字幕表 動画を再生する 英語字幕をプリント The job of this mRNA is to carry the gene's message from the DNA out of the nucleus to a ribosome for production of the particular protein that this gene codes for. There can be several million ribosomes in a typical eukaryotic cell. These complex catalytic machines use the mRNA copy of the genetic information to assemble amino acid building blocks into the three-dimensional proteins that are essential for life. Let's see how it works. The ribosome is composed of one large and one small sub unit that assemble around the messenger RNA, which then passes through the ribosome like a computer tape. The amino acid building blocks, that's the small glowing red molecules, are carried into the ribosome attached to specific transfer RNAs; that's the larger green molecules also referred to as tRNA. The small sub unit of the ribosome positions the mRNA so that it can be read in groups of three letters known as a codon. Each codon on the mRNA matches a corresponding anti-codon on the base of a transfer RNA molecule. The larger sub unit of the ribosome removes each amino acid and joins it onto the growing protein chain. As the mRNA is ratcheted through the ribosome, the mRNA sequence is translated into an amino acid sequence. There are three locations inside the ribosome designated the A-Site, the P-Site, and the E-Site. The addition of each amino acid is a three-step cycle; first the tRNA enters the ribosome at the A-Site, and is tested for a codon / anti-codon match with the mRNA. Next, provided there is a correct match, the tRNA is shifted to the P-Site, and the amino acid carries is added to the end of the amino acid chain. The mRNA is also ratcheted on three nucleotides, or one codon. Thirdly, the spent tRNA is moved to the E-Site, and then ejected from the ribosome to be recycled. As the protein synthesis precedes, the finished chain emerges from the ribosome; it folds up into a precise shape, determined by the exact order of amino acids. Thus the central dogma explains how the four-letter DNA code is, quite literally, turned into flesh and blood.