TRNA

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Transfer RNA molecules (tRNAs) are small RNA molecules of between 70 and 90 nucleotides in length, that function as adaptor molecules during the translation of mRNA into an amino acid sequence[1]. They are non-coding RNA sequences, that are created during transcription of DNA.

Structure

The structure of tRNA arises through the ability of RNA to fold into three-dimensional shapes using Watson and Crick base pairing. This folding leads to the formation of tRNA a tertiary RNA structure. If there are large enough regions of overlap tRNA will fold into a shape that resembles a cloverleaf. This will undergo further folding by hydrogen bonding, to form a compact L-shaped structure[2].

The cloverleaf structure of tRNA is composed of an anticodon (a triplet of nucleotides that is complementary to corresponding codons on mRNA molecules), a short single stranded region of nucleotides in the sequence C-C-A on the acceptor stem at a tRNAs 3' end where amino acids that correspond to the mRNA codon are attached[3], a D-arm  and a T-arm.

The primary structure of tRNA includes the modified bases; dihydrouridine, ribothymine, pseudouridine and inosine. The D loop of the tRNA molecule contains dihydrouridine, whereas the T loop has pseudouridine. Inosine nucleoside is formed in tRNA by the removal of an amino group from adenosine, whereas dihydrouridine and pseudouridine derive from uracil. The tRNA molecule has a 5' monophosphate rather than a 5' triphosphate, as well as having 15 invariant and 8 semi-variant residues within it[4]. Invariant bases are maintained throughout all tRNA molecules, and semi-variant bases are found in the majority of tRNAs. 

Function

The amino acids that bond to produce a protein do not bind to mRNA. They require an adaptor molecule to decode the base sequence of mRNA into the amino acid sequence of proteins. This adaptor molecule is tRNA.

The genetic code is described as redundant/degenerate as there are 64 codons coding for only 20 amino acids. There is more than one tRNA molecule for some of the amino acids. Some tRNAs can bind to more than one codon, and this theory is known as the Wobble hypothesis[5]. This theory, made by Francis Crick, suggests the 3' codon and 5' anticodon positions do not follow the standard codon-anticodon base pair, such as A-T and G-C. It allows some bases to pair with bases that they typically do not form links between, for example, Uracil can bind with Adenine or Guanine. Inosine is vital in the wobble hypothesis, able to form bonds with adenine, guanine and uracil, giving great flexibility to tRNA molecules with inosine at their 3' anticodon base position. These nonstandard base pairs are weaker than other common base pairs, hence "wobble" hypothesis[6].

A tRNA which is joined to an amino acid is called an aminoacyl tRNA or "charged" tRNA", which form using enzymes called aminoacyl-tRNA synthetases. Most cells have a synthetase for each amino acid and the reaction is coupled to the hydrolysis of ATP. The process begins with ATP being hydrolyzed and donating an AMP, which binds on the carboxyl group of the amino acid thus forming an adenylated amino acid. The AMP is then transferred to a hydroxyl group on either the 2' or 3' carbon of the 3'-end nucleotide on the tRNA molecule, which allows the formation of an ester bond with the tRNA, therefore finally forming the aminoacyl tRNA[7].

Each class of aminoacyl tRNA synthetases - class 1 and 2 - bind to different faces on the underside of the tRNA molecules, and each group includes enzymes specific for 10 out of the 20 fundamental amino acids.

References

  1. Snustad, D. Peter. (2010). Principles Of Genetics.Hobeken: Wiley and Sons
  2. Alberts, Bruce et al. (2009). New York: Garland Science
  3. Champe et al.(2008). Biochemistry. Baltimore: Lippincott Williams and; Wilkins
  4. Alberts, Johnson, Lewis, Raff, Roberts, Walter et al, 2008, 368-371
  5. Alberts, Bruce et al. (2008). Molecular Biology of the Cell. New York: Garland Science
  6. Alberts, Johnson, Lewis, Raff, Roberts, Walter et al., 2008, page 368-371
  7. Alberts, A.B, Johnson, A.J, Lewis, J.L, Raff, M.J, Roberts, K.R, Walter, P.W Et al. (2008). Molecular Biology of the Cell. 5th ed. New York, USA and Abingdon, UK: Jackie Harbor. p368-37
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