How is genetic information stored in the structure of DNA, and how is it copied so faithfully before a cell divides?
Describe the structure of DNA as a double helix of nucleotide base pairs and explain how complementary base pairing allows DNA to be copied accurately during replication (MA STE HS-LS1-1, HS-LS3-1, structure and function).
A standard-level answer on DNA structure and replication for the Massachusetts High School Biology MCAS: the double helix, the four bases and complementary pairing, and how DNA is copied accurately before cell division under HS-LS3.
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What this topic is asking
The Massachusetts STE framework (HS-LS3-1 and HS-LS1-1) wants you to connect the structure of DNA to its two jobs: storing genetic information and being copied accurately. On the High School Biology MCAS, this content is usually tested with a model of DNA or a base sequence, and you are asked to apply the base-pairing rule, complete a strand, or explain how the structure makes faithful copying possible. The crosscutting concepts are structure and function and stability and change.
The structure of DNA
DNA is shaped like a twisted ladder, the famous double helix. The two long sides of the ladder are made of alternating sugar and phosphate groups (the sugar-phosphate backbone). The rungs are pairs of bases reaching in from each side and joining in the middle. Because the structure is the same in every organism, DNA is a universal information store, which is one of the strongest pieces of evidence for common ancestry (see evidence for evolution).
The base-pairing rule
The four bases do not pair at random. They follow complementary base pairing:
- Adenine (A) pairs with thymine (T)
- Guanine (G) pairs with cytosine (C)
So if one strand reads A-G-C-T, the other strand must read T-C-G-A. This is the single most useful fact in the topic, because almost every DNA question on the MCAS asks you to use it: complete a strand, count bases, or explain accuracy. The pairing is specific because the shapes and bonding of the bases only fit one partner, another example of structure determining function.
How DNA is copied: replication
Replication uses the base-pairing rule to guarantee an accurate copy. The steps are:
- Unzip. The double helix unwinds and the two strands separate, breaking the bonds between the base pairs.
- Template. Each separated strand acts as a template. Free nucleotides in the nucleus pair with the exposed bases following the rule (A with T, G with C).
- Join. Enzymes link the new nucleotides into a new strand alongside each template.
- Result. Each new DNA molecule has one original strand and one new strand, and both molecules are identical to the starting molecule.
Because each strand specifies its partner exactly, the two copies match the original. This faithful copying keeps genetic information stable as cells divide, which is essential for growth and repair (covered in mitosis and the cell cycle).
Try this
Q1. State the base-pairing rule for DNA. [2]
- Cue. Adenine pairs with thymine (A-T); guanine pairs with cytosine (G-C).
Q2. Explain why each strand of DNA can act as a template during replication. [2]
- Cue. Each base pairs with only one partner, so an existing strand specifies the exact sequence of bases for the new strand built alongside it.
Exam-style practice questions
Practice questions written in the style of MA DESE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
HS Biology MCAS (style)3 marksA model shows one strand of DNA 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 item on structure and function with the practice of developing and using models.
(a) 1 point: T-A-C-G-G-T (A pairs with T, G pairs with C).
(b) 1 point: complementary base pairing: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).
(c) 1 point: because each base pairs with only one partner, each separated strand acts as a template that specifies the exact sequence of the new strand, so the copy matches the original. Markers reward linking the pairing rule to the template producing an exact copy.
HS Biology MCAS (style)2 marksBefore a cell divides, it copies its DNA. (a) Name this process. (b) Explain why it is important that the two new cells each receive an identical copy of the DNA.Show worked answer →
A 2-point item on stability and change.
(a) 1 point: DNA replication.
(b) 1 point: each new cell needs the full, identical set of genetic instructions so it can make the right proteins and function correctly; an identical copy keeps the genetic information stable across cell divisions. Markers reward the idea of a complete, identical set of instructions.
Related dot points
- Explain how a gene's base sequence is transcribed into messenger RNA and translated into a sequence of amino acids, and how this gene-to-protein pathway produces an organism's traits (MA STE HS-LS1-1, HS-LS3-1).
A standard-level answer on protein synthesis for the Massachusetts High School Biology MCAS: transcription of DNA into messenger RNA, translation into amino acids using codons, and how the gene-to-protein pathway produces traits under HS-LS3.
- Describe the cell cycle and mitosis as the process that produces two genetically identical daughter cells, and explain its role in growth, repair, and asexual reproduction (MA STE HS-LS1-4, HS-LS3-2 supporting).
A standard-level answer on mitosis and the cell cycle for the Massachusetts High School Biology MCAS: how a cell copies its DNA and divides into two genetically identical cells, and the role of mitosis in growth, repair, and asexual reproduction under HS-LS1.
- Explain how meiosis produces gametes with half the chromosome number and how meiosis and fertilization, together with mutation, create genetic variation among offspring (MA STE HS-LS3-2, HS-LS3-3).
A standard-level answer on meiosis for the Massachusetts High School Biology MCAS: how meiosis makes gametes with half the chromosome number, and how meiosis, fertilization, and mutation create genetic variation in offspring under HS-LS3.
- Use the rules of inheritance, including dominant and recessive alleles, genotype and phenotype, and Punnett squares, to predict the probability of traits in offspring and apply statistical reasoning to genetic crosses (MA STE HS-LS3-3, using mathematics).
A standard-level answer on inheritance for the Massachusetts High School Biology MCAS: dominant and recessive alleles, genotype and phenotype, how to use a Punnett square, and the probability reasoning behind genetic ratios under HS-LS3.
- Explain what a mutation is, how mutations change proteins and can be harmful, neutral, or beneficial, and describe examples of biotechnology such as selective breeding and genetic engineering (MA STE HS-LS3-2, HS-LS3-3 supporting).
A standard-level answer on mutations and biotechnology for the Massachusetts High School Biology MCAS: what a mutation is, how it changes proteins and can be harmful, neutral, or beneficial, and examples of selective breeding and genetic engineering under HS-LS3.
Sources & how we know this
- Massachusetts Science and Technology/Engineering Curriculum Framework (2016) — Massachusetts Department of Elementary and Secondary Education (2016)
- Science and Technology/Engineering (STE) Test Design and Development — Massachusetts Department of Elementary and Secondary Education (2024)