Transcription
Initially the DNA double helix is unwound and unzipped through the breaking of hydrogen bonds by the enzyme Helicase. This occurs in the nucleus of the cell. Another enzyme, RNA polymerase, then binds to one unwound chain and the gene is transcribed into RNA (transcription). During transcription the triplet-codes of DNA, consisting of three nucleotides, are copied and correspond to a codon on the RNA. The RNA then undergoes a Processing phase and the produced mRNA is transported out of a nuclear pore through the nuclear membrane and into the cytoplasm.
tRNA and Ribosomes
Once out of the nucleus a ribosome will bind to the mRNA in two parts, the small sub unit and the large sub unit. The mRNA is initially bound to the aminoacyl site, or site A. At point A the tRNA molecule carrying the correct amino acid binds to the ribosome. The tRNA then binds to site P or peptidyl site where the amino acid is prepared and bonded to the previous amino acid. The tRNA is then released and binds to site E where it will Exit the process. There are many tRNA molecules and are chosen specifically for each amino acid as coded for by the mRNA codon. This codon corresponds to the anti-codon found on the bottom of the tRNA. These molecules bring and release the correct amino acid and the amino acids form peptide links at site P on the ribosome. As more tRNA molecules are processed the peptide chain becomes longer and the primary structure of the amino acid order causes the protein to form a tertiary structure.
Specific Triplet codes
The triplet codes are specific to an amino acid required. The code consists of three nucleotides which allow for 64 amino acids due to there being 4 different nucleotides for DNA: Adenine Thymine Guanine and Cytosine. These correspond to the transcribed codons for mRNA and also tRNA anti-codons. The code is degenerate which means there are more codes possible than required and therefore there are often three or four codes for a single amino acid. There are also three triplet codes which signal to stop transcription and one which signals the start of transcription:
START:?? methionine ATG
STOP:? TAA????? TAG????? TGA
On the mRNA the codes are reversed to the paired nucleotides: A-T and G-C, although where thymine is required, Uracil is used instead. The anti-codon is then also reversed.
With over 100,000 different proteins to manufacture, how the heck does our body get it right? When one thinks of the amount of information the body needs to keep track of, - eye, hair and skin color, protein sequence, toenail size, etc. - it would seem a task for a supercomputer to record all of the necessary information. In essence it is. But not a supercomputer made of silicon wafers and TV screens, rather one made of an intricate biomolecule called DNA.
DNA (deoxyribonucleic acid) is in the family of molecules referred to as nucleic acids. One strand of DNA has a backbone consisting of a polymer of the simple sugar deoxyribose bonded to something called a phosphate unit. Very unimpressively then, the backbone of a strand of DNA resembles this:
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