How does the cell use the DNA code to build a protein?
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.
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What this topic is asking
The Tennessee LS3 standards ask you to explain how the information in DNA is used to build proteins, the molecules that carry out most of the cell's functions. For the Biology I EOC that means knowing the two steps, transcription (DNA to mRNA) and translation (mRNA to protein), what each produces, where each happens, the role of codons and tRNA, and the big idea that the base sequence determines the amino-acid sequence. Items may ask you to order the steps or work out how many amino acids a piece of mRNA codes for.
The flow of information
Keeping the order straight, transcription before translation, is worth memorizing because the EOC frequently asks you to sequence the steps or to identify which step makes mRNA versus which makes protein.
Transcription: DNA to mRNA
Transcription happens in the nucleus. The DNA of a gene unwinds, and one strand is used as a template to build a complementary strand of messenger RNA (mRNA). RNA differs from DNA in three ways: it is single-stranded, its sugar is ribose, and it uses the base uracil (U) instead of thymine. So when copying DNA into RNA, the pairing is A (DNA) to U (RNA), T to A, C to G, and G to C. The finished mRNA then leaves the nucleus through a nuclear pore and travels to a ribosome.
The genetic code: codons
The mRNA is read in codons, groups of three bases. Each codon specifies one amino acid (or a start or stop signal). Because there are four bases and three positions, there are possible codons, more than enough to code for the 20 amino acids (several codons can specify the same amino acid). This three-base code is why a piece of mRNA with a number of bases divisible by three tells you how many amino acids it codes for.
Translation: mRNA to protein
So tRNA is the adapter that connects the code (codons) to the building blocks (amino acids). When a stop codon is reached, the finished protein is released. The order of amino acids, set by the order of codons, set in turn by the order of DNA bases, determines the protein's shape and therefore its function, tying this topic back to the macromolecules standard and forward to mutations.
Try this
Q1. State the two steps of protein synthesis, what each produces, and where each occurs. [3]
- Cue. Transcription (in the nucleus) produces mRNA from DNA; translation (at the ribosome) produces a protein from the mRNA.
Q2. Explain why mRNA, not DNA, carries the message to the ribosome. [2]
- Cue. DNA stays in the nucleus as the master copy; a working mRNA copy of the gene is made and can leave the nucleus to reach the ribosome, protecting the original DNA.
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 marksDuring transcription, a DNA strand is used to make a molecule of: (A) DNA. (B) messenger RNA (mRNA). (C) protein. (D) glucose.Show worked answer →
A 1-point multiple-choice item on what transcription produces.
The correct answer is B. Transcription copies a gene's DNA sequence into messenger RNA (mRNA). Making more DNA (A) is replication, building a protein (C) is translation, and glucose (D) is unrelated. The order to remember is DNA, then mRNA (transcription), then protein (translation).
TN Biology I EOC (2024 released style)2 marksA section of mRNA reads A U G C G U. Use the rule that each group of three bases is a codon for one amino acid. (a) State how many amino acids this mRNA codes for. (b) Explain where translation occurs and what reads the codons.Show worked answer →
A 2-point item on codons and translation.
(a) 1 point: each codon is three bases, and there are six bases, so codons, coding for 2 amino acids.
(b) 1 point: translation occurs at the ribosome; transfer RNA (tRNA) molecules read the codons (each tRNA has an anticodon that matches a codon) and bring the matching amino acid, building the protein chain.
Markers reward dividing the bases into codons of three and naming the ribosome and tRNA.
Related dot points
- 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.
- 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.
- Construct an explanation that the essential functions of life are carried out by the four macromolecules (carbohydrates, lipids, proteins, and nucleic acids) built from monomers (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).
A standard-level answer on biological macromolecules for the Tennessee Biology I EOC: carbohydrates, lipids, proteins, and nucleic acids, their monomers, their functions, and why protein shape determines what a protein can do.
- 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.
- Obtain, evaluate, and communicate information about biotechnology, including genetic engineering, GMOs, DNA fingerprinting, and their applications and ethical considerations (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).
A standard-level answer on biotechnology for the Tennessee Biology I EOC: genetic engineering and GMOs, DNA fingerprinting and gel electrophoresis, selective breeding and cloning, modern tools such as CRISPR, and the applications and ethical considerations.
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)