Transcription: Non-coding DNA and mRNA Synthesis

Why a cell transcribes a gene

A gene is a section of DNA that carries the information for making a particular protein. The DNA remains as the cell's stored genetic information, so the relevant sequence is first transferred to a temporary, single-stranded molecule called messenger RNA, or mRNA. This first stage of protein synthesis is transcription.

Only the required gene needs to be transcribed. The resulting mRNA can later attach to a ribosome for translation, but that next stage is considered separately.

RNA polymerase finds the gene

Immediately in front of the gene is a region of non-coding DNA. It is described as non-coding because it does not code for the amino acid sequence of the protein. The enzyme RNA polymerase binds to this region, positioning it at the beginning of the gene.

This distinction matters: RNA polymerase first binds to non-coding DNA in front of the gene, then produces mRNA from the coding DNA of the gene.

The DNA opens

DNA normally consists of two strands held together in a double helix. During transcription, the helix unwinds and the two strands separate in the region being transcribed. This exposes the sequence of bases in the gene so that one DNA strand can act as the template for building mRNA.

A complementary mRNA strand is assembled

Free RNA nucleotides line up against exposed DNA bases by complementary base pairing. RNA uses uracil, U, in place of the thymine, T, found in DNA. The relevant pairings are therefore:

• DNA A pairs with RNA U
• DNA T pairs with RNA A
• DNA C pairs with RNA G
• DNA G pairs with RNA C

RNA polymerase moves along the gene and joins the RNA nucleotides together. This produces a single mRNA strand whose base sequence is complementary to the DNA sequence used as its template. For instance, the DNA sequence A T G C gives the complementary mRNA sequence U A C G.

The product of transcription

At the end of the gene, the newly formed mRNA separates from the DNA. The DNA strands can join together again. The mRNA now carries the transcribed genetic information; its later attachment to a ribosome begins translation.