**1. Introduction - Origin, biogenesis and functions of microRNAs**

MicroRNAs or miRNAs are a class of small non-coding RNA approximately 21–25 nucleotides that modulate on gene expression post-transcriptionally via binding to the 3′ untranslated region (3′-UTR) of the target messenger RNA (mRNA), resulting in mRNA degradation or translational repression. The first miRNA, lin-4, was discovered by Ambro and his research group in 1993 and it was found to be related with larva development in *Caenorhabditis elegans* [1]. Up to date, almost 2000 miRNAs have been identified in humans (http://www.miRbase.org – 7.3.2019) [1]. It has been estimated that 1–4% of human genes expression can be regulated by miRNAs, which is the largest of genomic regulator [2]. In mammals, miRNAs have been associated with various cellular pathways with the regulation of cell differentiation, cell cycle, proliferation, apoptosis, hematopoiesis, and other cellular functions. Recent studies have highlighted the importance of mRNA regulation mechanism by validation and differential miRNA expression in a variety of human pathological conditions, including chronic diseases.

miRNAs are normally transcribed by RNA polymerase II from miRNA genes. This transcription leads to generate a primary miRNA transcript (pri-miRNA). Then, pri-miRNA is further cleaved by a microprocessor complex, which consists of Drosha, the double-stranded RNase III enzyme and DiGeorge syndrome critical region 8 (DGCR8), important cofactor, into a hairpin structure precursor miRNA (pre-miRNA) in the nucleus (**Figure 1**). The double strand pre-miRNAs with 70 nucleotides are then exported to the cytoplasm by the process of nuclear export factor exportin-5. The pre-miRNA is then processed by RNase III, Dicer, thereby generating a mature miRNA:miRNA duplex approximately 22 nucleotides in length and without a hairpin structure. The helicase enzyme cleaves miRNA duplexes into single-stranded miRNAs and incorporated into the Argonaute (AGO), TRBP and PACT proteins to form the RNA-induced silencing complex (RISC). Usually, other single strand called passenger strand or the star (\*) strand will be degraded, while single strand mature miRNA is able to bind with its target mRNA and mediating translational inhibition or mRNA degradation, along with their sequence complementarity to the target [1, 3]. In plants, target mRNA will be degraded if miRNA has perfect or near-perfect complementarity to its target. In contrast to mammal, miRNAs bind to partially complementary sites in the 3′-UTRs of target mRNA, which leading to translational repression [4]. the target mRNA is either blocked (imperfect complementary) or degraded (perfect complementary) of the ribosomal translation, which sequentially impacts the cellular functions.

Phytochemicals are major plant-derived compounds that naturally found in vegetables, fruits, medicinal plants or other plants with medicinal properties including antioxidant, anti-diabetic, anti-inflammatory, antimicrobial, antidepressant, anticancer and prevention in other chronic non-communicable diseases [5–7]. Phenolic and flavonoid compounds are the most important group of bioactive compounds and second metabolites in plants which comprise of essential molecules

#### **Figure 1.**

*miRNA biogenesis. miRNA gene is transcribed by RNA polymerase II and then forming the primary miRNA transcript (pri-miRNA), which is further cleaved by the Drosha/DGCR8 complex to generate the precursor miRNA (pre-miRNA). Pre-miRNA is then exported into the cytoplasm by exportin 5/RAN-GTP and further processed by dicer to create the mature miRNA, which is loaded into RISC, which contains AGO, PACT and TRBP proteins. Mature miRNA that binding to its target mRNA by perfect complementary binding and resulting in gene suppression by mRNA degradation. The partially complementary binding of miRNA and its target mRNA, which in turn inhibit the protein translation.*

*The Impact of Dietary Compounds in Functional Foods on MicroRNAs Expression DOI: http://dx.doi.org/10.5772/intechopen.96746*

of human diet [6, 8]. It has been shown that bioactive compounds can modulate the endogenous miRNAs expression [1, 9–12]. Recently, some studies have revealed that plant-derived miRNAs (dietary miRNAs) as new bioactive compounds in plants can affect the synthesis of endogenous miRNAs [13–15]. Strikingly, miRNAs do not function only their origins but they are able to regulate the gene expression in cross-kingdom. Therefore, bioactive compounds present in functional foods are potentially regulate endogenous miRNAs expression.
