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You are an amino acid in the lumen of the small intestine of a newborn mammal. You are looking at intestinal epithelial cells that bring important maternal proteins (immunoglobulins) across their apical surfaces by receptor-mediated endocytosis. Your ambition is to be part of a receptor that does that job.

A. beginning from the challenge of entering the cell, until you have been loaded onto an appropriate tRNA.

Amino acids are the building blocks of protein. Within the small intestine is the only site in the digestive tube for absorption of amino acids. Absorption takes place on the surface of small intestinal epithelial cells. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. Diffusion is a type of passive transport, because molecules are always in random motion, and as a result, will naturally balance a difference in concentration, and move from an area of higher concentration to an area of relative lower concentration. Cell membranes are selectively permeable, which means that it allows some molecules to pass while preventing some molecules from entering. For example, molecules such as amino acids cannot cross the cell membrane without help. Carrier proteins are saturated throughout the cell membrane, and can facilitate movement by carrying small molecules, such as amino acids, across the cell membrane. Carrier proteins do not require energy because they facilitate movement from an area of high concentration to an area of lower concentration. Many carrier proteins can facilitate movement in both directions, and allow molecules to enter or exit. Direction is based on concentration gradient. For example, if the concentration is higher outside the cell, molecules would naturally move to na area of lower concentration. That is, they would enter the cell. Once the amino acid has entered the epithelial cell, it must be attached to its tRNA in order to be useful in protein synthesis. tRNA are free molecules that are found in the cell cytoplasm, and contain an attachment site where amino acids can bind. The attachment of an amino acid to a tRNA requires energy in the form of ATP. A tRNA that becomes covalently linked to an amino acid is called aminoacyl tRNA. The correct amino acid is added to its tRNA by a specific enzyme called an aminoacyl-tRNA synthetase. The process is called aminoacylation. There are 20 amino acids, there are 20 aminoacyl-tRNA synthetases. These aminoacyl tRNA synthetases are responsible for catalyzing the addition of amino acids to tRNAs. For each of the 20 major amino acids there is a different aminoacyl tRNA synthetase. That is, each aminoacyl tRNA is specific to one amino acid. The active site on an aminoacyl tRNA synthase binds ATP and amino acid. The resulting reaction binds the amino acid and AMP to the enzyme and two phosphate groups are released. The amino acid is activated and has a high potential energy. The amino acid is transferred from the tRNA synthetase to the tRNA specific to that amino acid, and AMP leaves. The amino acid is attached to its specific tRNA at the binding site, and the aminoacyl tRNA is ready to participate in translation.

B. beginning from initiation of translation, until your incorporation into a folded protein.  Transfer RNA allows amino acids to interact with an mRNA template. Messenger RNA is is a copy of a portion of DNA, and contains genetic information. Because DNA is so important to cellular functional integrity, it cannot leave the nucleus in case the caustic environment of the cytoplasm does harm to it. Therefore, mRNA must be employed to carry genetic information out of the nucleus. Nucleotides are the basic monomer building block units in the nucleic acids. tRNAs are made up of nucleotides, about 75 to 85 nucleotides in length. Some of the bases in the tRNA molecule can form hydrogen bonds with complementary bases in a different region of the same molecule, and as a result the tRNA...
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