Translation

This coding strand is later decoded by the ribosomes with the help of transfer RNA’s (tRNA‘s) that act as a decoder of the information and protein assembler in a process called translation. tRNA’s contain the necessary components to decode the mRNA and carry  individual amino acids to the ribosome. The pioneering work by Marshall Nirenberg’s group deciphered the way that tRNA’s interface with mRNA to facilitate the polymerization of proteins. The breakthrough from Nirenberg’s lab was made when utilizing artificial RNAs in cell free extracts where they supplied a single type of amino acid in radioactive form with 19 “cold” amino acids. By supplying cell free extract, they were able to monitor the production of “hot” poly-Phenylalanine in the presence of an RNA consisting of poly-Uracil. Through successive shortening of the length of the artificial RNA, they were able to deduce the minimum coding unit to consist of 3 nucleotides. Systematically altering the sequence of the RNA in these extracts permitted them to decipher the genetic code. The ribosome scans along the mRNA and recognizes nucleotides in batches of 3 . These batches of 3 can be translated into an amino acid and is known as a codon.

tRNAs can be represented two-dimensionally in what is called a cloverleaf configuration where there is a stem and loop structure. Stems occur due to the inherent self-base pairing within the molecule. A three-dimensional rendering of tRNA illustrates the accessibility of the sequence in the loop called the anticodon loop. This anticodon loop complementary base pairs with the mRNA codon to ensure the appropriate amino acid is brought to the ribosome complex.The acceptor stem at the 3′-end of the tRNA is where the specific amino acid is added by an aminoacyl-tRNA synthetase.

Since there are 4 types of bases and they are read as groups of 3, there are 4³ (or 64) combinations of these codons. However, there are only 20 amino acids used to build proteins. This indicates that there is room for redundancy. Three of these codons tell the ribosome to stop, like a period in a sentence. These are called stop codons. There is one special codon that performs double duty: ATG (AUG in RNA). The codon (ATG) that encodes the amino acid Methionine also acts as a start codon that tells the ribosome where to start translating. Like nucleic acids, proteins have a polarity and are synthesized in an amino (N) to carboxyl (C) direction. We abbreviate this by terming the beginning of the protein sequence, N-terminal, and the ending of the sequence as the C-terminal.

In Eukaryotes, secreted or transmembrane proteins are synthesized on the rough ER. These proteins are preceded by a small hydrophobic amino acid sequence called the signal peptide which attaches to a signal recognition particle (SRP). The SRP brings the translating ribosome/mRNA complex to the ER where the translation continues through the membrane of the rER.

Advanced video of Translation