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Transcript
2016-‐06-‐13 • In unicellular organisms, division of one cell reproduces the entire organism • Multicellular organisms depend on cell division for Cell cycle & Mitosis • Development from a fertilized cell • Growth • Repair Review • Cell division is part of the cell cycle, the life of a cell from formation to its own division Cellular Organization of the Genetic Material • All the DNA in a cell is called the genome • A genome can consist of a single DNA molecule (common in prokaryotic cellsbacteria) or a number of DNA molecules (common in eukaryotic cells-membrane bound organelles) • DNA molecules in a cell are packaged into chromosomes Figure 12.4 • Eukaryotic chromosomes consist of chromatin • a complex of DNA and protein that condenses during cell division • Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus • Somatic cells (body cells) have two (2n) sets of chromosomes • Gametes (reproductive cells) have half (n) as many chromosomes as somatic cells Figure 12.5-3 Chromosomes 1 • During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei • Once separate, the chromatids are called chromosomes Chromosomal DNA molecules Centromere Chromosome arm Chromosome duplication (including DNA replication) and condensation 2 Centromere Sister chromatids Separation of sister chromatids into two chromosomes 3 Sister chromatids 1 2016-‐06-‐13 Phases of the Cell Cycle • 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 • The cell cycle consists of • Mitotic (M) phase (mitosis and cytokinesis) • Interphase (cell growth and copying of chromosomes in preparation for cell division) Figure 12.6 INTERPHASE • Interphase (about 90% of the cell cycle) can be divided into subphases • The cell grows during all three phases, but chromosomes are duplicated only during the S phase S (DNA synthesis) G1 es kin to Mi to sis • G1 phase ( first gap ) • S phase ( synthesis ) • G2 phase ( second gap ) y is G2 M C (M) ITOTIC PHA SE Cell Cycle Control System • G1 Phase • make addi5onal cytoplasm and organelles • S Phase • Checkpoint = control point where stop/go signals regulate the cell cycle • DNA and centrisomes replicate • cell complement 2n • G2 Phase • cell growth con5nues • enzymes and proteins needed for cell division are created © 2011 Pearson Education, Inc. 2 2016-‐06-‐13 Major Checkpoints G1 Checkpoint 1. G1 checkpoint (Most important!) Controlled by cell size, growth factors, environment “Go” à completes whole cell cycle “Stop” à cell enters nondividing state (G0 Phase) • • • • Nerve, muscle cells stay at G0; liver cells called back from G0 2. G2 checkpoint • Controlled by DNA replication completion, DNA mutations, cell size 3. M-spindle (Metaphase) checkpoint • Check spindle fiber (microtubule) attachment to chromosomes at kinetochores (anchor sites) M-spindle Checkpoint: Mitotic spindle at metaphase • 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 Kinetochore = proteins associated with DNA at centromere Prophase • Mitosis is conventionally divided into five phases • • • • • Prophase Prometaphase Metaphase Anaphase Telophase • Cytokinesis overlaps the latter stages of mitosis • The chromosomes of the cell get ready to be moved around by coiling themselves up into 5ght liIle packages. (During interphase, DNA is spread throughout the nucleus of the cell in long thin strands). • The chromosome coil up and become visible • The nuclear membrane breaks down • The mito5c spindle forms and aIaches to the chromosome • The nucleoli break down and become visible © 2011 Pearson Educaton, Inc. 3 2016-‐06-‐13 Metaphase Anaphase • The chromosomes are tugged by the mito5c spindle un5l they are all lined up in the middle of the cell. • This phase is dis5nct with a metaphase plate. • The replicated chromosomes are separated so that the two sister chroma5ds (iden5cal halves) from each replicated chromosome go to opposite sides. This way each new cell has one copy of each DNA molecule from the parent cell when cell division is over. Telophase • The cell gets ready to divide into two by forming new nuclear membranes around the separate sets of chromosome. The two daughter nuclei each have a copy of every chromosome that was in the parent cell. • New nuclear membrane form around the two sets of chromosomes • The chromosomes uncoil and spread throughout the nucleus • The mito5c spindle breaks down • The nucleoli reform and become visible again • In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow • In plant cells, a cell plate forms during cytokinesis Animation: Cytokinesis Figure 12.12-4 Binary Fission in Bacteria • Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission • In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart • The plasma membrane pinches inward, dividing the cell into two Origin of replication E. coli cell 1 Chromosome replication begins. 2 Replication continues. Cell wall Plasma membrane Bacterial chromosome Two copies of origin Origin Origin 3 Replication finishes. 4 Two daughter cells result. 4 2016-‐06-‐13 Figure 12.10a (a) Cleavage of an animal cell (SEM) Cleavage furrow Contractile ring of microfilaments 100 µm Daughter cells 5