The genomic DNA content is the same in all cells of the same person and encodes all the genetic information for cellular function. Encoded in the DNA are the blueprints for all the RNA and protein molecules present in any type of cell. Different parts of the genetic information are used by different types of cells to accomplish each cell's specific function. DNA is used to produce RNA and protein molecules by processes called transcription and translation, respectively. The regions of DNA that encode RNA and protein molecules are called genes.
Replication requires an increase in building materials for the duplicated daughter cells. Highly condensed metaphase chromatin cannot produce gene products because proteins that initiate gene expression cannot bind to the chromosomes at this phase of replication. In contrast, partially condensed or unfolded chromatin permits the binding of specific proteins (e.g., RNA polymerases) that synthesize mRNA and tRNA. Ultimately, these molecules facilitate the production of gene products, specifically proteins.
RNA molecules function as the mediators between DNA and protein. These molecules essentially speak the same language as DNA because, as nucleic acids, they can base pair with complementary DNA sequences. Like transferring spoken language to a written form, this process of copying information from DNA to RNA is referred to as transcription. The transcription complex of proteins must unwind the double-stranded DNA at the specific gene site to be copied, locate the polymerase binding site on one of the DNA strands, and generate a primary (1°) transcript, which is one component of heterogeneous nuclear RNA (hnRNA) by reading the DNA strand in a 3' to 5' direction, with RNA synthesis proceeding in a 5' to 3' direction. The 1° RNA transcript is processed into mRNA, and finally the DNA in the region of the gene becomes double-stranded again. Numerous DNA sequences bind proteins that regulate and coordinate gene expression. These sequences can be used to identify the locations of genes within the entire human genome sequence. Since the generation of the first draft of the human genome, the interest in understanding gene structure has increased with the goal of identifying disease-associated genes.1214
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