Learn the story of the central dogma and how it relates to protein synthesis. We'll use a simple analogy to explore the roles of transcription and translation in building protein from the DNA code. In this lesson, we'll also introduce the concept of a gene.
The Truth About DNA
DNA: It's got to be the pinnacle of biology, right? Everyone's always talking about it - how powerful it is, how incredible it is, how scientists won Nobel Prizes for studying it. It has this amazing double-helical structure, and it determines the characteristics that make you who you are. DNA decides the color of your eyes, the tone of your skin, the likelihood of developing certain diseases, and even some aspects of your personality. DNA is passed down from one generation to the next, and with it, all the traits that we see in people, plants, and animals. DNA is amazing! By studying and working with DNA, we can clone sheep, improve our crops, determine the culprit in a crime, and maybe even bring back the dinosaurs!
All of these things are true about DNA. But, we don't tend to hear much about how all the wonderful things actually come from a tiny DNA molecule. We know that there's DNA, and we know about some of the possible outcomes. But we don't know what really happens between DNA and the final product. So in this lesson, we're going to lay out the basics of how DNA becomes something that we can see in real life.
Like other processes that happen inside our cells, it's a complicated set of interrelated steps. Scientists even invented a term to describe the basic flow of genetic information from DNA to our heritable traits. It's called the central dogma. It's a simple, general story that provides a framework for understanding all of the more intricate things that happen within. But before I go more into the central dogma, I'm going to tell you another kind of story.
Understanding the Central Dogma
Once upon a time, I made a trip to visit my mom. While I was there, she made me one of my favorite things for breakfast: French toast! Now obviously, I can get French toast anywhere, but there's something special about my mom's recipe. I really missed her French toast, so I wondered if I could learn to make it on my own. I asked Mom to give me the recipe, and she got out her giant cookbook. She opened it to the right page and handed me a pen and a recipe card so I could copy the recipe down. Once I got home, I took out the card and made my own French toast. I wasn't using Mom's giant cookbook, but it didn't matter. I still had the recipe, copied word for word, and I made my breakfast taste just like Mom's! The end.
The story I just told you is actually similar to the story of how DNA becomes what we see as living creatures, like animals, plants, and people. Just like my mom's cookbook, DNA is a set of instructions for how to make something. That something, in the case of my story, was the French toast; but in the case of DNA, it's a protein. Protein is the product, the biological molecule that DNA provides instructions for.
Now, both of my stories have an important first part. Remember how I had to copy down my mom's recipe in order to take it back to my house? That was because my mom couldn't give me her giant cookbook. My home, the place where I actually made the French toast, was too far away for me to read her cookbook. So I needed a copy to take with me. In the same way, the place where DNA 'lives' is really far away from the place where proteins are actually made.
DNA 'lives' inside the nucleus of a cell and proteins are made outside the nucleus, in the cytoplasm. So, just like my recipe had to be copied, the instructions in DNA have to be copied into some other form that can travel from the nucleus to the site where proteins are made. That other form is RNA.
Remember that RNA is another kind of nucleic acid. It's similar to DNA, but instead of being a double-stranded molecule, it's just a single strand of nucleotides. You can imagine it as one half of a ladder. RNA plays the role of my recipe card by carrying the genetic code outside of the cell's nucleus.
RNA carries the instructions found in DNA from the nucleus to the cytoplasm
Once in the cytoplasm, the instructions for making proteins are read by the cell's protein-building molecules. All the directions are carried out until the protein is made, just like how I carried out the directions on my recipe card. So whether we're making French toast or proteins, the point in this second part is that we have to carry out the instructions in order to build the final product.
The Central Dogma, Transcription, and Translation
So now let's go back to the term we used earlier, the central dogma. The central dogma is a framework for understanding the flow of genetic information. It states that DNA makes RNA, and RNA makes protein. Again, the process is way more complicated than this. But, when we talk about the steps that occur during any part of this sequence, we say that it's included in the central dogma.
Let's polish up some of the terms we use here. When we say that DNA makes RNA, we're really saying that the information in DNA is being transformed or, transcribed, into RNA. It's just like how I transcribed my mom's cookbook recipe onto my little recipe card. I physically wrote out a copy that I could then carry outside her house. Transcription is the transfer of genetic information from the form of DNA to the form of RNA. Transcription occurs inside the cell's nucleus, and then RNA leaves to go do the next step.
The next step is called translation. It's the process of using the code in RNA to put together the protein itself. In my French toast story, this would be the actual making of the French toast in my kitchen. The RNA code is like my recipe card, and the protein is like my French toast. Remember that proteins are large, complex molecules, or polymers, and they're made of smaller subunits, or monomers. The small units that make up a protein are the amino acids.
Amino acids are like the ingredients in my French toast recipe. Just like I have to put my ingredients together to make my dish, the amino acids are put together to make up a protein. Recall that a chain of amino acids is called a polypeptide, and the chain eventually folds into the shape of a protein. So, translation is the decoding of genetic information in RNA to form a chain of amino acids, which will become a protein.
DNA is transcribed into RNA, which is then translated into protein
It's easy to get mixed up about the difference between transcription and translation. So, here is one idea you can try. Transcription contains the word root 'scribe,' and a scribe is a person who writes copies of important documents. So, transcription relates to the copying of DNA into the form of RNA. On the other hand, translation is a word that describes the transfer of information from one language to another. You can think of RNA and amino acids as being different biological languages. So, translation is when we switch from the language of RNA to the language of amino acids.
The process of putting together the amino acids to build a protein molecule is called protein synthesis. To remember the term, just keep in mind that synthesis means 'putting together,' just like an electronic synthesizer puts together different types of sounds. So, protein synthesis refers to the making of proteins, but we have to be careful not to confuse protein synthesis with the process of translation.
When we talk about protein synthesis, we're including all of the cellular activities that are involved in making proteins. One of those activities is translation, but lots of other things happen that have nothing to do with the central dogma. Remember, the central dogma only refers to the pathway from DNA to RNA to protein. We can look at the central dogma as being separate from, but still related to, the process of protein synthesis.
The translation part of the central dogma is included in protein synthesis, but the transcription part is not. Likewise, there are many other processes that contribute to protein synthesis, but they're not directly related to the story of the central dogma. Protein synthesis is the cellular process of building proteins. Translation is just the decoding of RNA to make a chain of amino acids that will eventually become the protein.
It's easy to lose sight of the fact that the purpose of DNA is just to make proteins. Proteins are concrete, physical things that make up our hair, nails, muscle, and skin. But, we sometimes talk about DNA as though it can influence non-physical things, like human behavior or personality. While it is true that DNA plays a role in these abstract areas, the process of how that happens is way beyond anything that we need to discuss here. So, just keep in mind that when we're talking about the central dogma, we're saying that DNA is simply the instructions for building proteins.
You can see evidence of your own proteins by looking into a mirror. You can see the color of your eyes and the shades of your skin and hair. You can see the shapes of your chin, your cheeks, and your nose, which reflect the bone and muscle structure you have underneath. You may have been told that you got your features from your mother or your father. Some people say, 'It's in your genes.' It's true that many of the traits you have are inherited from your parents' DNA. But DNA is not the same thing as a gene.
There is one protein for every gene, just like there is one dish for every recipe
Genes are only sections of a DNA molecule. They only tell the instructions for how to make one of your proteins. An entire DNA strand is split up into genes, each gene consisting of a specific sequence of nucleotides. If the DNA code is like a giant cookbook, then genes are the individual recipes found within its pages. For every recipe, there is one dish. For every gene, there is one protein. So, for every protein that contributes to your overall appearance, there is one gene that provides the code for that protein. A gene is a distinct unit of hereditary information made of a specific nucleotide sequence in DNA.
So, that was the basic story of how DNA becomes protein. I'm sure you don't need to hear my French toast story again. But, let's review the steps involved in the story of the central dogma. The central dogma is a framework to describe the flow of genetic information from DNA to RNA to protein. The process of transferring genetic information from DNA to RNA is called transcription. Then, the RNA code is used as the instructions for building a chain of amino acids, and that process is called translation.
When amino acids are joined together to make a protein molecule, it's called protein synthesis. Each protein has its own set of instructions, which are encoded in sections of DNA, called genes. So, the overall story of the central dogma is this: Inside the nucleus of a cell, the genes in DNA are transcribed into RNA. Once RNA leaves the nucleus, it's translated in the cytoplasm, and the process of protein synthesis begins. The end result is a fully-formed protein, just like the end result of my recipe is a plate full of yummy French toast.
After this lesson, you will be able to:
- Describe the central dogma and how it relates to protein synthesis
- Explain the importance and function of genes