Nucleic acids are large biomolecules that store, transmit, and help express genetic information in living cells. The two main types are DNA and RNA, and both are built from smaller units called nucleotides. Their chemistry explains how traits are inherited, how proteins are made, and how cells copy information accurately.
Understanding nucleic acids connects chemistry to genetics, medicine, biotechnology, and evolution.
Each nucleotide contains a sugar, a phosphate group, and a nitrogenous base. In DNA, nucleotides form two antiparallel strands held together by specific base pairing, creating the double helix. In RNA, nucleotides usually form a single strand that can fold into shapes needed for functions such as carrying messages or helping build proteins.
The sugar-phosphate backbone gives nucleic acids direction, while the base sequence carries information.
Key Facts
- A nucleotide = phosphate group + 5-carbon sugar + nitrogenous base.
- DNA contains deoxyribose sugar, while RNA contains ribose sugar.
- DNA base pairing: A pairs with T, and G pairs with C.
- RNA base pairing: A pairs with U, and G pairs with C.
- Complementary DNA strands are antiparallel: one runs 5' to 3' and the other runs 3' to 5'.
- Chargaff's rule for double-stranded DNA: %A = %T and %G = %C.
Vocabulary
- Nucleotide
- A nucleotide is the basic building block of DNA and RNA, made of a phosphate group, a sugar, and a nitrogenous base.
- Sugar-phosphate backbone
- The sugar-phosphate backbone is the repeating chain of sugars and phosphate groups that forms the outer support structure of a nucleic acid strand.
- Nitrogenous base
- A nitrogenous base is a ring-shaped molecule in a nucleotide that helps encode genetic information through its sequence.
- Complementary base pairing
- Complementary base pairing is the specific matching of bases, such as A with T in DNA or A with U in RNA, and G with C.
- Antiparallel
- Antiparallel describes the opposite directions of the two DNA strands, with one strand oriented 5' to 3' and the other 3' to 5'.
Common Mistakes to Avoid
- Confusing DNA and RNA sugars: DNA has deoxyribose, while RNA has ribose with an extra oxygen-containing hydroxyl group.
- Pairing adenine with uracil in DNA: uracil is found in RNA, while thymine is used in DNA.
- Ignoring strand direction: base sequences must be read with 5' and 3' ends in mind because enzymes copy and build nucleic acids directionally.
- Thinking the backbone stores the genetic code: the sugar-phosphate backbone provides structure, but the order of nitrogenous bases carries the information.
Practice Questions
- 1 A DNA strand has the sequence 5'-ATGCCGTA-3'. Write the complementary DNA strand and label its direction.
- 2 A double-stranded DNA sample contains 28% adenine. What percentages of thymine, guanine, and cytosine are present?
- 3 Explain why RNA can leave the nucleus to help make proteins while DNA usually remains stored as the cell's long-term genetic archive.