Explain protein synthesis: how transcription copies DNA into mRNA and translation reads codons at the ribosome to build a protein, linking the DNA base sequence to the trait (Virginia 2018 Biology SOL BIO.5.a, supporting BIO.2.d).

What this topic is asking

Virginia Biology SOL standard BIO.5.a (with BIO.2.d) states that DNA is the foundation for protein synthesis. The Biology EOC expects you to explain the two stages, transcription and translation, to handle short sequences (remembering that RNA uses uracil), and to connect the DNA base sequence to the protein and therefore the trait. This is the molecular link between genotype and phenotype, a recurring idea across the genetics module.

From gene to protein

The big idea is that DNA does not build proteins directly. It first sends a copy of the instructions out as mRNA (transcription), and the ribosome then reads that copy to assemble the protein (translation). Keeping the two stages and their products straight is the key to this topic.

Transcription: DNA to mRNA

The mRNA carries the message out of the nucleus to the ribosome. The single most common error here is forgetting that RNA uses U instead of T, so a transcription question always tests whether you remember to write U where the DNA had A on the template.

Translation: mRNA to protein

In translation, the mRNA is read at a ribosome in groups of three bases called codons. Each codon specifies one amino acid. Transfer RNA (tRNA) molecules bring the matching amino acids, and the ribosome joins them in the order set by the codons, building a chain of amino acids that folds into the protein. Because codons are three bases long, the number of amino acids is the number of bases divided by three.

Linking base sequence to trait

The order of bases in DNA sets the order of codons in mRNA, which sets the order of amino acids in the protein. The amino acid order determines how the protein folds, and the folded shape determines the protein's function. Proteins (such as enzymes, structural proteins, and pigments) produce the organism's traits. So a change in the DNA base sequence can change the protein and therefore the trait, which is exactly why mutations matter (see mutations and genetic variation). This chain, DNA to mRNA to protein to trait, links genotype to phenotype.

Try this

Q1. Write the mRNA transcribed from the DNA template strand A-T-G-C-C-A. [2]

• Cue. U-A-C-G-G-U (uracil replaces thymine: A pairs with U, T with A, G with C, C with G, C with G, A with U).

Q2. Explain how the DNA base sequence determines a protein's function. [2]

• Cue. The base order sets the amino-acid order, which determines how the protein folds; the folded shape determines its function.