Nucleic acids, such as DNA and RNA, are polymers made up of nucleotide monomers linked by phosphodiester bonds. In DNA, the strands are anti-parallel, meaning they run in opposite directions. This is characterized by the 5' and 3' ends of the strands, where the 5' end has a phosphate group attached to the 5' carbon of the pentose sugar, and the 3' end has a hydroxyl group attached to the 3' carbon. The anti-parallel nature can be visualized as two arrows pointing in opposite directions, emphasizing the structural orientation of the strands.
RNA is generally less stable than DNA due to the presence of a reactive 2' hydroxyl group, which can lead to self-reactivity and strand breakage, making DNA the preferred genetic material for most life forms. While some viruses utilize RNA for genetic information, DNA's stability is a significant evolutionary advantage. The absorption of light by nucleotides occurs maximally at 260 nanometers, which is crucial for distinguishing between nucleic acids and proteins, as proteins absorb light at 280 nanometers. This property allows biochemists to isolate DNA from cellular samples effectively.
The two strands of DNA are complementary, adhering to specific base pairing rules: adenine pairs with thymine (forming two hydrogen bonds), and guanine pairs with cytosine (forming three hydrogen bonds). This specificity results in DNA strands with higher guanine-cytosine (GC) content having higher melting temperatures due to the increased number of hydrogen bonds. While RNA is typically single-stranded, it can form double-stranded structures and base pair with DNA during transcription, where adenine pairs with uracil instead of thymine.
In terms of nucleotide composition, DNA consists of 50% purines (adenine and guanine) and 50% pyrimidines (thymine and cytosine), ensuring a consistent width of the double helix. For example, if a double-stranded DNA molecule contains 35% adenine, it will also contain 35% thymine, while 15% cytosine will correspond to 15% guanine. This complementary nature of base pairing is fundamental to the transmission of genetic information within cells.