How does the structure of DNA let it store information and be copied accurately?
Describe the structure of DNA (the antiparallel double helix and base pairing) and explain how complementary base pairing allows DNA to be replicated accurately (Virginia 2018 Biology SOL BIO.5.a).
A SOL-level answer on DNA for the Virginia Biology EOC: the double helix, base pairing, why DNA is a stable information store, and how complementary base pairing allows accurate replication.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
Virginia Biology SOL standard BIO.5.a states that DNA has structure and is the foundation for protein synthesis. This page covers the structure and how it is copied; the next page covers how it directs protein synthesis. The Biology EOC expects you to describe the double helix and base pairing, and, above all, to explain how that structure lets DNA be replicated accurately so genetic information passes unchanged to new cells. A common technology-enhanced item gives one strand and asks for its complement.
The structure of DNA
Picture a twisted ladder: the sugar-phosphate backbones are the two sides, and the base pairs reaching across are the rungs. The two strands are antiparallel, meaning they run in opposite directions. The order of the bases along a strand is the genetic information, like letters spelling out instructions.
Base pairing
Complementary base pairing is the single most important fact about DNA for the EOC. It is the pattern that makes DNA a reliable information store and that allows it to be copied. When a question gives you one strand and asks for the other, pair each base by the rule, A to T, T to A, G to C, C to G.
Why DNA is a good information store
Because the molecule is double-stranded and the bases pair in only one way, the information is held twice, once on each strand. This makes it stable and means damage to one strand can be repaired using the intact strand as a guide. The sugar-phosphate backbone is chemically sturdy, suiting DNA to its long-term, archival role of storing the instructions for making proteins.
Replication: copying DNA accurately
Before a cell divides, its DNA must be replicated so each new cell gets a complete copy. The process relies entirely on base pairing:
- The double helix unzips as the hydrogen bonds between the paired bases break, separating the two strands.
- Each old strand acts as a template: free nucleotides pair with the exposed bases by the A-T, G-C rule.
- The new nucleotides are joined into a new strand, so each old strand now has a new complementary partner.
The result is two identical double helices, each with one old strand and one new strand. Because pairing is specific, the copies match the original, which is why genetic information passes on unchanged. This accurate copying must happen before mitosis (see the cell cycle and mitosis).
Try this
Q1. Write the complementary DNA strand for the sequence T-A-C-G-G-A. [2]
- Cue. A-T-G-C-C-T (T pairs with A, A with T, C with G, G with C).
Q2. Explain why complementary base pairing allows DNA to be copied accurately. [2]
- Cue. Each base pairs with only one partner, so each separated strand specifies its new partner exactly, producing two identical copies.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Biology SOL (2023 released style)1 marksIn a DNA molecule, adenine (A) always pairs with which base? (A) guanine. (B) cytosine. (C) thymine. (D) another adenine.Show worked answer →
A 1-point multiple-choice item on base pairing.
The correct answer is C. In DNA, adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). A and D pair the wrong bases, and B is the partner of guanine, not adenine.
The test rewards the base-pairing rule: A with T, G with C.
VA Biology SOL (2024 released style)2 marksOne strand of a DNA molecule reads A-T-G-C-C-A. (a) Write the base sequence of the complementary strand. (b) Explain how this base-pairing rule allows DNA to be copied accurately.Show worked answer →
A 2-point item on complementary strands and replication.
(a) 1 point: T-A-C-G-G-T (A pairs with T, T with A, G with C, C with G).
(b) 1 point: because each base pairs with only one partner, each separated strand acts as a template that specifies its new partner exactly, so the two new molecules are identical copies of the original.
Markers reward the correct complement and linking the pairing rule to templated, accurate copying.
Related dot points
- Explain protein synthesis: how transcription copies DNA into mRNA and translation reads codons at the ribosome to build a protein, linking the DNA base sequence to the trait (Virginia 2018 Biology SOL BIO.5.a, supporting BIO.2.d).
A SOL-level answer on protein synthesis for the Virginia Biology EOC: transcription of DNA into mRNA, translation of codons at the ribosome, and how the DNA base sequence determines the protein and the trait.
- Explain that a mutation is a change in the DNA base sequence with harmful, beneficial, or neutral effects, and that genetic variation (from mutation and sexual reproduction) is important to the survival of a species (Virginia 2018 Biology SOL BIO.5.c).
A SOL-level answer on mutations for the Virginia Biology EOC: what a mutation is, its harmful, beneficial, or neutral effects, the difference between body-cell and gamete mutations, and why genetic variation matters for survival.
- Describe the cell cycle and mitosis as the process that produces two genetically identical cells for growth and repair, and relate uncontrolled cell division to cancer (Virginia 2018 Biology SOL BIO.3.c, BIO.3.d).
A SOL-level answer on the cell cycle for the Virginia Biology EOC: interphase and the stages of mitosis, why the two daughter cells are identical, the role of growth and repair, and how loss of control leads to cancer.
- Describe the four classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids), their monomers, and their roles in maintaining life processes (Virginia 2018 Biology SOL BIO.2.b).
A SOL-level answer on biological macromolecules for the Virginia Biology EOC: carbohydrates, lipids, proteins, and nucleic acids, their monomers and functions, and how dehydration synthesis and hydrolysis build and break them.
- Use alleles, genotype and phenotype, dominant and recessive, and Punnett squares to predict the genotype and phenotype ratios and probabilities of monohybrid crosses (Virginia 2018 Biology SOL BIO.5.b).
A SOL-level answer on inheritance for the Virginia Biology EOC: alleles, genotype and phenotype, dominant and recessive traits, and using Punnett squares to predict ratios and probabilities of monohybrid crosses.
Sources & how we know this
- 2018 Science Standards of Learning (Biology) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)