codons

(noun)

The genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins (amino acid sequences) by living cells. Biological decoding is accomplished by the ribosome, which links amino acids in an order specified by mRNA, using transfer RNA (tRNA) molecules to carry amino acids and to read the mRNA three nucleotides at a time. The genetic code is highly similar among all organisms, and can be expressed in a simple table with 64 entries.

Related Terms

Examples of codons in the following topics:

  • Size Variation and ORF Contents in Genomes

    • The transcription termination pause site is located after the ORF, beyond the translation stop codon, because if transcription were to cease before the stop codon, an incomplete protein would be made during translation.
    • Normally, inserts which interrupt the reading frame of a subsequent region after the start codon cause frameshift mutation of the sequence and dislocate the sequences for stop codons.
    • For example, in a randomly generated DNA sequence with an equal percentage of each nucleotide, a stop-codon would be expected once every 21 codons.
    • Possible stop codons in DNA are "TGA", "TAA", and "TAG".
    • Thus, the last reading frame in this example contains a stop codon (TAA), unlike the first two.

  • Unsticking Stuck Ribosomes

    • This lack of a stop codon results a significant issue for cells.
    • Subsequently, the ribosome moves from the 3' end of the truncated messenger RNA onto the tmRNA where it translates the codons of the tmRNA until the tmRNA stop codon is encountered.
    • The tmRNP binds to the A-site, allowing the ribosome to switch templates from the broken message onto the open reading frame of the tmRNA via the resume codon (blue GCA).
    • Upon reaching the tmRNA stop codon (red UAA), a hybrid protein with a proteolysis tag (green beads) is released.

  • Amino Acid Synthesis

    • Selenocysteine is incorporated when the mRNA being translated includes a SECIS element, which causes the UGA codon to encode selenocysteine instead of a stop codon.
    • It is coded with the codon UAG, which is normally a stop codon in other organisms.
    • This UAG codon is followed by a PYLIS downstream sequence.

  • Slipped-Strand Mispairing

    • Start (ATG) is the start codon in which the ribosome initiates translation of nucleotide sequence into amino acids, and (-10 -35) is the promoter which is the binding site for the RNAP to initiate transcription of DNA into RNA.

  • Scarlet Fever

    • A transcriptional terminator is located 69 bases downstream from the translational termination codon.

  • Purifying Proteins by Affinity Tag

    • Eighteen bases coding six histidines are inserted right after the START codon or right before the STOP codon.

  • Prokaryotic Transcription and Translation Are Coupled

    • This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA.

  • Shuttle Vectors and Expression Vectors

    • Expression vectors must have expression signals such as a strong promoter, a strong termination codon, adjustment of the distance between the promoter and the cloned gene, and the insertion of a transcription termination sequence and a PTIS (portable translation initiation sequence).

  • The Incorporation of Nonstandard Amino Acids

    • Also, most nascent polypeptides start with the amino acid methionine because the "start" codon on mRNA also codes for this amino acid.

  • Regulation by Biosynthetic Enzymes

    • The attenuator sequence has its codons translated into a leader peptide, but is not part of the trp operon gene sequence.