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Coding strand
Coding strand
Coding strand
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Two linear DNA strands are separated by a blue oval, which is creating RNA by running along the template strand. The coding strand is above, not attached to RNA polymerase.
Position of the template and coding strands during transcription.

When referring to DNA transcription, the coding strand (or informational strand[1][2]) is the DNA strand whose base sequence is identical to the base sequence of the RNA transcript produced (although with thymine replaced by uracil). It is this strand which contains codons, while the non-coding strand contains anticodons. During transcription, RNA Pol II binds to the non-coding template strand, reads the anti-codons, and transcribes their sequence to synthesize an RNA transcript with complementary bases.

By convention, the coding strand is the strand used when displaying a DNA sequence. It is presented in the 5' to 3' direction.

Wherever a gene exists on a DNA molecule, one strand is the coding strand (or sense strand), and the other is the noncoding strand (also called the antisense strand,[3] anticoding strand, template strand or transcribed strand).

Strands in transcription bubble

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During transcription, RNA polymerase unwinds a short section of the DNA double helix near the start of the gene (the transcription start site). This unwound section is known as the transcription bubble. The RNA polymerase, and with it the transcription bubble, travels along the noncoding strand in the opposite, 3' to 5', direction, as well as polymerizing a newly synthesized strand in 5' to 3' or downstream direction. The DNA double helix is rewound by RNA polymerase at the rear of the transcription bubble.[3] Like how two adjacent zippers work, when pulled together, they unzip and rezip as they proceed in a particular direction. Various factors can cause double-stranded DNA to break; thus, reorder genes or cause cell death.[4]

RNA-DNA hybrid

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Where the helix is unwound, the coding strand consists of unpaired bases, while the template strand consists of an RNA:DNA composite, followed by a number of unpaired bases at the rear. This hybrid consists of the most recently added nucleotides of the RNA transcript, complementary base-paired to the template strand. The number of base-pairs in the hybrid is under investigation, but it has been suggested that the hybrid is formed from the last 10 nucleotides added.[5]

See also

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References

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Works cited

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Coding strand

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In molecular biology, the coding strand (also known as the sense strand or nontemplate strand) is one of the two strands of double-stranded DNA in a gene, characterized by having the same nucleotide sequence as the mature messenger RNA (mRNA) transcript produced during transcription, with the exception that it contains thymine (T) in place of uracil (U) found in RNA. This strand serves as the reference for the genetic code, containing the linear array of codons that specify the amino acid sequence of the encoded protein during translation. The coding strand plays a central but indirect role in gene expression, as it is not directly used as a template for RNA synthesis; instead, RNA polymerase II (in eukaryotes) or RNA polymerase (in prokaryotes) binds to the promoter region and transcribes the complementary template strand (also called the antisense strand) in the 3' to 5' direction, generating an mRNA molecule that is antiparallel and complementary to the template but identical in sequence to the coding strand (barring the T/U substitution). This process ensures that the mRNA carries the exact coding information from the coding strand to the ribosome for protein synthesis, maintaining the fidelity of genetic information transfer. The coding strand typically runs in the 5' to 3' direction relative to the transcription unit, aligning with the reading frame of the gene. Understanding the distinction between the coding and template strands is essential for annotating genomes, designing primers for PCR amplification, and interpreting sequencing data, as errors in strand identification can lead to misreading of genetic codes or regulatory elements. In prokaryotes, where transcription and are coupled, the coding strand's sequence directly influences immediate , while in eukaryotes, additional steps like splicing refine the mRNA to match the coding strand more precisely.

Fundamentals

Definition

In , the coding strand is the DNA strand whose is identical to that of the (mRNA) transcript produced during , except that (T) bases in DNA are replaced by uracil (U) bases in RNA. This direct correspondence allows the coding strand to serve as a reference for the genetic information that specifies the of proteins. It is also referred to as the or non-template strand, emphasizing its role in carrying the "sense" or readable akin to the mRNA. By standard convention, the coding strand is always written and depicted in the 5' to 3' direction, which aligns with the polarity of the mRNA molecule and the direction of during protein synthesis. This orientation facilitates straightforward sequence comparisons between DNA and RNA, as the 5' end corresponds to the start of the gene's and the 3' end to its termination. In genomic databases and diagrams, this 5' to 3' representation of the coding strand is the default for displaying gene sequences.

Comparison with Template Strand

The template strand, also referred to as the antisense or non-coding strand, is fully complementary to the coding strand in sequence and runs in an antiparallel orientation within the DNA double helix. Specifically, when the coding strand is aligned from its 5' to 3' end, the template strand extends in the opposite 3' to 5' direction, allowing the two strands to pair stably through bonds. This antiparallel arrangement is a fundamental property of double-stranded DNA, enabling the precise alignment of bases during replication and transcription. The complementarity between the strands follows standard Watson-Crick base pairing rules: (A) on the coding strand pairs with (T) on the template strand, while (G) pairs with (C). During transcription, this makes the template strand the direct blueprint that reads to synthesize , as the incorporates complementary ribonucleotides—uracil (U) opposite A, and so on—resulting in an mRNA sequence that matches the coding strand (with T replaced by U). In contrast, the coding strand itself is not used as a template for RNA synthesis but serves as the reference sequence for the gene's information content. Functionally, this distinction ensures that only the template strand is actively involved in directing RNA production, while the coding strand remains inert in the process, preserving the integrity of the for downstream applications like protein synthesis. Standard diagrams of the DNA double helix illustrate this by showing the two strands coiled together, with directional arrows marking the 5' to 3' polarity of each—typically depicting the coding strand on top (5' → 3' left to right) and the template below (3' ← 5' right to left)—to emphasize their complementary and antiparallel relationship.

Transcription Process

Overview of Transcription

Transcription is the biological process by which the nucleotide sequence of a gene in DNA is copied into a complementary RNA molecule, primarily messenger RNA (mRNA), serving as a template for protein synthesis. This process unfolds in three principal stages: initiation, where RNA polymerase binds to the promoter region of the DNA to form the transcription initiation complex; elongation, during which the polymerase moves along the DNA, unwinding the double helix and synthesizing RNA by adding nucleotides complementary to the template strand; and termination, where specific signals trigger the release of the newly synthesized RNA transcript and dissociation of the polymerase from the DNA. In prokaryotes, transcription is mediated by a core enzyme consisting of subunits α₂ββ'ω, which requires association with a (σ) factor to form the holoenzyme capable of promoter recognition, typically at -10 and -35 consensus sequences upstream of the . In eukaryotes, (Pol II) handles the transcription of protein-coding s, relying on general transcription factors such as TFIID, which binds to the in the promoter, to assemble the pre-initiation complex and facilitate Pol II recruitment. These mechanisms ensure precise start sites for synthesis, with the coding strand playing an indirect role by providing the sequence reference that matches the eventual mRNA (barring U/T differences), aiding in gene identification and annotation without direct physical interaction during the process. The directionality of transcription is antiparallel: the template (antisense) strand is read by in the 3' to 5' direction, while the growing chain is extended in the 5' to 3' direction, incorporating ribonucleotides that base-pair with the template. Consequently, the primary mRNA sequence is identical to that of the coding (sense) strand, except for the substitution of uracil for , allowing the coding strand to serve as a direct proxy for predicting the sequence of the encoded protein post-translation. This indirect involvement of the coding strand underscores its utility in bioinformatics and for mapping genes and interpreting transcripts, though the synthesis machinery engages solely with the template strand and associated factors.

Role in the Transcription Bubble

The transcription bubble is a transient unwound region of approximately 12-14 base pairs in the DNA double helix, formed and maintained by RNA polymerase as it progresses along the gene during transcription elongation. This localized separation of the DNA strands creates a single-stranded platform essential for RNA synthesis, with the bubble encompassing both the template strand, which pairs with the nascent RNA, and the coding strand on the opposite side. Within the transcription bubble, the coding strand, also known as the non-template strand, occupies the side opposite the RNA-DNA hybrid and remains predominantly single-stranded throughout the unwound region, except at the upstream and downstream edges where it reanneals with the template strand to form double-stranded DNA. This positioning allows the coding strand to interact dynamically with the enzyme, contributing to the stability of the bubble structure and facilitating the processive movement of the polymerase without dissociation from the DNA. The coding strand's separation from the template in this configuration ensures that the bubble's topology supports continuous addition, preventing premature collapse that could halt elongation. The transcription bubble originates at the promoter during and migrates downstream as advances, typically at a rate of 20-50 per second in prokaryotes. Behind the polymerase, the bubble rewinds rapidly, re-forming the double helix to minimize exposure of single-stranded to potential damage from nucleases or chemical modifications. This dynamic rewinding is crucial for maintaining genomic integrity, as prolonged single-stranded regions can lead to mutations or recombination events. The maintenance and propagation of the transcription bubble rely on energy derived from the of triphosphates (NTPs), including ATP, during RNA chain elongation by the . This NTP-driven mechanism powers the forward translocation of , which in turn promotes localized DNA unwinding at the leading edge of the bubble. Additionally, the nucleotide sequence of the coding strand influences bubble stability; regions with higher or specific motifs can modulate the ease of unwinding and rewinding, affecting overall transcription efficiency and fidelity.

RNA-DNA Hybrid

During transcription elongation, the RNA-DNA hybrid forms as the growing 3' end of the nascent RNA base-pairs with the complementary 5' region of the template DNA strand, typically spanning 8-9 base pairs, with minor variability up to 9-10 base pairs in eukaryotes depending on polymerase state and sequence. This hybrid structure is essential for maintaining the register of transcription, ensuring the RNA 3' terminus remains positioned at the polymerase active site for nucleotide addition. The RNA-DNA hybrid adopts an A-form helical conformation within the active site cleft of RNA polymerase, characterized by a widened minor groove and tilted base pairs that distinguish it from B-form DNA duplexes. In this configuration, the coding strand is displaced from the template strand but remains in close proximity upstream of the hybrid; its nucleotide composition indirectly modulates hybrid stability through base-pairing preferences that affect the overall energetics of the transcription bubble. For instance, higher GC content in the coding strand region can enhance the stability of the displaced single-stranded DNA, influencing the ease of hybrid formation and maintenance. Recent cryo-EM studies from 2022 to 2025 have revealed conformational changes in the hybrid, such as tilting during elongation and pausing in eukaryotic complexes. These changes, often involving a tilted hybrid in paused states, stabilize the polymerase at regulatory sites, such as promoter-proximal regions, to fine-tune . Prolonged hybrid persistence beyond normal elongation, however, raises the risk of DNA damage, as extended RNA-DNA pairing can lead to replication fork stalling and genomic instability if not resolved by helicases or nucleases. As transcription proceeds, the hybrid dissociates upon RNA exit from the exit channel, allowing the template strand to reanneal with the coding strand and restore the DNA duplex. This unwinding step, facilitated by polymerase translocation, ensures efficient progression without persistent strand separation.

Sequence Features

Nucleotide Composition

The composition of the coding strand varies significantly across organisms and genomic regions, influencing its biophysical properties such as temperature. In humans, coding regions typically exhibit a of approximately 40-50%, with an average plateau around 45% downstream of the transcription start site, higher than the genome-wide average of 41%. This composition can be AT-rich in certain prokaryotes or GC-rich in thermophilic organisms, where higher GC levels (up to 70%) correlate with elevated temperatures due to the three bonds in G-C base pairs compared to two in A-T pairs, enhancing DNA stability under high temperatures. The coding strand's sequence is identical to that of the mature mRNA, except for the replacement of (T) with uracil (U), enabling direct prediction of the protein sequence from the genomic coding strand without needing to infer from the template strand. This correspondence facilitates bioinformatics analyses, where sequencing reads are often aligned to the coding strand for gene annotation and functional prediction. In eukaryotes, nucleotide composition shows pronounced variations organized into isochores—large genomic segments with uniform base content ranging from 30% to 60% GC in humans. Exons on the coding strand tend to be GC-richer than surrounding introns, particularly in low-GC regions (by about 5-10%), while in GC-rich isochores their s are more similar, creating gradients that distinguish coding from non-coding regions and influence splicing efficiency. The percentage is calculated as (G+C)/(A+T+G+C)×100(G + C) / (A + T + G + C) \times 100, a standard metric used in genomic studies to quantify these patterns.

Codon Representation

The coding strand of DNA is read in the 5' to 3' direction, where its nucleotide sequence is organized into non-overlapping triplets known as codons, each specifying one of 20 standard amino acids or a stop signal during translation. This organization follows the standard genetic code, which consists of 64 possible codons—derived from four nucleotide bases (A, T, G, C) arranged in triplets—encoding 20 amino acids plus three stop codons (TAA, TAG, TGA), with the remaining codons being redundant due to the code's degeneracy. The on the coding strand is established by the ATG, which initiates and codes for , defining the correct grouping of subsequent triplets into codons. An (ORF) is thus the continuous sequence on the coding strand from the ATG to a (TAA, TAG, or TGA) in the same frame, without intervening stop codons, representing the translatable portion of a . Due to the degeneracy of the , most are specified by multiple synonymous codons on the coding strand, allowing sequence variations that do not alter the protein product. For instance, the is encoded by the codons TTT or TTC on the coding strand (corresponding to UUU or UUC in mRNA). A representative example is the beta-globin (HBB), where the coding strand sequence begins with the and proceeds in triplets that map directly to via the . The initial segment of the HBB coding sequence (5' to 3') is ATG GTG CAT CTG ACT CCT GAG GAG AAG TCT, translating to the Met-Val-His-Leu-Thr-Pro-Glu-Glu-Lys-Ser.
Codon PositionCoding Strand Codon
1ATGMet
2GTGVal
3CATHis
4CTGLeu
5ACTThr
6CCTPro
7GAGGlu
8GAGGlu
9AAGLys
10TCTSer

Biological Importance

Role in Gene Expression

In genomics, the coding strand serves as a critical for annotating genes by identifying open reading frames (ORFs), which are sequences starting with an ATG codon and ending with a , enabling predictions of exons and potential protein-coding regions for studying patterns. This annotation process relies on aligning genomic data to the coding strand to distinguish functional coding sequences from non-coding regions, facilitating expression studies across organisms. Promoter-proximal sequences, such as CpG islands in vertebrates, play a key role in regulating transcription rates by providing unmethylated regions that promote accessible structure and efficient recruitment. These elements enhance the initiation of transcription, influencing the overall efficiency of in a strand-specific manner. The sequence of the coding strand directly determines the mature mRNA sequence (with replaced by uracil), which undergoes splicing to remove introns and join exons, thereby shaping the final transcript available for . Additionally, this sequence influences , as variations in the coding strand can alter miRNA binding sites on the mRNA, modulating mRNA stability and translational repression. Evolutionarily, the coding strand experiences strong purifying selection to optimize codon usage, favoring codons that match abundant tRNAs and thereby enhancing speed and protein synthesis efficiency without altering the amino acid sequence. This conservation ensures that highly expressed genes maintain codon biases that support rapid and accurate protein production across species.

Implications for Mutations

Mutations in the coding strand directly alter the that corresponds to the mature mRNA (with replaced by uracil), thereby changing the transcribed and potentially the translated protein product. Point , such as substitutions, are common and can result in missense changes where a single codon is altered, leading to the incorporation of a different in the protein; for instance, in sickle cell anemia, a GAG to GTG substitution in the sixth codon of the HBB gene's coding strand replaces with in the beta-globin protein, causing polymerization and red blood cell sickling. Insertions or deletions (indels) in the coding strand typically cause frameshift by shifting the of downstream codons, often introducing premature stop codons and producing truncated or nonfunctional proteins. These coding strand mutations have profound functional impacts, as they directly disrupt codon representation and without requiring complementary changes on the template strand. In clinical contexts, such alterations drive diseases; for example, missense mutations in the TP53 gene's coding regions, occurring in approximately 50% of human cancers, impair the tumor suppressor's DNA-binding and transcriptional activation functions, promoting oncogenesis. Next-generation sequencing (NGS) technologies facilitate detection by targeting exonic regions of the coding strand for high-throughput variant calling, enabling identification of somatic and mutations with high sensitivity, even at low frequencies. Recent advancements in gene editing, particularly CRISPR-Cas9 and base editing, offer therapeutic implications for correcting coding strand mutations, as seen in approved therapies like Casgevy for , which indirectly mitigates the HBB mutation by enhancing production, while direct base editing approaches precisely revert point mutations without double-strand breaks. In cancer, CRISPR-based editing of TP53 coding variants holds promise for restoring wild-type function in preclinical models, with off-target effects remaining a key challenge for future clinical applications.

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