How is the structure of DNA suited to storing information and being copied accurately?
Identify the components of DNA, describe the structure of the double helix and base pairing, and explain how DNA is replicated accurately before cell division (TEKS Biology, Reporting Category 2; structure and function; patterns).
A TEKS-level answer on DNA for the Texas STAAR Biology EOC: the components of a nucleotide, the double helix and complementary base pairing, and how DNA replication produces two identical copies before a cell divides.
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What this topic is asking
The Biology TEKS ask you to identify the parts of DNA, describe the double helix and base pairing, and explain how DNA is replicated accurately. For STAAR Reporting Category 2 you need to relate the structure of DNA to its two jobs (storing information and being copied), and to use the base-pairing rule to complete a strand. This is a structure and function and patterns topic.
The components of DNA
The sugar and phosphate of each nucleotide join to form the two backbones of the molecule, and the bases stick inward, where they pair across the two strands.
The double helix and base pairing
The order of the bases along a strand is the genetic code, the information that ultimately determines an organism's traits. A common STAAR task gives you one strand and asks for the complementary strand, which simply means applying the pairing rule to each base.
The two strands also run in opposite directions (they are antiparallel), and the consistent A-T and G-C pairing keeps the helix a uniform width all the way along, because a larger base always pairs with a smaller one. This regular structure is what lets the molecule be both a stable store of information and an easy-to-copy template. In a eukaryotic cell, the long DNA molecule is wound up tightly with proteins into structures called chromosomes, which is how a large amount of DNA fits inside the nucleus and is organized for division.
How DNA is replicated
Before a cell divides it must copy all of its DNA so each new cell receives a complete set. Replication works like this:
- The double helix unzips, and the two strands separate.
- Each separated strand acts as a template.
- Free nucleotides pair with the exposed bases following the rule (A with T, G with C).
- The result is two identical DNA molecules, each with one original strand and one new strand.
Because each base pairs with only one partner, the copy is accurate. This accuracy is what lets genetic information pass unchanged from a cell to its descendants, and it is the reason mitosis and meiosis must be preceded by replication.
Try this
Q1. State the base-pairing rule in DNA. [1]
- Cue. Adenine pairs with thymine (A-T); guanine pairs with cytosine (G-C).
Q2. A DNA strand reads T-A-C-G-G-T. Write the complementary strand. [2]
- Cue. A-T-G-C-C-A (pair each base with its partner).
Exam-style practice questions
Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
STAAR Biology (2023 released style)1 marksOne strand of a DNA molecule has the base sequence A-T-G-C. What is the sequence of the complementary strand? (A) A-T-G-C. (B) T-A-C-G. (C) U-A-C-G. (D) G-C-A-T.Show worked answer →
A 1-point multiple-choice item on base pairing.
The correct answer is B. In DNA, adenine pairs with thymine and guanine pairs with cytosine, so A-T-G-C pairs with T-A-C-G. C is wrong because uracil belongs to RNA, not DNA. The other options do not follow the pairing rule.
Pair each base with its partner: A with T, G with C.
STAAR Biology (2024 SCR style)2 marksExplain how the structure of DNA allows it to be copied accurately during replication. Support your answer with reasoning about base pairing.Show worked answer →
A 2-point short constructed response on replication.
Full credit (2 points): DNA is made of two complementary strands held by base pairing (A with T, G with C). During replication the strands separate, and because each base pairs with only one partner, each strand acts as a template that specifies the exact sequence of a new strand, producing two identical double helices.
Partial credit (1 point): states base pairing without linking it to the accuracy of copying. The science is scored.
Related dot points
- Describe how the information in DNA is used to build proteins through transcription and translation, and explain how the order of bases determines the order of amino acids (TEKS Biology, Reporting Category 2; cause and effect; structure and function).
A TEKS-level answer on protein synthesis for the Texas STAAR Biology EOC: transcription of DNA into mRNA, translation of codons into amino acids at the ribosome, and how the base sequence determines the protein and the trait.
- Recognize the types of gene mutations and explain how a change in the DNA base sequence may be harmful, beneficial, or neutral and how it can be inherited (TEKS Biology, Reporting Category 2; cause and effect; stability and change).
A TEKS-level answer on mutations for the Texas STAAR Biology EOC: what a mutation is, substitution, insertion, and deletion, why an effect can be harmful, beneficial, or neutral, and how mutations in gametes are inherited and supply variation.
- Explain the role of meiosis in producing gametes with half the chromosome number and in generating genetic variation, and contrast meiosis with mitosis (TEKS Biology, Reporting Category 2; patterns; cause and effect).
A TEKS-level answer on meiosis for the Texas STAAR Biology EOC: chromosomes and the role of meiosis in halving the chromosome number, how crossing over and independent assortment create variation, and how meiosis differs from mitosis.
- Apply Mendel's laws and use Punnett squares to predict the genotype and phenotype ratios of monohybrid crosses, and identify patterns of inheritance including dominant, recessive, codominant, and incomplete dominance (TEKS Biology, Reporting Category 2; patterns; using mathematics).
A TEKS-level answer on inheritance for the Texas STAAR Biology EOC: alleles, genotype and phenotype, dominant and recessive traits, using Punnett squares to predict ratios and probabilities, and codominance and incomplete dominance.
- Describe applications of DNA technology, including gel electrophoresis, DNA fingerprinting, recombinant DNA, and genetically modified organisms, and evaluate their benefits and concerns (TEKS Biology, Reporting Category 2; cause and effect; structure and function).
A TEKS-level answer on biotechnology for the Texas STAAR Biology EOC: gel electrophoresis and DNA fingerprinting, recombinant DNA and genetic engineering, genetically modified organisms, and the benefits and concerns of these tools.
Sources & how we know this
- Texas Essential Knowledge and Skills for Science (Biology) — Texas Education Agency (2024)
- STAAR Biology Assessed Curriculum — Texas Education Agency (2024)