Protein synthesis consists of two stages.In the nucleus the DNA of a single gene is copied as a complementary mRNA strand. This leaves the nucleus through nuclear pores and associates with ribosomes in the cytoplasm.tRNA molecules attached to amino acids bind temporarily to the mRNA in the order determined by the base sequence on the mRNA.The enzymatic action on the large subunit of the ribosome binds the amino acids together with peptide bonds.Transcription occurs in the nucleus.An enzyme unwinds about 12 base pairs of DNA. Nucleotides from the pool of nucleotides in the nucleus attach to their complementary nucleotide residues on the DNA by hydrogen bonds. Cytosine nucleotides form three hydrogen bonds with guanine residues, guanine nucleotides form three hydrogen bonds with cytosine residues, thymine on the DNA binds with adenine nucleotides and form two hydrogen bonds and adenine binds with uracil forming two hydrogen bonds. In this way one DNA strand acts as a template strand.RNA polymerase joins the nucleotides in the 5' to 3' direction with phosphodiester bonds forming a single pre-mRNA strand with a sugar phosphate backbone and bases complementary to the bases on the DNA template strand. The DNA reforms behind the synthesised strand.When a stop codon is reached the enzyme, RNA polymerase, and the pre-mRNA detach.