DNA structure
Summary
DNA is a double helix of two antiparallel strands, each a sugar-phosphate backbone carrying the bases A, T, G and C. A pairs with T and G with C by hydrogen bonds, so the strands are complementary and, in double-stranded DNA, A = T and G = C.
Detail
Meaning: DNA is a polymer of deoxyribonucleotides, each made of deoxyribose, a phosphate, and a base (purines A and G; pyrimidines C and T). Phosphodiester bonds link the 3′ carbon of one sugar to the 5′ phosphate of the next, so each strand has a 5′ end and a 3′ end. Two strands run antiparallel and coil into a right-handed double helix (B-DNA is the common form). The charged backbone faces outward; the bases stack inside, paired by hydrogen bonds (A–T two, G–C three) and held by base stacking.
Equation: %A = %T and %G = %C (Chargaff's rules), where each term is the percent of all bases in double-stranded DNA (dimensionless).
On the MCAT: Given the percentage of one base, find the others: 20% A means 20% T and 30% each G and C. Compare melting temperatures: more G–C pairs means a higher Tm, and denaturation raises absorbance at 260 nm. Strand direction matters for replication: new DNA is built 5′ → 3′.
Common confusion: Strands are complementary, not identical, and they are antiparallel, not parallel. Heating breaks hydrogen bonds between bases, not the covalent phosphodiester bonds. Chargaff's rules hold for double-stranded DNA, not single-stranded DNA.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.