How Does DNA Make Proteins? The Genetic Code Explained

How does DNA make proteins? Your DNA is written in four letters and kept inside the nucleus. Your proteins are made of twenty kinds of beads, built outside it. So how does a recipe get out, and how do four letters spell twenty kinds of beads? Watch a cell copy a gene, see why the code has to use three-letter words, how scientists cracked the first word in 1961, and how a ribosome builds a protein bead by bead. No biology background needed. This is video 6 of a series that builds up cell biology from scratch. 0:00 Four letters, twenty beads 0:44 A working copy 1:55 A three-letter code 2:43 Cracking the code 3:42 The ribosome at work 4:55 One letter, one bead 5:36 One code for all life 6:23 Recap and what's next The fine print (things the video simplifies) The copy: in your cells, the first RNA copy of a gene is edited before it leaves the nucleus. Pieces that don't spell the protein (introns) are cut out, and a cap and a tail are added. The video shows the copy leaving as it was made. RNA polymerase reads one strand of the DNA (the template). The RNA gets the letters of the other strand, with U where that strand has T. "Its sugar is a little different": RNA's sugar is ribose, DNA's is deoxyribose. Pairing: a transfer RNA's three letters pair with the codon, but the third pair is allowed to be a looser fit (wobble). The enzymes that load each transfer RNA with the right bead are not shown. The first word: Marshall Nirenberg and Heinrich Matthaei, working at the US National Institutes of Health, fed a made-up RNA of only U letters to protein-building machinery taken from E. coli bacteria (published 1961). By 1966 the chart of all 64 words was complete. The table in the video is the standard genetic code. A few organisms, and mitochondria, read a few words differently: that is the "almost". At a stop word, proteins called release factors free the chain. Some proteins need helper proteins to fold. "About two beads a second" is the speed in human cells; bacterial ribosomes add about twenty. The sickle cell change is an A changed to a U in the messenger RNA of the gene for hemoglobin's beta chain (GAG to GUG), which puts valine where glutamate belongs. Insulin: human insulin was first made in E. coli bacteria in 1978. It is built from two short chains, which the video draws coming out of one bacterium. Sources OpenStax Biology 2e (free): https://openstax.org/details/books/bi... (sections 3.4, 4.3, 15.1 to 15.5, 17.1, 37.1, 37.3) Alberts et al., Molecular Biology of the Cell, 4th ed. (free on NCBI Bookshelf): "From DNA to RNA", "From RNA to Protein", "The Transport of Molecules between the Nucleus and the Cytosol" NCBI, "The Genetic Codes": https://www.ncbi.nlm.nih.gov/Taxonomy... Nirenberg and Matthaei, "The dependence of cell-free protein synthesis in E. coli upon naturally occurring or synthetic polyribonucleotides", PNAS 47 (1961) Marshall, "The genetic code", PNAS 111 (2014)