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Replication and transcription are taught one after the other in the same NCERT chapter, and they share so many words (template, polymerase, 5' and 3') that students mix them up in exams. The cure is to compare them directly. This post sets them side by side, works through the numbers NCERT expects you to handle, and finishes with a short practice routine.
Where is this in the syllabus?
The NEET (UG) 2026 syllabus lists, under molecular basis of inheritance, "DNA replication; Central dogma; Transcription, genetic code, translation". The content is Chapter 5 of the NCERT Class 12 Biology textbook. Check the current year's syllabus before you plan revision. For the rest of the chapter, including the lac operon, see our post on the molecular basis of inheritance.
How do replication and transcription compare?
Take each feature in turn.
What is copied. Replication: the total DNA of the organism, both strands. Transcription: only a segment of DNA, and only one strand of it.
Template. Replication: both strands serve as templates. Transcription: the strand with polarity 3'→5', called the template strand.
Enzyme. Replication: DNA-dependent DNA polymerase, helped by other enzymes such as DNA ligase. Transcription: DNA-dependent RNA polymerase.
Raw materials. Replication: deoxyribonucleoside triphosphates, which NCERT notes also supply the energy. Transcription: ribonucleoside triphosphates, with uracil in place of thymine.
Direction. Both enzymes add nucleotides only in the 5'→3' direction.
Where it starts. Replication: at a defined origin of replication. Transcription: at a promoter, ending at a terminator.
When. In eukaryotes, replication happens in the S phase of the cell cycle. Transcription happens whenever a gene is being expressed.
Product. Replication: two DNA double helices. Transcription: a single-stranded RNA.
Why is one strand made in pieces?
DNA polymerase can only build 5'→3'. The two strands of DNA run in opposite directions, so at the replication fork one new strand can grow continuously while the other must be made in short pieces that are later joined by DNA ligase. NCERT puts it this way: on the template with polarity 3'→5' replication is continuous, and on the template with polarity 5'→3' it is discontinuous. NCERT also notes that DNA polymerases cannot start replication on their own.
Why is only one strand transcribed?
NCERT gives two reasons. If both strands were copied, one stretch of DNA would code for two different RNAs and two different proteins. And the two RNAs would be complementary, so they would pair into double-stranded RNA that could not be translated.
Worked example 1: template, coding strand and mRNA
NCERT defines the coding strand as the strand with polarity 5'→3' whose sequence is the same as the RNA, except that it has T where RNA has U.
Question. The coding strand of a transcription unit reads 5'-ATGCATGC-3'. Write the template strand and the mRNA.
Template strand: pair each base and reverse the polarity: 3'-TACGTACG-5'.
mRNA: copy the coding strand with U for T: 5'-AUGCAUGC-3'.
Check: the mRNA pairs with the template strand, A with U and G with C.
Worked example 2: the Meselson and Stahl numbers
Meselson and Stahl (1958) grew E. coli in ¹⁵NH₄Cl, so all its DNA was heavy, then moved the cells to ordinary ¹⁴NH₄Cl. NCERT reports that after one generation (20 minutes) the DNA had hybrid density, and after two generations (40 minutes) it was equal amounts of hybrid and light DNA.
Question (from NCERT). After 80 minutes, what are the proportions of light and hybrid DNA?
80 minutes is four generations.
Start with one heavy double helix. Each generation doubles the number of molecules: 2, 4, 8, 16.
The two original heavy strands are never broken up, so they always end up in exactly two hybrid molecules.
After four generations: 2 hybrid and 14 light out of 16. Hybrid is 1/8 (12.5%) and light is 7/8 (87.5%).
Worked example 3: how fast is replication?
NCERT states that E. coli, with 4.6 × 10⁶ base pairs, replicates its DNA in 18 minutes, so polymerisation must run at about 2,000 bp per second.
18 minutes = 1,080 s.
4.6 × 10⁶ / 1,080 ≈ 4,300 bp per second for the whole chromosome.
E. coli replication is bidirectional: Prescott and Kuempel (1972) showed that the chromosome is copied in both directions from where replication begins. With two forks working at once, each handles about 4,300 / 2 ≈ 2,100 bp per second, close to NCERT's figure.
What happens to RNA after transcription in eukaryotes?
NCERT notes there are at least three RNA polymerases in the nucleus. RNA polymerase I makes the 28S, 18S and 5.8S rRNAs, III makes tRNA, 5S rRNA and snRNAs, and II makes the precursor of mRNA, called hnRNA. hnRNA is then processed: splicing removes introns and joins exons, capping adds a cap at the 5' end, and tailing adds 200 to 300 adenylate residues at the 3' end. In bacteria, mRNA needs no processing, and transcription and translation can run together.
Common mistakes
Writing the mRNA from the template strand without reversing polarity.
Saying replication copies "one gene". It copies the whole genome.
Using T in RNA, or U in DNA.
Mixing up the enzymes for prokaryotes (one RNA polymerase) and eukaryotes (three in the nucleus).
If you find strand directions hard to keep straight on paper, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
A practice routine
From memory, write the comparison list above, then check it against NCERT.
Do five strand questions: given one strand, write the other strand and the mRNA, with polarity.
Redo the Meselson and Stahl question for 3 and 5 generations.
Draw the replication fork and label both template strands, the continuous strand and the discontinuous strand.
Next, follow the mRNA into the ribosome in our post on the genetic code and translation.
