How is DNA structured, and how does it copy itself so accurately?
Develop and use a model of DNA's structure to explain how the sequence of nucleotides stores information and how DNA replicates (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).
A standard-level answer on DNA for the Tennessee Biology I EOC: the double-helix structure, nucleotides and base pairing (A-T, C-G), how the base sequence stores information, and how semiconservative replication copies DNA accurately.
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What this topic is asking
The Tennessee LS3 standards (heredity) ask you to model the structure of DNA and explain how that structure lets DNA store information and copy itself. For the Biology I EOC that means knowing the double helix, the nucleotide building blocks, the base-pairing rule (A-T, C-G), how the sequence of bases is the genetic code, and how replication uses each strand as a template. Items often give you one strand and ask for the complementary strand, so base pairing is the key skill.
The structure of DNA
There are four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). A useful way to remember the groups: A and G are purines (larger, double-ring), and C and T are pyrimidines (smaller, single-ring). But the most testable fact is how they pair.
Base pairing: the central rule
This rule is the most frequently tested DNA fact and the basis of both copying DNA and reading it to make proteins. Because A only ever pairs with T and C only ever with G, the amount of A in a DNA sample equals the amount of T, and the amount of C equals the amount of G.
The base sequence is the information
The order (sequence) of the bases along a strand is the genetic code. Just as the order of letters spells different words, the order of A, T, C, and G spells out the instructions for building the cell's proteins. A gene is a section of DNA that codes for a particular protein (or trait). This is the link to protein synthesis: the base sequence is read and translated to assemble a chain of amino acids.
Replication: copying DNA accurately
Before a cell divides, it must copy its DNA so each new cell gets a full set. This is replication, and it relies on base pairing:
- The double helix unwinds and the two strands separate, breaking the hydrogen bonds between the base pairs.
- Each separated strand acts as a template. Free nucleotides pair with the exposed bases following the A-T and C-G rule (an enzyme, DNA polymerase, builds the new strand).
- The result is two identical DNA molecules, each made of one original strand and one new strand.
Because each new molecule keeps one old strand, replication is described as semiconservative. The accuracy comes directly from base pairing: each template strand specifies exactly one possible new strand.
Try this
Q1. State the complementary base for each: A, C, G, T. [2]
- Cue. A pairs with T; C pairs with G; G pairs with C; T pairs with A.
Q2. A sample of DNA is found to be 30 percent adenine. State the percentage of thymine and explain why. [2]
- Cue. Thymine is also 30 percent, because A always pairs with T, so the amounts of A and T are equal.
Exam-style practice questions
Practice questions written in the style of TDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
TN Biology I EOC (2023 released style)1 marksIn a DNA molecule, adenine (A) always pairs with: (A) cytosine (C). (B) guanine (G). (C) thymine (T). (D) another adenine (A).Show worked answer →
A 1-point multiple-choice item on base pairing.
The correct answer is C. In DNA, the complementary base-pairing rule is A with T and C with G. So adenine always pairs with thymine. Cytosine pairs with guanine (which would make B and A wrong), and a base never pairs with the same base (D).
Base pairing is the most-tested DNA fact, so memorize A-T and C-G.
TN Biology I EOC (2024 released style)2 marksOne strand of a DNA molecule reads 5'-A T G C C A-3'. (a) Write the sequence of the complementary strand. (b) Explain how this complementary pairing lets DNA be copied accurately.Show worked answer →
A 2-point item applying base pairing and explaining replication.
(a) 1 point: pairing A-T and C-G, the complementary strand is T A C G G T (reading along the partner strand).
(b) 1 point: because each base pairs with only one partner, each original strand acts as a template that specifies exactly one new strand, so the two new DNA molecules are identical to the original. This is why replication is accurate.
Markers reward a correctly paired strand and the idea that each strand is a template that determines the new strand.
Related dot points
- Construct an explanation of how genetic information in DNA is expressed as proteins through transcription and translation (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).
A standard-level answer on protein synthesis for the Tennessee Biology I EOC: transcription of DNA into mRNA, the codon and the genetic code, translation at the ribosome using tRNA, and how the base sequence determines the amino-acid sequence.
- Construct an explanation of how mutations in DNA can change proteins and traits, and may be harmful, beneficial, or neutral (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).
A standard-level answer on mutations for the Tennessee Biology I EOC: what a mutation is, the types (substitution, insertion, deletion), how a change in DNA changes a protein, why mutations can be harmful, beneficial, or neutral, and their role as the source of new variation.
- Use a model of meiosis to explain how sexual reproduction halves the chromosome number and creates genetic variation through crossing over, independent assortment, and random fertilization (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).
A standard-level answer on meiosis for the Tennessee Biology I EOC: how meiosis produces four haploid gametes from one diploid cell, how it differs from mitosis, and the three sources of genetic variation it provides (crossing over, independent assortment, and random fertilization).
- Use mathematics and Punnett squares to predict the genotype and phenotype ratios and probabilities of monohybrid crosses (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).
A standard-level answer on inheritance for the Tennessee Biology I EOC: alleles, genotype and phenotype, dominant and recessive, and using Punnett squares to predict the ratios and probabilities of monohybrid crosses.
- Use a model of the cell cycle to explain how mitosis produces identical cells for growth and repair, and how a loss of cycle control leads to cancer (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).
A standard-level answer on the cell cycle for the Tennessee Biology I EOC: interphase and the phases of mitosis (PMAT), how mitosis makes two genetically identical cells for growth and repair, the role of cell-cycle checkpoints, and how a mutation that disables them leads to cancer.
Sources & how we know this
- Tennessee Academic Standards for Science — Tennessee Department of Education (2022)
- TNReady EOC Science Item Release (Biology and Chemistry) — Tennessee Department of Education (2018)