DNA stores and transmits the genetic information that guides the development, functioning, growth, and reproduction of organisms. Understanding its intricate structure, especially its inherent directionality, is important for comprehending how it operates within a cell.
Sugar Carbons and Numbering
The basic building block of DNA is a nucleotide, which consists of three main components: a nitrogenous base, a five-carbon sugar called deoxyribose, and a phosphate group. Deoxyribose is central to DNA’s directionality because its carbon atoms are numbered 1′ (one prime) through 5′ (five prime).
The phosphate group of a nucleotide attaches to the 5′ carbon of the deoxyribose sugar. Conversely, a hydroxyl (-OH) group is located at the 3′ carbon of the same sugar molecule. This arrangement, with a phosphate at the 5′ end and a hydroxyl group at the 3′ end, creates the molecular basis for DNA’s intrinsic directionality.
Building the DNA Strand
Individual nucleotides link together to form a single strand of DNA through covalent connections called phosphodiester bonds. These bonds form between the phosphate group attached to the 5′ carbon of one nucleotide and the hydroxyl group on the 3′ carbon of the adjacent nucleotide. This repetitive linkage establishes a sugar-phosphate backbone, giving each DNA strand a distinct 5′ end and a 3′ end.
DNA exists as a double helix, composed of two polynucleotide strands coiled around each other. The two strands run in opposite directions, an arrangement known as antiparallel. If one strand is oriented in the 5′ to 3′ direction, its complementary partner runs in the 3′ to 5′ direction. This antiparallel configuration is important for the stability of the DNA molecule and facilitates the precise pairing of nitrogenous bases, essential for maintaining genetic information.
The Biological Importance of Direction
The 3′ and 5′ directionality of DNA is fundamental to how cells handle genetic information. During DNA replication, enzymes called DNA polymerases can only add new nucleotides to the free hydroxyl group at the 3′ end of a growing DNA strand. This means that one new strand, known as the leading strand, can be synthesized continuously in the 5′ to 3′ direction, following the unwinding replication fork. The other new strand, the lagging strand, must be synthesized discontinuously in short segments, also in the 5′ to 3′ direction, but moving away from the replication fork.
Similarly, during gene expression, RNA polymerase enzymes read the DNA template strand in a specific direction. RNA polymerase moves along the DNA template in the 3′ to 5′ direction, synthesizing a new RNA molecule in the 5′ to 3′ direction. This directional reading and synthesis ensures that genes are accurately copied into RNA, which then guides protein synthesis. DNA repair mechanisms also rely on this inherent directionality to identify and correct damage, maintaining the integrity of the genetic code.