The nontemplate strand is one of two strands that form the DNA double helix. It plays a role in how genetic information is used within a cell, contributing to gene expression. This process converts DNA instructions into functional products, like proteins. While the template strand directly guides RNA synthesis, the nontemplate strand is important.
Understanding DNA Strands
DNA exists as a double helix, composed of two long, complementary strands. Each strand is a chain of nucleotides, containing a sugar, phosphate, and one of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), or thymine (T). The sugar and phosphate units form the backbone of each strand, with the bases extending inward.
The two DNA strands are held together by hydrogen bonds between specific base pairs: A always pairs with T, and G always pairs with C. This is known as complementary base pairing. These strands also run in opposite directions, known as antiparallel orientation. One strand runs from its 5′ end to its 3′ end, while the other runs from its 3′ end to its 5′ end.
Within a gene, one DNA strand is the “template strand” (also called antisense or non-coding), which guides RNA synthesis. The other is the “nontemplate strand” (also called coding or sense). Both strands are integral parts of the gene, but only the template strand is directly read by cellular machinery during RNA synthesis.
The Nontemplate Strand’s Role in Transcription
Transcription is the process where DNA’s genetic information is copied into an RNA molecule, primarily messenger RNA (mRNA). During this process, RNA polymerase binds to a DNA promoter region, signaling the start of a gene. It then unwinds the DNA double helix, separating the two strands.
RNA polymerase moves along the template strand, reading its nucleotide sequence from 3′ to 5′. It synthesizes a new mRNA molecule by adding complementary RNA nucleotides. For instance, if the template strand has an adenine (A), RNA polymerase adds a uracil (U) to the mRNA; if it has a guanine (G), it adds a cytosine (C). The nontemplate strand is not directly used as a guide by RNA polymerase.
However, the newly synthesized mRNA molecule will have a sequence almost identical to the nontemplate strand’s sequence. The only difference is that uracil (U) replaces thymine (T) in mRNA. This occurs because the template strand is complementary to both the nontemplate strand and the mRNA, causing the mRNA sequence to mirror the nontemplate strand.
Why It’s Called the Coding Strand
The nontemplate strand is referred to as the “coding strand” or “sense strand” because its nucleotide sequence, when read from 5′ to 3′, directly matches the sequence of the messenger RNA (mRNA) produced during transcription. The only distinction is that thymine (T) bases in the DNA’s nontemplate strand are replaced by uracil (U) bases in the mRNA. This direct correspondence gives it the “coding” designation.
The mRNA molecule carries genetic instructions for building proteins. These instructions are read in sets of three nucleotides, called codons. Each codon specifies a particular amino acid, which are the building blocks of proteins. Since the nontemplate strand’s sequence is essentially the same as the mRNA’s codons (barring the T to U substitution), it directly reflects the information that will be coded into a protein.