How does the cell turn the DNA code into a protein?
Construct an explanation, based on evidence, for how the structure of DNA determines the structure of proteins through transcription and translation (Louisiana Student Standards for Science, High School Biology, HS-LS1-1).
A standard-level answer on protein synthesis for Louisiana LEAP 2025 Biology: transcription of DNA into mRNA, translation at the ribosome using codons and tRNA, and how the base sequence determines the protein.
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What this topic is asking
Louisiana's LS1 standards (HS-LS1-1) ask you to explain how the structure of DNA determines the structure of proteins, which carry out the functions of life. For LEAP 2025 Biology you should know the two steps of protein synthesis, transcription (DNA to mRNA) and translation (mRNA to protein), the roles of mRNA, codons, tRNA, and the ribosome, and how the base sequence sets the amino-acid sequence. The test often asks you to count codons or trace how a base change affects the protein.
The big idea: DNA codes for proteins
The reason this matters is the LS3 cause-and-effect chain: DNA to protein to trait. Proteins (including enzymes) do most of the work in a cell, so by controlling which proteins are made, the DNA controls the organism's characteristics.
Transcription: DNA to mRNA
In transcription, which takes place in the nucleus:
- The DNA double helix opens at the gene.
- One strand acts as a template, and a complementary strand of mRNA is built, following the pairing rule but with uracil (U) in place of thymine (so DNA's A pairs with mRNA's U).
- The mRNA then leaves the nucleus and travels to a ribosome.
The mRNA is a portable copy of the gene's instructions that can be carried out of the nucleus to where proteins are made.
Translation: mRNA to protein
So the flow is: DNA base sequence sets the mRNA codons, the codons specify amino acids, and the chain of amino acids folds into the finished protein.
From base sequence to function
The whole point is that the order of bases in DNA determines the order of amino acids in the protein, and the order of amino acids determines the protein's shape, which determines its function. This is why a change in even a single DNA base (a mutation) can change a codon, swap an amino acid, alter the protein's shape, and so change a trait, the bridge to the mutations topic.
Try this
Q1. State what transcription produces and where in the cell it occurs. [2]
- Cue. Transcription produces messenger RNA (mRNA); it occurs in the nucleus.
Q2. A piece of mRNA has 15 bases. State how many amino acids it codes for and explain your reasoning. [2]
- Cue. amino acids, because each codon is three bases and codes for one amino acid.
Exam-style practice questions
Practice questions written in the style of LDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
LA LEAP 2025 Biology (style)1 marksDuring transcription, a gene's DNA is copied into: (A) more DNA. (B) messenger RNA (mRNA). (C) a protein directly. (D) glucose.Show worked answer →
A 1-point selected-response item on the first step of protein synthesis.
The correct answer is B. Transcription copies a gene's DNA into messenger RNA (mRNA) in the nucleus. Translation later builds the protein from the mRNA. RNA uses uracil in place of thymine. Copying DNA into more DNA is replication (A), not transcription.
Transcription makes mRNA; translation makes the protein.
LA LEAP 2025 Biology (style)2 marksA length of mRNA contains 9 bases. (a) State how many codons this is and how many amino acids it codes for. (b) Explain why a change in one DNA base can change the protein produced.Show worked answer →
A 2-point constructed-response item linking the code to the protein.
(a) 1 point: 9 bases is codons, coding for 3 amino acids, because each codon is three bases and codes for one amino acid.
(b) 1 point: the DNA base sequence sets the mRNA codons, which set the order of amino acids; changing a base can change a codon, so a different amino acid may be added, altering the protein's shape and function.
Markers reward the calculation in (a) and the gene-to-protein cause-and-effect chain in (b).
Related dot points
- Ask questions and construct an explanation about how the structure of DNA stores genetic information and is copied accurately by replication (Louisiana Student Standards for Science, High School Biology, HS-LS3-1).
A standard-level answer on DNA for Louisiana LEAP 2025 Biology: the double helix and nucleotides, the base-pairing rule (A-T, C-G), how the base sequence stores information, and how DNA replication copies it accurately.
- Make and defend a claim, based on evidence, that mutations and new genetic combinations are sources of inheritable variation (Louisiana Student Standards for Science, High School Biology, HS-LS3-2).
A standard-level answer on mutations for Louisiana LEAP 2025 Biology: substitution, insertion, and deletion, the frameshift effect, how mutations change proteins, and why mutations are the source of new alleles for evolution.
- Apply concepts of statistics and probability, using Punnett squares, to explain the variation and distribution of expressed traits from a genetic cross (Louisiana Student Standards for Science, High School Biology, HS-LS3-3).
A standard-level answer on inheritance for Louisiana LEAP 2025 Biology: alleles, genotype and phenotype, dominant and recessive, and using Punnett squares and probability to predict the ratios of a monohybrid cross.
- Construct and revise an explanation, based on evidence, for how carbon-based macromolecules (carbohydrates, lipids, proteins, and nucleic acids) are built from smaller subunits and carry out the functions of life (Louisiana Student Standards for Science, High School Biology, HS-LS1-6).
A standard-level answer on biological macromolecules for Louisiana LEAP 2025 Biology: carbohydrates, lipids, proteins, and nucleic acids, their monomers, and the functions each carries out in living things.
- Develop and use a model to explain how the structure of cell organelles relates to their functions within the cell (Louisiana Student Standards for Science, High School Biology, HS-LS1-2).
A standard-level answer on organelles for Louisiana LEAP 2025 Biology: the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, chloroplasts, vacuoles, and how each structure suits its function.
Sources & how we know this
- Louisiana Student Standards for Science — Louisiana Department of Education (2022)
- LEAP 2025 Assessment Guide for Biology — Louisiana Department of Education (2025)