 In this video, I will describe the steps involved in translation, including the enzymes involved where in the cell the process occurs and the roles of messenger RNA, ribosomal RNA, and transfer RNA, and I will describe the post-translational modification of polypeptides. The genetic code is a specific amino acid that will be incorporated into a polypeptide during the process of translation corresponding to each three-letter nucleotide sequence or each codon. I wouldn't expect you to memorize which amino acid is incorporated for each codon, but the basic idea that a three nucleotide sequence codes for one amino acid is what I want you to understand here. There's also a start codon and a stop codon. The start codon, AUG, also codes for the first amino acid incorporated during translation, methionine. Then the stop codons, UAA, UAG, and UGA do not accode for any amino acid when the ribosome reaches the stop codon, translation is terminated, and the polypeptide is released from the ribosome. Here we can see the structure of a transfer RNA or tRNA molecule. This tRNA is charged, meaning that this tRNA has an amino acid bound. Amino acids are covalently bound at the amino acid attachment site of the tRNA, and on the opposite end of the tRNA molecule is an anticodon, a three nucleotide sequence that is complementary to three nucleotide codon sequence of a messenger RNA. This anticodon will recognize the codon in the messenger RNA by forming complementary base pairing and hydrogen bonding to hold the tRNA to the codon of the mRNA. As the complementary base pairing will stabilize the association of the tRNA with the mRNA, the amino acid will be held into the ribosome, enabling the ribosome to catalyze the formation of a peptide bond attaching the amino acid from the tRNA onto the free end of the growing polypeptide. Translation has three steps, similar to DNA replication and transcription. The first step is initiation, the second step is elongation, and termination is the third and final step. During initiation, mRNA binds to the small ribosomal subunit, and a charged tRNA binds to the stark codon, the region within the ribosome where the first tRNA binds is known as the P site of the ribosome. The P site stands for the peptidal binding site, and this is where the growing polypeptide will be found inside of the ribosome. Then the large ribosomal subunit will bind to the messenger RNA and tRNA to complete the initiator complex. The elongation step of translation occurs when a tRNA enters the ribosome, bringing in the second amino acid. This tRNA will enter a site of the ribosome known as the A site. A stands for aminoacyl binding site, so the amino acid will enter the A site, and then the ribosome will catalyze the formation of a peptide bond between the amino acids found in the P site and the A site. As this occurs, the ribosome will move over so that the tRNA that was in the A site is now in the P site, and that tRNA will have the growing polypeptide chain attached to it. Then the tRNA that was in the P site is moved into another site of the ribosome known as the E site. The E stands for exit, so the E site is where the uncharged tRNA will exit the ribosome. A new charged tRNA will enter the A site, and then the process of elongation will continue in a cycle, transferring the polypeptide onto the tRNA at the A site as the ribosome moves over, so that the tRNA and the A site moves into the P site, and the uncharged tRNA that was in the P site moves into the E site to exit the ribosome. This mechanism of elongation will continue adding amino acids corresponding to the codons in the messenger RNA until the stop codon is reached. Once a stop codon is reached, the ribosome will release the newly formed polypeptide. Following translation, post-translational modifications include protein folding, where interactions between neighboring amino acids will cause the polypeptide to fold into its secondary structure, where hydrogen bonding between adjacent polar amino acid side chains will stabilize secondary structure formation into alpha helices and beta pleated sheets. Hydrophobic interactions of nonpolar amino acid side chains will push deep within the polypeptide structure, moving away from the watery environment surrounding the polypeptide, whereas polar side chains will push out to dissolve into the water, when two charged side chains have the same charge they will repel one another, whereas opposite charges can help to stabilize the folding structure as positive and negative charges are attracted to one another. Some amino acid side chains are capable of forming covalent bonds to further stabilize the folding of a polypeptide into its tertiary structure. The amino acids cysteine and methionine contain sulfur, and these sulfur atoms can bond together through a covalent bond, forming a disulfide linkage that stabilizes the tertiary structure of forming proteins. Multiple polypeptides can be joined together to give a protein quaternary structure. This quaternary structure can also be stabilized by disulfide bonds, covalent bonds between sulfur containing amino acids like cysteine. Another post-translational modification is the addition of a carbohydrate to a protein. This is known as glycosylation, and these carbohydrate tags make a protein into a glycoprotein, a protein that has carbohydrate attached. Glycoproteins are often signals on the surface of cells that function to help our immune system recognize the cells that belong in our body and distinguish those from pathogens. Glycosylation is also an important label that's added to proteins following translation as they're being sorted through the Golgi apparatus. Glycosylation will enable the packaging of multiple proteins into the same transport vesicle. Phosphorylation is another type of post-translational modification where the side chain of an amino acid is then covalently bound to a phosphate group. The amino acids cyrene, threonine, and tyrosine all have a hydroxyl group that can become phosphorylated. Phosphorylation is often a way of regulating the activity of proteins such as enzymes within cells. An enzyme known as a kinase is an enzyme that catalyzes the phosphorylation of another molecule. So a kinase is an enzyme that could activate another enzyme by phosphorylating it. And then a phosphatase is an enzyme that removes the phosphate group from a phosphorylated molecule. So a phosphatase could inactivate an enzyme that was turned on by phosphorylation. While it is common for phosphorylation to activate enzymes, some enzymes are instead inhibited by phosphorylation. Here's an example that we will study as we get further into cell signaling of how phosphorylation functions to regulate a cell signaling pathway controlling a metabolic function of a cell. This example is a hormone known as epinephrine, which is also commonly referred to as adrenaline. Epinephrine will stimulate a cascade of enzymes inside of a cell where one enzyme will then phosphorylate and activate another enzyme. Protein kinase A is named protein kinase A because it was the first kinase discovered that phosphorylates another protein. Protein kinase A phosphorylates GPK, which is known as an abbreviation for glycogen phosphorylase kinase. Glycogen phosphorylase kinase becomes activated when it's phosphorylated by protein kinase A. And then glycogen phosphorylase kinase will go on to function as a kinase phosphorylating another protein, the protein known as glycogen phosphorylase. Phosphorylation will then activate glycogen phosphorylase, which is an enzyme that will break down glycogen. Glycogen is a polysaccharide found in animal cells as a way of storing carbohydrate. When glycogen is broken down by glycogen phosphorylase, glucose molecules are released and that glucose can then be used as an energy source that can be broken down by a catabolic pathway.