Enduring Understanding 3B

Expression of genetic information involves cellular and molecular mechanisms.

Structure and function in biology result from the presence of genetic information and the correct expression of this information. The expression of the genetic material controls cell products, and these products determine the metabolism and nature of the cell. Most cells within an organism contain the same set of genetic instructions, but the differential expression of specific genes determines the specialization of cells. Some genes are continually expressed, while the expression of most is regulated; regulation allows more efficient energy utilization, resulting in increased metabolic fitness. Gene expression is controlled by environmental signals and developmental cascades that involve both regulatory and structural genes. A variety of different gene regulatory systems are found in nature. Two of the best studied are the inducible and the repressible regulatory systems (i.e., operons) in bacteria, and several regulatory pathways that are conserved across phyla use a combination of positive and negative regulatory motifs. In eukaryotes, gene regulation and expression are more complex and involve many factors, including a suite of regulatory molecules.

Multicellular organisms have developmental pathways from zygote to adult, yet all cells in the organism start with the same complement of DNA. The developmental sequences are predominately determined and programmed by differential gene expression. Which gene gets expressed and the level of expression are determined by both internal and external signals. In multicellular organisms, cell-to-cell interactions and cell-to-cell signaling via small molecules modulate and control gene expression and cell function. For example, morphogens help to determine spatial development, and hormones can influence cell metabolism. Developmental gene sequences have an evolutionary origin and are conserved across species; for example, HOX genes are present in genome sequences from Drosophila to humans. Errors or changes in regulation of genes involved in development often lead to severe, detrimental and even bizarre consequences.