Summary
Highlights
Transcription is the first step in using the genetic code in DNA to synthesize all the different proteins in your body. It's distinct from cell replication processes like DNA replication or mitosis. Proteins are made outside the nucleus in ribosomes, while DNA, containing the instructions, resides inside the nucleus. Transcription involves creating messenger RNA (mRNA) to carry the genetic code from the nucleus to the ribosome.
Transcription begins when RNA polymerase attaches to a gene, a segment of DNA containing the code to build a specific protein. A protein is a macromolecule made of a sequence of amino acids, and the order of nitrogenous bases in a gene dictates this sequence. Each group of three consecutive bases is called a codon, coding for a particular amino acid. RNA polymerase unwinds the DNA helix, separating it into two strands: a template strand (transcribed) and a non-template strand (not transcribed).
Using the template strand as a guide, RNA polymerase uses the base pair rule to assemble free nucleotides in the nucleus into a complementary strand of RNA. For instance, thymine on the DNA template binds to adenine, cytosine to guanine, and guanine to cytosine. Importantly, RNA never contains thymine; instead, adenine on the DNA template pairs with uracil in RNA. This process effectively transcribes the genetic code from the non-template DNA strand into mRNA.
Once transcription is complete, the mRNA, small enough to pass through a nuclear pore, carries the genetic code from the nucleus to the ribosome for protein synthesis. The actual protein building process at the ribosome is called translation. In summary, transcription is copying the genetic code for a protein into mRNA. A gene is a DNA segment with protein-building instructions, and a codon is a three-base group coding for a specific amino acid. RNA polymerase unwinds DNA, and the base pair rule guides mRNA assembly, making mRNA a copy of the non-template DNA strand with uracil substituting thymine.