Template strand vs coding strand, and what exactly happens to hnRNA before it becomes mRNA?
Mehak AroraDropper Sample
29 Sept 2026 · 1 min read
It's 1 am and I can't sleep so might as well ask this here. I keep going in circles with transcription.
Part 1: strands. DNA template strand is 3'-ATGCATGCA-5'. I wrote the mRNA as 5'-UACGUACGU-3' but my guide key says the same thing in a different way and I don't trust myself. NCERT says the strand with polarity 3' to 5' acts as template and the other (5' to 3') is called coding strand, and the mRNA sequence looks like the coding strand but with U. Which should I use for these questions?
Part 2: processing. In eukaryotes the primary transcript is hnRNA, with introns and exons. I know introns are removed by splicing, then capping and tailing. But which end gets which, and with what? I always switch them. And are there different RNA polymerases? NCERT says bacteria have just one but eukaryotes have I, II and III, and I can't remember which one makes what.
My brain is mush, any structured answer would help.
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[+][–]Nisha Verma12thSample· 21d ago
Your answer is correct. mRNA is complementary to the template, antiparallel. Template 3'-A T G C A T G C A-5' gives A→U, T→A, G→C, C→G: U A C G U A C G U, and the new strand is built 5' to 3', so it reads 5'-UACGUACGU-3'.
Shortcut: the coding strand is 5'-TACGTACGT-3' (complement of the template, written 5' to 3'). Just replace T with U and you get the same mRNA. So you can use either, if you remember that the mRNA is "a copy" of the coding strand.
[+][–]Ishita GoyalDropperSample· 21d ago
Processing: capping at the 5' end and tailing at the 3' end. Cap is an unusual nucleotide, methyl guanosine triphosphate, added at the 5' end. Tail = adenylate residues, about 200 to 300 of them, added at the 3' end by poly-A polymerase. The tailing makes it poly-A. I remember "CAP = Cranium (front) / head, TAIL is at the back", simple but works
[+][–]Mehak AroraDropperSample· 21d agoAuthor
took your advice zoya, slept 6 hrs. And thanks ishita, cap = 5', tail = 3', that part is finally clear
[+][–]Siddharth RaoDropperSample· 21d ago
RNA polymerases in eukaryotes, the exam-favourite:
- RNA pol I: rRNAs (28S, 18S, 5.8S)
- RNA pol II: precursor of mRNA, i.e. hnRNA
- RNA pol III: tRNA, 5S rRNA and snRNAs
Bacteria have a single RNA polymerase for all three RNAs, and it needs a sigma factor to start at the promoter and rho factor to terminate. The eukaryotic ones need other factors too, but NCERT doesn't ask for those names.
[+][–]Zoya Khan12thSample· 21d ago
Maybe one gentle reminder for tonight: this chapter isn't going to be learned at 1 am, and your brain will actually do better after sleep. But since you asked, the cleanest picture is: introns are the non-coding intervening sequences, exons are the coding ones that appear in the mature RNA. Splicing removes introns and joins exons in a defined order. Bacteria's genes are continuous (no introns, and their structural genes are mostly polycistronic), eukaryotic genes are split genes and mostly monocistronic. Go sleep, it will still be there tomorrow.
[+][–]Kavya NaikDropperSample· 21d ago
Adding the small thing that confuses people on the strand question: which strand is "template" is only a name based on what RNA polymerase reads, not that one strand always carries the gene on a given chromosome. Another gene elsewhere could use the other strand as its template. NCERT doesn't go that far, but it avoids a 'why' later.
[+][–]Siddharth RaoDropperSample· 21d ago
true, and that's why people say the "template strand" depends on the gene. Sometimes you will see the two strands being template for different genes.
