User:Dak/TRNA/simple

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Transfer RNA (tRNA) is a molecule made of RNA that plays a role in building protiens within cells.

The existance of tRNA was first hypothesysed by Francis Crick, as he (correctly) guessed that there must be a molecule, within cells, that can essentially 'grab' specific amino-acids (the building blocks of protiens), 'read' mRNA, and put the correct amino acid in place in the growing protien chain by lining up with the correct position on an mRNA.

Structure of tRNA

Structure of tRNA

tRNA can be thought of as a three-dimentional 'L' shape, made of RNA (including some unusual bases) with the following regions:

1. an area that acts as a 'label', allowing other molecules involved in protien-synthsys to recognise tRNA molecules.
2. an 'acceptor stem', to which amino-acids bind
3. the 'anti-codon', a sequence of three bases, used to identify which amino-acid the tRNA molecule carries.

tRNA label

The tRNA label is a sequence of RNA bases that goes CCACCACCACCA(etc).

acceptor stem

The acceptor stem is simply one end of the RNA chain, which has an alcohol (-OH) attached. amino-acids will be fused to this by an enzyme which removes the -OH from the RNA, and an -H from the amino acid (forming H-OH... water), and leaving both the amino acid and the tRNA with unused bonds, which are joined to each other.

tRNA-OH + H-amino acid --> tRNA- + -OH + H- + -aminoacid --> tRNA-aminoacid + H-OH

condensation reaction; so called, because the reaction creates water

the acceptor stem is also the end of the RNA chain that contains the label, thus allowing the enzyme that makes this reaction happen to recognise the tRNA acceptor stem, and join it to the amino-acid

anti-codon

see first: Codon/simple

Codons are sequences of three RNA bases that are 'code words' for a specific amino acid (for example, CCA stands for cantrememberine).

anti-codons are the opposite of a codon; eg, G is the opposite of C, and T is the opposite of A, so the anti-codon for cantrememberine is GGT (although, it's striclty speaking a little more complicated than that: see wobble bases/simple)

as opposite bases will bond to one-another, the anti-codon will bond to the codon in an mRNA molecule. The anti-codon also acts as a label, indicating which amino-acid the tRNA should carry (eg, a tRNA molecule with the anti-codon GGT should carry cantrememberine)

in addition, the enzyme tRNA-amino acid joinyase (the enzyme that catalyses the above reaction) recognises which tRNA to join a given amino-acid to by recognising the anti-codon (eg, would join cantrememberine to a tRNA molecule with the anti-codon 'GGT')

How it works

tRNA-amino acid joinyase will bind to a specific amino-acid (eg, cantrememberine), and will then bind to the tRNA for cantrememberine by finding the tRNA with the anticodon GGT.

tRNA-amino acid joinyase will then find the tRNA 'label', and fix the amino-acid to this point, creating one molecule that is made up of tRNA joined to an amino acid.

next, the tRNA will line up it's anti-codon with the opposite codon on a strand of mRNA. this is helped by ribosomes, and, as the order of codons on an mRNA dictates the order in which the amino acids should go, this 'drags' the amino-acids into the correct sequence.

finally, peptidase will break the bonds between the tRNA and the amino-acid, and join the aligned amino-acids up into a long string of amino-acids (ie, a protein), thus completing the first step of protien synthesys.

(better picture goes here)

===== <--mRNA. this dictates the sequence of amino-acids
  ||| <----tRNA. these grab amino-acids, and line up with the mRNA
  xxx <----amino-acids, put into the correct sequence by tRNA 'reading' mRNA
    O <----an enzyme joins the amino-acids, to create a protien