Before studying the details of protein synthesis, it will be helpful to consider the big picture. In brief, DNA contains a genetic code that specifies which proteins a cell can make. All the body's cells except the sex cells contain identical genes, but different genes are activated in different cells; for example, the genes for digestive enzymes are active in stomach cells but not in muscle cells. When a gene is activated, a molecule of messenger RNA (mRNA), a sort of mirror-image copy of the gene, is made. Most mRNA migrates from the nucleus to the cytoplasm, where its code is "read" by a ribosome. Ribo-somes are composed of ribosomal RNA (rRNA) and enzymes. Transfer RNA (tRNA) delivers amino acids to the ribosome, and the ribosome chooses from among these to assemble amino acids in the order directed by the mRNA.
In summary, you can think of the process of protein synthesis as DNA ^ mRNA ^ protein, with each arrow reading as "codes for the production of." The step from DNA to mRNA is called transcription, and the step from mRNA to protein is called translation. Transcription occurs in the nucleus, where the DNA is, and most translation occurs in the cytoplasm. Recent research has shown, however, that 10% to 15% of proteins are synthesized in the nucleus, with both steps occurring there.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.