Figure 1: Current paradigm of molecular biology.

DNA is packed in chromosomes in tertiary structures defined by the histones. Histone modifications and DNA methylation are epigenetic factors that can influence the transcription process. DNA is transcribed into pre-mRNA, which is spliced into different mRNA molecules by alternative splicing. Noncoding RNAs, such as siRNA and miRNA, are also transcribed from DNA, and these small RNA fragments affect mRNA stability and regulate the translation of mRNA into proteins. In addition, post-translational modifications of proteins (for example, folding, cleavage, and chemical modifications) contribute greatly to the diversity of the proteome. These proteins can function as enzymes, affecting DNA repair or replication, the transcription process or translation, or can act as a catalyst for metabolic reactions. Proteins are also involved in cell signaling and ligand binding, and as structural elements. Each level interacts closely with the other level: protein–DNA and RNA–DNA interactions and chemical modifications of DNA that do not affect the primary DNA sequence also affect the molecular biological processes in cell systems. Each of these molecular biological processes can be studied with specialized omics tools—genomics/epigenomics, transcriptomics, proteomics and metabolomics—which study the complete sets of genes, transcripts, proteins and metabolites. Abbreviations: dsRNA, double-stranded RNA; Me, methylation; miRNA, microRNA; mRNA, messenger RNA; pre-mRNA, precursor messenger RNA; siRNA, small interfering RNA.