ATG and
AUG denote sequences of
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
and
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
respectively that are the
start codonATG and AUG denote sequences of DNA and RNA respectively that are the start codon or initiation codon encoding the amino acid methionine in eukaryotes and a modified Met in prokaryotes....
or initiation codon encoding the
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
methionineMethionine is an α-amino acid with the chemical formula HO
2CCHCH
2CH
2SCH
3. This essential amino acid is classified as nonpolar.-Function:...
(Met) in
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
s and a modified Met (fMet) in prokaryotes.
The principle called the
Central dogma of molecular biologyThe central dogma of molecular biology was first enunciated by Francis Crick in 1958 and re-stated in a Nature paper published in 1970:In other words, 'once information gets into protein, it can't flow back to nucleic acid.'...
describes the process of translation of a
geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
to a
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
. Specific sequences of
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
act as a template to synthesize mRNA in a process termed "
transcriptionTranscription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA in the presence of the correct enzymes...
" in the
nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
. This mRNA is exported from the nucleus into the cytoplasm of the cell and acts as a template to synthesize protein in a process called "translation."
Three nucleotide bases specify one amino acid in the
genetic codeThe genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells. A more precise term for the concept might be "genetic cipher". The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids...
, a mapping encoded in the tRNA of the organism.
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ATG and
AUG denote sequences of
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
and
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
respectively that are the
start codonATG and AUG denote sequences of DNA and RNA respectively that are the start codon or initiation codon encoding the amino acid methionine in eukaryotes and a modified Met in prokaryotes....
or initiation codon encoding the
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
methionineMethionine is an α-amino acid with the chemical formula HO
2CCHCH
2CH
2SCH
3. This essential amino acid is classified as nonpolar.-Function:...
(Met) in
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
s and a modified Met (fMet) in prokaryotes.
The principle called the
Central dogma of molecular biologyThe central dogma of molecular biology was first enunciated by Francis Crick in 1958 and re-stated in a Nature paper published in 1970:In other words, 'once information gets into protein, it can't flow back to nucleic acid.'...
describes the process of translation of a
geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
to a
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
. Specific sequences of
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
act as a template to synthesize mRNA in a process termed "
transcriptionTranscription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA in the presence of the correct enzymes...
" in the
nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
. This mRNA is exported from the nucleus into the cytoplasm of the cell and acts as a template to synthesize protein in a process called "translation."
Three nucleotide bases specify one amino acid in the
genetic codeThe genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells. A more precise term for the concept might be "genetic cipher". The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids...
, a mapping encoded in the tRNA of the organism. The first three bases of the coding sequence (CDS) of mRNA to be translated into protein are called a start codon or initiation codon. The start codon is almost always preceded by an untranslated region 5' UTR.
The start codon is typically AUG (or ATG in DNA; this also encodes methionine). Very rarely in higher organisms (eukaryotes) are non AUG start codons used.
In addition to AUG, alternative start codons, mainly GUG and UUG are used in prokaryotes. For example E. coli uses 83% ATG (AUG), 14% GTG (GUG), 3% TTG (UUG) and one or two others.
Interestingly when an alternative start codon is used, it gets translated as methionine - even if the codon might normally encode a different amino acid. This is because a separate [tRNA] is used for initiation.
Well known coding regions that do not have ATG initiation codons are those of
lacI (GTG) and
lacA (TTG) in the
E. coli lac operonThe lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria. It consists of three adjacent structural genes, a promoter, a terminator, and an operator. The lac operon is regulated by several factors including the availability...
.
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