The template strand serves as a master pattern for creating new nucleic acid molecules. Think of it as a stencil; it provides the precise guide needed to produce a new, corresponding structure. The sequence of chemical units, known as nucleotides, on this strand dictates the sequence of the new molecule being built. This guiding function ensures that genetic information is copied with a high degree of accuracy.
The Template Strand in DNA Replication
When a cell prepares to divide, its DNA must be duplicated. This process begins with the unwinding of the DNA double helix, separating its two intertwined strands. Each of these original strands then becomes a template for the construction of a new partner strand. The process is governed by complementary base pairing, where adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C).
An enzyme called DNA polymerase moves along each template strand, reading its sequence of nucleotides. It reads the template in a specific direction, from the 3′ end to the 5′ end, which guides the assembly of the new strand. As the enzyme reads each nucleotide on the template, it adds the corresponding complementary nucleotide to the growing new strand.
The result is the formation of two complete DNA molecules, each identical to the original. Each new DNA double helix is a hybrid, consisting of one original template strand and one newly synthesized strand. This method of replication is described as semi-conservative and ensures each daughter cell receives an accurate copy of the genetic blueprint.
The Template Strand in Transcription
Creating a temporary RNA message from a DNA gene, a process called transcription, also relies on a template. Unlike in replication, only one of the two DNA strands serves as the template for any given gene. This specific strand is often referred to as the antisense strand. Its sequence is read by an enzyme named RNA polymerase to build a messenger RNA (mRNA) molecule.
The other DNA strand, not used as the template, is called the coding strand. Its nucleotide sequence is nearly identical to the new mRNA molecule. The primary difference is that in RNA, uracil (U) is used in place of thymine (T). The mRNA transcript is effectively a copy of the coding strand, with U substituted for T.
RNA polymerase binds to a specific region of the DNA near the gene, known as the promoter, which signals the start of transcription. The enzyme then unwinds a small section of the DNA double helix, exposing the template strand. It moves along the template, reading its nucleotide sequence and synthesizing a complementary RNA strand. This mRNA molecule then carries the gene’s instructions out of the nucleus.
How the Template Ensures Accuracy
The use of a template strand is important to maintaining genetic fidelity. The rules of complementary base pairing provide a mechanism for accurate information transfer. This system ensures that a new DNA or RNA molecule is a faithful copy of the information stored in the template. The structural properties of these nucleotide pairs make incorrect pairings energetically unfavorable, acting as a first line of defense against errors.
This accuracy is important for cellular function and heredity. During DNA replication, precise copying ensures that each new cell inherits the same genetic information as its parent. This allows for the stable transfer of traits. In transcription, an accurate RNA copy is necessary to produce a functional protein. An incorrect RNA message can result in a protein with the wrong amino acid sequence, causing it to lose its function.
While the system is highly reliable, errors can occasionally occur. An incorrect nucleotide might be inserted, or a section might be skipped. Such errors in the copying process are known as mutations. The template mechanism, combined with proofreading activities of enzymes like DNA polymerase, minimizes the frequency of these mutations.