Essay !11 - Number of genes encoding tRNA and deducting why people should have that amount of tRNA gene Code : KSI0023

<Genomics Essay#11>

 Sangin Kim

 

 

(1) Number of genes encoding tRNA

According to tRNAscan-SE anaylsis of complete genomes, the number of tRNAs which are decoding standard 20 amino acids are 597. the number of selenoccysteine which is the 21st proteinogenic amino acid’s tRNAs(TCA) are 3. The number of possible suppressor tRNAs (CTA, TTA) are 4 and tRNAs with undetermined or unknown isotypes 6. So, total tRNAs are 610. Among them, there are 34 tRNAs which with introns.

 

(2) Why people should have that amount of tRNA gene?

According to wobble theory, rules of base pairing are relaxed at the third position. As a result, a base can pair with more than one complementary base. Also, some anticodons have ‘inosine’ at 3rd position. It can pair with U, C, or A. From this theory, we can speculate that we don’t need to have 61 different tRNA molecules(except stop codon). However, even the wobble theory, the minimal 32 anticodons are needed to recognize all of the sense-codons in mRNA(Not stop. Stop tRNA doens’t exist). Amino acids are attached to the specific tRNA through a specific aminoacyl-tRNA synthetase or a tRNA-dependent amino acid modification. So, lots of tRNA genes are redundant. From this information, we can speculate several reasons why Homo sapiens should have that amount of tRNA genes. I can organize the reasons largely into two. First reason is wobble theory and tRNA expression. We have 20 standard amino acids, also, proteinogenic such as selenoccystein. If we have 1 to 1 tRNA with amino acid, then it will be disaster when one tRNA can’t function. Finally, it can leads to we can’t make that amino acid. So there exist isotypes and it’s a kind of ‘spare mechanism’. Second, there’re lots of translational regulation mechanisms. To regulate in the translational level, there should have suppressor tRNAs like CTA or TTA.