[Music] In this video, we are going to take a look at two very important processes that occur within the cell to create proteins. We all know that proteins are extremely important for cell functions. So, the process of synthesizing these proteins is just as important to understand as the role of the proteins themselves. This along with DNA replication is commonly referred to as the central dogma of molecular biology. And there are two important parts to outline protein synthesis which include transcription and translation. We'll discuss how both of these work in this video. Starting with transcription. Transcription is a process in which a DNA template is read to produce a messenger RNA sequence. This process is performed by an enzyme called RNA polymerase which carries out many functions. It first attaches to the DNA at the gene location and splits the doublestranded DNA molecule at the hydrogen bonds between the bases creating two open singlestranded pieces of DNA. The RNA polymerase then moves along the template strand of DNA and allows for the attachment of RNA nucleotides. This creates a growing strand of messenger RNA along the template strand and the RNA polymerase facilitates the linking of the backbone of the messenger RNA so that it is one complete molecule. When the process is done, the RNA polymerase detaches the messenger RNA from the template and allows the DNA to zip back up. So at the end of transcription, we now have created a strand of messenger RNA which carries with it the genetic code from the DNA template. The process of transcription follows the complimentary base pairing rules that we already know like guanine pairs with cytosine and adanine pairs with thymine with one exception. RNA molecules do not carry the base thymine and instead carry the base uricil. So if this template strand of the DNA was transcribed by RNA polymerase, it would read a U G C U G U AU GCA as the messenger RNA sequence. Notice that all of the A bases in the template strand pair with a U in the messenger RNA strand. There are a few important terminology terms to understand with the process of transcription. The section of DNA that is transcribed is called a gene. The section of DNA that we want to make a messenger RNA copy of is called the coding strand or the sense strand. But because pairing only works with complimentary bases, the RNA polymerase is going to read the opposite strand of what we want to code to get the strand that we need. This is called the template strand or the anti-sense strand. You can see as a final result of the process, the messenger RNA sequence is identical to the coding strand that we wanted. Of course, with the exception of uricil bases replacing thymine bases. As we discussed, during transcription, the doublest stranded DNA molecule must be split in order for the RNA polymerase to copy the template strand. During this process, the template strand of the DNA is very stable and to a high degree does not undergo any changes. This is important so that if genes need to be transcribed again, it will always create the correct messenger RNA sequence without any mutations. Meaning the same protein gets made every time. Of course, there is a very small chance that a mutation could occur, but the DNA being very stable and the process of transcription only having the DNA unwound for a brief period of time means that mutations are very, very unlikely. Transcription of a gene only occurs when that gene needs to be expressed and subsequently create a protein for the cell. This process is controlled based on each cell and because the human body has many different cell types, different genes are expressed in each cell to achieve its desired phenotype. Let's look at an example of two different cells like a skin cell and a neuron. Both of these cells have the same set of DNA and therefore the same set of genes. But each cell expresses different genes within the genome so they can function properly. The neuron will express certain genes to build neurotransmitters and the skin cells will express certain genes to create keratin proteins. We would not see this the other way around because neurons do not need keratin and skin cells do not need neurotransmitters. So instead each respective cell will turn off those genes and not transcribe them. This process of controlling gene expression is important for cell differentiation and [Music] development. Moving on from transcription, we are now going to talk about the process of translation. This process occurs in the cytoplasm of the cell and includes a few components which are messenger RNA, ribosomes, and tRNA molecules that hold amino acids. We'll talk in more detail about how the process works on the next slide, but the overall gist is that the ribosome reads the messenger RNA sequence in order to create a polyeptide chain. It does this by pulling in transfer RNA molecules that are carrying amino acids. The messenger RNA code will pull in specific tRNA molecules carrying specific amino acids that need to be added to the chain in an order specified by the code. Remember that the messenger RNA sequence was copied from the DNA in the nucleus from transcription. So the DNA is still the main code here that is controlling which of the 20 amino acids are placed in order to make the polyeptide. The messenger RNA is just the messenger molecule carrying the code because the DNA cannot leave the nucleus. Let's go through a step-by-step animation of how translation works. Before we start the animation, let's label the components we have here, which are the messenger RNA, the small subunit of the ribosome, the large subunit of the ribosome, and the transfer RNA molecules carrying one amino acid each. You can see that each tRNA molecule is already carrying an amino acid, which was connected there by an enzyme, which we will talk about a bit more later. In addition, you will notice that the large ribosomeal subunit has three specific spots on it that allow it to hold tRNA molecules in which it can only hold two at a time. These sites are abbreviated EPA and stand for the amino acil site, the pepidal site and the exit site. The process of translation will begin when the messenger RNA binds to the small ribosomal subunit. This will be followed by the attachment of the large ribosomeal subunit. With everything in place, the ribosome will move along the messenger RNA strand while it is being pulled through and read in sets of three bases at a time. These sets of three bases are called codons. The process waits for a specific start codon to begin. And once it hits that sequence, the tRNA molecule that is carrying a specific amino acid will be pulled into the A site. This will have three bases that are complimentary to the bases of the codon allowing for a perfect fit. We call this code on the tRNA molecule the anti-codon. As this is the first amino acid in the growing chain, this tRNA molecule will simply hold onto the amino acid and be moved to the P site. As the ribosome shifts to the next codon with the A site now open again, a new codon will pull in a new tRNA molecule with the proper anti-codon. The amino acid from the tRNA on the P site will be moved and linked to the new amino acid on the tRNA molecule at the A site. As we already know, two amino acids together form a peptide bond. the ribosome will shift again kicking out the first tRNA molecule that has given up the amino acid. The polyeptide chain will shift to the open P site and when the new tRNA molecule comes in to attach to the next codon at the A site, the complete amino acid chain is linked to the new one again forming another peptide bond between the amino acids that are connected to both tRNA molecules. This process continues until a stop codon is reached which will allow the final protein structure to detach and fold into its final form to perform the function that it was designed for. This completely ends the process of translation and the ribosome subunits will detach and be available for another messenger RNA sequence to transcribe. As we saw on the last slide, the genetic code is read by a ribosome in groups of three nucleotides at a time which we call a codon. Each codon is matched up with a complimentary anti-codon on the tRNA molecule that carries a specific amino acid. Because DNA and RNA have four possible bases in their code and a codon is made up of three of those bases, there are 64 total combinations that are possible. But even though there are 64 possible codon combinations to code for amino acids, there are actually only 20 possible amino acids. So what ends up happening here is that there are multiple codon configurations that code for the same amino acid. For example, the codon AU will pair with a tRNA molecule that holds the amino acid isolucine. And the codon A will pair with a different tRNA molecule that also holds the amino acid isolucine. This concept of having more than one codon code for the same amino acid is called degeneracy. So we can say that the genetic code is degenerate because of the product of these overlapping codons. And this is not true only for humans. But this is true for every living organism as all living things use DNA as their genetic code. So we can also say that the genetic code and the basis for how it works is universal for all life that we know of. To make sense of the code on possibilities, all 64 options are put into a table which you will very likely see on the exam. Using the table, you can read the first, second, and third letters used to make up the codon. This will then lead you to one of the 20 amino acids that the specific codon codes for. Again, there is a lot of overlap here because there are 64 possible codon configurations with only 20 amino acids. You can use this information to answer questions about the process of transcription and translation to deduce the protein sequence that is created for a specific genetic code. As an example, if you were given this specific code sequence, could you tell me what the protein would be? Pause the video and give it a shot. So, first off, we can tell that this is a DNA sequence because we have thymine bases. So, before we can translate the sequence, we need to transcribe it. We take each base and pair it with the messenger RNA sequence that it would create as seen here. Now with the messenger RNA, we can use the codon chart to identify each amino acid. You can see the final result here. Be ready to answer questions similar to this for the exam. We have seen from this process that the DNA code provides the instructions to create the messenger RNA and the messenger RNA code provides the instructions to create the protein. So if the DNA code were to change, let's say through a mutation, this could have an impact on the final protein. For example, if there was a single point mutation in the DNA code that we used on the previous slide, and this thymine was mutated to instead be guanine, this would impact the messenger RNA structure as the new codon would read AAC. Taking a look at the codon table, AAC would code for the asperagene amino acid. This would impact the final structure of the protein and could make the protein not function properly. All because there was a mutation within the original DNA sequence. A real example of a single point mutation impacting a protein can be seen with cickle cell disease. In this case, the gene that is used to create the hemoglobin protein has a single point mutation. This then leads to the creation of improper hemoglobin structure which when it fills up a red blood cell will cause an alteration in the cell to make it look crescent moon- shaped. These cickle cells can get caught in capillaries which can block blood flow. But while this is an example of a single point mutation being negative, that is not always the case. Sometimes if the DNA gets mutated, nothing will happen because a new codon could code for the same amino acid than it did before, which is all due to the degeneracy of the genetic code. [Music] [Music] [Music]