How is the structure of DNA suited to storing and copying genetic information?
Describe the molecular structure of DNA and explain how complementary base pairing allows it to be copied accurately during replication (Ohio's Learning Standards for Science, Biology, B.H.4).
A standard-level answer on DNA structure and replication for Ohio's Biology EOC: the double helix, nucleotides, complementary base pairing (A-T, C-G), the antiparallel strands, and how semi-conservative replication copies DNA accurately.
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What this topic is asking
Ohio standard B.H.4 states that "the molecular structure of DNA determines the genetic information encoded in DNA." Ohio's Biology EOC turns this into items on the structure of the DNA molecule, the base-pairing rule, and how that structure lets DNA be copied accurately before a cell divides. The crosscutting idea is structure and function: the shape of DNA is what makes it a reliable store of information that can be replicated.
The structure of DNA
A single nucleotide has three parts:
- a sugar (deoxyribose),
- a phosphate group,
- one of four nitrogen bases: adenine (A), thymine (T), guanine (G), or cytosine (C).
The sugar and phosphate of each nucleotide link to form the two "rails" of the ladder (the sugar-phosphate backbone), and the bases stick inward to form the "rungs."
Complementary base pairing
The rungs of the ladder are pairs of bases held together by hydrogen bonds, and the pairing is fixed.
- Adenine (A) always pairs with thymine (T).
- Guanine (G) always pairs with cytosine (C).
This is the single most useful fact on the topic. Because each base has exactly one partner, the sequence of one strand completely determines the sequence of the other. If one strand reads A-T-G-C, its partner must read T-A-C-G. The two strands are therefore complementary, and they run in opposite directions (antiparallel), which the EOC may show as 5' and 3' ends.
How DNA is replicated
Before a cell divides (during interphase, see the cell cycle and mitosis), it must copy its DNA so each new cell gets a full set. The process is replication.
- The double helix unzips: the two strands separate as the hydrogen bonds between bases break.
- Each old strand acts as a template. Free nucleotides pair with the exposed bases following the base-pairing rule (A with T, G with C).
- The new nucleotides are joined into a new strand, so each old strand now has a new partner.
- The result is two identical DNA molecules, each with one old strand and one new strand.
Because each new molecule keeps one original strand, replication is called semi-conservative. The base-pairing rule is what makes it accurate: each template can only specify one correct complementary strand.
Why accurate copying matters
Accurate replication is what lets a body grow from one fertilized egg into trillions of cells that all carry the same instructions. It also sets up the contrast with mutations (covered in mutations and genetic variation): a mutation is a rare error or change in the DNA sequence, and because the sequence is normally copied so faithfully, such changes stand out and can be passed on.
Try this
Q1. Name the three parts of a DNA nucleotide. [3]
- Cue. A sugar (deoxyribose), a phosphate group, and a base (adenine, thymine, guanine, or cytosine).
Q2. Explain why DNA replication is described as semi-conservative. [2]
- Cue. The helix unzips and each old strand acts as a template for a new strand, so each new DNA molecule contains one old (conserved) strand and one new strand.
Exam-style practice questions
Practice questions written in the style of ODEW exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Ohio Biology EOC (style)1 marksIn a DNA molecule, which bases pair together? (A) Adenine with cytosine and guanine with thymine. (B) Adenine with thymine and guanine with cytosine. (C) Adenine with guanine and cytosine with thymine. (D) All four bases pair with each other equally.Show worked answer →
A 1-point item on complementary base pairing.
The correct answer is B. In DNA, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This A-T, G-C rule is why the two strands are complementary and why DNA can be copied accurately. The other options break the pairing rule.
A useful memory aid: the letters that look "alike" pair (A with T, the two straight-line letters; G with C, the two curved letters).
Ohio Biology EOC (style)2 marksOne strand of a DNA molecule reads 3'-TAC GGT ACA-5'. (a) Write the sequence of the complementary strand. (b) Explain how base pairing makes accurate copying possible.Show worked answer →
A 2-point item applying base pairing.
(a) 1 point: the complementary strand is ATG CCA TGT (each base paired: T-A, A-T, C-G, G-C, G-C, T-A, A-T, C-G, A-T). Any clear correct complement earns the mark.
(b) 1 point: because each base pairs with only one partner (A with T, G with C), each original strand acts as a template that specifies exactly one matching strand, so the two new molecules are identical copies of the original.
Related dot points
- Explain that genes are segments of DNA located on chromosomes, and distinguish between genes, alleles, genotype, and phenotype (Ohio's Learning Standards for Science, Biology, B.H.1).
A standard-level answer on chromosomes, genes, and alleles for Ohio's Biology EOC: how DNA is packaged into chromosomes, the difference between a gene and an allele, homologous chromosomes, and the meaning of genotype and phenotype.
- Use models to explain how the structure of DNA determines the structure of proteins through transcription and translation (Ohio's Learning Standards for Science, Biology, B.H.5).
A standard-level answer on protein synthesis for Ohio's Biology EOC: transcription of DNA into mRNA, translation of mRNA into a protein at the ribosome, codons and the genetic code, and the role of tRNA and amino acids.
- Explain that gene expression is regulated so different cells use different genes, and that traits result from inherited genes interacting with the environment (Ohio's Learning Standards for Science, Biology, B.H.3).
A standard-level answer on gene expression for Ohio's Biology EOC: how regulation lets cells with the same DNA specialize, why genes are switched on and off, and how the environment interacts with genes to shape the phenotype.
- Explain how mutations change the DNA sequence and therefore proteins and traits, and how they can be harmful, neutral, or beneficial (Ohio's Learning Standards for Science, Biology, B.H.4 and B.H.5).
A standard-level answer on mutations for Ohio's Biology EOC: what a mutation is, the main types (substitution, insertion, deletion), how a changed base can change a protein, mutagens, and why mutations can be harmful, neutral, or beneficial.
- Use a model of the cell cycle to explain how cell division and differentiation support growth, maintenance, and repair, and how a loss of control leads to cancer (Ohio's Learning Standards for Science, Biology, B.C.1).
A standard-level answer on the cell cycle and mitosis for Ohio's Biology EOC: interphase and the phases of mitosis (PMAT), how mitosis supports growth and repair, cell differentiation, and how a mutation in cell-cycle genes leads to cancer.
Sources & how we know this
- Ohio's Learning Standards and Model Curriculum for Science — Ohio Department of Education and Workforce (2022)
- Biology State-Tested Course Resources — Ohio Department of Education and Workforce (2024)