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Unit Chunk 2A: Mitosis, the Cell Cycle, Gene Expression & Cell Differentiation Day One: The Cell Cycle - Interphase • • • • Cells do not continue to grow bigger as organisms do. Cells continue to produce more cells to foster the growth of an organism. Cell division is how eukaryotic cells reproduce to make new cells. Mitotic cell division—MITOSIS—produces two new clone daughter eukaryotic cells with the same DNA as the original parent cell. Eukaryotes divide via the Cell Cycle. • There are two Stages for the Cell Cycle: 1) Interphase – G1, S, and G2 2) Mitosis EQ: Describe the Interphase Stage of the Cell Cycle. • • • • First Stage – Interphase – G1, S, & G2 Growth 1 (G1) - the phase in which a cell spends most of its time growing, synthesizing proteins, and performing its job for the organism Towards the very end of this phase, the cell gets ready to divide by reproducing its organelles. Synthesis (S) – DNA Replication: in this phase the cell makes an exact duplicate of DNA in its nucleus. The chromatin now appears as a dense mass in the nucleus. Growth 2 (G2) – a cell checkpoint occurs; if there are no errors in the replicated DNA and the cell is ready, “GO” proteins signal the cell to proceed into mitosis. If DNA errors are detected or the cell is not ready, “Stop” proteins signal the cell to NOT proceed with mitotic division and to make repairs. G2 – If the cell cannot repair the damaged DNA, proteins will signal the cell to undergo programmed death – Apoptosis. If the cell does not die, uncontrolled cell growth may lead to cancer. If a “Stop” or “Go” protein gets damaged, this may also result in uncontrolled cell division or cancer. Day Two: The Cell Cycle - Mitosis EQ: Describe the Second Stage of the Cell Cycle--Mitosis. • • • • • • • • • • • Second Stage – Mitosis – M Cell’s nucleus divides into 2 new nuclei Cell divides to form two new cells Chromatin condenses to form distinct X chromosomes and the nuclear membrane breaks down X chromosomes line up in the middle of cell X chromosomes separate and move to opposite ends of the cell. New nuclear membrane forms around each of the two new sets of chromosomes. Chromosomes unwind to become chromatin Cytoplasm divides to become two new daughter cells with same DNA as the parent EQ: What three functions does Mitosis perform for a multicellular organism? Mitosis produces: New cells for the growth of a multicellular organism New cells to replace dead cells to maintain a multicellular organism New cells that can differentiate to create different types of cells that do different jobs for a multicellular organism Day Three: Gene Expression & Cell Differentiation In the development of most multicellular organisms, a single cell--a fertilized egg called a zygote—reproduces to make identical cells called embryonic stem cells. Each stem cell has the potential to become a specialized body cell, with a different structure for a specific function. A zygote produces a large number of embryonic cells through mitosis. Mitosis results in two new daughter cells with the same DNA--two clone cells. So the process of cell division alone only leads to increasing numbers of identical cells. * Through gene expression, embryonic cells express, activate, or repress, do not activate, specific groups of genes. This, in turn, controls what proteins each cell will make. It also controls what proteins a cell will NOT make. * This causes cell differentiation, the production of cells that are different in structure and function. These embryonic cells have made different specialized cells --BUT ALL THESE CELLS STILL HAVE THE SAME DNA!!! EQ: HOW does gene expression regulate cell differentiation??? Gene expression regulates cell differentiation by determining what groups of genes are expressed or repressed in a cell, and this controls what proteins are made or not made for that cell. This makes cells different and specialized in both their structure and function-cell differentiation. Day Four: Gene Expression & Cell Differentiation Cell Development and Differentiation in Humans 1) Sperm and egg--the fusion of two different sets of DNA; 23 chromosomes in each 2) The product: one fertilized cell, a zygote with 46 chromosomes 3) Zygote divides into two clone stem cells, which divide into four identical stem cells, which divide into eight stem clones and so forth. 4) Cells on the outside, inside, and middle initiate gene expression and begin to differentiate. 5) Cell reproduction and differentiation continues, and the specialized cells begin to organize into groups that will form the tissues, organs, and organ systems of the most complex multicellular organism—a human being. • • Embryonic cells, which have not yet differentiated into various cell types, are called Embryonic Stem Cells - ESCs. Stem cells found in adult organisms, for instance in bone marrow, are called Adult Stem Cells - ASCs. All these cells--zygote, stem cells, and all the specialized cells--have the SAME genes with the same DNA! But the genes in these cells express themselves differently making different cells with different structures and functions, or cell differentiation. EQ: Distinguish between a stem cell and a specialized differentiated cell like a blood cell or neuron. Stem cells are unspecialized cells that can reproduce to make more clone stem cells, and, they also have the ability to differentiate into one or more types of specialized body cells through gene expression. Specialized body cells that have differentiated, like a blood cell or a neuron, can only reproduce clone cells like themselves. Once a cell differentiates, the process cannot be reversed! The process of cell differentiation is the basis for the hierarchical organization of multicellular organisms and their cells, tissues, organs, and organ systems. EQ: Develop and label a model to illustrate the role of mitosis and cell differentiation in producing and maintaining complex multicellular organisms.