DNA is the molecule that stores genetic information in nearly all living organisms. Its structure explains how cells copy instructions, pass traits to offspring, and build proteins. The famous double helix shape is made from two long strands that twist around each other.
Understanding DNA structure helps connect chemistry to heredity, evolution, biotechnology, and medicine.
Each DNA strand has a sugar-phosphate backbone on the outside and nitrogen bases pointing inward. The bases pair in a specific way: adenine pairs with thymine, and guanine pairs with cytosine. The two strands run in opposite directions, called antiparallel orientation, which is important for DNA replication.
The twisting helix creates major and minor grooves where proteins can bind and read the genetic code.
Key Facts
- DNA stands for deoxyribonucleic acid.
- A nucleotide = phosphate group + deoxyribose sugar + nitrogen base.
- Complementary base pairing: A pairs with T, and G pairs with C.
- A-T has 2 hydrogen bonds, while G-C has 3 hydrogen bonds.
- DNA strands are antiparallel: one strand runs 5' to 3', and the other runs 3' to 5'.
- Chargaff's rule for double-stranded DNA: %A = %T and %G = %C.
Vocabulary
- Double helix
- The twisted ladder shape of DNA formed by two strands wrapped around each other.
- Nucleotide
- The basic building block of DNA, made of a phosphate group, a deoxyribose sugar, and one nitrogen base.
- Sugar-phosphate backbone
- The repeating chain of sugars and phosphate groups that forms the outside support structure of each DNA strand.
- Complementary base pairing
- The rule that adenine pairs only with thymine and guanine pairs only with cytosine in DNA.
- Antiparallel
- The arrangement in which the two DNA strands run in opposite directions, one 5' to 3' and the other 3' to 5'.
Common Mistakes to Avoid
- Pairing A with G or C with T is wrong because DNA bases pair by shape and hydrogen bonding rules: A pairs with T, and G pairs with C.
- Putting the bases on the outside is wrong because the sugar-phosphate backbones form the outer rails while the bases point inward to form the rungs.
- Drawing both strands in the same direction is wrong because DNA strands are antiparallel, with one strand running 5' to 3' and the other running 3' to 5'.
- Treating all base pairs as equally strong is wrong because A-T has 2 hydrogen bonds, while G-C has 3, making G-C-rich regions harder to separate.
Practice Questions
- 1 A DNA strand has the sequence 5'-A T G C C A T-3'. Write the complementary strand and label its direction.
- 2 A double-stranded DNA molecule contains 30% adenine. What percentages of thymine, guanine, and cytosine does it contain?
- 3 Explain why the specific pairing of A with T and G with C allows DNA to be copied accurately during replication.