RNA Interference - Online Tutor, Practice Problems & Exam Prep

RNA interference (RNAi) is a crucial mechanism for regulating gene expression post-transcriptionally, primarily through microRNAs (miRNAs) and small interfering RNAs (siRNAs). miRNAs are single-stranded and can target multiple transcripts, while siRNAs are double-stranded and specific to one transcript. Both types utilize the Dicer enzyme for processing and the Argonaute protein within the RNA-induced silencing complex (RISC) to bind and mark RNA transcripts for degradation, preventing their translation into proteins. This regulation is vital for maintaining cellular function and responding to environmental changes.

RNA Interference

Okay. So RNA interference is a way to regulate transcripts. The cell creates a lot of transcripts, but not all of them are wanted by the cell to produce proteins. So there are a few different ways that it can stop this, and one of them is through RNA interference. RNA interference is called that because it interferes with the transcript and because it is a type of post-transcriptional regulation occurring through RNA. Other than that, there's really no DNA involved here. It's all RNA that's being processed in a certain way, and then binding to transcripts to stop their production into proteins and to prevent any further processing of them.

Now, there are many types of RNA interference. The two main ones, which account for the overwhelming majority of RNA interference, are performed by what are called microRNAs or miRNAs and siRNAs. The 'si' in siRNAs stands for 'short interfering,' although that term is not commonly used, but the terms microRNAs and siRNAs are frequently mentioned. These RNAs are different and must go through different processing in order to be able to stop the translation of a transcript. You can tell them apart because microRNAs are single-stranded, while siRNAs are double-stranded, which is a significant difference. Another main difference is that microRNAs target a wide range of different transcripts; they are not specific to just one. When they are created, they can inhibit transcription of multiple different RNAs. On the other hand, siRNAs are specific; they are used to halt the creation or translation of one particular transcript.

MicroRNAs are produced from genes and are found in non-coding regions such as introns or upstream or downstream of the start codon, parts that would not typically be processed. These RNAs are transcribed, and then an enzyme is required to cut out the non-coding sections. Inside these sections, a microRNA may be included. This long piece of RNA, which contains the microRNA, is attacked by an enzyme called Dicer. Dicer cuts off all the excess nucleotides, reducing the structure to approximately 22 nucleotides in length, forming what is recognized as a microRNA. Before being cut, it is known as pre-microRNA, and after being cut, it is simply called microRNA.

This 22-nucleotide long microRNA interacts with another enzyme called RISC. Once bound to RISC, the complex can bind to the transcript it will inhibit. The purpose here is to find various different transcripts, bind to them, and mark them for destruction. The microRNA, with its nucleotides complementary to those of the RNA transcripts, joins RISC in identifying and binding to these complementary transcripts, thereby marking them for destruction.

SIRNAs are initially double-stranded RNAs that often start out looking like a hairpin loop, as RNA can form secondary structures. These are also processed by Dicer, which identifies and cleaves the pre-siRNA, forming the siRNA. Here, RISC plays a different role compared to its interaction with microRNAs. It converts the double-stranded siRNA into a single-stranded form by digesting one of the strands. The remaining strand, known as the guide strand, and RISC then find an RNA with a complementary sequence to bind to and mark it for destruction. Once bound, this transcript will not be translated.

So, that is how RNA interference works. With that explanation, let's move on.

siRNAs target a variety of different RNA transcripts for degradation.

• A. True
• B. False

Which of the following enzymes are responsible for cleaving and processing miRNAs and siRNAs?

• A. DICER
• B. RISC
• C. Guide RNA enzymes
• D. RNA polymerase II

MiRNAs and siRNAs target degradation of RNA transcripts by binding to what?

• A. RNA polymerase II
• B. The gene promoter
• C. A gene transcript
• D. The spliceosome

Here’s what students ask on this topic:

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

RNA interference (RNAi) is a biological process where RNA molecules inhibit gene expression by neutralizing targeted mRNA molecules. It primarily involves two types of small RNA molecules: microRNAs (miRNAs) and small interfering RNAs (siRNAs). miRNAs are single-stranded and can target multiple mRNA transcripts, while siRNAs are double-stranded and specific to one mRNA transcript. Both types are processed by the enzyme Dicer and incorporated into the RNA-induced silencing complex (RISC). RISC uses these RNA molecules to bind to complementary mRNA sequences, marking them for degradation and preventing their translation into proteins. This post-transcriptional regulation is crucial for maintaining cellular function and responding to environmental changes.

What are the main differences between microRNAs (miRNAs) and small interfering RNAs (siRNAs)?

MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) are both involved in RNA interference but have key differences. miRNAs are single-stranded and can target multiple mRNA transcripts, making them versatile in gene regulation. They are often found in non-coding regions of genes and require the enzyme Dicer for processing. siRNAs, on the other hand, are double-stranded and specific to one mRNA transcript. They also require Dicer for processing but start as double-stranded RNA that forms a hairpin loop. Both miRNAs and siRNAs are incorporated into the RNA-induced silencing complex (RISC), which helps them bind to complementary mRNA sequences and mark them for degradation.

How does the enzyme Dicer function in RNA interference?

The enzyme Dicer plays a crucial role in RNA interference by processing precursor RNA molecules into their active forms. For microRNAs (miRNAs), Dicer cleaves long, single-stranded RNA precursors into shorter, approximately 22-nucleotide miRNAs. For small interfering RNAs (siRNAs), Dicer processes double-stranded RNA precursors, also around 22 nucleotides in length, by cutting them into smaller fragments. These processed RNA molecules are then incorporated into the RNA-induced silencing complex (RISC). Dicer's precise cleavage ensures that the resulting miRNAs and siRNAs can effectively bind to complementary mRNA sequences, marking them for degradation and preventing their translation into proteins.

What role does the RNA-induced silencing complex (RISC) play in RNA interference?

The RNA-induced silencing complex (RISC) is essential in RNA interference, acting as the effector complex that mediates gene silencing. Once small RNA molecules like microRNAs (miRNAs) or small interfering RNAs (siRNAs) are processed by the enzyme Dicer, they are incorporated into RISC. RISC uses these RNA molecules to identify and bind to complementary mRNA sequences. The binding of RISC to the target mRNA marks it for degradation, effectively silencing the gene by preventing its translation into a protein. This mechanism allows cells to regulate gene expression post-transcriptionally, maintaining cellular function and responding to environmental changes.

How are microRNAs (miRNAs) processed and activated in RNA interference?

MicroRNAs (miRNAs) are processed and activated through a series of steps in RNA interference. Initially, miRNAs are transcribed as long, single-stranded RNA precursors found in non-coding regions of genes. These precursors are then cleaved by the enzyme Dicer into shorter, approximately 22-nucleotide miRNAs. The processed miRNAs are incorporated into the RNA-induced silencing complex (RISC). Within RISC, miRNAs guide the complex to complementary mRNA sequences. The binding of miRNAs to these mRNA targets marks them for degradation, preventing their translation into proteins. This post-transcriptional regulation is crucial for controlling gene expression and maintaining cellular function.