Chapter 13, Problem 1P

Devoting a few sentences to each, describes the following structures or complexes and their effects on eukaryotic gene expression:

a. Promoter

b. Enhancer

c. Silencer

d. RISC

e. Dicer

Summary Introduction

To analyze:

The following structures and their effects on eukaryotic gene expression

Promoter

Enhancer

Silencer

RISC

Dicer

Introduction:

Gene regulation is an important feature of all living organisms. A regulatory sequence is a stretch of the DNA that can increase or decrease the expression of the genes. These genes can control the expression of one or more genes. Controlling genes can be negative or positive regulators. Negative regulators control gene regulation by inhibiting transcription or translation. These gene regulatory elements include insulators, enhancers, promoters, silencers.

Explanation of Solution

Promoter:

It is the regulatory part of the DNA that is responsible for the initiation of the gene. It can be $\text{100-1000}$ bps long. It is present near the transcription start site, upstream of the gene, that is, towards the $\text{5'}$ region. This region recognizes the transcription factors and binds to them. These factors recruit the RNA polymerase that synthesizes mRNA from the coding segment of the DNA. It consists of a core promoter and proximal promoter. The core promoter is necessary for the proper initiation of the transcription. It includes a binding site for RNA polymerase; the binding site for general transcription factors, transcription start site. The proximal promoter contains primary regulatory elements. It includes the binding site for specific transcription factors.

The eukaryotic promoter contains a TATA box that provides a binding site for TATA-binding protein. This protein helps in the formation of the RNA polymerase transcriptional complex.

Enhancer:

It is another regulatory sequence that can be bound by the activator proteins. It surges the transcription of the gene. It is the cis-acting gene. It can be existing upstream or downstream of the gene it regulates. It may be present near or very far from the genes they regulate.

Silencer:

It is a cis-acting sequence that binds to the regulatory sequence known as a repressor. By binding to the silencer, repressor prevents the binding of RNA polymerase and transcription of DNA sequence. Therefore, it does not allow the expression of the gene into proteins. Silencer sequence can be present upstream or downstream of the promoter sequence. In eukaryotes, silencer controls the gene regulation at the transcription level.

RISC:

It is also known as the RNA induced silencing complex. It is ribonucleoprotein that is involved in gene silencing. When a cell contains double-stranded RNA, the enzyme dicer cuts the double-stranded RNA into fragments. These fragments attach to the RISC which results in denaturation of double-stranded RNA into single-stranded RNA. One of the strands is a biologically active guide strand and second is the passenger strand. RISC binds to the guide strand that directs gene silencing.

Dicer:

It is an enzyme that converts double-stranded RNA to small interfering RNA. It consists of one double-stranded RNA binding site and two domains that have RNase activity. This RNase activity breaks the RNA. Dicer activates RISC that is crucial for RNA interference. Dicer also converts pre-miRNA to micro-RNA.

Conclusion

Transcriptional regulatory elements are nucleic acid sequences that are important in gene regulation.