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Oxford Tutorial Oral.docx

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BIOL 130
Dragana Miskovic

Oxford Tutorial Question: You are an amino acid in the lumen of the small intestine. You are looking at transporter proteins protruding from the apical surfaces of intestinal epithelial cells. Your ambition is to be part of one of those membrane transporters. You will be asked to describe your adventures during one of three phases of your journey: A. beginning from the challenge of entering the epithelial cell, until you have been loaded onto an appropriate tRNA. The small intestine is the site where most of the nutrients from ingested food are absorbed and this is where proteins are decomposed to single amino acids by digestion. Amino acids are the building blocks of proteins, which give this amino acid hope in its quest to becoming a membrane transport protein. So, the goal of the amino acid is to get from the lumen of the intestines to the internal environment of the epithelial cells. The epithelium is a monolayer sheet of epithelial cells joined together, and this sheet lines the interior of the small intestine. The epithelium has two faces, one free and exposed to the lumen of the intestine called the apical surface, and the other surface is resting and attached to some kind of tissue. This surface is called the basal surface. For now we just need to know about the apical surface, as this is the surface that the amino acid needs to become a part of. So similar to the plasma membrane of a single cell, the epithelial sheet creates a barrier to keep some molecules in and other out. This allows it to can take in nutrients and export waste. An amino acid contains an amino group, carboxylic acid and a r chain. It is a large, polar and uncharged molecule, preventing it from crossing the membrane through passive transportation. Therefore the absorption of the amino acids into the internal environment of the epithelial cells is done by active transport. It moves across the plasma membrane by a sodium-dependent amino acid transporter. The buildup of sodium ions outside the cell creating a concentration gradient. The sodium-dependent amino acid transporter and the hydrolysis of ATP transport Sodium or hydrogen ions into the cell, against the concentration gradient, while symporting amino acids. Once the amino acid is in the intracellular environment of the epithelial cell, it can start protein synthesis. To continue its journey towards becoming a membrane transport protein, it must be picked up by transfer RNA (tRNA). In the cytoplasm, free­floating transfer RNA gets attached to an amino acid using the  enzyme aminoacyl­t­RNA synthetase to form a temporary complex called aa­tRNA  complex The ‘aa’ stands for the particular amino acid being used, as there is a specific tRNA for  each amino acid. met For example: if the amino acid was methionine, the tRNA would be tRNA B. beginning from initiation of translation, until your incorporation into a folded protein. At this point, the amino acid has entered the cell, and is loaded onto transfer RNAs. The  tRNA transports the amino acids to the ribosomes, which are the driving force behind  assembling amino acids into proteins.  In the cytoplasm, free­floating transfer RNA gets attached to an amino acid using the  enzyme aminoacyl­t­RNA synthetase to form a temporary complex called aa­tRNA  complex The ‘aa’ stands for the particular amino acid being used, as there is a specific tRNA for  each amino acid. A tRNA carrying an amino acid is said to be a “charged” tRNA So for example: one kind of tRNA, called tRNA , joins with the amino acid methionine  met (met), which creates a temporary amino acid complex called met­TNA  complex. This complex is considered the initiator tRNA and is used to position the first amino acid  in the polypeptide chain. met Now the met­tRNA  complex (initiator tRNA) attaches to the ribosome at the AUG  ‘start’ codon. It attaches here because the 3 nucleotide sequence UAG at the bottom of  the tRNA molecule is the complement to the AUG codon on the mRNA Each active ribosome has four binding sites: one for mRmettranscript, and three sites E,  P and A for binding tRNA molecules. The met­tRNA  attaches to the P site about the  start codon on the mRNA. Once the ribosome is fully set up on the mRNA transcript, the synthesis of the  polypeptide can begin. This next stage is known as elongation, which is the process of attaching amino acids  together into a polypeptide chain.  This involves four steps: 1. new amino acid is brought to the A site by its aa­tRNA complex, which is
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