[Caitlin:] Let's take a closer look at protein molecules and what they do. Humans have 23 pairs of chromosomes packed tightly inside the nucleus of a cell. Each of those chromosomes is made up of thousands of genes, and the base pairs in each of those genes contains a genetic code required to build one protein. [Felicia:] Right!, our genes contain the instructions required to build about 20 000 different proteins. [Caitlin:] Proteins are amazing! Why do you ask? Because at the end of the day, proteins carry out all of the functions of our cells. [Felicia:] For example, some proteins build the walls of the cell. [Caitlin:] Other proteins generate energy for the cell. [Felicia:] There are proteins called enzymes that accelerate chemical reactions. Remember when we talked about how helicase, primase, and DNA polymerase are involved in replication of DNA? Those are all enzymes! Here's a little trick for spotting an enzyme: their name ends in -ase. [Caitlin:] And don't get confused when we say that something is a protein and also an enzyme. Sometimes we describe these molecules by saying what they are and what they do. Same thing with us humans. You could describe me as a human because, uh, well, I am. But you could also describe me by my job. I'm a professor. My job is to teach. Either way works. [Felicia:] Or we could also describe you as short and me as tall -- [Caitlin:] Thanks a lot! [Felicia:] -- because you can also describe protein molecules by their shape. See what I did there? Proteins are polymers. They are long chains of smaller molecules called monomers. [Caitlin:] Now where have I heard that before? [Felicia:] Remember when we made a DNA molecule? I said that DNA is a polymer that is made up of monomers. [Caitlin:] And like DNA, the basic building block of polymers are linked together through a covalent -- [Felicia:] or strong -- [Caitlin:] Right -- or strong -- bond that's extremely difficult to break. [Felicia:] There is one really big difference though. We built our DNA polymer using nucleotides as the building blocks. But the building blocks for a protein are called amino acids. There are 20 different amino acids. Think about it! That's almost a full alphabet of amino acids! [Caitlin:] Very cool! But how are we going to encode that many amino acids with only four base pairs? Isn't that like being able to spell all the words in the dictionary by using just the first four letters of the alphabet? [Felicia:] We'll find out how to decode DNA this week on... [Both:] DNA Decoded. [MUSIC] [Felicia:] Remember when we stuffed all of our DNA into the nucleus? Well, the nucleus is only one compartment in a cell. Let's see, if we think of the entire cell as a company, each compartment is like a department that has a different function. The furnace of the cellular factory is the mighty mitochondria. It provides all the energy and power a cell needs to carry out its function. [Caitlin:] I'm calling for a pause on all science related puns. [Felicia:] And I'm ignoring that. Moving on! The endoplasmic reticulum, is the production centre, where proteins are made and synthesized. The processing centre in this factory is the Golgi body, AKA the Golgi apparatus. It's like the distribution centre. It helps to package, label, and deliver proteins to the various departments of the factory. [Caitlin:] Got it. [Felicia:] So, you know how we keep saying that DNA's the blueprint of us, the genetic information for constructing life? Well, let's take a closer look at what that actually means. The blueprints for building proteins are stored in our genes within the nucleus. In order to actually build proteins, you have to send the blueprints from the office down to the factory floor. But our chromosomes are too big to make it out of the nucleus. So, we need to use a messenger. [Caitlin:] You mean Messenger RNA? [Felicia:] Correct! The blueprint for building a protein is copied, or transcribed, from the DNA molecule into a short lived piece of messenger RNA, also known mRNA. [Felicia:] That little mRNA has one job and one job only: to exit the nucleus and carry the decoded DNA message to another compartment. When it gets there, the blueprint encoded on the RNA molecule will be read and a protein will physically be assembled. [Felicia:] Right! Making proteins is a two-part process. First, DNA makes RNA. That's called transcription. And second, RNA makes protein. That's called translation. This process is so important that it's called Central dogma. [Caitlin:] Let's tackle that first part. Transcription: DNA makes RNA. But, before we do, I feel like I need to nerd out a little. [Felicia:] If you must. [MUSIC] [Caitlin:] Okay, so what is RNA? Like DNA, RNA is a nucleic acid that is present in all living cells. However, there are four main differences between DNA and RNA. First, RNA only has one strand, not two like DNA. Second, DNA is tightly packed in chromosomes within the nucleus. Because it's so small, RNA can move in and out of the nucleus into other compartments in the cell. Third, Watson and Crick rules apply to RNA, but with one difference. In DNA, adenine pairs with T for thymine but in RNA, A pairs with U for uracil. And finally, four, RNA is not as stable as DNA, so it won't sit around as long as DNA will. It lasts just long enough to get the message across. Caitlin out. [Felicia:] Are you done now? [Caitlin:] I guess so. You may continue. [Felicia:] Okay, where were we? Yes, transcription! DNA makes RNA. Let's start at the beginning... Molecular processes need signals to start and to stop. You can think of these as molecular green lights and red lights. Conveniently each gene contains a sequence of base pairs that kick off the transcription process. This region called a promoter acts as a molecular green light. A protein know as RNA polymerase binds to the promoter region. [Caitlin:] Wait. RNA polymerase, that sounds familiar... [Felicia:] It sure should, because we already learned about DNA polymerase. DNA polymerase uses DNA as a template to make more DNA copies. RNA polymerase does a similar job, it uses DNA as a template to make an RNA molecule. [Caitlin:] RNA polymerase binds to the start signal or promoter region of DNA. This creates what we like to call the transcription bubble. Using Watson Crick rules the RNA polymerase starts adding nucleotides, moving in the five to three prime direction. [Felicia:] RNA polymerase will continue to add RNA nucleotides, until it reaches a region that we call the termination signal. When it reaches this stop signal, the RNA polymerase stops. Remember when we said in week one that the non-covalent bonds between base pairs that make up the rungs of a ladder of DNA aren't very strong? Well, neither are the bonds between the newly-synthesized mRNA and the DNA template. The bonds holding the mRNA and the DNA together break apart, leaving a single strand of messenger RNA. [Caitlin:] And that brings us to the end of Part One: Transcription. But before we go, let's briefly recap... Central dogma describes the process that uses the information stored in our genes to create proteins. The first stage in this process is transcription. The genetic code of our DNA is transcribed onto an mRNA molecule. Let me continue my recap... Each gene contains a promoter region that basically serves as a green light and says: Start here. The process of transcription begins when RNA polymerase binds to the promoter region of a gene. THe RNA polymerase creates a transcription bubble, or small segment where the strands of DNA are separated. Using Watson and Crick rules, the RNA polymerase starts to add complementary RNA nucleotides to the template DNA. The RNA polymerase proceeds along the gene, adding RNA nucleotides, until it reaches the termination signal. The termination signal's like a red light that causes the RNA polymerase to stop. And then, the mRNA breaks away from DNA template, exits the nucleus through a pore, and heads out into the great unknown -- the world outside the nucleus. [Felicia:] Time for a break. [Caitlin:] I agree. We'll pick up where we left off in our next video. Stay tuned for Part Two of Central Dogma: Translation. [MUSIC]
