Protein Synthesis By Reiss Amoah What is a
Protein Synthesis By Reiss Amoah
What is a Protein? A chain of Amino Acids linked through Peptide Bonds. Can catalyse reactions, form structures or aid in the transport of substances throughout the body. All of the chemistry in living organisms is regulated by proteins.
The two stages of Protein Synthesis Stage 1: Transcription Stage 2: Translation
Stage 1: Transcription An enzyme (RNA Polymerase in humans) unzips a section of the DNA double helix structure by breaking the Hydrogen Bonds between complimentary base pairs on opposite strands. This causes DNA nucleotides on both single strands to become exposed. Free-floating RNA nucleotides (NTPs) align with complimentary exposed DNA nucleotides on one Antisense / Template Strand(Exposed DNA strand) to form a DNA-RNA Hybrid Zone. During this stage, m. RNA Splicing can occur, in which Introns (non-coding sections of DNA) in the section being transcribed are removed in order to alter the m. RNA strand produced. RNA nucleotides link up as Phosphate groups form Ether Bonds with Ribose sugars. The m. RNA strand then leaves the Nucleus, where Stage 2 is initiated in the cytoplasm.
Stage 2: Translation In this stage, a recently produced m. RNA strand encounters a protein structure called a Ribosome which reads the m. RNA and causes the formation of a chain of corresponding Amino Acids in order to create a Protein.
Step 2 a: Initiation A free-floating m. RNA strand encounters a lone Ribosome in the cytoplasm / Rough Endoplasmic Reticulum. The m. RNA Initially fuses with the Small Ribosome subunit, with an Amino Acid-carrying t. RNA molecule in order to form an Initiation Complex. The larger Ribosomal subunit then joins the complex, using Initiation Factor Proteins to stabilise the Ribosome structure. The 70 S Ribosome consists of two sites where t. RNA binds, the Peptidyl (P) and Acceptor (A) sites. Initially, there is a t. RNA molecule bonded to the P Site which has the corresponding Anticodon for the first Codon (Base Triplet) on the m. RNA strand. The A site at this point in empty.
Step 2 b: Elongation Another free-floating Amino acid carrying t. RNA molecule enters the complex by aligning with the complimentary m. RNA codon in the A site of the Ribosome. The ribosome then moves along the m. RNA strand by one codon, moving the initial t. RNA molecule out of the complex whilst the new t. RNA molecule takes its place. As this happens, the t. RNA that was initially in the P site transfers its Amino acid to the new t. RNA molecule , where the two Amino Acids link through a Condensation Reaction to form a Peptide Bond. This process continues up to step 2 c: Termination
Step 2 c: Termination The Amino Acid chain continues to elongate until a Stop Codon on the m. RNA strand enters the A site position. A stop codon is a base Triplet which does not correspond with the Anticodon of an Amino Acid-carrying t. RNA molecule, instead it causes the Ribosome to terminate elongation. The last t. RNA molecule in the P site transfers its Amino Acid to the chain, which is then released by the Ribosome where it is free to travel through the cytoplasm to where it is required.
After Protein Synthesis After leaving the ribosome, the newly formed chain may fold over itself numerous times due to the Hydrogen Bonds and Disulphide Bridges which are present. The position of these are determined by the sequence of Amino Acids. Therefor each species of protein has a specific structure as it has an individual sequence. However, the protein may not be completed after Translation as it may require the addition / removal of other molecules In order to make them suitable for the role they perform. The process of removing molecules on a protein is called Proteolysis.
The Function of the Golgi Apparatus This organelle is required for the modification and excretion of proteins out of the cell. In this structure, simple molecules are added to newly-formed proteins in order to modify them. After modification, the protein is then enveloped in a Vesicle (a small sack composed of the Golgi membrane) which then transports the protein to the Inner Cell Membrane where it is excreted out of the cell through Exocytosis. Once out of the cell (if required) the protein can be transported to its designated environment by diffusing through the blood stream / tissue.
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