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The below mentioned article provides short notes on anticodons.
The base sequence of tRNA which pairs with codon of mRNA during translation is called anticodon. Codons are written in 5 to 3 direction, whereas anticodons are usually written in 3 to 5 direction. In a codon, the first letter is at the 5 end while in anticodon the first letter is at the 3 end. There are 61 codons which code for amino acids.
However, the number of tRNA molecules [anticodons] is always much lesser than codons in an organism. Hence, anticodon of some tRNA molecules has to pair with more than one codon. There are four main differences between codons and anticodons (Table 28.4). For AUG codon the anticodon is UAC as given below.
There are two types of base pairing, viz:
(1) Normal base pairing, and
(2) Unusual base pairing.
These are explained as follows:
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1. Normal Base Pairing:
In case of normal base pairing, A pairs with U and G with C. This type of base pairing is also known as Watson and Crick base pairing, because it was originally proposed by Watson and Crick.
2. Wobble Base Pairing:
The Wobble hypothesis was proposed by Crick in 1966 to explain for degeneracy of code and also unusual base pairing between codons and anticodons. There are 61 codons which code for amino acids, but the number of tRNA molecules which act as anticodon is always much lesser than the number of codons.
Hence, the anticodons of some tRNA molecules have to pair with more than one mRNA codon. According to Wobble hypothesis, only the first two bases of a codon must pair in a usual manner with the bases of tRNA anticodon.
The third base of the codon can pair in a different way (i.e. G with U). For example, the anticodon UCG of serine tRNA pairs with two codons, viz. AGC and AGU which differ only in the third base. The pairing between UCG and AGC is normal (Watson Crick pairing).
When only two codons specify an amino acid, the third letters of the codon are both purines (CU) or both pyrimidines (AG) but never one purine and one pyrimidine.
A new nucleotide called inosine (I) has been discovered in yeast which is found on the third position and can pair with A, U and C and thus helps in Wobble base pairing. Inosine is a deamination product of adenosine which occupies first position in an anticodon. Inosine in other position remains unaffected.
