According to the Human Genome Project, the total number of protein-coding genes is about 20,000 with about 13 genes encoded in the mitochondrial genome. Only about 12 percent of the entire genome consists of protein-coding genes, with the rest being introns, noncoding RNAs, and things called retrotransposons. Essential genes are believed to be critical for the survival of the organism. This number is small in bacteria and represents only about 250-400 genes—less than 10 percent of existing genes. Most are involved in protein synthesis. Humans are believed to have 2000 essential genes. A synthetic organism has been created that has a minimal genome, consisting of about 473 essential genes in the organism. Essential genes are used for basic cell functions and for the life cycle of the organism. Most proteins come from transcribed messenger RNA. Some genes are called RNA genes because the genes’ end products are the actual RNA molecules. These include transfer RNA and ribosomal RNA. Some RNA molecules are referred to as ribozymes, which are actually capable of enzymatic functionality, while microRNA is made to serve as regulatory molecules. The genes that make these products are called non-coding RNA genes.
REGULATION OF GENE EXPRESSION Gene regulation is the process of controlling which genes get expressed at different times. Not all genes can be expressed in the different cells of the organism, even though all cells have the same copy of DNA. The different set of genes that get expressed determines what properties the cell has. There are many different regulatory steps in gene regulation—most of them affiliated with the transcription of genes. The cell will regulate its gene expression depending on what it perceives are the external environmental factors. These include the temperature, environmental stresses, and available nutrients. Internal signals, such as whether or not the cell is infected and metabolic needs, will also determine gene expression. Gene expression can happen in transcription (most common), in RNA processing, in translation, and in posttranslational modification.
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