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Cell Cycle

Pearson
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In this lesson, we will explore the cell cycle. Specifically, we will answer the following questions: what are the phases of the cell cycle, what happens during each phase, and what happens during cell division, or cell reproduction? Like us, our cells have life spans. According to the cell theory, cells come from preexisting cells. Cells are born, so to speak, through the division of previous cells. Each new cell then grows up, if you will, conducting its usual cell jobs and living its life. And finally, it reproduces, dividing to produce two new daughter cells, and the process begins anew. All cells in your body began with one common cell, the zygote, that arose when dad's sperm fertilized mom's egg. All the trillions of cells in your body with all their diversity are derived from that one original cell, which itself came from your parents' cells. Once born, cells go through specific stages in their existence. These are the phases of the cell cycle. Most simply, the cell cycle is divided into two parts: interphase and cell division, which is also called cell reproduction. Which part do you think takes up most of a typical cell cycle? Here's a hint. Think about humans. We spend far more time living our lives and working than we do reproducing, right? It's the same for cells. Most of their time is spent in interphase. In fact, some cells stay in interphase and never reproduce. New cells begin in interphase, and interphase itself is usually divided into three phases: G1, or gap 1; s, or synthesis; and G2, or gap 2. During G1 of interphase a cell grows to its full size and does its usual cellular work. It also begins preparing for cell reproduction. Two notable events during the synthesis phase of interphase are centriole duplication and DNA replication. We'll come back to the centrioles later. Recall that DNA is your genetic material. It contains all of the genes that control your cell functions and make you who you are. With the exception of the egg or sperm and red blood cells that lack nuclei, each human cell contains a complete set of the individual's total DNA. Human DNA is organized into 46 chromosomes, as shown here. Rather like socks our chromosomes come in pairs, 23 pairs. Because each cell needs a full set of DNA, the original cell's DNA must be copied before the cell divides. Let's go back to socks. Let's say you start with one pair of brown socks and one pair of blue socks and you want to pack two suitcases that each contain the original number of socks. For that to happen you would need to double the number of socks before packing. You would need two pairs of each color, right? Our chromosomes work the same way. They must double before cell reproduction so both daughter cells can get a full set, all 23 pairs. DNA replication during the synthesis phase of interphase is the process by which a cell's DNA is copied, or reproduced, so that each daughter cell can receive a full set of DNA. Recall that a DNA molecule is shaped somewhat like a twisted ladder. During DNA replication, the sides of the ladder separate so the steps are exposed, and then each side of the ladder is rebuilt from DNA building-block molecules called nucleotides. The end result is two complete DNA ladders, each identical to the original. After the DNA is replicated, the cell enters the G2 phase of interphase to continue preparing for cell reproduction. Cell reproduction, or cell division, includes two processes: mitosis, which is nuclear division, and cytokinesis, which is cytoplasmic division. Because DNA is so important to our cells, division of the nucleus is its own process, mitosis, and it is a very precise process, to be sure that the DNA is divided equally into the daughter cells. Mitosis includes four phases: prophase, metaphase, anaphase, and telophase. We will hit the high points of each of these. During interphase, the DNA is stretched out in thin strands called chromatin. Imagine the nucleus looking like a bowl of long, tangled stands of angel hair pasta. During prophase, the first phase of mitosis, the chromatin condenses into rod-like structures called chromosomes. They are somewhat like short, wide lasagna noodles and are more easily separated. The centrioles begin to move apart, and delicate spindle fibers extend from them and attach to each chromosome, to direct their movements during the rest of mitosis. The nuclear envelope breaks down during prophase so the chromosomes can move freely, and the nucleolus dismantles as well. The next phase of mitosis is metaphase. Meta means middle, and during metaphase the chromosomes align very precisely along the midline of the cell. Anaphase is next. Ana means away, and during anaphase the replicated chromosomes separate. The spindle fibers pull the chromosomes apart, and one complete set goes to each side of the cell. The final stage of mitosis is telophase. The chromosomes finish migrating to opposite sides of the cell, and this stage is rather like a reverse of prophase. The nuclear envelope reappears as does the nucleolus, and the chromosomes relax back into stretched-out chromatin strands. Mitosis is a very precise division of the nucleus. But there is more to a cell than its nucleus. We must still divide the rest of the cell, the cytoplasm. Cytoplasmic division is called cytokinesis. It overlaps the latter part of mitosis. Because the goal of mitosis is to get a full set of DNA into each new cell, cytokinesis doesn't start until anaphase, after the chromosomes separate. It concludes at the end of telophase, finally splitting the original cell into two new daughter cells. And these cells begin their lives in interphase, and the cell cycle begins all over again. The cell cycle allows for central processes such as growth and development, repair, and basic maintenance that keeps our bodies going.
In this lesson, we will explore the cell cycle. Specifically, we will answer the following questions: what are the phases of the cell cycle, what happens during each phase, and what happens during cell division, or cell reproduction? Like us, our cells have life spans. According to the cell theory, cells come from preexisting cells. Cells are born, so to speak, through the division of previous cells. Each new cell then grows up, if you will, conducting its usual cell jobs and living its life. And finally, it reproduces, dividing to produce two new daughter cells, and the process begins anew. All cells in your body began with one common cell, the zygote, that arose when dad's sperm fertilized mom's egg. All the trillions of cells in your body with all their diversity are derived from that one original cell, which itself came from your parents' cells. Once born, cells go through specific stages in their existence. These are the phases of the cell cycle. Most simply, the cell cycle is divided into two parts: interphase and cell division, which is also called cell reproduction. Which part do you think takes up most of a typical cell cycle? Here's a hint. Think about humans. We spend far more time living our lives and working than we do reproducing, right? It's the same for cells. Most of their time is spent in interphase. In fact, some cells stay in interphase and never reproduce. New cells begin in interphase, and interphase itself is usually divided into three phases: G1, or gap 1; s, or synthesis; and G2, or gap 2. During G1 of interphase a cell grows to its full size and does its usual cellular work. It also begins preparing for cell reproduction. Two notable events during the synthesis phase of interphase are centriole duplication and DNA replication. We'll come back to the centrioles later. Recall that DNA is your genetic material. It contains all of the genes that control your cell functions and make you who you are. With the exception of the egg or sperm and red blood cells that lack nuclei, each human cell contains a complete set of the individual's total DNA. Human DNA is organized into 46 chromosomes, as shown here. Rather like socks our chromosomes come in pairs, 23 pairs. Because each cell needs a full set of DNA, the original cell's DNA must be copied before the cell divides. Let's go back to socks. Let's say you start with one pair of brown socks and one pair of blue socks and you want to pack two suitcases that each contain the original number of socks. For that to happen you would need to double the number of socks before packing. You would need two pairs of each color, right? Our chromosomes work the same way. They must double before cell reproduction so both daughter cells can get a full set, all 23 pairs. DNA replication during the synthesis phase of interphase is the process by which a cell's DNA is copied, or reproduced, so that each daughter cell can receive a full set of DNA. Recall that a DNA molecule is shaped somewhat like a twisted ladder. During DNA replication, the sides of the ladder separate so the steps are exposed, and then each side of the ladder is rebuilt from DNA building-block molecules called nucleotides. The end result is two complete DNA ladders, each identical to the original. After the DNA is replicated, the cell enters the G2 phase of interphase to continue preparing for cell reproduction. Cell reproduction, or cell division, includes two processes: mitosis, which is nuclear division, and cytokinesis, which is cytoplasmic division. Because DNA is so important to our cells, division of the nucleus is its own process, mitosis, and it is a very precise process, to be sure that the DNA is divided equally into the daughter cells. Mitosis includes four phases: prophase, metaphase, anaphase, and telophase. We will hit the high points of each of these. During interphase, the DNA is stretched out in thin strands called chromatin. Imagine the nucleus looking like a bowl of long, tangled stands of angel hair pasta. During prophase, the first phase of mitosis, the chromatin condenses into rod-like structures called chromosomes. They are somewhat like short, wide lasagna noodles and are more easily separated. The centrioles begin to move apart, and delicate spindle fibers extend from them and attach to each chromosome, to direct their movements during the rest of mitosis. The nuclear envelope breaks down during prophase so the chromosomes can move freely, and the nucleolus dismantles as well. The next phase of mitosis is metaphase. Meta means middle, and during metaphase the chromosomes align very precisely along the midline of the cell. Anaphase is next. Ana means away, and during anaphase the replicated chromosomes separate. The spindle fibers pull the chromosomes apart, and one complete set goes to each side of the cell. The final stage of mitosis is telophase. The chromosomes finish migrating to opposite sides of the cell, and this stage is rather like a reverse of prophase. The nuclear envelope reappears as does the nucleolus, and the chromosomes relax back into stretched-out chromatin strands. Mitosis is a very precise division of the nucleus. But there is more to a cell than its nucleus. We must still divide the rest of the cell, the cytoplasm. Cytoplasmic division is called cytokinesis. It overlaps the latter part of mitosis. Because the goal of mitosis is to get a full set of DNA into each new cell, cytokinesis doesn't start until anaphase, after the chromosomes separate. It concludes at the end of telophase, finally splitting the original cell into two new daughter cells. And these cells begin their lives in interphase, and the cell cycle begins all over again. The cell cycle allows for central processes such as growth and development, repair, and basic maintenance that keeps our bodies going.