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.
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.
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.Here, initiation is the first phase of transcription, during which RNA polymerase attaches to the promoter and begins to build mRNA.
The promoter for our keratin gene only exists on one strand of the DNA, which is how we know which strand is the antisense strand. For the next gene, the promoter might be on the same strand, or it could be on the opposite strand. It doesn’t really matter. Every gene has its promoter on one or the other strand, and it varies from gene to gene.Once initiation is done, then RNA polymerase moves down the length of the gene and transcribes it into a strand of mRNA. The enzyme works by matching up the complementary nucleotide bases.
Do you remember what the names of those bases are? In DNA, the bases are adenine, guanine, cytosine and thymine. However, in RNA, the nucleotide bases are adenine, guanine, cytosine and uracil.In general, the RNA strand is built just like in DNA replication; for every cytosine, RNA polymerase lays down a guanine RNA nucleotide. For every guanine, it lays down a cytosine. For every thymine, it lays down an adenine.
But whenever it encounters an adenine on the antisense strand, it lays down a uracil instead of a thymine because it’s RNA. The result is a length of mRNA built on top of a DNA strand.
It’s a pretty intricate dance here, because the DNA molecule has to split open to allow transcription to occur, and it has to come back together as soon as that part is transcribed. So, DNA splits open, gets transcribed and zips back together all within a very short distance.
There’s no room for extra molecules in there. So, what happens to the mRNA that’s still growing longer? Well, it just peels away from the DNA template and hangs off to the side.You can think of it like a person knitting a scarf. As the knitter creates more and more length of fabric, the front of the scarf trails off to the side of the person’s lap. Anyway, this whole phase of transcription is called elongation, because the mRNA strand elongates as transcription continues down the length of the gene. So, elongation is the middle phase of transcription, during which mRNA grows longer with each additional nucleotide.Now we’re at the end of the gene, and it’s time to finish up.
Can you guess the name of the last phase here? I’ll give you a hint: it has something to do with the RNA polymerase finally reaching the terminator at the end of the gene. That’s right, it’s called termination.RNA polymerase sees the signal that it’s time to stop transcription. It stops adding RNA nucleotides and comes off from the DNA gene. It also drops the mRNA that it was building because it’s finally done.
Then it floats off to do more transcription somewhere else. So, termination is the final phase of transcription, during which the RNA polymerase reaches the terminator and detaches from the gene and the mRNA.
All three phases of transcription occur inside a cell’s nucleus. Remember, everything we’ve discussed in this lesson is related to transcription, which is only the first part of the central dogma. The purpose of transcription is to provide a copy of genetic information that can travel outside the nucleus in preparation for translation. We start with the genes in the original DNA, and we end with the genes in the mRNA.
Just to be sure that we’ve got all the steps here, let’s walk through transcription from the top.Transcription begins when RNA polymerase attaches to a promoter within DNA. This is called initiation. The DNA molecule splits open and allows RNA polymerase to add RNA nucleotides onto the DNA template. The DNA strand that serves as the template is called the antisense strand.
Elongation is the phase in which the RNA molecule grows longer as transcription continues down the length of the gene. Termination occurs when RNA polymerase reaches the terminator and detaches from both strands. The RNA that’s produced at the end of transcription can be one of three different types.
The type that we’re concerned with here, which encodes the sequence for amino acids, is messenger, or mRNA. Once the mRNA is complete, then it can leave the nucleus and continue on to the cytoplasm, where it helps with protein synthesis.
Achieve the following goals by studying this lesson:
- Describe the three phases of transcription and remember where they occur
- Determine when the process of transcription begins
- Identify the DNA strand that acts as the template
- Discuss the purposes of a promoter, polymerase, and mRNA
- Understand when initiation, elongation and termination occur