Difference between revisions of "Nucleic acid" - New World Encyclopedia
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[[Image:DNA123.png|thumb|right|150px|A space-filling model of a section of a DNA molecule.]] | [[Image:DNA123.png|thumb|right|150px|A space-filling model of a section of a DNA molecule.]] | ||
| − | A '''nucleic acid''' is a complex, high-molecular-weight [[macromolecule]] composed of [[nucleotide]] chains whose sequence of bases conveys [[genetic information]]. Nucleic acids are found in all living [[cell (biology)|cells]] and in [[virus]]es, and the flow of genetic information is essentially the same in all | + | A '''nucleic acid''' is a complex, high-molecular-weight [[macromolecule]] composed of [[nucleotide]] chains whose sequence of bases conveys [[genetic information]]. Nucleic acids are found in all living [[cell (biology)|cells]] and in [[virus]]es, and the flow of genetic information is essentially the same in all [[organism]]s. |
| − | The most common nucleic acids are '''[[DNA|deoxyribonucleic acid]]''' (DNA) and '''[[RNA|ribonucleic acid]]''' (RNA). The main role of DNA | + | The most common nucleic acids are '''[[DNA|deoxyribonucleic acid]]''' (DNA) and '''[[RNA|ribonucleic acid]]''' (RNA). The main role of DNA is the long-term storage of genetic information. DNA is often compared to a [[blueprint]], since it contains instructions for constructing other components of the cell, such as [[protein]]s and RNA molecules. The DNA segments that carry genetic information are called ''[[gene]]s'', but other DNA sequences have structural purposes or are involved in regulating the expression of genetic information. |
| − | Although RNA may serve as a genetic blueprint for certain [[virus]]es, it plays a diversity of roles. | + | Although RNA may serve as a genetic blueprint for certain [[virus]]es, it plays a diversity of roles. RNA may be thought of as the intermediate between the DNA blueprint and the actual workings of the cell, serving as the template for the synthesis of proteins from the genetic information stored in DNA. Some RNA molecules (called [[ribozymes]]) are also involved in the catalysis of biochemical reactions. |
| − | The nucleic | + | The nucleic acids DNA and RNA can be found in the [[nuclei]] of [[eukaryotic]] cells and the [[cytoplasm]]s of [[prokaryote]]s (which lack a nucleus). In eukaryotes, DNA is also present in other cellular compartments (called [[organelles]]), such as [[mitochondria]] and [[chloroplast]]s. |
==The chemical structure of nucleic acids== | ==The chemical structure of nucleic acids== | ||
| − | === | + | ===Nucleic acids are composed of repeating nucleotide units=== |
[[Image:Nucleic acid.png|frame|A schematic diagram of a double-stranded nucleic acid. Covalent bonds (solid lines) connect pentose sugars (green-hatched circles) to phosphates (dotted circles) to form the sugar-phosphate backbone, which provides structural support. The interior of the molecule contains nitrogenous bases (red-slashed circles) joined together via hydrogen bonds (dotted lines).]] | [[Image:Nucleic acid.png|frame|A schematic diagram of a double-stranded nucleic acid. Covalent bonds (solid lines) connect pentose sugars (green-hatched circles) to phosphates (dotted circles) to form the sugar-phosphate backbone, which provides structural support. The interior of the molecule contains nitrogenous bases (red-slashed circles) joined together via hydrogen bonds (dotted lines).]] | ||
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#One of the four major nucleobases in RNA is uracil (U) instead of thymine (T). | #One of the four major nucleobases in RNA is uracil (U) instead of thymine (T). | ||
| − | === | + | ===Nucleic acids form single or double-stranded structures=== |
| − | [[Image:NA-comparedto-DNA thymineAndUracilCorrected.png|left| | + | [[Image:NA-comparedto-DNA thymineAndUracilCorrected.png|left|300px|thumb|A comparison of the DNA double-helix and a single-stranded RNA, showing the chemical structures of the bases.]] |
| + | Nucleic acids are constructed from chains of nucleotides attached by [[phosphodiester bond]]s, which are formed between the phosphate residue of one nucleotide and one of two possible carbons on the sugar molecule of an adjacent nucleotide. These sugar-phosphate interactions play a primarily structural role, forming what is sometimes referred to as the "backbone" of the nucleic acid. | ||
| − | Nucleic acids | + | Nucleic acids organize into single-stranded or double-stranded molecules. The DNA of many chromosomes and DNA-containing viruses forms long, [[unbranched]], double-helical threads, in which two strands of DNA coil around a common axis. The strands run in opposite directions, and are held together by [[hydrogen bond]]s between pairs of bases from each strand. The base adenine is always paired with thymine, and guanine with cytosine (i.e., a purine pairs with a pyrimidine). The stability created by the hydrogen-bonding between these complementary ''base pairs'' makes DNA a sturdy form of genetic storage. |
| − | + | The DNA of many viruses and the DNA found in mitochondria are circular; in some cases, they also twist into a supercoiled form. RNA is usually single-stranded, but it may contain double-helical regions where a given strand has folded back on itself. | |
==Nucleic acids store and transmit genetic information== | ==Nucleic acids store and transmit genetic information== | ||
| − | ===DNA encodes | + | ===DNA encodes instructions for the synthesis of proteins=== |
| − | DNA contains the genetic [[information]] that allows living things to function, grow and reproduce. This information is encoded in the biochemical composition of the molecule itself; specifically, in its particular sequence of nucleobases (which are the variable part of the DNA molecule). | + | DNA contains the genetic [[information]] that allows living things to function, grow and reproduce. This information is encoded in the biochemical composition of the molecule itself; specifically, in its particular sequence of nucleobases (which are the variable part of the DNA molecule). A particular sequence of nucleotides along a segment of the DNA strand (i.e., a gene) defines a messenger RNA sequence, which in turn defines a protein. The relationship between the nucleotide sequence and the [[amino acid|amino-acid]] sequence of the protein is determined by simple cellular rules of [[translation]], known collectively as the [[genetic code]]. The genetic code is the relation between the sequence of bases in DNA (or its RNA transcript) and the sequence of amino acids in [[protein]]s. Amino acids are coded by groups of three bases (called [[codon]]s) starting from a fixed point (e.g. ACT, CAG, TTT). These codons can then be translated with messenger RNA and then transfer RNA from the chemical language of nucleic acids to that of amino acids, with each codon corresponding to a particular amino acid. |
| − | ===The structure of DNA | + | ===The double-helical structure of DNA facilitates its own replication=== |
| − | The double-helical structure of DNA is also crucial for understanding the simple mechanism of [[DNA replication]]. [[Cell division]] is essential for an organism | + | The double-helical structure of DNA is also crucial for understanding the simple mechanism of [[DNA replication]]. [[Cell division]] is essential for an organism's growth and development, but when a cell divides, it must replicate its DNA so that it may transmit the characteristics of the parent to the two daughter cells. During DNA replication, the two strands are first separated, and then each strand's complementary DNA sequence is recreated by an enzyme called [[DNA polymerase]]. This enzyme synthesizes the complementary strand by finding the correct base through complementary base pairing and bonding it to the original strand. In this way, the base on the original strand dictates which base appears on the new strand, and the cell ends up with an perfect copy of its DNA. |
===Three types of RNA are involved in protein synthesis=== | ===Three types of RNA are involved in protein synthesis=== | ||
| − | RNA has a greater variety of possible structures and properties than DNA | + | RNA has a greater variety of possible structures and chemical properties than DNA due to the diversity of roles it performs in the cell. Three principal types of RNA are involved in protein synthesis: |
*'''Messenger RNA (mRNA)''' serves as the template for the synthesis of a protein. It carries information from DNA to the [[ribosome]], a specialized structure where the message is translated into a protein. | *'''Messenger RNA (mRNA)''' serves as the template for the synthesis of a protein. It carries information from DNA to the [[ribosome]], a specialized structure where the message is translated into a protein. | ||
| − | *'''Transfer RNA (tRNA)''' is a small chain of about 70-90 nucleotides that transfers a specific [[amino acid]] to a growing [[ | + | *'''Transfer RNA (tRNA)''' is a small chain of about 70-90 nucleotides that transfers a specific [[amino acid]] to a growing [[polypeptide]] chain at the ribosomal site of synthesis. It pairs the amino acid to the appropriate codon on the mRNA molecule. |
*'''Ribosomal RNA (rRNA)''' molecules are extremely abundant and make up at least 80% of the RNA molecules found in a typical eukaryotic cell. In the cytoplasm, rRNA molecules combine with proteins to perform a structural role, as components of the ribosome. | *'''Ribosomal RNA (rRNA)''' molecules are extremely abundant and make up at least 80% of the RNA molecules found in a typical eukaryotic cell. In the cytoplasm, rRNA molecules combine with proteins to perform a structural role, as components of the ribosome. | ||
| − | ===RNA | + | ===RNA serves as a genetic blueprint in some viruses=== |
| − | Some viruses contain either single-stranded or double-stranded RNA as their source of genetic information. [[Retrovirus]]es, for example, store their genetic information as RNA, though they replicate in their hosts via a [[DNA]] intermediate. Once in the host's cell, the RNA strands undergo [[reverse transcription]] to DNA in the [[cytosol]] and are integrated into the host's genome. [[Human immunodeficiency virus (or HIV) | + | Some viruses contain either single-stranded or double-stranded RNA as their source of genetic information. [[Retrovirus]]es, for example, store their genetic information as RNA, though they replicate in their hosts via a [[DNA]] intermediate. Once in the host's cell, the RNA strands undergo [[reverse transcription]] to DNA in the [[cytosol]] and are integrated into the host's [[genome]], the complete DNA sequence of one set of chromosomes. [[Human immunodeficiency virus]] (or HIV) is a retrovirus that causes [[acquired immune deficiency syndrome]] (AIDS), a condition in which the human immune system begins to fail, leading to life-threatening opportunistic infections. |
==Some RNA molecules function as enzymes== | ==Some RNA molecules function as enzymes== | ||
| − | In the 1980s that certain RNA molecules (called ''ribozymes'') may function as enzymes, whereas previously only proteins were believed to have catalytic ability | + | In the 1980s, scientists discovered that certain RNA molecules (called ''ribozymes'') may function as enzymes, whereas previously only proteins were believed to have catalytic ability. Many natural ribozymes catalyze either their own [[Bond cleavage|cleavage]] or the cleavage of other RNAs, but they have also been found to catalyze the aminotransferase activity of the ribosome. |
| − | The discovery of ribozymes | + | The discovery of ribozymes provides a possible explanation for how early RNA molecules might have first catalyzed their own replication and developed a range of enzymatic activities. Known as the ''RNA world hypothesis'', this explanation posits that RNA evolved before DNA and proteins from free-floating nucleotides in the early "primordial soup." In their function as enzymes, RNA molecules might have begun to catalyze the synthesis of proteins, which are more versatile than RNA, from amino acid molecules. Next, DNA might have been formed by reverse transcription of RNA, with DNA eventually replacing RNA as the storage form of genetic material. There are remaining difficulties with the RNA world hypothesis; however, the multifunctional nature of nucleic acids does suggest the interconnectedness of life and its common origins. |
==References== | ==References== | ||
Revision as of 00:35, 20 December 2006
A nucleic acid is a complex, high-molecular-weight macromolecule composed of nucleotide chains whose sequence of bases conveys genetic information. Nucleic acids are found in all living cells and in viruses, and the flow of genetic information is essentially the same in all organisms.
The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The main role of DNA is the long-term storage of genetic information. DNA is often compared to a blueprint, since it contains instructions for constructing other components of the cell, such as proteins and RNA molecules. The DNA segments that carry genetic information are called genes, but other DNA sequences have structural purposes or are involved in regulating the expression of genetic information.
Although RNA may serve as a genetic blueprint for certain viruses, it plays a diversity of roles. RNA may be thought of as the intermediate between the DNA blueprint and the actual workings of the cell, serving as the template for the synthesis of proteins from the genetic information stored in DNA. Some RNA molecules (called ribozymes) are also involved in the catalysis of biochemical reactions.
The nucleic acids DNA and RNA can be found in the nuclei of eukaryotic cells and the cytoplasms of prokaryotes (which lack a nucleus). In eukaryotes, DNA is also present in other cellular compartments (called organelles), such as mitochondria and chloroplasts.
The chemical structure of nucleic acids
Nucleic acids are composed of repeating nucleotide units
Nucleic acids are polymers of repeating units (called monomers). Specifically, they consist of long chains of nucleotide monomers connected by covalent chemical bonds. RNA molecules may contain as few as 75 nucleotides to more than 5000 nucleotides, while a DNA molecule may be composed of more than 1,000,000 nucleotide units.
A nucleotide is a chemical compound with three components: a nitrogen-containing base, a pentose (five-carbon) sugar, and one or more phosphate groups. The nitrogen-containing base of a nucleotide (also called the nucleobase) is typically a derivative of either purine or pyrimidine. The most common bases in nucleotides are the purines adenine and guanine and the pyrimidines cytosine and thymine (or uracil in RNA).
The sugar component is either deoxyribose or ribose. (“Deoxy” simply indicates that the sugar lacks an oxygen atom present in ribose, the parent compound.)
There are two major compositional differences between RNA and DNA:
- The sugar units in RNA molecules are riboses, while DNA is built of nucleotides with a deoxyribose sugar.
- One of the four major nucleobases in RNA is uracil (U) instead of thymine (T).
Nucleic acids form single or double-stranded structures
Nucleic acids are constructed from chains of nucleotides attached by phosphodiester bonds, which are formed between the phosphate residue of one nucleotide and one of two possible carbons on the sugar molecule of an adjacent nucleotide. These sugar-phosphate interactions play a primarily structural role, forming what is sometimes referred to as the "backbone" of the nucleic acid.
Nucleic acids organize into single-stranded or double-stranded molecules. The DNA of many chromosomes and DNA-containing viruses forms long, unbranched, double-helical threads, in which two strands of DNA coil around a common axis. The strands run in opposite directions, and are held together by hydrogen bonds between pairs of bases from each strand. The base adenine is always paired with thymine, and guanine with cytosine (i.e., a purine pairs with a pyrimidine). The stability created by the hydrogen-bonding between these complementary base pairs makes DNA a sturdy form of genetic storage.
The DNA of many viruses and the DNA found in mitochondria are circular; in some cases, they also twist into a supercoiled form. RNA is usually single-stranded, but it may contain double-helical regions where a given strand has folded back on itself.
Nucleic acids store and transmit genetic information
DNA encodes instructions for the synthesis of proteins
DNA contains the genetic information that allows living things to function, grow and reproduce. This information is encoded in the biochemical composition of the molecule itself; specifically, in its particular sequence of nucleobases (which are the variable part of the DNA molecule). A particular sequence of nucleotides along a segment of the DNA strand (i.e., a gene) defines a messenger RNA sequence, which in turn defines a protein. The relationship between the nucleotide sequence and the amino-acid sequence of the protein is determined by simple cellular rules of translation, known collectively as the genetic code. The genetic code is the relation between the sequence of bases in DNA (or its RNA transcript) and the sequence of amino acids in proteins. Amino acids are coded by groups of three bases (called codons) starting from a fixed point (e.g. ACT, CAG, TTT). These codons can then be translated with messenger RNA and then transfer RNA from the chemical language of nucleic acids to that of amino acids, with each codon corresponding to a particular amino acid.
The double-helical structure of DNA facilitates its own replication
The double-helical structure of DNA is also crucial for understanding the simple mechanism of DNA replication. Cell division is essential for an organism's growth and development, but when a cell divides, it must replicate its DNA so that it may transmit the characteristics of the parent to the two daughter cells. During DNA replication, the two strands are first separated, and then each strand's complementary DNA sequence is recreated by an enzyme called DNA polymerase. This enzyme synthesizes the complementary strand by finding the correct base through complementary base pairing and bonding it to the original strand. In this way, the base on the original strand dictates which base appears on the new strand, and the cell ends up with an perfect copy of its DNA.
Three types of RNA are involved in protein synthesis
RNA has a greater variety of possible structures and chemical properties than DNA due to the diversity of roles it performs in the cell. Three principal types of RNA are involved in protein synthesis:
- Messenger RNA (mRNA) serves as the template for the synthesis of a protein. It carries information from DNA to the ribosome, a specialized structure where the message is translated into a protein.
- Transfer RNA (tRNA) is a small chain of about 70-90 nucleotides that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of synthesis. It pairs the amino acid to the appropriate codon on the mRNA molecule.
- Ribosomal RNA (rRNA) molecules are extremely abundant and make up at least 80% of the RNA molecules found in a typical eukaryotic cell. In the cytoplasm, rRNA molecules combine with proteins to perform a structural role, as components of the ribosome.
RNA serves as a genetic blueprint in some viruses
Some viruses contain either single-stranded or double-stranded RNA as their source of genetic information. Retroviruses, for example, store their genetic information as RNA, though they replicate in their hosts via a DNA intermediate. Once in the host's cell, the RNA strands undergo reverse transcription to DNA in the cytosol and are integrated into the host's genome, the complete DNA sequence of one set of chromosomes. Human immunodeficiency virus (or HIV) is a retrovirus that causes acquired immune deficiency syndrome (AIDS), a condition in which the human immune system begins to fail, leading to life-threatening opportunistic infections.
Some RNA molecules function as enzymes
In the 1980s, scientists discovered that certain RNA molecules (called ribozymes) may function as enzymes, whereas previously only proteins were believed to have catalytic ability. Many natural ribozymes catalyze either their own cleavage or the cleavage of other RNAs, but they have also been found to catalyze the aminotransferase activity of the ribosome.
The discovery of ribozymes provides a possible explanation for how early RNA molecules might have first catalyzed their own replication and developed a range of enzymatic activities. Known as the RNA world hypothesis, this explanation posits that RNA evolved before DNA and proteins from free-floating nucleotides in the early "primordial soup." In their function as enzymes, RNA molecules might have begun to catalyze the synthesis of proteins, which are more versatile than RNA, from amino acid molecules. Next, DNA might have been formed by reverse transcription of RNA, with DNA eventually replacing RNA as the storage form of genetic material. There are remaining difficulties with the RNA world hypothesis; however, the multifunctional nature of nucleic acids does suggest the interconnectedness of life and its common origins.
ReferencesISBN links support NWE through referral fees
- Stryer, Lubert. 1995. Biochemistry, 4th edition. New York, N.Y.: W.H. Freeman.
External links
Template:ChemicalSources
| Nucleic acids edit |
|---|
| Nucleobases: Adenine - Thymine - Uracil - Guanine - Cytosine - Purine - Pyrimidine |
| Nucleosides: Adenosine - Uridine - Guanosine - Cytidine - Deoxyadenosine - Thymidine - Deoxyguanosine - Deoxycytidine |
| Nucleotides: AMP - UMP - GMP - CMP - ADP - UDP - GDP - CDP - ATP - UTP - GTP - CTP - cAMP - cGMP |
| Deoxynucleotides: dAMP - dTMP - dUMP - dGMP - dCMP - dADP - dTDP - dUDP - dGDP - dCDP - dATP - dTTP - dUTP - dGTP - dCTP |
| Nucleic acids: DNA - RNA - LNA - PNA - mRNA - ncRNA - miRNA - rRNA - siRNA - tRNA - mtDNA - Oligonucleotide |
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