Source: NOVA: "Secret of Life"
This narrated animation illustrates the process of protein synthesis, starting with chromosomal DNA inside the cell's nucleus. A gene is transcribed into messenger RNA, which leaves the nucleus and binds to a ribosome in the cytoplasm. There, transfer RNAs attach and build a polypeptide chain that will fold into a functional protein. Adapted from Interactive NOVA: "Secret of Life."
How can millions of different and complex structures be built using only a few simple building blocks? Just ask your DNA. DNA (short for deoxyribonucleic acid) spells out the genetic codes of millions of species, using just four molecules: adenine (A), guanine (G), thymine (T), and cytosine (C). These molecules are called nucleotide bases. Different sequences of nucleotide bases are what define each species.
But DNA sequences are only templates for building and maintaining organisms. Alone they can't do anything. A living cell must provide the energy and "machinery" to extract and carry out DNA's instructions. Inside the living cell, DNA directs cellular activities -- growth, division, movement, maintenance, and even death -- in a surprisingly simple way. It just tells the cell to build the right proteins at the right times.
DNA contains discrete sequences, called genes, each of which holds the code for one or more proteins. But the DNA exists in a unique form that ensures that the codes are used only at the appropriate times. Two sugar-phosphate backbones, connected by "rungs" of nucleotide bases, are wound around each other to form a double helix. They are bound together by chemical bonds between the strands: adenines pair up with thymines, and cytosines with guanines.
When a cell needs to make a particular protein, an activation signal stimulates an enzyme (DNA helicase) to unwind the DNA in the region of the appropriate gene. As the two strands of DNA separate, the so-called "coding strand" becomes a template for building a molecule called messenger RNA (mRNA). RNA is similar to DNA, except that it is single-stranded and contains uracil (U) in place of DNA's thymine. An enzyme called RNA polymerase "transcribes" DNA into mRNA by piecing together complementary mRNA bases along the DNA template.
Once the DNA code has been transcribed into mRNA, it moves out of the nucleus and into the cytoplasm. There, two kinds of molecules interact with the mRNA to "translate" it into a protein: ribosomes and transfer RNA (tRNA).
Each transfer RNA contains a triplet of nucleotide bases (CGG, for example) and the corresponding amino acid (for example, CGG goes with alanine). The tRNAs need to link up in an order that complements the mRNA code, but tRNAs can't bind to mRNA directly; they need ribosomes to pair them up. Ribosomes are large RNA-protein complexes that can hold on to an mRNA strand plus two tRNAs at a time. As the ribosome ratchets along the mRNA, it brings together tRNAs, whose amino acids bind. This growing string of amino acid building blocks becomes (finally!) the protein.
While this ends the process of protein synthesis, it's only the beginning for the protein, which will pass through a series of organelles that may chop it apart and/or add sugars to it before finally sending it on to its final destination. Proteins may be used inside the cell, sent out to its surface, or excreted from the cell entirely.
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