ï»¿ This student is cramming for her biology test with only a frosting-covered donut for lunch. Specialized cells in her pancreas respond to the increasing amount of sugar in her blood by releasing insulin, a small protein that regulates blood sugar levels. Let's go inside one of these cells to see how this protein is manufactured. The instructions for making insulin are coded by a segment of DNA in the nucleus. In transcription, an enzyme zips along the DNA, forming RNA, shown here in red. RNA nucleotides line up with their complementary DNA partners, transcribing the information in DNA into RNA. As the RNA grows, it is processed in several ways. First, a modified guanine nucleotide is added to the beginning as a cap. Also, segments of the RNA strand that do not actually code for the protein are removed, and the remaining segments of RNA are reconnected. Finally, extra adenine nucleotides are added to the end of the RNA strand, forming a tail. The completed messenger RNA (mRNA) now leaves the nucleus. The message in mRNA is translated into a protein in the cytoplasm. First, a transfer RNA (tRNA) arrives, carrying a specific amino acid. The small subunit of a ribosome attaches to the mRNA. Now the large sub-unit of the ribosome attaches. A second tRNA docks, bringing another amino acid. The ribosome helps to form a covalent bond between the two amino acids. The mRNA shifts, and the first tRNA leaves. A new tRNA brings another amino acid. The ribosome helps to form a new bond, and the process is repeated. Notice that one end of a tRNA molecule has a set of three bases, called an anticodon, that pairs with complementary bases on the mRNA. The other end of the tRNA carries a specific amino acid. Different types of tRNAs carry different amino acids. In this way, the message in mRNA is translated into a specific sequence of amino acids. For proteins that will be secreted from the cell, like insulin, the ribosome docks on the rough ER, and the protein grows into the ER compartment. The new protein molecules are packaged in a vesicle that is transported to the Golgi apparatus, where many proteins are processed. However, insulin is packaged in a vesicle that leaves the Golgi and is then processed. Proteins secreted from the cell are shipped to the plasma membrane. Here, insulin is secreted from the pancreas and begins to regulate the student's rising blood sugar levels.