<P> The basic process of protein production is addition of one amino acid at a time to the end of a protein . This operation is performed by a ribosome . A ribosome is made up of two subunits, a small subunit and a large subunit . these subunits come together before translation of mRNA into a protein to provide a location for translation to be carried out and a polypeptide to be produced . The choice of amino acid type to add is determined by an mRNA molecule . Each amino acid added is matched to a three nucleotide subsequence of the mRNA . For each such triplet possible, the corresponding amino acid is accepted . The successive amino acids added to the chain are matched to successive nucleotide triplets in the mRNA . In this way the sequence of nucleotides in the template mRNA chain determines the sequence of amino acids in the generated amino acid chain . Addition of an amino acid occurs at the C - terminus of the peptide and thus translation is said to be amino - to - carboxyl directed . </P> <P> The mRNA carries genetic information encoded as a ribonucleotide sequence from the chromosomes to the ribosomes . The ribonucleotides are "read" by translational machinery in a sequence of nucleotide triplets called codons . Each of those triplets codes for a specific amino acid . </P> <P> The ribosome molecules translate this code to a specific sequence of amino acids . The ribosome is a multisubunit structure containing rRNA and proteins . It is the "factory" where amino acids are assembled into proteins . tRNAs are small noncoding RNA chains (74 - 93 nucleotides) that transport amino acids to the ribosome . tRNAs have a site for amino acid attachment, and a site called an anticodon . The anticodon is an RNA triplet complementary to the mRNA triplet that codes for their cargo amino acid . </P> <P> Aminoacyl tRNA synthetases (enzymes) catalyze the bonding between specific tRNAs and the amino acids that their anticodon sequences call for . The product of this reaction is an aminoacyl - tRNA . This aminoacyl - tRNA is carried to the ribosome by EF - Tu, where mRNA codons are matched through complementary base pairing to specific tRNA anticodons . Aminoacyl - tRNA synthetases that mispair tRNAs with the wrong amino acids can produce mischarged aminoacyl - tRNAs, which can result in inappropriate amino acids at the respective position in protein . This "mistranslation" of the genetic code naturally occurs at low levels in most organisms, but certain cellular environments cause an increase in permissive mRNA decoding, sometimes to the benefit of the cell . </P>

Where does the second step of protein synthesis take place