Protein Synthesis

A protein is synthesised during two processes. The first is transcription.

Transcription begins with the specified DNA strand coding for the gene dipping into the nucleolus. This `unzips` the DNA (by breaking hydrogen bonds between complementary bases). The half-DNA acquired is the template strand . Free floating nucleotides line up and bind to the template strand (via Hydrogen bonds). In the mRNA strand about to be created, the base pairs are complementary to one another, but here U-nucleotides bind to A-Template strand nucleotides. The bindings are catalysed by RNA polymerase.

The mRNA produced is complementary to the nucleotide base sequence on the template DNA strand and is therefore a copy of the base sequence on the coding strand of DNA.  The mRNA is the released and goes through a nuclear pore.

Translation is the second stage of protein synthesis, when amino acids are lined up to create the protein.

The mRNA strand is able to fit into a groove in a ribosome. When this happens one can move along the other. There is space for two amino acid-tRNA complexes on the ribosome, where there are six bases (two codons) to bind to. The first base sequence is always AUG.

An amino acid-tRNA complex attaches to the ribosome. The tRNA has an anti-codon, which can bind to a complementary codon on the mRNA. Another complex binds to the adjacent base sequence. The amino acids bound to the tRNA molecules are placed together using peptide bond. The first tRNA molecule is now free to move (it has given up its Amino Acid), and the Ribosome or mRNA shifts along. Another complex binds to the next codon that’s just been revealed. The new amino acid bind to the other amino acids with a peptide bond. This is repeated till the end of the mRNA strand. The chain of amino acids is the coded protein.

A few pictures showing the full process.

 A picture showing Translation across a Ribosome.