{"id":131,"date":"2021-12-07T06:43:02","date_gmt":"2021-12-07T06:43:02","guid":{"rendered":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/?post_type=chapter&p=131"},"modified":"2021-12-07T06:47:11","modified_gmt":"2021-12-07T06:47:11","slug":"2-5-nucleic-acid","status":"publish","type":"chapter","link":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/chapter\/2-5-nucleic-acid\/","title":{"raw":"2.5 Nucleic Acid","rendered":"2.5 Nucleic Acid"},"content":{"raw":"
\r\n

Learning Objectives<\/strong><\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nBy the end of this section, you will be able to:\r\n
    \r\n \t
  • Describe nucleic acids\u2019 structure and define the two types of nucleic acids<\/li>\r\n \t
  • Explain DNA structure and role<\/li>\r\n \t
  • Explain RNA structure and roles<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\nNucleic acids\u00a0are the most important macromolecules for the continuity of life. They carry the cell\u2019s genetic blueprint and carry instructions for its functioning.\r\n

    DNA and RNA<\/h2>\r\nThe two main types of nucleic acids are\u00a0deoxyribonucleic acid<\/strong>\u00a0(DNA)<\/strong>\u00a0and\u00a0ribonucleic acid (RNA).<\/strong>\u00a0DNA is the genetic material in all living organisms, ranging from single-celled bacteria to multicellular mammals. It is in the nucleus of eukaryotes and in the organelles, chloroplasts, and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope.\r\n\r\nThe cell\u2019s entire genetic content is its genome, and the study of genomes is genomics. In eukaryotic cells but not in prokaryotes, DNA forms a complex with histone proteins to form chromatin, the substance of eukaryotic chromosomes. A chromosome may contain tens of thousands of genes. Many genes contain the information to make protein products. Other genes code for RNA products. DNA controls all the cellular activities by turning the genes \u201con\u201d or \u201coff.\u201d\r\n\r\nThe other type of nucleic acid, RNA, is mostly involved in protein synthesis. The DNA molecules never leave the nucleus but instead use an intermediary to communicate with the rest of the cell. This intermediary is the\u00a0messenger RNA (mRNA).<\/strong>\u00a0Other types of RNA\u2014like rRNA, tRNA, and microRNA\u2014are involved in protein synthesis and its regulation.\r\n\r\nDNA and RNA are comprised of monomers that scientists call\u00a0nucleotides<\/strong>. The nucleotides combine with each other to form a\u00a0polynucleotide, DNA or RNA. Three components comprise each nucleotide: a nitrogenous base, a pentose (five-carbon) sugar, and a phosphate group (Figure 2.5.1). Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to one or more phosphate groups.\r\n\r\n \r\n
    \"Nucleotide
    Figure\u00a02.5.1.<\/strong>\u00a0Nucleotide components.\u00a0<\/strong>Three components comprise a nucleotide: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Carbon residues in the pentose are numbered 1\u2032 through 5\u2032 (the prime distinguishes these residues from those in the base, which are numbered without using a prime notation). The base is attached to the ribose\u2019s 1\u2032 position, and the phosphate is attached to the 5\u2032 position. When a polynucleotide forms, the incoming nucleotide\u2019s 5\u2032 phosphate attaches to the 3\u2032 hydroxyl group at the end of the growing chain. Two types of pentose are in nucleotides, deoxyribose (found in DNA) and ribose (found in RNA). Deoxyribose is similar in structure to ribose, but it has an H instead of an OH at the 2\u2032 position. We can divide bases into two categories: purines and pyrimidines. Purines have a double ring structure, and pyrimidines have a single ring.<\/figcaption><\/figure>\r\nThe nitrogenous bases, important components of nucleotides, are organic molecules and are so named because they contain carbon and nitrogen. They are bases because they contain an amino group that has the potential of binding an extra hydrogen, and thus decreasing the hydrogen ion concentration in its environment, making it more basic. Each nucleotide in DNA contains one of four possible nitrogenous bases: adenine (A), guanine (G) cytosine (C), and thymine (T).\r\n\r\nScientists classify adenine and guanine as\u00a0purines<\/strong>. The purine\u2019s primary structure is two carbon-nitrogen rings. Scientists classify cytosine, thymine, and uracil as\u00a0pyrimidines<\/strong>\u00a0which have a single carbon-nitrogen ring as their primary structure (Figure 2.5.1). Each of these basic carbon-nitrogen rings has different functional groups attached to it. In molecular biology shorthand, we know the nitrogenous bases by their symbols A, T, G, C, and U. DNA contains A, T, G, and C; whereas, RNA contains A, U, G, and C.\r\n\r\nThe pentose sugar in DNA is deoxyribose, and in RNA, the sugar is ribose (Figure 3.5.1). The difference between the sugars is the presence of the hydroxyl group on the ribose\u2019s second carbon and hydrogen on the deoxyribose\u2019s second carbon. The carbon atoms of the sugar molecule are numbered as 1\u2032, 2\u2032, 3\u2032, 4\u2032, and 5\u2032 (1\u2032 is read as \u201cone prime\u201d). The phosphate residue attaches to the hydroxyl group of the 5\u2032 carbon of one sugar and the hydroxyl group of the 3\u2032 carbon of the sugar of the next nucleotide, which forms a 5\u2032\u20133\u2032\u00a0phosphodiester\u00a0linkage. A simple dehydration reaction like the other linkages connecting monomers in macromolecules does not form the\u00a0phosphodiester<\/strong>\u00a0linkage. Its formation involves removing two phosphate groups. A polynucleotide may have thousands of such phosphodiester linkages.\r\n

    DNA Double-Helix Structure<\/h2>\r\n
    \"Native
    Figure\u00a02.5.2.\u00a0Native DNA is an antiparallel double helix<\/strong>. The phosphate backbone (indicated by the curvy lines) is on the outside, and the bases are on the inside. Each base from one strand interacts via hydrogen bonding with a base from the opposing strand. (Credit: Jerome Walker\/Dennis Myts).<\/figcaption><\/figure>\r\nDNA has a double-helix structure (Figure 2.5.2). The sugar and phosphate lie on the outside of the helix, forming the DNA\u2019s backbone. The nitrogenous bases are stacked in the interior, like a pair of staircase steps. Hydrogen bonds bind the pairs to each other. Every base pair in the double helix is separated from the next base pair by 0.34 nm. The helix\u2019s two strands run in opposite directions, meaning that the 5\u2032 carbon end of one strand will face the 3\u2032 carbon end of its matching strand. (Scientists call this an antiparallel orientation and is important to DNA replication and in many\u00a0nucleic acid<\/strong>\u00a0interactions.)\r\n\r\nOnly certain types of base pairing are allowed. For example, a certain purine can only pair with a certain pyrimidine. This means A can pair with T, and G can pair with C, as\u00a0Figure 2.5.3\u00a0shows. This is the base complementary rule. In other words, the DNA strands are complementary to each other. If the sequence of one strand is AATTGGCC, the complementary strand would have the sequence TTAACCGG. During DNA replication, each strand copies itself, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesised strand.\r\n
    \"Hydrogen
    Figure\u00a02.5.3.\u00a0Hydrogen bonds in a double stranded DNA molecule.\u00a0<\/strong>In a double stranded DNA molecule, the two strands run antiparallel to one another so that one strand runs 5\u2032 to 3\u2032 and the other 3\u2032 to 5\u2032. The phosphate backbone is located on the outside, and the bases are in the middle. Adenine forms hydrogen bonds (or base pairs) with thymine, and guanine base pairs with cytosine.<\/figcaption><\/figure>\r\nA mutation occurs, and adenine replaces cytosine. What impact do you think this will have on the DNA structure?\r\n

    RNA<\/h2>\r\nRibonucleic acid, or RNA, is mainly involved in the process of protein synthesis under the direction of DNA. RNA is usually single-stranded and is comprised of ribonucleotides that are linked by phosphodiester bonds. A ribonucleotide in the RNA chain contains ribose (the pentose sugar), one of the four nitrogenous bases (A, U, G, and C), and the phosphate group.\r\n\r\nThere are four major types of RNA: messenger RNA (mRNA),\u00a0ribosomal RNA (rRNA)<\/strong>,\u00a0transfer RNA (tRNA)<\/strong>, and microRNA (miRNA). The first, mRNA, carries the message from DNA, which controls all the cellular activities in a cell. If a cell requires synthesising a certain protein, the gene for this product turns \u201con\u201d and the messenger RNA synthesises in the nucleus. The RNA base sequence is complementary to the DNA\u2019s coding sequence from which it has been copied. However, in RNA, the base T is absent and U is present instead. If the DNA strand has a sequence AATTGCGC, the sequence of the complementary RNA is UUAACGCG. In the cytoplasm, the mRNA interacts with ribosomes and other cellular machinery (Figure 2.5.4).\r\n
    \"Diagram
    Figure\u00a02.5.4.<\/strong>\u00a0Ribosome<\/strong>.\u00a0A ribosome has two parts: a large subunit and a small subunit. The mRNA sits in between the two subunits. A tRNA molecule recognises a codon on the mRNA, binds to it by complementary base pairing, and adds the correct amino acid to the growing peptide chain.<\/figcaption><\/figure>\r\nThe mRNA is read in sets of three bases known as codons. Each codon codes for a single amino acid. In this way, the mRNA is read and the protein product is made.\u00a0Ribosomal RNA (rRNA)\u00a0is a major constituent of ribosomes on which the mRNA binds. The rRNA ensures the proper alignment of the mRNA and the Ribosomes. The ribosome\u2019s rRNA also has an enzymatic activity (peptidyl transferase) and catalyses\u00a0peptide bond<\/strong>\u00a0formation between two aligned amino acids.\u00a0Transfer RNA (tRNA)\u00a0is one of the smallest of the four types of RNA, usually 70\u201390 nucleotides long. It carries the correct amino acid to the protein synthesis site. It is the base pairing between the tRNA and mRNA that allows for the correct amino acid to insert itself in the\u00a0polypeptide<\/strong>\u00a0chain. MicroRNAs are the smallest RNA molecules and their role involves regulating gene expression by interfering with the expression of certain mRNA messages.\u00a0Table 2.5.1\u00a0summarises DNA and RNA features.\r\n\r\nTable 2.5.1\u00a0<\/strong>DNA and RNA Features\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
    <\/td>\r\nDNA<\/strong><\/td>\r\nRNA<\/strong><\/td>\r\n<\/tr>\r\n
    Function<\/td>\r\nCarries genetic information<\/td>\r\nInvolved in protein synthesis<\/td>\r\n<\/tr>\r\n
    Location<\/td>\r\nRemains in the nucleus<\/td>\r\nLeaves the nucleus<\/td>\r\n<\/tr>\r\n
    Structure<\/td>\r\nDouble helix<\/td>\r\nUsually single-stranded<\/td>\r\n<\/tr>\r\n
    Sugar<\/td>\r\nDeoxyribose<\/td>\r\nRibose<\/td>\r\n<\/tr>\r\n
    Pyrimidines<\/td>\r\nCytosine, thymine<\/td>\r\nCytosine, uracil<\/td>\r\n<\/tr>\r\n
    Purines<\/td>\r\nAdenine, guanine<\/td>\r\nAdenine, guanine<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nEven though the RNA is single stranded, most RNA types show extensive intramolecular base pairing between complementary sequences, creating a predictable three-dimensional structure essential for their function.\r\n\r\nAs you have learned, information flow in an organism takes place from DNA to RNA to protein. DNA dictates the structure of mRNA in a process scientists call\u00a0transcription<\/strong>, and RNA dictates the protein\u2019s structure in a process scientists call\u00a0translation<\/strong>. This is the Central Dogma of Life, which holds true for all organisms; however, exceptions to the rule occur in connection with viral infections.\r\n
    \r\n

    Section Review<\/strong><\/p>\r\n\r\n<\/header>\r\n

    \r\n\r\nNucleic acids are molecules comprised of nucleotides that direct cellular activities such as cell division and protein synthesis. Pentose sugar, a nitrogenous base, and a phosphate group comprise each nucleotide. There are two types of nucleic acids: DNA and RNA. DNA carries the cell\u2019s genetic blueprint and passes it on from parents to offspring (in the form of chromosomes). It has a double-helical structure with the two strands running in opposite directions, connected by hydrogen bonds, and complementary to each other. RNA is a single-stranded polymer composed of linked nucleotides made up of a pentose sugar (ribose), a nitrogenous base, and a phosphate group. RNA is involved in protein synthesis and its regulation. Messenger RNA (mRNA) copies from the DNA, exports itself from the nucleus to the cytoplasm, and contains information for constructing proteins. Ribosomal RNA (rRNA) is a part of the ribosomes at the site of protein synthesis; whereas, transfer RNA (tRNA) carries the amino acid to the site of protein synthesis. The microRNA regulates using mRNA for protein synthesis.\r\n\r\n<\/div>\r\n<\/div>\r\n
    \r\n

    Review Questions<\/strong><\/p>\r\n\r\n<\/header>\r\n

    \r\n
    \r\n
    [h5p id=\"45\"]<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
    \r\n

    Critical Thinking Questions<\/strong><\/p>\r\n\r\n<\/header>\r\n

    \r\n
    \r\n
    [h5p id=\"46\"]<\/div>\r\n<\/div>\r\n
    \r\n
    [h5p id=\"47\"]<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nClick the drop down below to review the terms learned from this chapter.\r\n
    \r\n
    [h5p id=\"48\"]<\/div>\r\n<\/div>","rendered":"
    \n
    \n

    Learning Objectives<\/strong><\/p>\n<\/header>\n

    \n

    By the end of this section, you will be able to:<\/p>\n

      \n
    • Describe nucleic acids\u2019 structure and define the two types of nucleic acids<\/li>\n
    • Explain DNA structure and role<\/li>\n
    • Explain RNA structure and roles<\/li>\n<\/ul>\n<\/div>\n<\/div>\n

      Nucleic acids\u00a0are the most important macromolecules for the continuity of life. They carry the cell\u2019s genetic blueprint and carry instructions for its functioning.<\/p>\n

      DNA and RNA<\/h2>\n

      The two main types of nucleic acids are\u00a0deoxyribonucleic acid<\/strong>\u00a0(DNA)<\/strong>\u00a0and\u00a0ribonucleic acid (RNA).<\/strong>\u00a0DNA is the genetic material in all living organisms, ranging from single-celled bacteria to multicellular mammals. It is in the nucleus of eukaryotes and in the organelles, chloroplasts, and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope.<\/p>\n

      The cell\u2019s entire genetic content is its genome, and the study of genomes is genomics. In eukaryotic cells but not in prokaryotes, DNA forms a complex with histone proteins to form chromatin, the substance of eukaryotic chromosomes. A chromosome may contain tens of thousands of genes. Many genes contain the information to make protein products. Other genes code for RNA products. DNA controls all the cellular activities by turning the genes \u201con\u201d or \u201coff.\u201d<\/p>\n

      The other type of nucleic acid, RNA, is mostly involved in protein synthesis. The DNA molecules never leave the nucleus but instead use an intermediary to communicate with the rest of the cell. This intermediary is the\u00a0messenger RNA (mRNA).<\/strong>\u00a0Other types of RNA\u2014like rRNA, tRNA, and microRNA\u2014are involved in protein synthesis and its regulation.<\/p>\n

      DNA and RNA are comprised of monomers that scientists call\u00a0nucleotides<\/strong>. The nucleotides combine with each other to form a\u00a0polynucleotide, DNA or RNA. Three components comprise each nucleotide: a nitrogenous base, a pentose (five-carbon) sugar, and a phosphate group (Figure 2.5.1). Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to one or more phosphate groups.<\/p>\n

       <\/p>\n

      \"Nucleotide
      Figure\u00a02.5.1.<\/strong>\u00a0Nucleotide components.\u00a0<\/strong>Three components comprise a nucleotide: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Carbon residues in the pentose are numbered 1\u2032 through 5\u2032 (the prime distinguishes these residues from those in the base, which are numbered without using a prime notation). The base is attached to the ribose\u2019s 1\u2032 position, and the phosphate is attached to the 5\u2032 position. When a polynucleotide forms, the incoming nucleotide\u2019s 5\u2032 phosphate attaches to the 3\u2032 hydroxyl group at the end of the growing chain. Two types of pentose are in nucleotides, deoxyribose (found in DNA) and ribose (found in RNA). Deoxyribose is similar in structure to ribose, but it has an H instead of an OH at the 2\u2032 position. We can divide bases into two categories: purines and pyrimidines. Purines have a double ring structure, and pyrimidines have a single ring.<\/figcaption><\/figure>\n

      The nitrogenous bases, important components of nucleotides, are organic molecules and are so named because they contain carbon and nitrogen. They are bases because they contain an amino group that has the potential of binding an extra hydrogen, and thus decreasing the hydrogen ion concentration in its environment, making it more basic. Each nucleotide in DNA contains one of four possible nitrogenous bases: adenine (A), guanine (G) cytosine (C), and thymine (T).<\/p>\n

      Scientists classify adenine and guanine as\u00a0purines<\/strong>. The purine\u2019s primary structure is two carbon-nitrogen rings. Scientists classify cytosine, thymine, and uracil as\u00a0pyrimidines<\/strong>\u00a0which have a single carbon-nitrogen ring as their primary structure (Figure 2.5.1). Each of these basic carbon-nitrogen rings has different functional groups attached to it. In molecular biology shorthand, we know the nitrogenous bases by their symbols A, T, G, C, and U. DNA contains A, T, G, and C; whereas, RNA contains A, U, G, and C.<\/p>\n

      The pentose sugar in DNA is deoxyribose, and in RNA, the sugar is ribose (Figure 3.5.1). The difference between the sugars is the presence of the hydroxyl group on the ribose\u2019s second carbon and hydrogen on the deoxyribose\u2019s second carbon. The carbon atoms of the sugar molecule are numbered as 1\u2032, 2\u2032, 3\u2032, 4\u2032, and 5\u2032 (1\u2032 is read as \u201cone prime\u201d). The phosphate residue attaches to the hydroxyl group of the 5\u2032 carbon of one sugar and the hydroxyl group of the 3\u2032 carbon of the sugar of the next nucleotide, which forms a 5\u2032\u20133\u2032\u00a0phosphodiester\u00a0linkage. A simple dehydration reaction like the other linkages connecting monomers in macromolecules does not form the\u00a0phosphodiester<\/strong>\u00a0linkage. Its formation involves removing two phosphate groups. A polynucleotide may have thousands of such phosphodiester linkages.<\/p>\n

      DNA Double-Helix Structure<\/h2>\n
      \"Native
      Figure\u00a02.5.2.\u00a0Native DNA is an antiparallel double helix<\/strong>. The phosphate backbone (indicated by the curvy lines) is on the outside, and the bases are on the inside. Each base from one strand interacts via hydrogen bonding with a base from the opposing strand. (Credit: Jerome Walker\/Dennis Myts).<\/figcaption><\/figure>\n

      DNA has a double-helix structure (Figure 2.5.2). The sugar and phosphate lie on the outside of the helix, forming the DNA\u2019s backbone. The nitrogenous bases are stacked in the interior, like a pair of staircase steps. Hydrogen bonds bind the pairs to each other. Every base pair in the double helix is separated from the next base pair by 0.34 nm. The helix\u2019s two strands run in opposite directions, meaning that the 5\u2032 carbon end of one strand will face the 3\u2032 carbon end of its matching strand. (Scientists call this an antiparallel orientation and is important to DNA replication and in many\u00a0nucleic acid<\/strong>\u00a0interactions.)<\/p>\n

      Only certain types of base pairing are allowed. For example, a certain purine can only pair with a certain pyrimidine. This means A can pair with T, and G can pair with C, as\u00a0Figure 2.5.3\u00a0shows. This is the base complementary rule. In other words, the DNA strands are complementary to each other. If the sequence of one strand is AATTGGCC, the complementary strand would have the sequence TTAACCGG. During DNA replication, each strand copies itself, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesised strand.<\/p>\n

      \"Hydrogen
      Figure\u00a02.5.3.\u00a0Hydrogen bonds in a double stranded DNA molecule.\u00a0<\/strong>In a double stranded DNA molecule, the two strands run antiparallel to one another so that one strand runs 5\u2032 to 3\u2032 and the other 3\u2032 to 5\u2032. The phosphate backbone is located on the outside, and the bases are in the middle. Adenine forms hydrogen bonds (or base pairs) with thymine, and guanine base pairs with cytosine.<\/figcaption><\/figure>\n

      A mutation occurs, and adenine replaces cytosine. What impact do you think this will have on the DNA structure?<\/p>\n

      RNA<\/h2>\n

      Ribonucleic acid, or RNA, is mainly involved in the process of protein synthesis under the direction of DNA. RNA is usually single-stranded and is comprised of ribonucleotides that are linked by phosphodiester bonds. A ribonucleotide in the RNA chain contains ribose (the pentose sugar), one of the four nitrogenous bases (A, U, G, and C), and the phosphate group.<\/p>\n

      There are four major types of RNA: messenger RNA (mRNA),\u00a0ribosomal RNA (rRNA)<\/strong>,\u00a0transfer RNA (tRNA)<\/strong>, and microRNA (miRNA). The first, mRNA, carries the message from DNA, which controls all the cellular activities in a cell. If a cell requires synthesising a certain protein, the gene for this product turns \u201con\u201d and the messenger RNA synthesises in the nucleus. The RNA base sequence is complementary to the DNA\u2019s coding sequence from which it has been copied. However, in RNA, the base T is absent and U is present instead. If the DNA strand has a sequence AATTGCGC, the sequence of the complementary RNA is UUAACGCG. In the cytoplasm, the mRNA interacts with ribosomes and other cellular machinery (Figure 2.5.4).<\/p>\n

      \"Diagram
      Figure\u00a02.5.4.<\/strong>\u00a0Ribosome<\/strong>.\u00a0A ribosome has two parts: a large subunit and a small subunit. The mRNA sits in between the two subunits. A tRNA molecule recognises a codon on the mRNA, binds to it by complementary base pairing, and adds the correct amino acid to the growing peptide chain.<\/figcaption><\/figure>\n

      The mRNA is read in sets of three bases known as codons. Each codon codes for a single amino acid. In this way, the mRNA is read and the protein product is made.\u00a0Ribosomal RNA (rRNA)\u00a0is a major constituent of ribosomes on which the mRNA binds. The rRNA ensures the proper alignment of the mRNA and the Ribosomes. The ribosome\u2019s rRNA also has an enzymatic activity (peptidyl transferase) and catalyses\u00a0peptide bond<\/strong>\u00a0formation between two aligned amino acids.\u00a0Transfer RNA (tRNA)\u00a0is one of the smallest of the four types of RNA, usually 70\u201390 nucleotides long. It carries the correct amino acid to the protein synthesis site. It is the base pairing between the tRNA and mRNA that allows for the correct amino acid to insert itself in the\u00a0polypeptide<\/strong>\u00a0chain. MicroRNAs are the smallest RNA molecules and their role involves regulating gene expression by interfering with the expression of certain mRNA messages.\u00a0Table 2.5.1\u00a0summarises DNA and RNA features.<\/p>\n

      Table 2.5.1\u00a0<\/strong>DNA and RNA Features<\/p>\n\n\n\n\n\n\n\n\n\n
      <\/td>\nDNA<\/strong><\/td>\nRNA<\/strong><\/td>\n<\/tr>\n
      Function<\/td>\nCarries genetic information<\/td>\nInvolved in protein synthesis<\/td>\n<\/tr>\n
      Location<\/td>\nRemains in the nucleus<\/td>\nLeaves the nucleus<\/td>\n<\/tr>\n
      Structure<\/td>\nDouble helix<\/td>\nUsually single-stranded<\/td>\n<\/tr>\n
      Sugar<\/td>\nDeoxyribose<\/td>\nRibose<\/td>\n<\/tr>\n
      Pyrimidines<\/td>\nCytosine, thymine<\/td>\nCytosine, uracil<\/td>\n<\/tr>\n
      Purines<\/td>\nAdenine, guanine<\/td>\nAdenine, guanine<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

      Even though the RNA is single stranded, most RNA types show extensive intramolecular base pairing between complementary sequences, creating a predictable three-dimensional structure essential for their function.<\/p>\n

      As you have learned, information flow in an organism takes place from DNA to RNA to protein. DNA dictates the structure of mRNA in a process scientists call\u00a0transcription<\/strong>, and RNA dictates the protein\u2019s structure in a process scientists call\u00a0translation<\/strong>. This is the Central Dogma of Life, which holds true for all organisms; however, exceptions to the rule occur in connection with viral infections.<\/p>\n

      \n
      \n

      Section Review<\/strong><\/p>\n<\/header>\n

      \n

      Nucleic acids are molecules comprised of nucleotides that direct cellular activities such as cell division and protein synthesis. Pentose sugar, a nitrogenous base, and a phosphate group comprise each nucleotide. There are two types of nucleic acids: DNA and RNA. DNA carries the cell\u2019s genetic blueprint and passes it on from parents to offspring (in the form of chromosomes). It has a double-helical structure with the two strands running in opposite directions, connected by hydrogen bonds, and complementary to each other. RNA is a single-stranded polymer composed of linked nucleotides made up of a pentose sugar (ribose), a nitrogenous base, and a phosphate group. RNA is involved in protein synthesis and its regulation. Messenger RNA (mRNA) copies from the DNA, exports itself from the nucleus to the cytoplasm, and contains information for constructing proteins. Ribosomal RNA (rRNA) is a part of the ribosomes at the site of protein synthesis; whereas, transfer RNA (tRNA) carries the amino acid to the site of protein synthesis. The microRNA regulates using mRNA for protein synthesis.<\/p>\n<\/div>\n<\/div>\n

      \n
      \n

      Review Questions<\/strong><\/p>\n<\/header>\n

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      \n
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      \n