Transcribe DNA coding (sense) and template (antisense) strands into 5'→3' mRNA transcripts, tRNA anticodons, and translated amino acid protein sequences. Includes reverse transcription, 6-frame ORF analysis, and GC% metrics.
Proposed by Francis Crick in 1958, the Central Dogma of Molecular Biology governs the directional flow of genetic information: $\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}$. In eukaryotic nuclei, protein-coding genes are transcribed into messenger RNA by the 12-subunit enzyme complex RNA Polymerase II (RNAP II) through a tightly coordinated multi-stage process:
The TATA-binding protein (TBP) subunit of TFIID recognizes the upstream TATA box ($-25\text{ to }-30\text{ bp}$ from the transcription start site), bending DNA by $\sim 80^\circ$. Sequential recruitment of TFIIA, TFIIB, TFIIE, TFIIF, and the ATP-dependent helicase TFIIH unwinds $12–14\text{ base pairs}$ to form the open transcription bubble.
TFIIH phosphorylates Serine-5 on the C-terminal domain (CTD) heptapeptide repeat of RNAP II, clearing the promoter. Processive elongation at $\sim 20–50\text{ nucleotides/second}$ is maintained as Serine-2 is phosphorylated, synthesizing RNA exclusively in the $5' \rightarrow 3'$ direction via attack of the $3'\text{-OH}$ on incoming ribonucleoside triphosphates (rNTPs).
A $5'\text{-to-}5'$ triphosphate bridge adds an $m^7G$ cap, protecting mRNA against $5'\rightarrow 3'$ XRN1 exoribonucleases and facilitating eIF4E translation initiation binding.
U1, U2, U4/U6, and U5 snRNPs excise non-coding intronic sequences at invariant $5'\text{-GU}$ donor, branchpoint Adenosine, and $3'\text{-AG}$ acceptor sites.
CPSF recognizes the $5'\text{-AAUAAA-3'}$ polyadenylation signal; cleavage occurs $10–30\text{ nt}$ downstream, and Poly(A) Polymerase (PAP) adds $200–250$ Adenines.
Understanding which DNA strand is provided is the most critical requirement for accurate transcription. Because DNA is double-stranded and anti-parallel, the two strands serve completely different biochemical roles:
This strand is NOT read by RNA Polymerase. However, because it is complementary to the template strand, its sequence is identical to the resulting mRNA transcript, except that every Thymine ($T$) in DNA is replaced by Uracil ($U$) in mRNA. Bioinformatics databases (NCBI, GenBank, Ensembl) standardly report gene sequences using the Coding Strand convention.
This strand is the physical substrate read by RNA Polymerase in the $3' \rightarrow 5'$ direction. The polymerase synthesizes an anti-parallel, complementary mRNA transcript in the $5' \rightarrow 3'$ direction according to Watson-Crick base pairing: Adenine ($A$) pairs with Uracil ($U$), Thymine ($T$) pairs with Adenine ($A$), Cytosine ($C$) pairs with Guanine ($G$), and Guanine ($G$) pairs with Cytosine ($C$).
The standard nuclear genetic code comprises $4^3 = 64$ triplet codons specifying 20 standard amino acids and 3 termination signals. The code is degenerate (redundant)—multiple codons specify the same amino acid—which provides evolutionary buffering against deleterious single-nucleotide mutations:
Ribosomal translation begins at AUG (Methionine). In eukaryotes, the 40S subunit scans for the Kozak consensus sequence ($(GCC)GCCRCCAUGG$, where $R = \text{Purine } A/G$). In bacteria, the 30S subunit binds the Shine-Dalgarno motif ($5'\text{-AGGAGGU-3'}$).
The three stop codons—UAA (Ochre), UAG (Amber), and UGA (Opal)—do not bind tRNAs. Instead, eukaryotic Release Factor 1 (eRF1) mimics tRNA shape to hydrolyze the peptidyl-tRNA ester bond, releasing the polypeptide.
Non-standard base pairing at the 3rd codon position ($5'$ base of tRNA anticodon) allows modified bases like Inosine ($I$) to pair with $U, C,$ or $A$, permitting as few as 31–45 tRNA species to decode all 61 sense codons.
Discovered independently by David Baltimore and Howard Temin in 1970, Reverse Transcriptase (RNA-dependent DNA polymerase) synthesizes first-strand complementary DNA (cDNA) from an mRNA template. Because eukaryotic genomic DNA contains non-coding introns, cDNA represents only mature, spliced coding exons:
Oligo(dT) primers anneal specifically to the eukaryotic $3'\text{-poly(A)}$ tail to capture full-length mRNAs, whereas random hexamers prime across the entire transcript (ideal for degraded or bacterial RNA lacking poly(A) tails).
By designing qPCR primers spanning exon-exon junctions in cDNA, researchers prevent amplification of residual genomic DNA (gDNA) contaminants.
Because double-stranded DNA contains two anti-parallel strands that can each be read in three triplet offsets, every unannotated genomic locus possesses six potential reading frames:
Frame +1 starts at base 1; Frame +2 starts at base 2; Frame +3 starts at base 3 of the $5' \rightarrow 3'$ forward coding sequence.
Frames -1, -2, -3 scan the antiparallel complementary strand in the opposite direction, locating genes transcribed from the opposite strand.
Insertion or deletion of nucleotides in numbers not divisible by three ($\Delta \text{bp} \neq 3n$) shifts the entire triplet reading phase downstream. This scrambles all subsequent amino acids and almost universally creates a Premature Termination Codon (PTC), which flags the transcript for destruction via the cellular surveillance pathway known as Nonsense-Mediated mRNA Decay (NMD).
During cellular transcription, RNA Polymerase binds to and reads the Template Strand (also termed the Antisense or Non-Coding strand) in the 3' to 5' direction to synthesize a complementary mRNA molecule in the 5' to 3' direction. In contrast, the Coding Strand (also termed the Sense or Non-Template strand) is identical in sequence to the newly synthesized mRNA transcript, except that every Thymine (T) in the DNA coding strand corresponds to a Uracil (U) in the mRNA transcript.
Transcription follows Watson-Crick complementary base-pairing rules between deoxyribonucleotides and ribonucleotides: (1) Adenine (A) in DNA pairs with Uracil (U) in mRNA; (2) Thymine (T) in DNA pairs with Adenine (A) in mRNA; (3) Cytosine (C) in DNA pairs with Guanine (G) in mRNA; and (4) Guanine (G) in DNA pairs with Cytosine (C) in mRNA. RNA utilizes Uracil instead of Thymine because Uracil lacks a 5-methyl group and is energetically less costly for transient RNA synthesis.
Translation occurs at the ribosome, where mRNA is read sequentially in non-overlapping nucleotide triplets called codons in the 5' to 3' direction. Transfer RNA (tRNA) molecules matching each codon via complementary anticodons deliver the corresponding amino acid according to the universal genetic code (comprising 64 codons encoding 20 standard amino acids and 3 termination signals). For example, codon 5'-AUG-3' specifies Methionine (Start), 5'-UUU-3' specifies Phenylalanine, and 5'-GGG-3' specifies Glycine.
Reverse transcription is the enzymatic process catalyzed by Reverse Transcriptase (an RNA-dependent DNA polymerase originally discovered in retroviruses like HIV and M-MLV) that synthesizes single-stranded complementary DNA (cDNA) from an mRNA template. In laboratory workflows (RT-qPCR and RNA-seq), an Oligo(dT) primer anneals to the mRNA poly(A) tail, and reverse transcriptase extends cDNA in the 5' to 3' direction, converting unstable RNA into stable DNA for molecular analysis.
In the standard nuclear genetic code: (1) The universal Start codon is 5'-AUG-3', which codes for Methionine (N-formylmethionine in prokaryotes) and defines the reading frame; (2) The three termination (Stop or Nonsense) codons are UAA (Ochre), UAG (Amber), and UGA (Opal). Stop codons do not recruit aminoacyl-tRNAs; instead, they bind eukaryotic Release Factors (eRF1/eRF3) or prokaryotic Release Factors (RF1/RF2) to trigger peptide hydrolysis and ribosomal subunit dissociation.
An Open Reading Frame (ORF) is a continuous stretch of nucleotide triplets starting with an initiation codon (AUG) and ending with a stop codon (UAA, UAG, or UGA) without internal in-frame stop codons, signifying a potential protein-coding sequence (CDS). Because double-stranded DNA can be transcribed in either direction and read in three possible triplet offsets (frames +1, +2, +3 on the forward strand and frames -1, -2, -3 on the reverse strand), 6-frame translation scans all possible reading phases to accurately identify functional coding genes.
