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Molecular Basis of Heredity: Part 1. Nucleic Acids

Author(s): Raye L. Alford, PhD

Transcription

Genes-the instructions for making the RNA and proteins needed to build and maintain cells and organisms-are embedded within the nucleotide sequence of the DNA double helix. DNA serves as the repository of these biologic instructions which consist of a specific series of nucleotides that are handed down from cell to cell and parent to offspring.

In order to implement the instructions encoded within the genes, DNA uses RNA as a messenger. RNA is formed through a process called transcription. During transcription, an enzyme called RNA polymerase II utilizes the precise base pairing between DNA and RNA nucleotides to synthesize an RNA molecule that is a complementary copy of one of the DNA strands. However, instead of incorporating deoxynucleotides into the nascent strand like the DNA polymerase does during replication, an RNA polymerase incorporates ribonucleotides into the growing nucleotide strand. The RNA molecule is synthesized in a 5′ to 3′ direction.

Genes have distinct starting and stopping points that are encoded within the DNA molecule. These signals specify where transcription of a particular gene should start and where it should stop. The coding strand of the DNA molecule is the strand that bears the same sequence as the RNA that will be made (with the substitution of uracil for thymine). As such, RNA polymerase "reads" the non-coding strand of the DNA molecule, so that when it synthesizes the complementary molecule, the sequence matches that of the coding strand of the DNA. The coding strand is so-called because it is the sequence that specifies the amino acid composition of the protein to be made.