How is the structure of DNA suited to storing genetic information and copying it accurately?
Describe the structure of DNA (the antiparallel double helix and base pairing) and explain how complementary base pairing allows DNA to be copied accurately during replication (NYSSLS LS3, structure and function; patterns).
A NYSSLS-level answer on DNA for the New York Life Science: Biology Regents: the double-helix structure, base pairing, why DNA is a stable store of information, and how complementary base pairing allows accurate replication.
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What this topic is asking
NYSSLS LS3 (Heredity) begins with the molecule that carries genetic information: DNA. The Life Science: Biology Regents wants you to describe its structure and, above all, to explain how that structure lets DNA be copied accurately. The crosscutting concepts are structure and function and patterns (the regular base-pairing rule).
The structure of DNA
The molecule is a double helix, like a twisted ladder. The sides of the ladder are sugar-phosphate backbones, and the rungs are pairs of bases reaching across from the two strands. The two strands are antiparallel, running in opposite directions.
Base pairing
This complementary pairing is the single most important fact about DNA for the exam. It is the pattern that makes the molecule a reliable information store and that allows it to be copied. A common cluster task gives you one strand and asks for its partner: pair each base by the rule.
Why DNA is a good information store
The order of the bases along a strand is the genetic information, like letters spelling out instructions. Because the molecule is double-stranded and the bases pair in only one way, the information is held twice (once on each strand), which makes it stable and allows damage to be repaired using the intact strand. The sugar-phosphate backbone is chemically stable, suiting DNA to its long-term, archival role.
Replication: copying DNA accurately
Before a cell divides, its DNA must be replicated so that each new cell gets a full, correct copy. The process relies entirely on base pairing:
- The double helix unzips as the hydrogen bonds between the bases break, separating the two strands.
- Each 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 is essential before mitosis (see mitosis and the cell cycle).
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 identical copies.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Life Science sample, 2024)3 marksA diagram shows one strand of a DNA molecule with the base sequence A-T-G-C-C-A. (a) Write the base sequence of the complementary strand. (b) State the rule you used. (c) Explain how this base-pairing rule allows DNA to be copied accurately.Show worked answer →
A 3-point constructed-response item assessing patterns and structure and function.
(a) 1 point: T-A-C-G-G-T (A pairs with T, G pairs with C).
(b) 1 point: complementary base pairing, adenine with thymine and guanine with cytosine.
(c) 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.
Markers reward the correct complement and linking the pairing rule to templated, accurate copying.
Regents (Life Science CR, 2025)2 marksDNA is described as a stable molecule that stores genetic information. (a) State what holds the two strands of the DNA double helix together. (b) Explain why it is important that DNA is copied accurately before a cell divides.Show worked answer →
A 2-point item on DNA structure and the importance of accurate replication.
(a) 1 point: hydrogen bonds between the complementary bases (which pair A with T and G with C).
(b) 1 point: each new cell must receive a complete and correct copy of the genetic information so it has the right instructions to make its proteins and function normally; errors could disrupt those instructions.
Markers reward "hydrogen bonds between complementary bases" and the need for each new cell to get a correct copy.
Related dot points
- Explain how genes are expressed through transcription and translation, how the sequence of DNA bases codes for the sequence of amino acids in a protein, and why this links genotype to phenotype (NYSSLS LS3, structure and function; cause and effect).
A NYSSLS-level answer on protein synthesis for the New York Life Science: Biology Regents: how transcription makes mRNA from DNA, how translation reads codons to build a protein, and how the base sequence of a gene determines a protein and so a trait.
- Explain how mitosis and the cell cycle produce two genetically identical cells, describe its role in growth, repair and asexual reproduction, and explain how uncontrolled division leads to cancer (NYSSLS LS1 and LS3, stability and change; cause and effect).
A NYSSLS-level answer on mitosis for the New York Life Science: Biology Regents: the cell cycle, how mitosis produces two identical cells, its role in growth, repair and asexual reproduction, and what happens when division is not controlled.
- Explain how meiosis produces gametes with half the chromosome number and generates genetic variation through crossing over and independent assortment, and how fertilization restores the chromosome number (NYSSLS LS3, patterns; cause and effect).
A NYSSLS-level answer on meiosis for the New York Life Science: Biology Regents: how meiosis halves the chromosome number to make gametes, how crossing over and independent assortment create variation, and how fertilization restores the chromosome number.
- Explain how mutations change the DNA sequence and their possible effects, and describe how genetic technologies such as selective breeding and genetic engineering are used (NYSSLS LS3, cause and effect; structure and function).
A NYSSLS-level answer on mutations and biotechnology for the New York Life Science: Biology Regents: what mutations are and their effects, how they create variation, and how selective breeding and genetic engineering are used and assessed.
- Explain how carbohydrates, lipids, proteins and nucleic acids are constructed from monomers and how the structure of each macromolecule relates to its function (NYSSLS LS1, structure and function).
A NYSSLS-level answer on the chemistry of life for the New York Life Science: Biology Regents: the role of water, the four classes of biological molecule, how monomers join into polymers, and how structure relates to function.
Sources & how we know this
- New York State P-12 Science Learning Standards (Life Science) — New York State Education Department (2016)
- Educator Guide to the Regents Examination in Life Science: Biology — New York State Education Department (2025)