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DNA, a nucleic acid contains the genetic code, located within the nucleus. It has the instructions to manufacture proteins outside the nucleus at the ribosomes. Another nucleic acid, called RNA is capable of copying the DNA message and carry it to a ribosome to allow protein synthesis to occur. This type of RNA is called messenger RNA. This process of copying the genetic code is called transcription. Once the messenger RNA has a copy of the DNA code, it leaves via pores in the nuclear membrane. It attaches to a ribosome. There, another type of RNA called transfer RNA brings the appropriate amino acids according to the messenger RNA instructions. This is the process called translation and results in the production of a protein. Amino acids are building blocks of proteins. As each new amino acid is linked to the previous one, by peptide bonds, a long chain of amino acids forms. This is called a polypeptide. Eventually, the translation process stops and the polypeptide detaches from the ribsome. It will then fold into a three dimensional shape and become a functional protein.
The cistron DNA is transcribed into a variety of RNA intermediates. The last version is used as a template in synthesis of a polypeptide chain. Proteins can often be synthesized directly from genes by translating mRNA. When a protein must be available on short notice or in large quantities, a protein precursor is produced. A proprotein is an inactive protein containing one or more inhibitory peptides that can be activated when the inhibitory sequence is removed by proteolysis during posttranslational modification. A preprotein is a form that contains a signal sequence (an N-terminal signal peptide) that specifies its insertion into or through membranes, i.e., targets them for secretion. The signal peptide is cleaved off in the endoplasmic reticulum.Preproproteins have both sequences (inhibitory and signal) still present.
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