Organization of Genes

Inside the nucleus, genetic material is tightly packaged, and human genes have a distinct and organized structure.

Human cells contain over 20,000 genes arranged along chromosomes. These genes make up a small fraction of the DNA. They are separated by long stretches of noncoding DNA that do not code for proteins.

Each gene contains coding regions along with regulatory DNA sequences that control when, where, and how much the gene is expressed. A key component of this regulatory region is the promoter, a specific DNA sequence that marks the start site of transcription. It provides a binding site for proteins required for RNA synthesis.

Transcription factors bind to the promoter first. They help position RNA polymerase at a nearby transcription start site, where RNA synthesis begins using the DNA strand as a template.

RNA polymerase reads the DNA template strand in the 3 prime to 5 prime direction and builds a complementary RNA strand in the 5 prime to 3 prime direction as it moves along the DNA. It continues until it reaches a termination sequence. At this point, RNA synthesis stops.

Between the start and end of the gene are regions called exons and introns. Both regions are copied into the initial RNA transcript during transcription.

Introns are removed from the RNA transcript through RNA splicing. The remaining exons are joined together to form a mature messenger RNA that encodes proteins.

Other noncoding DNA elements, such as silencers and enhancers, also help control gene expression. Repressor proteins bind to silencer sequences and block RNA polymerase from binding, which inhibits transcription. Enhancers increase transcription by binding activator proteins, which interact with promoter-bound transcription factors to help RNA polymerase bind.

So, each gene includes promoters, exons, introns, and regulatory elements that together help control protein expression in a cell.