1.6 Nucleic Acids
Identify the three structural components of a nucleotide monomer.
A nucleotide consists of a five-carbon sugar (deoxyribose or ribose), a phosphate group, and a nitrogenous base.
Explain how nucleic acids encode biological information.
Biological information is encoded in the specific linear sequence of nucleotide monomers. Different arrangements of the four nitrogenous bases along the strand create different genetic messages, so the order of bases determines what information is stored and transmitted.
Describe what defines the 3' and 5' ends of a nucleic acid strand.
The 5' end has an exposed phosphate group attached to the fifth carbon of the sugar, while the 3' end has an exposed hydroxyl group on the third carbon of the sugar. These chemically distinct ends give the strand directionality.
Explain why nucleic acid strands are said to grow in the 5' to 3' direction.
During synthesis, each new nucleotide is added to the 3' hydroxyl end of the existing strand through a covalent bond. Because addition always occurs at the 3' end, the strand extends in the 5' to 3' direction.
Describe the antiparallel arrangement of DNA.
The two strands of the DNA double helix run in opposite directions — one oriented 5' to 3' and the other oriented 3' to 5'. This antiparallel arrangement allows complementary base pairing between the two strands.
Explain why knowing the sequence of one DNA strand allows you to determine the sequence of the other.
Complementary base pairing rules dictate that adenine always pairs with thymine and cytosine always pairs with guanine. Therefore, each base on one strand specifies the identity of the base across from it on the complementary strand.
Predict the mRNA sequence that would be produced from the DNA template strand 3'-TACGGA-5'.
Using RNA base pairing rules (A pairs with U, T pairs with A, C pairs with G, G pairs with C), the mRNA sequence produced would be 5'-AUGCCU-3'. Thymine in DNA is replaced by uracil in RNA, so the adenine on the template pairs with uracil in the mRNA.
Identify three structural differences between DNA and RNA.
DNA contains deoxyribose sugar while RNA contains ribose, DNA uses thymine while RNA uses uracil, and DNA is typically double stranded while RNA is typically single stranded.
Explain how the double-stranded structure of DNA relates to its function in information storage.
The double-stranded structure provides stability because hydrogen bonds between complementary base pairs hold the two strands together, protecting the encoded genetic information. Additionally, having two complementary strands means that if one strand is damaged, the other can serve as a template for repair, making DNA a reliable long-term storage molecule.