1. Introduction

The term gene silencing is commonly used to describe the "switching off" of a gene by a mechanism without genetic modification. The term gene silencing refers to an epigenetic phenomenon, the heritable inactivation of gene expression that does not involve any changes to the deoxyribonucleic acid (DNA) sequence. While this phenomenon has initially been studied in transgenic plants, its relevance in the regulation of endogenous plant genes has become increasingly apparent. Below some cellular components are mentioned where gene silencing occurred:

• Chromatin and heterochromatin

• Dicer

• dsRNA

• Histones

• MicroRNA

• siRNA

• Transposons

Gene silencing has following two major subdivisions by which genes are regulated:

1.  Transcriptional gene silencing (TGS) and

2. Posttranscriptional gene silencing (PTGS)

 

2. Transcriptional gene silencing (TGS)

Transcriptional gene silencing is the product of chromosomal histone modifications, creating an environment of heterochromatin, which is surrounded to a gene that makes it inaccessible to transcriptional machinery (RNA polymerase, transcription factors, etc.). TGS blocks primary transcription from nuclear DNA and is in most cases associated with DNA methylation and chromatin condensation in nearly all organisms that possess a DNA methylation system (Figure 29.1).