By Dianne Hermance | Updated Aug 30 2022
Image: DNA double helix (© GettyImages)
Deoxyribonucleic acid (DNA) is the master blueprint for life, encoding the instructions that govern cell division, reproduction, and the production of thousands of proteins. Two key processes—transcription and translation—translate that blueprint into functional molecules.
During protein synthesis, messenger RNA (mRNA) is copied from a DNA strand known as the template strand. The complementary strand, the coding strand, mirrors the mRNA sequence (except that thymine is replaced by uracil) and is often called the sense strand.
Transcription is the first step toward building a protein. An enzyme called RNA polymerase binds to a specific segment of DNA and, together with transcription factors, reads the template strand in a 3′→5′ direction. It assembles a single‑stranded pre‑mRNA molecule by adding nucleotides that are complementary to the DNA bases, with uracil (U) substituting thymine (T). The resulting mRNA runs in a 5′→3′ orientation and is released once transcription ends.
Once produced, the mRNA travels to a ribosome where translation occurs. Codons—sets of three nucleotides (A, C, G, U)—are read sequentially and matched to specific amino acids. The ribosome links these amino acids together, forming a polypeptide chain that folds into a functional protein.
The DNA strand used as a template during transcription is called the template strand, or antisense strand. It runs in the 3′→5′ direction and provides the sequence from which mRNA is synthesized.
Also referred to as the sense or coding strand, this DNA strand is not transcribed. Its sequence is identical to that of the mRNA (with T replaced by U), running in the 5′→3′ direction. It effectively “codes” for the protein by matching the mRNA codon sequence.
Both strands of the DNA double helix are essential: the template strand guides transcription, while the coding strand defines the genetic message that the cell uses to build proteins.
Key Takeaway: In protein synthesis, the template strand supplies the actual mRNA template, and the coding strand provides the sequence that mirrors the mRNA, ensuring accurate protein production.
