What makes tRNA? a) Amino acid which is specific to each tRNA. (b) CCA base sequence to which the amino acid is attached by the 'Activating Enzyme'. (c) Complementary base pairing sequence. Helical in shape. (d) 8 free bases non-pairing giving one loop of RNA. (e) 7 free bases non-pairing giving second loop of RNA. (f) Small open loop of RNA which is variable in shape between different tRNA. (g) Anti-codon (3 bases) which binds to the mRNA codon (3 bases) this is specific to the amino acids being carried. The anti-codon is complementary to the sense DNA.
3.5.3 Genetic code 'You can treat the genetic code like a dictionary in which sixty- four words in one language (the sixty-four possible triplets of a four-letter alphabet) are mapped onto twenty-one words in another language (twenty amino acids plus a punctuation mark). The odds of arriving at the same 64:21 mapping are less than one in a million million million million million. Yet the genetic code is in fact literally identical in all animals, plants and bacteria that have ever been looked at. All living things are certainly descended from a single ancestor' R.Dawkins, (1995),River out of Eden.
The Genetic Code: Polypeptide is a sequence of bases Bases are either A U G or C There are 4 bases if placed in sets of 3 (a triplet).= 4 3 possible triplets of RNA = 64 triplets There are 20 common amino acids Therefore 64 triplets are mapped to 20 amino acids However there is a 'punctuation' triplet Therefore the mapping of the code is 64: 21
The genetic code is first transcribed into mRNA The mRNA codons can be mapped to a specific amino acid. Degenerate code: – DNA is a degenerate code since there are more than one triplet or codon that maps to an amino acid or punctuation. – mRNA codon AUG codes for Methionine and is a START signal for translation. – mRNA codon UAA, UAG, UGA are all stop codons punctuating the code. – GGU, GGC, GGA and GGG all code for amino acid glycine. Consider what the benefits of a degenerate code might be from an evolutionary perspective?