Difference between revisions of "Nucleic acid" - New World Encyclopedia
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===RNA is the molecule of genetic storage in some viruses=== | ===RNA is the molecule of genetic storage 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)]] is a retrovirus that causes [[acquired immune deficiency syndrome (AIDS)]], a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections. | |
==Some RNA molecules function as enzymes== | ==Some RNA molecules function as enzymes== | ||
| − | A '''ribozyme''' (from '''ribo'''nucleic acid en'''zyme''', also called [[RNA]] [[enzyme]] or catalytic RNA) is an [[RNA]] [[molecule]] that catalyzes a [[chemical reaction]]. 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]]. Although RNA contains only four bases, in comparison to the twenty amino acids commonly found in proteins, some RNAs are still able to catalyse chemical reactions. These include cutting and [[ligase|ligating]] other RNA molecules and also the catalysis of [[peptide bond]] formation in the [[ribosome]] | + | In the 1980s that certain RNA molecules (called ''ribozymes'') may function as enzymes, whereas previously only proteins were believed to have catalytic ability. A '''ribozyme''' (from '''ribo'''nucleic acid en'''zyme''', also called [[RNA]] [[enzyme]] or catalytic RNA) is an [[RNA]] [[molecule]] that catalyzes a [[chemical reaction]]. 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]]. Although RNA contains only four bases, in comparison to the twenty amino acids commonly found in proteins, some RNAs are still able to catalyse chemical reactions. These include cutting and [[ligase|ligating]] other RNA molecules and also the catalysis of [[peptide bond]] formation in the [[ribosome]]. |
| + | |||
| + | The discovery of ribozymes provided 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 [[protein]]s 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 from [[amino acid]] molecules. Proteins are more versatile than nucleotides, as they can be built from 20 amino acids with unique side chains versus the four bases of nucleotides. Next, DNA might have been formed by reverse transcription of RNA, with DNA eventually replacing RNA as the storage form of genetic material because of the greater stability and dependability of its double helical structure. There are remaining difficulties with the RNA world hypothesis; however, the multifunctional nature of nucleotides does suggest the interconnectedness of life and its common origins. | ||
==References== | ==References== | ||
Revision as of 20:13, 19 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 in the cell is the long-term storage of information. It is often compared to a blueprint, since it contains the instructions to construct 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. RNA serves as the template for translation of genes into proteins, transferring amino acids to the ribosome to form proteins, and also translating the transcript into proteins. While both types of nucleic acids are involved in the storage and transmission of genetic information, some RNA molecules (called ribozymes) are also involved in the catalysis of biochemical reactions.
DNA is a major component of the chromosomes in the nuclei of eukaryotic cells and can also be found in other cellular components (e.g., mitochondria). The cytoplasms of prokaryotes (which lack a nucleus) also contain DNA, andlRNA is found in both the nucleus and cytoplasm of many cells.
significance
The chemical structure of nucleic acids
Components
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).
Organization
To build the nucleic acid polymer out of individual nucleotides, phosphodiester bonds 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 may 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.
In contrast, 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 folden back on itself.
Nucleic acids store and transmit genetic information
DNA encodes genetic 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). Within a gene, the sequence of nucleotides along a DNA strand 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 structure of DNA also contains a mechanism for 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 to grow, but when a cell divides it must replicate the DNA in its genome so that the two daughter cells have the same genetic information as their parent. 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 onto the original strand. In this way, the base on the old 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 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.
- Transfer RNA (tRNA) is a small chain of about 74-93 nucleotides that attaches to a specific amino acid and 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 and protein combine to form a nucleoprotein called a ribosome, which is the site at which.
RNA is the molecule of genetic storage 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. Human immunodeficiency virus (or HIV) is a retrovirus that causes acquired immune deficiency syndrome (AIDS), a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections.
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. A ribozyme (from ribonucleic acid enzyme, also called RNA enzyme or catalytic RNA) is an RNA molecule that catalyzes a chemical reaction. 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. Although RNA contains only four bases, in comparison to the twenty amino acids commonly found in proteins, some RNAs are still able to catalyse chemical reactions. These include cutting and ligating other RNA molecules and also the catalysis of peptide bond formation in the ribosome.
The discovery of ribozymes provided 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 from amino acid molecules. Proteins are more versatile than nucleotides, as they can be built from 20 amino acids with unique side chains versus the four bases of nucleotides. Next, DNA might have been formed by reverse transcription of RNA, with DNA eventually replacing RNA as the storage form of genetic material because of the greater stability and dependability of its double helical structure. There are remaining difficulties with the RNA world hypothesis; however, the multifunctional nature of nucleotides 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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