TRNA

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Transfer RNA molecules (tRNAs) are small RNA molecules usually approximately 80 nucleotides in length, that function as adaptor molecules during the translation of mRNA into an amino acid  sequence [1].

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 nolecules. tRNAS also have a short single stranded region at a tRNAs 3' end where amino acids that match an mRNA codon are attached [3]

The primary structure of tRNA consists of modified bases; dihydrouridine, ribothymine, pseudouridine and inosine. The D loop of the tRNA molecule contains dihydrouridine, whereas the T loop has pseudouridine. Inosine is a result of deamination of guanine, 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. (Alberts, Johnson, Lewis, Raff, Roberts, Walter et al, 2008, 368-371)

Function

The amino acids that bond to produce a protein  do not bind to mRNA. They require an adaptor molecule to bind to mRNA at one point and to the amino acid at another. 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. The theory of one tRNA being able to bind to more than one codon is known as the Wobble hypothesis [4]. 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. These nonstandard base pairs are weaker than other common base pairs, hence "wobble" hypothesis [5].

A tRNA which is joined to an amino acid is called an aminoacyl tRNA, which form via an enzyme called aminoacyl-tRNA synthetase. 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 hydrolysed and donating a 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 the sugar of the tRNA molecule, which allows formation of an ester bond with the tRNA, therefore finally forming a aminoacyl tRNA [6].

References

  1. Snustad, D. Peter. (2010). Principles Of Genetics.Hobeken: Wiley & Sons
  2. Alberts, Bruce et al. (2009). New York: Garland Science
  3. Champe et al.(2008). Biochemistry. Baltimore: Lippincott Williams & Wilkins
  4. Alberts, Bruce et al. (2008). Molecular Biology of the Cell. New York: Garland Science
  5. Alberts, Johnson, Lewis, Raff, Roberts, Walter et al., 2008,page 368-371
  6. 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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