, in which crowded cells stop dividing Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide Cancer Cells Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence Cancer cells do not respond to signals that normally regulate the cell cycle Cancer cells do not need growth factors to grow and divide They may make their own growth factor They may convey a growth factor’s signal without the presence of the growth factor They may have an abnormal cell cycle control system A normal cell is converted to a cancerous cell by a process called transformation Cancer cells that are not eliminated by the immune system form tumors, masses of abnormal cells within otherwise normal tissue If abnormal cells remain only at the original site, the lump is called a benign tumor Malignant tumors invade surrounding tissues and undergo metastasis, exporting cancer cells to other parts of the body, where they may form additional tumors
