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2.31 Cell Division

Where do cells come from? Cell

• basic unit of structure and function of living things.
• all cells come from preexisting cells through the process of cell division. The cell may be the simplest bacterium or a complex muscle, bone, or blood cell. -The cell may comprise the whole organism, or be just one cell of trillions. Cell Division -You consist of a great many cells, but like all other organisms, you started life as a single cell. How did you develop from a single cell into an organism with trillions of cells? The answer is cell division. After cells grow to their maximum size, they divide into two new cells. These new cells are small at first, but they grow quickly and eventually divide and produce more new cells. This process keeps repeating in a continuous cycle. -Cell division is the process in which one cell, called the parent cell , divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic or eukaryotic. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no nucleus, and few other cell structures. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus, and many other organelles. All of these cell parts must be duplicated and then separated when the cell divides. A chromosome is a coiled structure made of DNA and protein, and will be the focus of a subsequent concept. Cell Division in Prokaryotes Most prokaryotic cells divide by the process of binary fission. A bacterial cell dividing this way is depicted in Binary Fission -method of asexual reproduction that occurs in prokaryotic cells in which a parent cell divides into two identical daughter cells. Binary Fission in a Bacterial Cell. Cell division is relatively simple in prokaryotic cells. The two cells are dividing by binary fission. Green and orange lines indicate old and newly-generated bacterial cell walls, respectively. Eventually the parent cell will pinch apart to form two identical daughter cells. Left, growth at the center of bacterial body. Right, apical growth from the ends of the bacterial body. Bacterium - single-celled prokaryotic organism that is a member of the Bacteria domain. Bone - hard tissue consisting of the protein collagen and minerals such as calcium that generally makes up most of the vertebrate endoskeleton. Organism - an individual living thing. Parent cell - Starting cell prior to cell division. Daughter cells - a new cell that results from cell division; divides from parent cell. Eukaryotes - organism that has cells containing a nucleus and other membrane- bound organelles. Prokaryotes - single-celled organism that lacks a nucleus and other membrane- bound organelles. Chromosome - coiled structure made of DNA and protein that is present during cell division and visible with a light microscope DNA - double-stranded nucleic acid that stores genetic information; also known as deoxyribonucleic acid. Protein - organic compound made up of amino acids.

Binary fission can be described as a series of steps, although it is actually a continuous process. They include DNA replication, chromosome segregation, and finally the separation into two daughter cells. Step 1: DNA Replication. Just before the cell divides, its DNA is copied in a process called DNA replication. This results in two identical chromosomes instead of just one. This step is necessary so that when the cell divides, each daughter cell will have its own chromosome. Step 2 : Chromosome Segregation. The two chromosomes segregate, or separate, and move to opposite ends (known as "poles") of the cell. This occurs as each copy of DNA attaches to different parts of the cell membrane. Step 3: Separation. A new plasma membrane starts growing into the center of the cell, and the cytoplasm splits apart, forming two daughter cells. As the cell begins to pull apart, the new and the original chromosomes are separated. The two daughter cells that result are genetically identical to each other and to the parent cell. New cell wall must also form around the two cells. Cell Division in Eukaryotes Cell division is more complex in eukaryotes than prokaryotes. Prior to dividing, all the DNA in a eukaryotic cell’s multiple chromosomes is replicated. Its organelles are also duplicated. Then, when the cell divides, it occurs in two major steps: Step 1: Mitosis , a multi-phase process in which the nucleus of the cell divides. During mitosis, the nuclear membrane breaks down and later reforms. The chromosomes are also sorted and separated to ensure that each daughter cell receives a diploid number (2 sets) of chromosomes. In humans, that number of chromosomes is 46 (23 pairs). Mitosis is described in greater detail in a subsequent concept. Step 2: Cytokinesis. As in prokaryotic cells, the cytoplasm must divide. Cytokinesis is the division of the cytoplasm in eukaryotic cells, resulting in two genetically identical daughter cells. Summary  Cell division is part of the life cycle of virtually all cells. Cell division is the process in which one cell divides to form two new cells.  Most prokaryotic cells divide by the process of binary fission.  In eukaryotes, cell division occurs in two major steps: mitosis and cytokinesis. DNA replication -process occurring before cell division in which DNA is copied. Cell membrane - thin coat of phospholipids that surrounds a cell and controls what enters and leaves the cell Cytoplasm - material inside the cell membrane, including the watery cytosol and other cell structures except the nucleus if one is present. Cell wall - rigid layer that surrounds the cell membrane in plants, fungi, and bacteria that helps support and protect the cell. Mitosis - division of the nucleus in a eukaryotic cell, which occurs in four phases: prophase, metaphase, anaphase, and telophase. Diploid - referring to the total number of chromosomes in a species; having two sets of chromosomes. Cytokinesis - last event in cell division, when the cytoplasm divides, and daughter cells form.

2.32 Cell Cycle

The sequence of phases leading up to cell division and then ending with cell division itself is called the cell cycle. The Cell Cycle The cell cycle is a repeating series of events that includes growth, DNA synthesis, and cell division. The eukaryotic cell spends most of its "life" in interphase of the cell cycle, which can be subdivided into the three phases, G1, S and G2. Cell division is just one of several stages that a cell goes through during its lifetime. The cell cycle in prokaryotes is quite simple: the cell grows, its DNA replicates, and the cell divides. In eukaryotes, the cell cycle is more complicated. The Eukaryotic Cell Cycle The cell cycle of a eukaryotic cell is represented below. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase. During interphase , the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below. Interphase Interphase of the eukaryotic cell cycle can be subdivided into the following three phases: Gap 1 (G1): during this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes proteins needed for DNA replication and copies some of its organelles in preparation for cell division. This phase is sometimes referred to as Growth Phase 1. Synthesis Phase (S): during this phase, the cell’s DNA is copied in the process of DNA replication. Gap 2 (G2): during this phase, the cell makes final preparations to divide. For example, it makes additional proteins and organelles. This phase is sometimes referred to as Growth Phase 2. Mitosis- The four phases of mitosis are prophase, metaphase, anaphase and telophase Control of the Cell Cycle The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in the Figure below. There are a number of main checkpoints. The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide. The S checkpoint determines if the DNA has been replicated properly. The mitotic spindle checkpoint occurs at the point in metaphase where all the chromosomes should have aligned at the mitotic plate. (Checkpoints in the eukaryotic cell cycle ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.)

Cancer and the Cell Cycle Cancer is a disease that occurs when the cell cycle is no longer regulated. This may happen because a cell’s DNA becomes damaged. Damage can occur due to exposure to hazards such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. They may form a mass of abnormal cells called a tumor. The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death. Summary  The cell cycle is a repeating series of events that cells go through. It includes growth, DNA synthesis, and cell division. In eukaryotic cells, there are two growth phases, and cell division includes mitosis and cytokinesis.  The cell cycle is controlled by regulatory proteins at three key checkpoints in the cycle. The proteins signal the cell to either start or delay the next phase of the cycle.  Cancer is a disease that occurs when the cell cycle is no longer regulated. Cancer cells grow rapidly and may form a mass of abnormal cells called a tumor.