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codon

(KOH-don)
A sequence of three consecutive nucleotides in a DNA or RNA molecule that codes for a specific amino acid. Certain codons signal the start or end of translation. These are called start or stop (or termination) codons.
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Transcription and translation; the top part of the drawing shows transcription occurring in the nucleus of a cell: a piece of DNA that codes for a specific gene is copied into mRNA; bases in the copied DNA, adenine (A), cytosine (C), guanine (G), and thymine (T), form specific pairs with the bases in the mRNA, except adenine (A) in the DNA pairs with uracil (U) in the mRNA (also shown is a sequence of 3 bases called a codon in the mRNA); the mRNA then carries the genetic information from the DNA to the cytoplasm. The bottom part of the drawing shows translation occurring in the cytoplasm of a cell: the mRNA attaches to and passes through the ribosome; tRNA carries an amino acid to the ribosome, where it binds to a matching codon sequence in the mRNA; the amino acid joins with the other amino acids to form a growing protein chain; the completed protein is then released from the ribosome.Enlarge this image in new window
Transcription and translation are processes a cell uses to make all proteins the body needs to function from information stored in the sequence of bases in DNA. The four bases (C, A, T/U, and G in the figure) are the building blocks of DNA and RNA. During transcription, a piece of DNA that codes for a specific gene is copied into messenger RNA (mRNA) in the nucleus of the cell. The mRNA then carries the genetic information from the DNA to the cytoplasm, where translation occurs. During translation, proteins are made using the information stored in the mRNA sequence. The mRNA attaches to a structure called a ribosome that can read the genetic information. As the mRNA passes through the ribosome, another type of RNA called transfer RNA (tRNA) carries a protein building block called an amino acid to the ribosome. The tRNA carrying the amino acid binds to a matching sequence in the mRNA. As each tRNA binds to the mRNA strand, the amino acid it carried joins with the other amino acids to form a chain of amino acids. Once all of the amino acids coded for in the piece of mRNA have been linked, the completed protein is released from the ribosome.
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