The genetic code is a set of rules defining how the four-letter code of DNA is translated into the 20-letter code of amino acids, which are the building blocks of proteins. The genetic code is a set of three-letter combinations of nucleotides called codons, each of which corresponds to a specific amino acid or stop signal. The concept of codons was first described by Francis Crick and his colleagues in 1961. During the same year, Marshall Nirenberg and Heinrich Matthaei performed experiments that began deciphering the genetic code. They showed that the RNA sequence UUU specifically coded for the amino acid phenylalanine. Following this discovery, Nirenberg, Philip Leder, and Gobind Khorana identified the rest of the genetic code and fully described each three-letter codon and its corresponding amino acid. Show
There are 64 possible permutations, or combinations, of three-letter nucleotide sequences that can be made from the four nucleotides. Of these 64 codons, 61 represent amino acids, and three are stop signals. Although each codon is specific for only one amino acid (or one stop signal), the genetic code is described as degenerate, or redundant, because a single amino acid may be coded for by more than one codon. It is also important to note that the genetic code does not overlap, meaning that each nucleotide is part of only one codon-a single nucleotide cannot be part of two adjacent codons. Furthermore, the genetic code is nearly universal, with only rare variations reported. For instance, mitochondria have an alternative genetic code with slight variations.
The standard RNA codon table organized in a wheel A codon table can be used to translate a genetic code into a sequence of amino acids.[1][2] The standard genetic code is traditionally represented as an RNA codon table, because when proteins are made in a cell by ribosomes, it is messenger RNA (mRNA) that directs protein synthesis.[2][3] The mRNA sequence is determined by the sequence of genomic DNA.[4] In this context, the standard genetic code is referred to as translation table 1.[3] It can also be represented in a DNA codon table. The DNA codons in such tables occur on the sense DNA strand and are arranged in a 5′-to-3′ direction. Different tables with alternate codons are used depending on the source of the genetic code, such as from a cell nucleus, mitochondrion, plastid, or hydrogenosome.[5] There are 64 different codons in the genetic code and the below tables; most specify an amino acid.[6] Three sequences, UAG, UGA, and UAA, known as stop codons,[note 1] do not code for an amino acid but instead signal the release of the nascent polypeptide from the ribosome.[7] In the standard code, the sequence AUG—read as methionine—can serve as a start codon and, along with sequences such as an initiation factor, initiates translation.[3][8][9] In rare instances, start codons in the standard code may also include GUG or UUG; these codons normally represent valine and leucine, respectively, but as start codons they are translated as methionine or formylmethionine.[3][9] The first table—the standard table—can be used to translate nucleotide triplets into the corresponding amino acid or appropriate signal if it is a start or stop codon. The second table, appropriately called the inverse, does the opposite: it can be used to deduce a possible triplet code if the amino acid is known. As multiple codons can code for the same amino acid, the International Union of Pure and Applied Chemistry's (IUPAC) nucleic acid notation is given in some instances. Translation table 1[edit]Standard RNA codon table[edit]
Inverse RNA codon table[edit]Inverse table for the standard genetic code (compressed using IUPAC notation)[13]
Standard DNA codon table[edit]
Inverse DNA codon table[edit]Inverse table for the standard genetic code (compressed using IUPAC notation)[13]
Alternative codons in other translation tables[edit]The genetic code was once believed to be universal:[16] a codon would code for the same amino acid regardless of the organism or source. However, it is now agreed that the genetic code evolves,[17] resulting in discrepancies in how a codon is translated depending on the genetic source.[16][17] For example, in 1981, it was discovered that the use of codons AUA, UGA, AGA and AGG by the coding system in mammalian mitochondria differed from the universal code.[16] Stop codons can also be affected: in ciliated protozoa, the universal stop codons UAA and UAG code for glutamine.[17][note 4] The following table displays these alternative codons.
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Which nucleotides sequence code for an amino acid?The nucleotide triplet that encodes an amino acid is called a codon.
What is an amino acid sequence called?The sequence of amino acids in a protein is called the primary structure. The specific sequence of amino acids will give the protein its properties later that allow it to interact with other molecules and carry out catalytic functions.
What is a sequence of nucleotides that codes for a protein?A gene is an ordered sequence of nucleotides located in a particular position within the genome that encodes a specific functional product (i.e., a protein or RNA molecule).
What are 3 bases that code for an amino acid called?The three consecutive DNA bases, called nucleotide triplets or codons, are translated into amino acids (GCA to alanine, AGA to arginine, GAT to aspartic acid, AAT to asparagine, and TGT to cysteine in this example).
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What do we call the sequence of nucleotides that codes for an amino acid?
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