DNAからタンパク質まで

DNA contains genes, sequences of nucleotides, some of which are instructions that code for the series of amino acids in a protein. The flow of genetic information from DNA to RNA to protein is a process known as The Central Dogma. The first step of this process is transcription, where an RNA polymerase enzyme synthesizes an RNA based copy, or transcript of the gene. The DNA is used as a template where each new RNA base added to the transcript is complementary to the original strand of DNA. Some transcripts, called messenger or mRNA, code for proteins, while non-coding ones participate in other cellular processes. For example, ribosomal rRNA and transfer tRNA participate in protein synthesis. The next step is translation, where mRNA is decoded to synthesize a chain of amino acids. A set of instructions known as the genetic code is used to read the mRNA. Most organisms use this same universal code composed of three nucleotide groups called codons that translate to specific amino acids. There are 64 different nucleotide triplets but only 20 standard amino acids in proteins making the code degenerate, that is, multiple codon sets can give the same instruction. Sixty-one sets code for amino acids, and three signal the stop of translation. Translation occurs at the ribosome, a large complex of rRNAs and proteins, with the help of tRNA.  tRNA has a three hairpin loop structure. One loop contains a sequence called the anticodon, which has complementary bases to the codon. An amino acid corresponding to this sequence is attached at the end of the tRNA, which transports it into the ribosome. Proteins called initiation factors bring together the small ribosome unit, an initiator tRNA and the mRNA. After the assembly of the complex, the ribosome glides along the mRNA in search of the translation start site.   Here, the initiator tRNA anticodon binds to the complementary codon; the large ribosome unit binds to the assembly,  and translation is initiated. When the next tRNA comes in, the amino acid from the initiator is detached and transferred to the neighboring amino acid resulting in a growing polypeptide chain.    The addition of amino acids continues until a stop codon is detected in the mRNA. The ribosome then releases the chain so that it can fold into a functional protein.