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6. Protein Factory - Background
There are 2 major steps in the protein synthesis process. The first is the synthesis of messenger RNA in a process known as transcription. This process is similar to DNA replication, except that only a tiny portion of one strand is copied and it is copied into a single-stranded RNA molecule, not a double stranded DNA molecule.
To start transcription, RNA polymerase binds to a specific DNA sequence known as a promotor. Promotors sequences are very diverse, however, generally are found in the stretch of DNA in front of the gene and contain a place for RNA polymerase to bind as well as a transcriptional start sequence that indicates where transcription should begin. They range in length from less than a hundred base pairs to several thousand base pairs. Many promotor sequences contain the sequence TATAAA, known as a TATA box by biologists. This TATAAA sequence is used in this activity to indicate where the RNA polymerase should bind and begin transcription.
Once, the RNA polymerase binds to the promotor, it follows along the DNA, unzipping the base pairs, reading one of the two DNA strands, matching an RNA nucleotide to each DNA nucleotide, and assembling a messenger RNA molecule. The RNA polymerase continues moving along the DNA until it reaches a specific terminator sequence, at which point it releases the messenger RNA and disassembles. Messenger RNA molecules may extend over 2 million bases in length. At this point, the messenger RNA travels out of the nucleus to the ribosome where proteins are actually made.
This second step of the protein synthesis process is known as translation. First, a ribosome assembles around the messenger RNA molecule. Translation always begins at the messenger RNA sequence AUG. The messenger RNA then feeds its way through the ribosome like a tape. As it proceeds, each codon on the messenger RNA is matched to a transfer RNA. The ribosome forms bonds between the amino acids carried by the transfer RNAs and the empty transfer RNA molecules detach and float away. Gradually, the amino acid chain grows longer and longer until a stop sequence (UAG, UAA, or UGA) is reached. At that point, the protein is released.
From here, the protein may go through many stages of further processing. Depending on the sequence of amino acids, some parts of the protein like water and some curl away from it. Thus, the protein will fold itself up to protect the water-hating parts of the protein from the surrounding cytosol. In addition, proteins may be cut, spliced, joined together, packaged and reshaped into a final functional protein.