Eukaryotic Post-Transcriptional Regulation - Online Tutor, Practice Problems & Exam Prep

Eukaryotic post-transcriptional regulation involves three key mechanisms: alternative RNA splicing, mRNA protection, and RNA interference (RNAi). Alternative RNA splicing allows different proteins to be produced from the same pre-mRNA by removing introns and joining exons. mRNA protection is achieved through the addition of a 5' cap and a poly-A tail, safeguarding mRNA from degradation in the cytoplasm. RNAi utilizes small non-coding RNAs to block translation or degrade target mRNA, effectively regulating gene expression. Understanding these processes is crucial for grasping gene regulation in eukaryotic cells.

Eukaryotic Post-Transcriptional Regulation

In this video, we're going to begin our lesson on eukaryotic post-transcriptional regulation. Eukaryotes regulate gene expression at the post-transcriptional level in three different ways that you can see numbered down below 1 2 3. The first way they regulate their gene expression at the post-transcriptional level, after transcription is complete, is through alternative RNA splicing. Alternative RNA splicing is going to result in different protein products coming from the same mRNA transcript or the same gene, and we'll talk more about alternative RNA splicing as we move forward.

The second level of post-transcriptional regulation in eukaryotes is RNA processing by adding a 5' cap and a poly-A tail to the mRNA in order to protect the RNA from degrading enzymes. Again, this is an idea that we'll talk more about moving forward as well.

The third level of post-transcriptional regulation in eukaryotes is that the mRNA can actually be tagged for degradation or transcription can be blocked, from the mRNA. Transcription of the mRNA can be blocked by small noncoding RNA molecules. We'll talk more about these small noncoding RNA molecules as we move forward as well.

This here concludes our brief introduction to eukaryotic post-transcriptional regulation, and we'll be able to talk about all three of these different levels as we move forward. So I'll see you all in our next video.

1) Alternative RNA Splicing

In this video, we're going to talk about the first level of post-transcriptional regulation in eukaryotes, and that is alternative RNA splicing. Recall that way back in our previous lesson videos, we already covered alternative RNA splicing. If you don't remember anything about alternative RNA splicing, then make sure to go back and watch those older videos before you continue here. Eukaryotes require post-transcriptional modifications like RNA splicing, which can alter gene expression. Alternative splicing is really when different mRNA molecules are produced from the same premature RNA molecule or the same pre-mRNA. Because there are different mRNA molecules produced, that's going to lead to different proteins being made from the same gene. Different mRNA molecules will lead to different proteins being made. Recall that the spliceosome is the complex RNA-protein complex, that is going to remove introns from the pre-mRNA and splice together the exons.

If we take a look at our image down below, we have our miniature map and you can see that we're focusing on RNA processing, which is a form of post-transcriptional modifications—the splicing here that we're referring to. At the top, we have the DNA, the specific gene of interest. Within the gene, the red regions represent exons that are going to be spliced together, and the blue regions represent introns that are going to be removed. When the gene is first transcribed through transcription, a pre-mRNA molecule or a pre-mRNA transcript is formed. The pre-mRNA molecule is not going to be the fully mature mRNA. It must undergo splicing. We are zooming into the spliceosome formation, which is an assembly of a complex of RNA and protein that will come together to remove the introns, the blue regions, and splice together the red regions, the exons. That's what we're referring to here as RNA splicing.

Alternative RNA splicing is when RNA splicing can occur in multiple different ways. There are alternative pathways for RNA splicing to occur. In this image, we're focusing on just 2 different alternative pathways for RNA splicing to occur. The first RNA pathway RNA splicing pathway is over here on the left-hand side, showing you exons 1, 2, 3 all being expressed. This is the mature mRNA that's ready for translation. This is one possibility for the RNA to get spliced, and that would create this particular circular protein product upon translation. But the alternate RNA splicing pathway over here, notice, has a different mature mRNA transcript where exon 3, notice, is not available over here. It's not present. Exon 3 acted as an intron, in this alternative RNA splicing pathway. So, only exons 1, 2, and 4 are available over here. That creates a shorter polypeptide chain and that ultimately leads to a different protein. By filtering and controlling alternative RNA splicing, a specific gene product can be redirected to create a new gene product. This is a way of regulating gene expression.

This is a post-transcriptional method because the alternative RNA splicing is occurring after transcription has occurred. This here concludes our brief review and introduction of alternative RNA splicing. We'll be able to get some practice applying this as we move forward and talk about the other forms of post-transcriptional regulation. So I'll see you all in our next video.

Alternative RNA splicing has been estimated to occur in more than 95% of multi-exon genes. Which of the following is not an evolutionary advantage of alternative RNA splicing?

• A. Alternative RNA splicing increases diversity without increasing genome size.
• B. Different protein variants can be expressed by the same gene in different tissues.
• C. Alternative RNA splicing creates shorter mRNA transcripts.
• D. Different protein variants can be expressed by the same gene during different stages of development.

2) mRNA Protection in the Cytoplasm

In this video, we're going to talk about the second level of post-transcriptional regulation, which is mRNA protection in the cytoplasm. mRNA transcripts must be transported to the cytoplasm of the cell in order for the mRNA transcript to be translated by ribosomes and create a protein product. However, the cytoplasm actually has many RNA-degrading enzymes. When mRNA transcripts enter the cytoplasm, they are at risk of being degraded by these RNA-degrading enzymes. These enzymes are there to help destroy foreign viral RNA molecules, acting as a defense mechanism against viral RNAs.

In order for the mRNA transcripts to be protected, the mRNA needs to be processed so that it has a 5' cap added, as well as a poly A tail added at the 3' end of the molecule. This 5' cap and poly A tail protect the mRNA from degradation by enzymes. The process of degradation and protection is a way to regulate gene expression.

Let's take a look at our image down below to get a better understanding. Notice the example here says that mRNA is protected from degradation by cytoplasm enzymes with a 5' cap and a poly A tail. RNA processing, which is the addition of the 5' cap and poly A tail, occurs within the nucleus. But of course, the RNA needs to be transported outside, into the cytoplasm of the cell.

Here, within the cell, we have the nucleus. Inside the nucleus, we have our mature mRNA, which must be transported out of the nucleus. If the mRNA is not protected, if it does not have a 5' cap and a poly A tail, then it is unprotected mRNA, and that leads to degradation. The unprotected mRNA is degraded, and as a result, the final gene product, the protein, is not created. This is a way of turning off the gene.

Conversely, if the RNA is properly protected with the 5' cap and the poly A tail, then we have protected mRNA that will not be degraded. This protected mRNA can then be translated into a protein, and when it's translated into a protein, the final gene product is being made. This is a way of turning on the gene. You can see the off and on states, which are ways of regulating gene expression to turn genes on or off.

This concludes our introduction to this second level of post-transcriptional regulation. We'll be able to get some practice applying these concepts as we move forward, as well as talk about the third and final level of post-transcriptional regulation. So, I'll see you all in that video.

3) RNA Interference

In this video, we're going to talk about the third level of post-transcriptional regulation in eukaryotes, and that is RNA interference. And so RNA interference is commonly abbreviated as just RNA I with a lowercase I here. And so RNA interference or RNAi is really just the process of small non-coding RNAs blocking translation of target mRNA molecules. And so these small non-coding RNAs are really just short strands of RNA that have a complementary sequence to an mRNA target. And so we'll be able to see more and learn more about them down below in our image.

Now really there are two possible scenarios that are going to turn gene expression off when it comes to RNA interference. And so the first possible scenario is mRNA is going to be degraded and targeted for degradation. And then, the second possible scenario is going to be that the ribosome is going to be blocked from binding and that's going to prevent translation. And so let's take a look at our image down below where you can see in our example, RNA interference can block ribosome binding or recruit cellular enzymes for mRNA degradation. And so over here on the left-hand side, notice that we're showing you our miniature version of the map, and you can see that again mRNA degradation and translational control is going to occur in the cytoplasm of the cell outside of the nucleus.

And so up here, notice in this image where this image we're focusing on RNA interference or RNAi. And, notice that it is going to require small non-coding RNAs, like this little short orange RNA molecule that you see at the top. And this short small non-coding RNA is complementary to a small sequence on the mRNA itself, the messenger RNA. And so, there are two levels here. There is the mRNA degradation scenario, and then there is the translational control scenario. So in the mRNA degradation scenario, what happens is the small noncoding RNA is going to complementary bind to the mRNA, and the mRNA is going to be degraded. It's going to be degraded by enzymes. And so the small noncoding RNA here is basically marking the mRNA for degradation. And so you can see that this enzyme over here is degrading the mRNA into small tiny pieces. And of course, the gene product will not be made if the mRNA is being degraded. And so this is a way of turning off the gene expression.

Now if we take a look at the translational control scenario, what happens is the mRNA, the small noncoding RNA complementary binds to the mRNA. But in this scenario, when the small noncoding RNA binds to the mRNA, the mRNA is not going to be degraded. Instead, the ribosome is not going to be able to bind to the mRNA as it normally would to translate it. And so in this scenario, the ribosome is blocked from binding to the mRNA. And of course, that is also going to prevent the gene product, the protein from being made. And so that is also going to be turning off gene expression. And so, this here RNA interference is basically interfering with the mRNA and turning off the expression of the mRNA. And so this here concludes our introduction to RNA interference, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Which of the following statements best describes the function of RNAi?

• A. Small RNA molecules interfere with translation by targeting ribosomes for degradation.
• B. Small DNA molecules interfere with mRNA molecules by blocking their ability to bind to a ribosome.
• C. Small RNA molecules interfere with translation by targeting specific tRNA molecules
• D. Small RNA molecules interfere with translation by blocking a target mRNA's ability to bind to a ribosome.

Types of Small Noncoding RNAs

In this video, we're going to introduce 2 different types of small non-coding RNAs. And so again, there are 2 classes of RNAs that are involved in RNAi or RNA interference. And so the first class is going to be the microRNAs, and the second class are going to be the small interfering RNAs or, for short, the siRNAs. And so both types of RNA, the microRNAs and the small interfering RNAs are going to bind to a target mRNA by complementary base pairing and it's going to turn off the expression of a gene, through RNA interference. So both microRNAs and small interfering RNAs will turn off the expression of a gene. And so what is the difference between the two? Well, the only difference between microRNAs and siRNAs is the structure of their precursor form. And so microRNAs have a single-stranded precursor, and siRNAs have a double-stranded precursor. And so we can take a look at this, in our image down below. And so notice in this example, we're showing you RNA interference by 2 types of small non-coding RNAs, the microRNAs and the siRNAs. And so once again, over here on the far left, we have our miniature version of the map of our lesson, and, notice that, RNA interference is really taking place here with mRNA degradation, which is going to occur in the cytoplasm, but it also is going to block translation as well, and so it is a type of translational regulation as well here. And so, over here on the left-hand side, notice that we're focusing on the microRNAs. The microRNAs are going to have a single-stranded precursor, and so this is the precursor for microRNA. And then the microRNA is going to complementary bind to the mRNA itself, and it is going to block, transcription or I'm sorry. It's going to block, the next step, either mark the molecule for degradation or block the ribosome from binding and block translation. And so it is a way of turning off the gene. Now over here on the right-hand side, we're focusing on the siRNAs, the small interfering RNAs, and they have a double-stranded precursor. And so notice that up above, we're showing you a double-stranded precursor molecule. And so, ultimately, it will be, converted into a single-stranded siRNA, and it's going to be very, very similar, under both conditions. They both are going to be complementary binding to the mRNA. And again, it can either mark the mRNA for degradation or it can block the ribosome from translating. And so in either case, it is going to be interfering with the RNA. It is a type of RNA interference, and it will be turning off the gene. And so really, the main learning objective here is that the microRNAs are going to have single-stranded precursors whereas the siRNAs are going to have double-stranded precursors. But other than that, they're going to be very, very similar in their functions. And so this here concludes our introduction to the different types of small non-coding RNAs, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Which of the following best describes siRNA?

• A. A short double-stranded RNA with one strand that can complimentarily bind to and inactivate an mRNA.
• B. A single-stranded RNA with internal complementary base pairs that allow it to fold into a cloverleaf pattern.
• C. A portion of rRNA which is a component of the large and small ribosomal subunits.
• D. A molecule, known as Dicer, that can degrade or cut RNA sequences.

Here’s what students ask on this topic:

What is alternative RNA splicing and how does it affect gene expression?

Alternative RNA splicing is a process by which different mRNA molecules are produced from the same pre-mRNA transcript. This is achieved by selectively including or excluding certain exons during the splicing process. The spliceosome, a complex of RNA and proteins, removes introns and splices together exons in various combinations. This results in the production of different protein isoforms from a single gene, thereby increasing the diversity of proteins that a cell can produce. By regulating which exons are included in the final mRNA, cells can control the type of protein produced, thus affecting gene expression and allowing for complex regulation of cellular functions.

How does the addition of a 5' cap and a poly-A tail protect mRNA in eukaryotic cells?

The addition of a 5' cap and a poly-A tail to mRNA molecules is crucial for their stability and function. The 5' cap, added to the 5' end of the mRNA, consists of a modified guanine nucleotide that protects the mRNA from degradation by exonucleases. The poly-A tail, a stretch of adenine nucleotides added to the 3' end, also helps in protecting the mRNA from enzymatic degradation. Together, these modifications enhance the stability of the mRNA in the cytoplasm, facilitate its export from the nucleus, and aid in the initiation of translation by ribosomes. This protection ensures that the mRNA can be efficiently translated into proteins, thereby regulating gene expression.

What is RNA interference (RNAi) and how does it regulate gene expression?

RNA interference (RNAi) is a biological process in which small non-coding RNAs, such as microRNAs (miRNAs) and small interfering RNAs (siRNAs), regulate gene expression by targeting mRNA molecules. These small RNAs bind to complementary sequences on target mRNAs, leading to either degradation of the mRNA or inhibition of its translation. In the degradation pathway, the bound mRNA is marked for destruction by cellular enzymes. In the translational inhibition pathway, the binding of small RNAs prevents ribosomes from attaching to the mRNA, thereby blocking protein synthesis. RNAi is a crucial mechanism for controlling gene expression, maintaining cellular homeostasis, and defending against viral infections.

What are the differences between microRNAs (miRNAs) and small interfering RNAs (siRNAs) in RNA interference?

Both microRNAs (miRNAs) and small interfering RNAs (siRNAs) are involved in RNA interference (RNAi) and function to regulate gene expression by binding to target mRNAs. The primary difference between them lies in their precursor structures. miRNAs are derived from single-stranded RNA precursors that form hairpin structures, while siRNAs originate from double-stranded RNA precursors. Despite this difference, both miRNAs and siRNAs undergo processing to become single-stranded and then bind to complementary sequences on target mRNAs. This binding can lead to mRNA degradation or inhibition of translation, effectively turning off gene expression. Both types of small RNAs play essential roles in gene regulation, development, and defense mechanisms.

How does mRNA degradation contribute to post-transcriptional regulation in eukaryotes?

mRNA degradation is a critical component of post-transcriptional regulation in eukaryotes. It involves the breakdown of mRNA molecules, thereby preventing their translation into proteins. This process can be triggered by various mechanisms, including the binding of small non-coding RNAs (such as miRNAs and siRNAs) to the mRNA, marking it for degradation by cellular enzymes. Additionally, unprotected mRNAs lacking a 5' cap or a poly-A tail are more susceptible to degradation. By controlling the stability and lifespan of mRNA molecules, cells can finely tune the levels of specific proteins, ensuring that gene expression is appropriately regulated in response to developmental cues and environmental changes.