Transcription of Messenger RNA (mRNA) from DNA

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  • 0:01 Transcription
  • 2:07 Sense & Antisense Strands
  • 2:56 RNA Polymerase & mRNA
  • 4:23 Promoter & Terminator
  • 5:48 Initiation, Elongation…
  • 9:30 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

In this lesson, you will gain a thorough understanding of how transcription works. We will investigate how DNA is transcribed into RNA with the help of a promoter and RNA polymerase. Learn the purpose of messenger RNA and explore the three phases of transcription.

Introduction to Transcription

Everyone knows that DNA contains the instructions for living things. But if you've ever been confused about how DNA actually turns into a living creature, then you're not alone. Most people, even scientists, get overwhelmed at the details involved in all the steps of the central dogma. The central dogma describes the entire flow of genetic information from DNA to RNA to the final product, a protein.

To simplify it all in my head, I think of it like I'm following a recipe. DNA is like the master cookbook and RNA is like the card that I copy a recipe onto. I imagine that I have to make a recipe card because my mom owns the cookbook, and I can't take it with me. The cookbook is similar to DNA, which lives inside the cell's nucleus and can't be taken out. So, DNA has to be copied into RNA, which can be taken outside the nucleus.

Why do we need to take the genetic information out of the nucleus? It's because we're going to use it to make a protein, which we can only do in the cytoplasm. The cytoplasm contains all the machinery, or the equipment, that is used to make the proteins. It's just like how my kitchen contains all the equipment for cooking my recipe. If I'm going to make my food inside my kitchen, then I need my recipe card. If we're going to make proteins in the cytoplasm, then we need the genetic recipe. That recipe comes in the form of RNA.

In this lesson, we're only going to be talking about transcription. That means we won't be getting through the entire central dogma. We'll only get to the point where we end up with RNA. In order to investigate the steps involved, we'll have to take a look at some close-up images that show how all of the molecules are arranged.

Sometimes in biology, when you're learning the tiny details, it's easy to lose sight of the big picture. You may begin to wonder exactly where we are in the context of a cell. But remember that transcription starts with DNA, and DNA lives in the nucleus. So, throughout this entire lesson, keep in mind that all of it happens inside the nucleus of a cell.

Sense Strand and Antisense Strand

Transcription is the copying of genetic information from the form of DNA to the form of RNA. Remember that RNA is a single-stranded molecule; it doesn't have a complementary strand like DNA does. It's only half a ladder, or a single strand of nucleotides. In order to make the RNA strand, we only need one of the original DNA strands. We'll talk later about how to know which DNA strand to use, but for now, we just need to know that one will be called the sense strand, and the other will be called the antisense strand. The antisense strand is the strand of DNA which serves as the template during the process of transcription. The RNA will become a complement of the antisense strand. This means that with a minor exception we'll address later, it will essentially be a copy of the sense stand.

RNA Polymerase and mRNA

The RNA strand has to be built one nucleotide at a time. Does that sound familiar? It should, because a similar process happens in DNA replication. And just like in DNA replication, we need the help of an enzyme to bus in all the nucleotides. You may recall that DNA polymerase was the enzyme that constructed the DNA. Well, in this case, we have RNA polymerase that constructs the RNA. RNA polymerase is the enzyme that assembles the individual nucleotides to create the RNA strand based on the DNA template.

The RNA that is built here can actually be one of three different kinds. You may recall that the three different RNA types are mRNA, rRNA and tRNA. The type that we're concerned with here is mRNA. The 'm' stands for 'messenger,' because this type of RNA serves as a messenger to carry genetic information from the nucleus to the cytoplasm. mRNA contains the codes for making a sequence of amino acids.

As you know, a chain of amino acids will eventually become a protein when we continue on with protein synthesis. Remember that for every protein, there is one gene that provides the code for making it. Genes, of course, exist inside the DNA but are transcribed into mRNA. So, messenger RNA is the type of RNA that codes for amino acid sequences.

mRNA carries genetic information to the cytoplasm from the nucleus
mRNA to Cytoplasm Diagram

Promoter and Terminator

Let's take a look at a hypothetical gene that we could find within the original DNA. We'll say it's a gene that codes for a familiar protein named keratin. So, here's the keratin gene, just sitting here in the middle of the DNA molecule, with other genes to its left and other genes to its right. The keratin gene has a starting point and an ending point. It's just like a recipe that flows in a linear sequence. It has to be read in the correct way or else the amino acid sequences won't come out right.

So, to make sure that transcription goes in the right direction, there is an extra chunk of DNA that marks where transcription should begin. This piece of DNA is called a promoter. It's sort of like a big 'start here' sign that tells the RNA polymerase to begin transcription at that point. The promoter itself is not actually part of the keratin gene, and it doesn't code for any amino acids. It's simply a signal for transcription to begin.

Another chunk of DNA sits at the end of the keratin gene. It's sort of like the finish line, and it's called the terminator. Now, don't be afraid of the terminator! It's only a stop signal. 'Termination' means to stop, or end, a process. So, the promoter is the nucleotide sequence in front of the gene that signals the beginning of transcription, and the terminator is the nucleotide sequence at the end of the gene that signals the end of transcription.

Promoters signal transcription to begin while terminators signal to end it
Promoter Terminator Diagram

Initiation, Elongation and Termination

RNA polymerase recognizes the promoter and binds to it on the DNA molecule. This is the official beginning of transcription, and we call this phase initiation. That's not hard to remember; 'initiation' just means 'the beginning of something.' If I was copying a recipe from my mom's giant cookbook, then initiation would be the moment where I sit down at the table with my pen and recipe card.

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