Nucleic acids are large biological molecules that store, transmit, and help express genetic information. The two main types are DNA and RNA, which are built from smaller units called nucleotides. These molecules matter because they carry the instructions cells use to make proteins and pass traits from one generation to the next.
Their structure explains how information can be copied accurately and used when needed.
Key Facts
- A nucleotide = phosphate group + 5-carbon sugar + nitrogenous base.
- DNA uses deoxyribose sugar, while RNA uses ribose sugar.
- DNA bases are A, T, C, and G; RNA bases are A, U, C, and G.
- Base-pairing rules in DNA: A pairs with T, and C pairs with G.
- Base-pairing rules in RNA synthesis: DNA A pairs with RNA U, DNA T pairs with RNA A, DNA C pairs with RNA G, and DNA G pairs with RNA C.
- The central dogma is DNA → RNA → protein.
Vocabulary
- Nucleic acid
- A polymer such as DNA or RNA that stores or transfers genetic information in cells.
- Nucleotide
- The monomer of nucleic acids, made of a sugar, a phosphate group, and a nitrogenous base.
- Sugar-phosphate backbone
- The repeating chain of sugars and phosphate groups that forms the outer support structure of DNA or RNA.
- Complementary base pairing
- The specific matching of nitrogenous bases, such as A with T in DNA and C with G.
- Transcription
- The process in which an RNA copy is made from a DNA template.
Common Mistakes to Avoid
- Calling a nucleotide just a base is wrong because a nucleotide includes three parts: a sugar, a phosphate group, and a base.
- Pairing A with C or G with T is wrong because DNA base pairing depends on specific hydrogen bonding: A pairs with T, and C pairs with G.
- Saying RNA contains thymine is wrong because RNA usually uses uracil instead of thymine.
- Thinking DNA and RNA have the same sugar is wrong because DNA contains deoxyribose, while RNA contains ribose with one more oxygen atom.
Practice Questions
- 1 A DNA segment has 24 nucleotides total. If 6 of the bases are adenine, how many thymine bases are present, and how many cytosine plus guanine bases are present?
- 2 A DNA template strand has the sequence TAC GGA CTT. Write the complementary mRNA sequence produced during transcription.
- 3 Explain why complementary base pairing is important for accurate DNA replication and for making RNA from DNA.