The Cell Cycle (Cellular Organization)

The Cell Cycle MITOSIS      In unicellular organisms, division of one cell reproduces the entire organism Multicellular eukaryotes depend on cell division for Growth, repair, and development of a fertilized egg Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division Most cell divisions result in two daughter cells with identical genetic information (DNA) The exception is meiosis, a special type of division that can produce sperm and egg cells Cellular Organization of Genetic Material    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 few DNA molecules (common in eukaryotic cells) DNA molecules in a cell are packaged into chromosomes. DNA      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 (NONREPRODUCTIVE CELLS) have two sets of chromosomes (46 chromosomes in total) Gametes (REPRODUCTIVE CELLS: SPERM AND EGGS) have half as many chromosomes as somatic cells (23 chromosomes in total) Chromatin looks like spaghet Distribution of Chromosomes During Eukaryotic Cell Division    In preparation for cell division, DNA is replicated, and the chromosomes condense Each duplicated chromosome has two sister chromatids (joined copies of the original chromosome) attached along their lengths by cohesion The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached. Insert picture of sister chromatids Divisions    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 half as many chromosomes as the parent cell Phases of the cell cycle The cell cycle consists of  Mitotic phase (M phase) – mitosis and cytokinesis take place  Interphase – cell growth and copying of chromosomes in preparation for cell division Interphase (about 90% of the cell cycle) is divided into 3 parts  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.    The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is like a clock 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 Checkpoints    For many cells, the G1 check point is the most important If it receives a go ahead signal at the G1 checkpoint, it usually completes the following phases and divides If it does NOT receive the go- ahead signal, it will exit the cell cycle, switching into a nondividing state called the G0 phase External Factors     Specific growth factors influence cell division Growth factors are released by certain cells and stimulate other cells to divide Platelet derived growth factor (PDGF) is made by blood cell fragments called platelets In density-dependent inhibition, crowded cells will stop dividing Example: Epithelial Cells Cancer Cells  Cancer cells do not respond normally to the body’s control mechanisms Cancer cells do not need growth factors to grow and divide 1. They may make their own growth factor 2. They may convey a growth factor’s signal without the presence of the growth factor 3. They may have an abnormal cell cycle control system Tumors  Stages of Mitosis       Prophase Pro-metaphase Metaphase Anaphase Telophase Cytokinesis The Mitotic Spindle  The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis        In animal cells, assembly of spindle microtubules begins in the centrosome, the 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 is a radial array of short microtubules that 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 associated with centromeres At metaphase, the chromosomes are all lined up at the metaphase plate, a plane midway between the spindle’s two poles. Cytokinesis  In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow They pinch the cell until it splits into two.  In plant cells, a cell plate forms during cytokinesis Instead of pinching, there are vesicles that come together to create a new well wall in the center of the cell 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 or replication) and the two daughter chromosomes actively move apart The plasma membrane pinches inward, dividing the cell into two