The video above is a really good animation to help you visualise what is happening 100 trillion times per second in your body. The process can be split into two main parts; transcription and translation, which are summarised below.
- DNA-helicase unzips the DNA, exposing it.
- The exposed base sequence is used as a template for free RNA nucleotides. Activated RNA nucleotides (ones with 2 extra phosphoryl groups) temporarily hydrogen bond onto the template strand of DNA (leaving the coding strand unchanged). RNA polymerase catalyses this reaction, and the extra phosphoryl groups are released, producing energy for the bonding of adjacent nucleotides.
- The mRNA, which is a copy of the coding strand (with T replaced by U), passes out of the nucleus, via a pore in the nuclear envelope, to a ribosome.
- The mRNA molecule binds with a ribosome. Two codons are attatched to the smaller subunit of the ribosome, and are therefore exposed to the larger subunit. The first codon is always AUG, and so a tRNA molecule with anticodon UAC and amino acid mathionine hydrogen bonds to the codon.
- A second tRNA molecule with a different amino acid and complementary anticodon binds to the second codon.
- The two adjacent amino acids are joined together by a peptide bond, the reaction being catalysed by an enzyme in the small ribosomal subunit.
- The ribosome moves along the mRNA, and a third tRNA molecule brings another amino acid, whoch forms a peptide bond with the dipeptide. The first tRNA is then released to bring another amino acid to the ribosome.
- The polypeptide chain continues to grow in this way until a stop codon is reached. The stop codon works because there is no tRNA for the codons UAA, UAC or UGA.
- The polypeptide is released and assumes its secondary and tertiary structure.