How does the cell use the code in DNA to build a protein?
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.
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What this topic is asking
Ohio standard B.H.5 states that "the structure of DNA determines the structure of proteins." Ohio's Biology EOC turns this into items on protein synthesis: how a gene's DNA sequence is first copied into mRNA (transcription) and then read to build a chain of amino acids (translation). The crosscutting idea is structure and function: the order of bases in DNA sets the order of amino acids, which sets the shape and job of the protein. Many items give a short DNA or mRNA sequence and ask you to apply the code.
The central idea: DNA codes for protein
A gene is a stretch of DNA that codes for a protein, and proteins do most of the work in a cell (enzymes, structural fibers, receptors). The information flows in one direction: DNA, to mRNA, to protein. Ohio standard B.H.5 is the claim that the sequence of bases in DNA determines the sequence of amino acids in the protein, and therefore the protein's shape and function.
Step 1: Transcription
In transcription, the DNA of a gene unzips and one strand acts as a template. Free RNA nucleotides pair with the exposed DNA bases using the base-pairing rule, with one key change: RNA uses uracil (U) instead of thymine (T), so DNA's adenine pairs with RNA's uracil. The result is a single strand of mRNA that carries a copy of the gene's code. The mRNA then leaves the nucleus and travels to a ribosome in the cytoplasm.
| DNA template base | mRNA base |
|---|---|
| A | U |
| T | A |
| G | C |
| C | G |
Step 2: Translation
The ribosome reads the mRNA three bases at a time. Each group of three bases is a codon, and each codon specifies one amino acid. Molecules of transfer RNA (tRNA) each carry one amino acid and have an anticodon that matches a codon, so they deliver the right amino acid to the right place. As the ribosome moves along the mRNA, the amino acids are joined into a growing chain. A start codon (AUG) begins the protein and a stop codon ends it. The finished chain of amino acids folds into a working protein.
Why the order matters
Because the order of amino acids determines how a protein folds, a change to even one base can change one amino acid and alter the protein. This is the link to mutations and genetic variation: a mutation in DNA can change a codon, change an amino acid, and so change the protein and the trait. It is also why the topic sits under Heredity: the protein-coding instructions are what get passed from parent to offspring.
Try this
Q1. State where transcription and translation each take place. [2]
- Cue. Transcription takes place in the nucleus; translation takes place at a ribosome in the cytoplasm.
Q2. Explain what a codon is and what it specifies. [2]
- Cue. A codon is a group of three mRNA bases; each codon specifies one amino acid in the protein.
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 marksWhich process makes a molecule of messenger RNA (mRNA) from a DNA template? (A) Translation. (B) Replication. (C) Transcription. (D) Respiration.Show worked answer →
A 1-point item on naming the steps.
The correct answer is C. Transcription copies a gene's DNA sequence into mRNA in the nucleus. Translation (A) is the next step, where mRNA is read to build a protein. Replication (B) copies DNA into DNA, and respiration (D) is unrelated.
Order to remember: DNA, then transcription makes mRNA, then translation makes protein. This is Ohio standard B.H.5: DNA structure determines protein structure.
Ohio Biology EOC (style)2 marksA section of DNA template reads 3'-TAC-5'. (a) Write the mRNA codon transcribed from it. (b) Explain how a codon specifies part of a protein.Show worked answer →
A 2-point item applying the code.
(a) 1 point: the mRNA codon is AUG (T pairs with A, A pairs with U because RNA uses uracil, C pairs with G). AUG is the start codon.
(b) 1 point: a codon is a group of three mRNA bases that codes for one amino acid; the order of codons sets the order of amino acids, and the order of amino acids determines the protein's structure and function.
Related dot points
- 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.
- 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.
- 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.
- Describe the major organelles of prokaryotic and eukaryotic cells and explain how each cell structure corresponds to its function (Ohio's Learning Standards for Science, Biology, B.C.3).
A standard-level answer on cell structure for Ohio's Biology EOC: the major organelles as structure-and-function pairs, the difference between prokaryotic and eukaryotic cells, and the extra structures that plant cells have but animal cells do not.
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)