{"id":1696,"date":"2014-10-21T03:46:47","date_gmt":"2014-10-21T03:46:47","guid":{"rendered":"https:\/\/courses.candelalearning.com\/apvccs\/?post_type=chapter&#038;p=1696"},"modified":"2017-07-03T21:47:25","modified_gmt":"2017-07-03T21:47:25","slug":"the-nucleus-and-dna-replication","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-ap1\/chapter\/the-nucleus-and-dna-replication\/","title":{"raw":"The Nucleus and DNA Replication","rendered":"The Nucleus and DNA Replication"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul>\r\n \t<li>Describe the structure and features of the nuclear membrane<\/li>\r\n \t<li>List the contents of the nucleus<\/li>\r\n \t<li>Explain the organization of the DNA molecule within the nucleus<\/li>\r\n \t<li>Describe the process of DNA replication<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe nucleus is the largest and most prominent of a cell\u2019s organelles (Figure 1a). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus (Figure 1b), which is known as multinucleated. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body (Figure 2). Without nuclei, the life span of RBCs is short, and so the body must produce new ones constantly.\r\n\r\n[caption id=\"attachment_3230\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-3230\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1512\/2017\/02\/23223300\/0318_Nucleusab-1024x399.jpg\" alt=\"Figure (a) shows the structure of the nucleus. The nucleolus is inside the nucleus, surrounded by the chromatin and covered by the nuclear envelope.\u00a0Micrograph (b) shows a muscle cell with multiple nuclei.\" width=\"1024\" height=\"399\" \/> Figure 1.\u00a0The Nucleus in Muscle Cells. (a)\u00a0The nucleus is the control center of the cell. The nucleus of living cells contains the genetic material that determines the entire structure and function of that cell. (b)\u00a0Unlike cardiac muscle cells and smooth muscle cells, which have a single nucleus, a skeletal muscle cell contains many nuclei, and is referred to as \u201cmultinucleated.\u201d These muscle cells are long and fibrous (often referred to as muscle fibers). During development, many smaller cells fuse to form a mature muscle fiber. The nuclei of the fused cells are conserved in the mature cell, thus imparting a multinucleate characteristic to mature muscle cells. LM \u00d7 104.3. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]\r\n\r\n<div class=\"textbox\">View the <a href=\"http:\/\/141.214.65.171\/Histology\/Basic%20Tissues\/Muscle\/058thin_HISTO_83X.svs\/view.apml\" target=\"_blank\" rel=\"noopener\">University of Michigan WebScope<\/a> to explore the tissue sample in greater detail.<\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"899\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180942\/0320_RBC_Extruding_Nucleus_Micrograph.jpg\" alt=\"This set of micrographs shows a red blood cell extruding its nucleus. In the left panel, the nucleus is partially extruded from the red blood cell and in the right panel, the nucleus is completely extruded from the cell.\" width=\"899\" height=\"309\" \/> <strong>Figure 2.\u00a0Red Blood Cell Extruding Its Nucleus.<\/strong>\u00a0Mature red blood cells lack a nucleus. As they mature, erythroblasts extrude their nucleus, making room for more hemoglobin. The two panels here show an erythroblast before and after ejecting its nucleus, respectively. (credit: modification of micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]\r\n\r\n<div class=\"textbox\">View the <a href=\"http:\/\/virtualslides.med.umich.edu\/Histology\/EMsmallCharts\/3%20Image%20Scope%20finals\/139%20-%20Erythroblast_001.svs\/view.apml\" target=\"_blank\" rel=\"noopener\">University of Michigan WebScope<\/a>\u00a0to explore the tissue sample in greater detail.<\/div>\r\nInside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA. The nucleus sends \u201ccommands\u201d to the cell via molecular messengers that translate the information from DNA. Each cell in your body (with the exception of germ cells) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that the each new cell receives a full complement of DNA. The following section will explore the structure of the nucleus and its contents, as well as the process of DNA replication.\r\n<h2>Organization of the Nucleus and Its DNA<\/h2>\r\nLike most other cellular organelles, the nucleus is surrounded by a membrane called the\u00a0<strong>nuclear envelope<\/strong>. This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A\u00a0<strong>nuclear pore<\/strong>\u00a0is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.\r\n\r\nInside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a\u00a0<strong>nucleolus<\/strong>\u00a0(plural = <em>nucleoli<\/em>). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes.\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"451\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180945\/0321_DNA_Macrostructure.jpg\" alt=\"This diagram shows the macrostructure of DNA. A chromosome and its component chromatin are shown to expand into nucleosomes with histones, which further unravel into a DNA helix and finally into a DNA ladder.\" width=\"451\" height=\"354\" \/> <strong>Figure 3.\u00a0DNA Macrostructure.<\/strong>\u00a0Strands of DNA are wrapped around supporting histones. These proteins are increasingly bundled and condensed into chromatin, which is packed tightly into chromosomes when the cell is ready to divide.[\/caption]\r\n\r\nOnce synthesized, newly made ribosomal subunits exit the cell\u2019s nucleus through the nuclear pores. The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of\u00a0<strong>chromatin<\/strong>\u00a0composed of DNA and associated proteins (Figure 3).\r\n\r\nAlong the chromatin threads, the DNA is wrapped around a set of\u00a0<strong>histone<\/strong>\u00a0proteins. A\u00a0<strong>nucleosome<\/strong>\u00a0is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones. When a cell is in the process of division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the \u201cdaughter cells.\u201d\r\n\r\nThe\u00a0<strong>chromosome<\/strong>\u00a0is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.\r\n<h2>DNA Replication<\/h2>\r\nIn order for an organism to grow, develop, and maintain its health, cells must replicated themselves by dividing to produce two new daughter cells, each with the full complement of DNA as found in the original cell. Billions of new cells are produced in an adult human every day. There are a few cell types in the body do not divide, including nerve cells, skeletal muscle fibers, and cardiac muscle cells. The division time of different cell types varies. Epithelial cells of the skin and gastrointestinal lining, for instance, divide very frequently to replace those that are constantly being rubbed off of the surface by friction.\r\n\r\nA DNA molecule is made of two strands that \u201ccomplement\u201d each other: the molecules that compose the strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups (Figure 4).\r\n\r\n[caption id=\"attachment_3773\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-3773\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1512\/2014\/10\/03214607\/0322_DNA_Nucleotides2-1024x298.jpg\" alt=\"This figure shows the DNA double helix on the far left panel. The different nucleotides are color-coded. In the center panel, the interaction between the nucleotides through the hydrogen bonds and the location of the sugar-phosphate backbone is shown. In the far right panel, the structure of a nucleotide is described in detail.\" width=\"1024\" height=\"298\" \/> <strong>Figure 4.\u00a0Molecular Structure of DNA.<\/strong>\u00a0The DNA double helix is composed of two complementary strands. The strands are bonded together via their nitrogenous base pairs using hydrogen bonds.[\/caption]\r\n\r\nThe two sides of the ladder are not identical, but are complementary. These two backbones are bonded to each other across pairs of protruding bases, each bonded pair forming one \u201crung,\u201d or cross member. The four DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine always binds with thymine, and cytosine always binds with guanine. The particular sequence of bases along the DNA molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could infer the order of the bases in one strand from the bases in the other, complementary strand. For example, if one strand has a region with the sequence AGTGCCT, then the sequence of the complementary strand would be TCACGGA.\r\n\r\n<strong>DNA replication<\/strong>\u00a0is the copying of DNA that occurs before cell division can take place. After a great deal of debate and experimentation, the general method of DNA replication was deduced in 1958 by two scientists in California, Matthew Meselson and Franklin Stahl. This method is illustrated in\u00a0Figure 5\u00a0and described below.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"700\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180949\/0323_DNA_Replication.jpg\" alt=\"This image shows the process of DNA replication. A chromosome is shown expanding into the original template DNA and unwinding at the replication fork. The helicase is present at the replication fork. DNA polymerases are shown adding nucleotides to the leading and lagging strands.\" width=\"700\" height=\"433\" \/> <strong>Figure 5.\u00a0DNA Replication.<\/strong>\u00a0DNA replication faithfully duplicates the entire genome of the cell. During DNA replication, a number of different enzymes work together to pull apart the two strands so each strand can be used as a template to synthesize new complementary strands. The two new daughter DNA molecules each contain one pre-existing strand and one newly synthesized strand. Thus, DNA replication is said to be \u201csemiconservative.\u201d[\/caption]\r\n<h3>Stage 1: Initiation<\/h3>\r\nThe two complementary strands are separated, much like unzipping a zipper. Special enzymes, including\u00a0<strong>helicase<\/strong>, untwist and separate the two strands of DNA.\r\n<h3>Stage 2: Elongation<\/h3>\r\nEach strand becomes a template along which a new complementary strand is built.\u00a0<strong>DNA polymerase<\/strong>\u00a0brings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand.\r\n<h3>Stage 3: Termination<\/h3>\r\nOnce the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete. Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term for this mode of replication is \u201csemiconservative,\u201d because half of the original DNA molecule is conserved in each new DNA molecule. This process continues until the cell\u2019s entire\u00a0<strong>genome<\/strong>, the entire complement of an organism\u2019s DNA, is replicated. As you might imagine, it is very important that DNA replication take place precisely so that new cells in the body contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and corrects them. Once the process of DNA replication is complete, the cell is ready to divide. You will explore the process of cell division later in the chapter.\r\n<div class=\"textbox\">\r\n\r\nWatch this\u00a0video\u00a0to learn about DNA replication. DNA replication proceeds simultaneously at several sites on the same molecule. What separates the base pair at the start of DNA replication?\r\n\r\nhttps:\/\/youtu.be\/FBmO_rmXxIw\r\n\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul>\n<li>Describe the structure and features of the nuclear membrane<\/li>\n<li>List the contents of the nucleus<\/li>\n<li>Explain the organization of the DNA molecule within the nucleus<\/li>\n<li>Describe the process of DNA replication<\/li>\n<\/ul>\n<\/div>\n<p>The nucleus is the largest and most prominent of a cell\u2019s organelles (Figure 1a). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus (Figure 1b), which is known as multinucleated. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body (Figure 2). Without nuclei, the life span of RBCs is short, and so the body must produce new ones constantly.<\/p>\n<div id=\"attachment_3230\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3230\" class=\"size-large wp-image-3230\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1512\/2017\/02\/23223300\/0318_Nucleusab-1024x399.jpg\" alt=\"Figure (a) shows the structure of the nucleus. The nucleolus is inside the nucleus, surrounded by the chromatin and covered by the nuclear envelope.\u00a0Micrograph (b) shows a muscle cell with multiple nuclei.\" width=\"1024\" height=\"399\" \/><\/p>\n<p id=\"caption-attachment-3230\" class=\"wp-caption-text\">Figure 1.\u00a0The Nucleus in Muscle Cells. (a)\u00a0The nucleus is the control center of the cell. The nucleus of living cells contains the genetic material that determines the entire structure and function of that cell. (b)\u00a0Unlike cardiac muscle cells and smooth muscle cells, which have a single nucleus, a skeletal muscle cell contains many nuclei, and is referred to as \u201cmultinucleated.\u201d These muscle cells are long and fibrous (often referred to as muscle fibers). During development, many smaller cells fuse to form a mature muscle fiber. The nuclei of the fused cells are conserved in the mature cell, thus imparting a multinucleate characteristic to mature muscle cells. LM \u00d7 104.3. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/p>\n<\/div>\n<div class=\"textbox\">View the <a href=\"http:\/\/141.214.65.171\/Histology\/Basic%20Tissues\/Muscle\/058thin_HISTO_83X.svs\/view.apml\" target=\"_blank\" rel=\"noopener\">University of Michigan WebScope<\/a> to explore the tissue sample in greater detail.<\/div>\n<div style=\"width: 909px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180942\/0320_RBC_Extruding_Nucleus_Micrograph.jpg\" alt=\"This set of micrographs shows a red blood cell extruding its nucleus. In the left panel, the nucleus is partially extruded from the red blood cell and in the right panel, the nucleus is completely extruded from the cell.\" width=\"899\" height=\"309\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 2.\u00a0Red Blood Cell Extruding Its Nucleus.<\/strong>\u00a0Mature red blood cells lack a nucleus. As they mature, erythroblasts extrude their nucleus, making room for more hemoglobin. The two panels here show an erythroblast before and after ejecting its nucleus, respectively. (credit: modification of micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/p>\n<\/div>\n<div class=\"textbox\">View the <a href=\"http:\/\/virtualslides.med.umich.edu\/Histology\/EMsmallCharts\/3%20Image%20Scope%20finals\/139%20-%20Erythroblast_001.svs\/view.apml\" target=\"_blank\" rel=\"noopener\">University of Michigan WebScope<\/a>\u00a0to explore the tissue sample in greater detail.<\/div>\n<p>Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA. The nucleus sends \u201ccommands\u201d to the cell via molecular messengers that translate the information from DNA. Each cell in your body (with the exception of germ cells) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that the each new cell receives a full complement of DNA. The following section will explore the structure of the nucleus and its contents, as well as the process of DNA replication.<\/p>\n<h2>Organization of the Nucleus and Its DNA<\/h2>\n<p>Like most other cellular organelles, the nucleus is surrounded by a membrane called the\u00a0<strong>nuclear envelope<\/strong>. This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A\u00a0<strong>nuclear pore<\/strong>\u00a0is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.<\/p>\n<p>Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a\u00a0<strong>nucleolus<\/strong>\u00a0(plural = <em>nucleoli<\/em>). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes.<\/p>\n<div style=\"width: 461px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180945\/0321_DNA_Macrostructure.jpg\" alt=\"This diagram shows the macrostructure of DNA. A chromosome and its component chromatin are shown to expand into nucleosomes with histones, which further unravel into a DNA helix and finally into a DNA ladder.\" width=\"451\" height=\"354\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 3.\u00a0DNA Macrostructure.<\/strong>\u00a0Strands of DNA are wrapped around supporting histones. These proteins are increasingly bundled and condensed into chromatin, which is packed tightly into chromosomes when the cell is ready to divide.<\/p>\n<\/div>\n<p>Once synthesized, newly made ribosomal subunits exit the cell\u2019s nucleus through the nuclear pores. The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of\u00a0<strong>chromatin<\/strong>\u00a0composed of DNA and associated proteins (Figure 3).<\/p>\n<p>Along the chromatin threads, the DNA is wrapped around a set of\u00a0<strong>histone<\/strong>\u00a0proteins. A\u00a0<strong>nucleosome<\/strong>\u00a0is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones. When a cell is in the process of division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the \u201cdaughter cells.\u201d<\/p>\n<p>The\u00a0<strong>chromosome<\/strong>\u00a0is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.<\/p>\n<h2>DNA Replication<\/h2>\n<p>In order for an organism to grow, develop, and maintain its health, cells must replicated themselves by dividing to produce two new daughter cells, each with the full complement of DNA as found in the original cell. Billions of new cells are produced in an adult human every day. There are a few cell types in the body do not divide, including nerve cells, skeletal muscle fibers, and cardiac muscle cells. The division time of different cell types varies. Epithelial cells of the skin and gastrointestinal lining, for instance, divide very frequently to replace those that are constantly being rubbed off of the surface by friction.<\/p>\n<p>A DNA molecule is made of two strands that \u201ccomplement\u201d each other: the molecules that compose the strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups (Figure 4).<\/p>\n<div id=\"attachment_3773\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3773\" class=\"size-large wp-image-3773\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1512\/2014\/10\/03214607\/0322_DNA_Nucleotides2-1024x298.jpg\" alt=\"This figure shows the DNA double helix on the far left panel. The different nucleotides are color-coded. In the center panel, the interaction between the nucleotides through the hydrogen bonds and the location of the sugar-phosphate backbone is shown. In the far right panel, the structure of a nucleotide is described in detail.\" width=\"1024\" height=\"298\" \/><\/p>\n<p id=\"caption-attachment-3773\" class=\"wp-caption-text\"><strong>Figure 4.\u00a0Molecular Structure of DNA.<\/strong>\u00a0The DNA double helix is composed of two complementary strands. The strands are bonded together via their nitrogenous base pairs using hydrogen bonds.<\/p>\n<\/div>\n<p>The two sides of the ladder are not identical, but are complementary. These two backbones are bonded to each other across pairs of protruding bases, each bonded pair forming one \u201crung,\u201d or cross member. The four DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine always binds with thymine, and cytosine always binds with guanine. The particular sequence of bases along the DNA molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could infer the order of the bases in one strand from the bases in the other, complementary strand. For example, if one strand has a region with the sequence AGTGCCT, then the sequence of the complementary strand would be TCACGGA.<\/p>\n<p><strong>DNA replication<\/strong>\u00a0is the copying of DNA that occurs before cell division can take place. After a great deal of debate and experimentation, the general method of DNA replication was deduced in 1958 by two scientists in California, Matthew Meselson and Franklin Stahl. This method is illustrated in\u00a0Figure 5\u00a0and described below.<\/p>\n<div style=\"width: 710px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/18\/2014\/07\/19180949\/0323_DNA_Replication.jpg\" alt=\"This image shows the process of DNA replication. A chromosome is shown expanding into the original template DNA and unwinding at the replication fork. The helicase is present at the replication fork. DNA polymerases are shown adding nucleotides to the leading and lagging strands.\" width=\"700\" height=\"433\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 5.\u00a0DNA Replication.<\/strong>\u00a0DNA replication faithfully duplicates the entire genome of the cell. During DNA replication, a number of different enzymes work together to pull apart the two strands so each strand can be used as a template to synthesize new complementary strands. The two new daughter DNA molecules each contain one pre-existing strand and one newly synthesized strand. Thus, DNA replication is said to be \u201csemiconservative.\u201d<\/p>\n<\/div>\n<h3>Stage 1: Initiation<\/h3>\n<p>The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including\u00a0<strong>helicase<\/strong>, untwist and separate the two strands of DNA.<\/p>\n<h3>Stage 2: Elongation<\/h3>\n<p>Each strand becomes a template along which a new complementary strand is built.\u00a0<strong>DNA polymerase<\/strong>\u00a0brings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand.<\/p>\n<h3>Stage 3: Termination<\/h3>\n<p>Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete. Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term for this mode of replication is \u201csemiconservative,\u201d because half of the original DNA molecule is conserved in each new DNA molecule. This process continues until the cell\u2019s entire\u00a0<strong>genome<\/strong>, the entire complement of an organism\u2019s DNA, is replicated. As you might imagine, it is very important that DNA replication take place precisely so that new cells in the body contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and corrects them. Once the process of DNA replication is complete, the cell is ready to divide. You will explore the process of cell division later in the chapter.<\/p>\n<div class=\"textbox\">\n<p>Watch this\u00a0video\u00a0to learn about DNA replication. DNA replication proceeds simultaneously at several sites on the same molecule. What separates the base pair at the start of DNA replication?<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-1\" title=\"DNA Replication\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/FBmO_rmXxIw?feature=oembed&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-1696\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Chapter 3. <strong>Authored by<\/strong>: OpenStax College. <strong>Provided by<\/strong>: Rice University. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.1@7.1.\">http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.1@7.1.<\/a>. <strong>Project<\/strong>: Anatomy &amp; Physiology. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/content\/col11496\/latest\/.<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":74,"menu_order":4,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Chapter 3\",\"author\":\"OpenStax College\",\"organization\":\"Rice University\",\"url\":\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.1@7.1.\",\"project\":\"Anatomy & Physiology\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/content\/col11496\/latest\/.\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1696","chapter","type-chapter","status-publish","hentry"],"part":1687,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/pressbooks\/v2\/chapters\/1696","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/wp\/v2\/users\/74"}],"version-history":[{"count":10,"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/pressbooks\/v2\/chapters\/1696\/revisions"}],"predecessor-version":[{"id":3774,"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/pressbooks\/v2\/chapters\/1696\/revisions\/3774"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/pressbooks\/v2\/parts\/1687"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/pressbooks\/v2\/chapters\/1696\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/wp\/v2\/media?parent=1696"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/pressbooks\/v2\/chapter-type?post=1696"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/wp\/v2\/contributor?post=1696"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-ap1\/wp-json\/wp\/v2\/license?post=1696"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}