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The process of protein synthesis occurs in two major steps driven by enzymes inside a cell. First, deoxyribonucleic acid (DNA) is transcribed to ribonucleic acid (RNA) with the enzyme RNA polymerase. Second, the RNA is then translated into a protein molecule by ribosomes in the cell. Transcription of DNA and translation of RNA are the key steps in the central process of protein biosynthesis.
Transcription is the first step in the process, and it is usually initiated by various signaling molecules in the cell's nucleus. To begin, the enzyme DNA helicase unzips the two strands of DNA, exposing the template strand, which will code for the RNA that will be transcribed. Next, the enzyme RNA polymerase binds to the template strand, moving along it and synthesizing a strand of messenger RNA (mRNA) that is complementary to the template strand of DNA. Each single nucleotide of DNA will code for one nucleotide of RNA to be added to the mRNA strand.
In eukaryotic cells, the mRNA will usually be modified after it is made. This step in the process of protein synthesis involves adding a cap to the front, which is usually a methylated guanine nucleotide, and a poly-adenine tail (poly-A tail) to the back. The mRNA will also be spliced, because enzymes in the cell remove any mRNA segments that are not directly involved in coding for the target protein. These segments are known as introns, while the segments that are involved in coding for the protein are known as exons.
The next step in the process of protein synthesis is translation, in which the RNA codes for specific amino acids. This process is catalyzed outside of the nucleus by ribosomes, small organelles that are made of ribosomal RNA (rRNA) and protein. Ribosomes bind to both the mRNA strand and the amino acids that will make up the final protein. Every set of three mRNA nucleotides will code for one specific amino acid. The ribosomes travel down the mRNA strand, adding one amino acid at a time, until they reach the poly-A tail and complete the protein translation.
Sometimes the process of protein synthesis involves additional steps after the polypeptide has been created. Proteins can begin to fold into their native structure, or most stable three-dimensional conformation, with hydrophobic interactions. Since the cell is an aqueous, or water-based, environment, it is quite polar, and hydrophobic amino acids will gather together to avoid being exposed to this environment. This inward grouping of hydrophobic residues gives the protein more energetic stability, and helps it to fold.
Frequently, proteins cannot fold into their native structure of their own accord. In this case, they need the help of a chaperonin, a protein enzyme that binds to the newly synthesized polypeptide and folds it into the correct shape. Chaperonins and other enzymes can also repair denatured, misfolded, or other damaged proteins.
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