Transcription: mRNA Synthesis - Online Tutor, Practice Problems & Exam Prep

Transcription: mRNA Synthesis Concept 1

Hey, everyone. Before we talk about transcription, it's important to go over certain key terms when it comes to this idea of mRNA synthesis. Now, here, the key terms we need to discuss are genes and pre-mRNA. Now, a gene is just a DNA segment containing the code for protein synthesis. Now remember, DNA is pretty large, and we're not trying to copy the entire thing to make our pre-mRNA or mRNA. We're just copying segments of DNA to make the protein that we need. Now, when we say pre-mRNA, this is the precursor of mRNA that's processed later on into mature mRNA. Now, with this out of the way, let's talk about transcription. Transcription copies genetic information from a gene, so what we just talked about, to RNA. And we're going to say here that RNA polymerase binds to the DNA and then it's going to unwind the double helix. So, if we take a look here, we have our DNA double helix. Notice how it is anti parallel to one another. This is 5' down to 3' with this orange strand, and then 3' is here and 5' is here with this bluish strand. We're going to save for step 1, RNA polymerase, which we're going to show as this dark cloud part here, this grayish part is going to bind to the DNA and unwind it. And it's going to open it up. It's breaking the hydrogen bonds between the nitrogenous bases, exposing them so that our mRNA can start being created. Now here, we have this grayish RNA polymerase. We have an initiation sequence which we'll talk about, we have our template strand, and we have our informational strand. So let's talk about these things. When we say our informational strand, we say that our informational strand from the first image it goes 5' to 3'. We can see that here, this orange strand, 5' and then down here is 3'. Our template strand, which we've marked in blue, runs anti parallel to it, so it would be 3' to 5'. Now, here, we're going to say that our pre-mRNA, also called our hnRNA, so these two terms are synonymous. Your professor may use one, may use the other, may use them interchangeably. They mean the same thing. So this pre-mRNA is synthesized on the template strand using complementary bases. And we're going to say here, this would be our step 2, and we're going to say here that transcription starts from the initiation sequence, so that's our start sequence, And we're going to say, transcribed pre-mRNA is a copy of the informational strand, except that all our uracils have replaced our thymines. Remember, DNA uses thymine, RNA uses uracil. So, coming here, if we look here we're going to say that this is our pre-mRNA in this magenta color. Right? And it is basically starting to copy through the use of complementary bases the template strand here. And because it's doing that, it has to run, basically anti parallel to our template strand. So this would have to be the 5' end and this would have to be the 3' end here. And we're going to say here that since this is G, then this would have to be a C. This is RNA, so this is an A so this has to be a U. This is an A so this has to be a U. This is C, so this will be a G. Now, here we'd have our free RNA nucleotides, sometimes that happens floating around. We have what's called our termination sequence down here, and that'll lead us into step 3. We're going to say that transcription stops when RNA polymerase reaches a termination sequence. Our termination sequence acts as a stop. It tells pre-mRNA, okay. We've copied enough of this particular segment of DNA. We no longer need to go any further. And, we're going to say here that pre-mRNA is released and DNA rewinds into the double helix. So, if we take a look here, the pre-mRNA has been made. Our DNA is rewound back to what it was originally. Again, remember it runs anti parallel to the template strand that it copied, so this would have to be the 5' and the 3' here. It hasn't been fully processed yet so this exists as pre-mRNA. Now, one more thing, we said that the transcribed pre-mRNA is a copy of the informational strand. So if we take a look, here is our informational strand that runs from 5' down to 3'. It's the orange strand. And this one runs 5' to 3'. And they're copies of each other. The difference though is that the informational strand comes from DNA, so it has thymine. And the pre-mRNA since it's RNA, those thymines have been changed into uracil. So if you were to look, you would see how things basically match up. So this is a C and this is a C. This is a T on the informational strand, but RNA doesn't use thymine, it uses uracil, so here's a uracil instead. And then, here we have another T, so it'd be a U here. Here we have a G with a G. So just remember that our pre-mRNA, we copied it, we made it by using the template strand, and we use complementary base pairing to do that. The informational strand and the pre-mRNA strand are copies of each other. The informational strand is a DNA copy or version, and the RNA, the pre-mRNA, is the RNA version. The differences are just in their bases. The informational DNA strand uses thymine. The pre-mRNA strand uses uracil. So keep this in mind when we're talking about these important steps of transcription.

Transcription: mRNA Synthesis Example 1

In this example, it says, "Write the sequence of pre-mRNA produced from the following DNA template strand." Now remember, it must be complementary to this, so it means that we need to be antiparallel in terms of our orientation, so we have our 3' here and our 5' here. Remember that since this is RNA, A is linked up with U and G is linked up with C. So here we'd have C, G, G, U, A, C, there's an A here so, UGUC A U. This is the sequence of our pre-mRNA.

Now, if we take a look, we have 3', 3', 3', 3', so it should be CGG initially. Here, put T. Remember, this is RNA, so it should not have Thymine involved. This is out. This would be the correct answer. Here, we made it a little trickier. This is the way we would see the pre-mRNA, but here we reversed it when we have our 5' end here and our 3' end here. But just go from 3' to 5'. Look at how the bases match up with the DNA template strand to find the correct answer. Again, the answer here would be option D.

Write the sequence of pre-mRNA produced from the following DNA informational strand.

                                                           5’ AATCAGTGACGT 3’

• A. 5’ UUAGUCACUGUA 3’
• B. 5’ AAUCAGUGACGU 3’
• C. 3’ UGCAGUGACUAA 5’
• D. 5’ AAUCAGTGACGU 3’