U

no wobble

The most frequent modification is methylation. Methyl or dimethyl versions of A, U, G or C all exist. Methylation prevents pairing of certain bases and also aids binding of ribosomal proteins. Note that thymine (= 5-methyl uracil), which is normally only found in DNA, is also found in the TyC-loop of tRNA, where it is attached to ribose and is made by methylation of uracil after transcription.

In pseudouridine, the uracil itself is not altered, but is attached to ribose by carbon-5 instead of nitrogen-1, as in normal uridine. The base found in the nucleoside inosine is actually named hypoxanthine. However, it is written as "I" in sequences and often called I-base to avoid confusion. Similarly, the bases of queuosine and wyosine are referred to as Q-base and Y-base.

Some tRNA Molecules Read More Than One Codon

Each transfer RNA carries only a single amino acid, so at least 20 different tRNAs are needed for the 20 different amino acids. On the other hand (excluding the stop codons), there are 61 codons to be recognized, as some amino acids have more than one codon. In fact, some tRNAs can read more than one codon, though, of course, these must all code for the same amino acid. The minimum set of different tRNA molecules needed to read all 61 codons is 31.The actual number found is usually slightly higher and varies a little from species to species.

Since only complementary bases can pair, how does a tRNA with one anticodon read more than one codon? Remember that the standard base pairing rules apply to bases that form part of a DNA double helix. Since the codon and anticodon do not form a standard double helix, slightly different rules for base pairing apply. The last two bases of the tRNA anticodon, which pair with the first two bases of the mRNA codon, pair strictly according to normal rules. However, the first base of the tRNA anticodon (which pairs with the third base of the mRNA codon) can wobble around a little because it is not squeezed between other bases as in a helix structure. Consequently, the codon/anticodon base pairing rules are known as the wobble rules (see Table. 8.01).

If the first anticodon base is G it can pair with C, as usual, or, in wobble mode, with U. For example, tRNA for histidine, with GUG as anticodon, can recognize both the CAC and CAU codons. Similarly, if the first anticodon base is U, it can pair with A or G. Whenever an amino acid is encoded by a pair of codons, the third codon bases are U and C (e.g., histidine, tyrosine) or A and G (e.g., lysine, glutamic acid), but never other combinations. Similarly, those privileged amino acids with four or six codons may be regarded as having two or three such pairs. Due to wobble pairing, only a single tRNA is needed to read each such pair of codons. It is possible for a single tRNA to inosine An unusual modified nucleoside derived from guanosine wobble rules Rules allowing less rigid base pairing but only for codon/anticodon pairing

Some bases are chemically modified after RNA has been transcribed. Modified bases are especially common in tRNA.

Wobbling between the anticodon and codon allows some tRNA molecules to read more than one codon.

Some tRNA Molecules Read More Than One Codon 203

Normal rna bases o

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