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 splicing allows different proteins to be produced from the same gene by removing introns and joining exons. mRNA is protected in the cytoplasm by a 5' cap and poly A tail, preventing degradation by enzymes. RNAi utilizes small non-coding RNAs to block translation or degrade target mRNA, effectively silencing gene expression. Understanding these processes is crucial for grasping gene regulation and expression 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 that 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.

Now the second level of post-transcriptional regulation in eukaryotes is RNA processing by adding a 5′ prime 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.

And then 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 non-coding RNA molecules. We'll talk more about these small non-coding 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 discuss all three of these different levels as we move forward. So, I'll see you all in our next video.

Eukaryotic Post-Transcriptional Regulation

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. Recall that 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. Another way to phrase alternative splicing is, 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 and these different mRNA molecules are made from the same premature mRNA or the same gene, if you will. Now 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, over here again, we have our miniature map, and you can see that we're focusing on RNA processing and this is going to be a form of post-transcriptional modifications and regulation. RNA processing here, is also going to include, the splicing here that we're referring to. At the top here, what we have is the DNA, the specific gene of interest. Within the gene, the red regions here represent exons that are gonna be spliced together, and the blue regions represent introns that are going to be removed. Notice that when the gene is first mRNA molecule or a pre-mRNA transcript that is formed. And the pre-mRNA molecule is not going to be the fully mature mRNA. It must undergo splicing. Over here, what we are zooming into is the spliceosome formation, which is going to be an assembly of complex of RNA and protein that will come together to remove the introns, remove the blue regions, and splice together the red regions, the exons. That's what we're referring to here as RNA splicing. And 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 two different alternative pathways for RNA splicing to occur. The first RNA pathway RNA splicing pathway is over here on the left-hand side, which is showing you exons 1, 2, 3, and 4 all being expressed, and this is going to be the fully 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. And so exon 3 acted as an intron, in this alternative RNA splicing pathway. And so exon 3 was removed as an intron. And so only exons 1, 2, and 4 are available over here, and 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 here 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.

Eukaryotic Post-Transcriptional Regulation

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. But the problem is that 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 help destroy foreign viral RNA molecules, acting as a defense mechanism against viruses.

To counteract this degradation, the mRNA is processed to have a 5' prime cap added, as well as a PolyA tail. The 5' prime cap and the PolyA tail, added to the 3' end of the molecule, protect the mRNA from degradation by these enzymes. This mechanism of degradation and protection is a method of regulating gene expression.

Let’s examine an image below to gain a better understanding. In it, mRNA is portrayed as protected from degradation by cytoplasmic enzymes with a 5' prime cap and a PolyA tail. RNA processing, which includes the addition of the 5' prime cap and PolyA tail, occurs within the nucleus. However, the RNA must be transported outside the nucleus into the cytoplasm of the cell.

If the mRNA is not protected with a 5' prime cap and a PolyA tail, it becomes unprotected and is likely to be degraded, turning off the gene. Conversely, if the RNA is properly protected, the protected mRNA will not be degraded and can thus be translated into a protein, creating the final gene product and turning on the gene.

This process of turning genes on or off is a form of regulating gene expression. This concludes our introduction to this second level of post-transcriptional regulation. As we move forward, we will get some practice applying these concepts, as well as discuss the third and final level of post-transcriptional regulation in the next video. I’ll see you all in that video.

Eukaryotic Post-Transcriptional Regulation

In this video, we're going to talk about the 3rd level of post-transcriptional regulation in eukaryotes, and that is RNA interference. RNA interference is commonly abbreviated as just RNAi with a lowercase 'i' here. RNA interference or RNAi is really just the process of small non-coding RNAs blocking the translation of target mRNA molecules. These small non-coding RNAs are short strands of RNA that have a complementary sequence to an mRNA target, and we'll be able to see more and learn more about them down below in our image.

There are 2 possible scenarios that are going to turn gene expression off when it comes to RNA interference. The first possible scenario is that mRNA is going to be degraded and targeted for degradation. The second possible scenario is that the ribosome is going to be blocked from binding, which will prevent translation. 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. 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 are the focuses, representing RNAi. Notice that this process requires small non-coding RNAs, like the little short orange RNA molecule seen at the top.

This short, small non-coding RNA is complementary to a small sequence on the mRNA itself, the messenger RNA. There are two levels here: the mRNA degradation scenario and the translational control scenario. 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 by enzymes. This small noncoding RNA here is essentially marking the mRNA for degradation. You can see that this enzyme over here is degrading the mRNA into small tiny pieces. Of course, the gene product will not be made if the mRNA is being degraded, turning off the gene expression.

If we look at the translational control scenario, what happens is the small noncoding RNA complementary binds to the mRNA. But in this scenario, when the small noncoding RNA binds to the mRNA, it is not going to be degraded. Instead, the ribosome is blocked from binding to the mRNA as it normally would to translate it. In this scenario, the ribosome is blocked from binding to the mRNA, and this also prevents the gene product, the protein, from being made, turning off gene expression. RNA interference is basically interfering with the mRNA and turning off the expression of the mRNA.

This 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.

Eukaryotic Post-Transcriptional Regulation

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 micro RNAs, and the second class are going to be the small interfering RNAs or, for short, the siRNAs. And so both types of RNA, the micro RNAs 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 micro RNAs and small RNA will turn off expression of a gene. And so what is the difference between the 2? Well, the only difference between micro RNAs and siRNAs is the structure of their precursor form. And so micro RNAs 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 micro RNAs 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 micro RNAs. And the micro RNAs are going to have a single-stranded precursor, and so this is the precursor for the 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 micro RNAs 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 joins 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. Alternative splicing allows a single gene to encode multiple proteins, which can have different, sometimes even opposing, functions. This mechanism is crucial for the regulation of gene expression and the adaptability of eukaryotic organisms.

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

The addition of a 5' cap and a poly A tail to mRNA molecules is a crucial post-transcriptional modification that protects mRNA from degradation in the cytoplasm. The 5' cap is a modified guanine nucleotide added to the 5' end of the mRNA, while the poly A tail is a stretch of adenine nucleotides added to the 3' end. These modifications prevent RNA-degrading enzymes in the cytoplasm from degrading the mRNA. The 5' cap also facilitates ribosome binding for translation, and the poly A tail aids in the stability and export of the mRNA from the nucleus to the cytoplasm. Together, these modifications ensure that the mRNA remains intact long enough to be translated into a protein.

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 specific mRNA molecules. These small RNAs bind to their complementary mRNA sequences, leading to either the degradation of the mRNA or the 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 and maintaining cellular homeostasis.

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 silence gene expression by binding to target mRNA. The primary difference between them lies in their precursor forms. 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 their target mRNA through complementary base pairing. Once bound, they can either mark the mRNA for degradation or block its translation, effectively silencing the gene.

How does mRNA degradation contribute to post-transcriptional regulation?

mRNA degradation is a key mechanism of post-transcriptional regulation that controls the stability and lifespan of mRNA molecules in the cytoplasm. By selectively degrading specific mRNAs, cells can rapidly decrease the levels of certain proteins in response to changing conditions. This process is often mediated by small non-coding RNAs, such as miRNAs and siRNAs, which bind to target mRNAs and recruit cellular enzymes that degrade the mRNA. This ensures that the mRNA is broken down into smaller fragments, preventing its translation into protein. mRNA degradation allows cells to fine-tune gene expression and maintain homeostasis.