How does the sequence of bases in DNA end up determining a protein and a trait?
Describe how the information in DNA is used to build proteins through transcription and translation, and explain how the order of bases determines the order of amino acids (TEKS Biology, Reporting Category 2; cause and effect; structure and function).
A TEKS-level answer on protein synthesis for the Texas STAAR Biology EOC: transcription of DNA into mRNA, translation of codons into amino acids at the ribosome, and how the base sequence determines the protein and the trait.
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What this topic is asking
The Biology TEKS ask you to describe how the information in DNA is used to build proteins, through transcription and translation, and to explain how the base order determines the amino-acid order. For STAAR Reporting Category 2 you need both stages in order, the role of mRNA, codons, and the ribosome, and the chain of cause and effect from gene to trait. This is also a structure and function topic, because a protein's shape (set by its amino acids) determines its job.
From gene to protein
Proteins do most of the work in a cell: enzymes, structural parts, and signals are all proteins. Because a protein's job depends on its shape, and its shape depends on its amino-acid order, the base sequence of a gene ultimately controls the trait.
Transcription: DNA to mRNA
In transcription, the DNA of a gene is copied into a strand of messenger RNA. The rules are like base pairing in DNA, with one change: RNA uses uracil (U) instead of thymine.
- DNA A pairs with RNA U
- DNA T pairs with RNA A
- DNA G pairs with RNA C
- DNA C pairs with RNA G
The mRNA is a working copy of the gene that can leave the nucleus and travel to a ribosome. A frequent STAAR task is to transcribe a short DNA sequence, where the single most common error is writing T instead of U.
Translation: mRNA to protein
Because there are three bases per codon, a sequence of mRNA is read three bases at a time. A short stretch of mRNA such as A-U-G-C-C-U is two codons (A-U-G and C-C-U) and so codes for two amino acids. The finished chain of amino acids folds into a specific shape, and that shape determines the protein's function.
Why the base sequence matters
This is the central cause-and-effect chain of molecular genetics:
Change the base sequence (a mutation) and you can change an amino acid, the protein's shape, its function, and the trait it produces. This is why gene mutations can have effects, and it is the molecular link between genotype and phenotype.
Try this
Q1. State what is made during transcription and where it is read during translation. [2]
- Cue. Transcription makes mRNA; it is read at the ribosome during translation.
Q2. A DNA template strand reads A-A-A. Write the mRNA codon and state how many amino acids it codes for. [2]
- Cue. mRNA is U-U-U (one codon), coding for one amino acid.
Exam-style practice questions
Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
STAAR Biology (2023 released style)1 marksDuring transcription, a DNA template strand with the sequence T-A-C produces which mRNA codon? (A) A-T-G. (B) A-U-G. (C) T-A-C. (D) U-A-G.Show worked answer →
A 1-point multiple-choice item on transcription.
The correct answer is B. In transcription the DNA template is copied into mRNA, and RNA uses uracil instead of thymine, so T pairs with A, A pairs with U, and C pairs with G, giving A-U-G. C just copies the template, and A and D use thymine, which RNA does not contain.
Remember U replaces T when reading DNA into RNA.
STAAR Biology (2024 SCR style)2 marksA mutation changes one base in a gene, which changes one amino acid in the protein the gene codes for. Explain how a change in the DNA base sequence can change a protein and therefore a trait. Support your answer with reasoning.Show worked answer →
A 2-point short constructed response linking gene to trait.
Full credit (2 points): the order of bases in DNA sets the order of codons in mRNA, and each codon specifies one amino acid, so the base sequence determines the amino-acid sequence of the protein. The protein's amino-acid order sets its folded shape, which determines its function and therefore the trait; changing a base can change an amino acid, the shape, the function, and the trait.
Partial credit (1 point): links bases to amino acids without carrying it through to protein function or trait. The science is scored.
Related dot points
- Identify the components of DNA, describe the structure of the double helix and base pairing, and explain how DNA is replicated accurately before cell division (TEKS Biology, Reporting Category 2; structure and function; patterns).
A TEKS-level answer on DNA for the Texas STAAR Biology EOC: the components of a nucleotide, the double helix and complementary base pairing, and how DNA replication produces two identical copies before a cell divides.
- Recognize the types of gene mutations and explain how a change in the DNA base sequence may be harmful, beneficial, or neutral and how it can be inherited (TEKS Biology, Reporting Category 2; cause and effect; stability and change).
A TEKS-level answer on mutations for the Texas STAAR Biology EOC: what a mutation is, substitution, insertion, and deletion, why an effect can be harmful, beneficial, or neutral, and how mutations in gametes are inherited and supply variation.
- Apply Mendel's laws and use Punnett squares to predict the genotype and phenotype ratios of monohybrid crosses, and identify patterns of inheritance including dominant, recessive, codominant, and incomplete dominance (TEKS Biology, Reporting Category 2; patterns; using mathematics).
A TEKS-level answer on inheritance for the Texas STAAR Biology EOC: alleles, genotype and phenotype, dominant and recessive traits, using Punnett squares to predict ratios and probabilities, and codominance and incomplete dominance.
- Describe applications of DNA technology, including gel electrophoresis, DNA fingerprinting, recombinant DNA, and genetically modified organisms, and evaluate their benefits and concerns (TEKS Biology, Reporting Category 2; cause and effect; structure and function).
A TEKS-level answer on biotechnology for the Texas STAAR Biology EOC: gel electrophoresis and DNA fingerprinting, recombinant DNA and genetic engineering, genetically modified organisms, and the benefits and concerns of these tools.
- Identify the four major classes of biological macromolecules and their functions, and explain how enzymes act as biological catalysts affected by temperature and pH (TEKS Biology, Reporting Category 4; structure and function; cause and effect).
A TEKS-level answer on biomolecules and enzymes for the Texas STAAR Biology EOC: carbohydrates, lipids, proteins, and nucleic acids and their functions, and how enzymes catalyze reactions and are affected by temperature and pH.
Sources & how we know this
- Texas Essential Knowledge and Skills for Science (Biology) — Texas Education Agency (2024)
- STAAR Biology Assessed Curriculum — Texas Education Agency (2024)