Paper Topic:

Biology

SYNTHESIS AND SECRETION OF FUNCTIONAL PROTEINS

Many cells secrete proteins into their surrounding extracellular fluid Some cells , for example , secrete regulatory molecules such as hormones and neurotransmitters . Others release digestive enzymes , antibodies , or mucus . And throughout the body , cells such as fibroblasts secrete collagen and other structural proteins to provide strength and hold the body together . All proteins are synthesized at cellular structures called ribosomes

This process can be divided into two parts . Before the synthesis of a protein begins , the corresponding RNA molecule is produced by RNA transcription . Next

is translation . The ribosome binds to the mRNA at the start codon that is recognized only by the initiator tRNA . The ribosome proceeds to the elongation phase of protein synthesis . During this stage , complexes , composed of an amino acid linked to tRNA sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA anticodon . The ribosome moves from codon to codon along the mRNA . Amino acids are added one by one translated into polypeptidic sequences dictated by DNA and represented by mRNA . At the end , a release factor binds to the stop codon terminating translation and releasing the complete polypeptide from the ribosome (Voet , 25

But as a protein begins to be assembled , the very first amino acids in the polypeptide chain indicate whether the protein is to be secreted or used in the cytosol . If a protein is destined for secretion , the first amino acids are a special sequence called the signal sequence . When a protein begins to be synthesized with this sequence , the synthesis stops until the ribosome docks at the rough endoplasmic reticulum . Without the signal sequence , the protein remains for use inside the cell . Once docked , the synthesis continues , with the new protein threading into the rough endoplasmic reticulum . The signal sequence is then cleaved from the polypeptide chain . Often , too , further enzymes cut the protein in other places . Most secreted proteins are modified before secretion

The endoplasmic reticulum (ER ) is a system of membranous tunnels and sacs , which often lies near the nucleus . The endoplasmic reticulum comes in two forms : rough and smooth . Rough endoplasmic reticulum gets its name from its bumpy shape . The bumps are actually ribosomes attached to the surface of the membrane assisting the protein in synthesis . Rough ER 's are usually more frequent in cells that produce proteins for export . As a protein is made , it is pushed into the interior of this sac , known as the lumen where substances are kept separate from the cytoplasm . Here , enzymes modify the protein and get it ready to move on to the next step of synthesis . Smooth endoplasmic reticulum is ER without ribosomes attached to it . They contain enzymes needed for the construction of molecules such as carbohydrates and lipids . Smooth ER 's are responsible for most of the phospholipid production in the cell , and they increase the surface area efficiently Patches of ER membrane bud off to form membranous sacs called vesicles that transport the protein to...

4 pages

32.5 KB

Free sing-up