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Every time a cell divides, it copies billions of DNA letters. Your NCERT book names the enzyme that does this, DNA-dependent DNA polymerase, in one line. This post tells how that enzyme was found, why the experiment was hard, and how Arthur Kornberg's results let you check base pairing with simple arithmetic.
Who was Arthur Kornberg?
Kornberg was born in Brooklyn, New York, in 1918 and died at Stanford in 2007, according to his Nobel Prize facts page. His Nobel biography says he trained as a doctor (MD, University of Rochester, 1941), worked at the US National Institutes of Health, and learned enzyme chemistry with Severo Ochoa and Carl Cori before moving to Washington University in St. Louis and then founding the biochemistry department at Stanford in 1959.
That same year he shared the 1959 Nobel Prize in Physiology or Medicine with Ochoa "for their discovery of the mechanisms in the biological synthesis of ribonucleic acid and deoxyribonucleic acid". The facts page records that he isolated DNA polymerase from bacteria in 1956.
How do you find an enzyme that builds DNA?
The problem was noise. Cell extracts are full of enzymes that break DNA down, so any building was swamped. In his Nobel lecture, Kornberg describes adding a radioactive (carbon-14) nucleotide to an extract of *Escherichia coli* and looking for radioactivity that ended up in long, acid-insoluble chains. The first positive result was "50 or so counts out of a million added".
That is 50 ÷ 1,000,000 = 0.005% of the label. Kornberg wrote that "through this tiny crack we tried to drive a wedge, and the hammer was enzyme purification". His purest preparations were several thousand-fold purer than the crude extract.
What did the purified enzyme need?
The lecture lists the requirements, and each one taught something about how DNA is copied:
All four deoxynucleoside triphosphates (dATP, dTTP, dGTP, dCTP). Leaving one out cut the reaction by a factor of more than 10,000.
Triphosphates, not diphosphates. Inorganic pyrophosphate was released in amounts equal to the nucleotides added. This matches the NCERT point that the two terminal phosphates provide the energy for polymerisation.
Existing DNA. With no DNA added, no reaction took place at all. Kornberg argued the DNA acts as a template, not just a starter.
The product could be 20 or more times the amount of DNA added, so most of it was new.
Worked example: does the copy match the template?
Kornberg's third line of evidence was base composition. He used DNA from different sources as templates (*Mycobacterium phlei*, *E. coli*, calf thymus and phage T2). In every product, adenine equalled thymine, guanine equalled cytosine, and the template's ratio of A-T pairs to G-C pairs was "imposed rather faithfully on the product". This work was published as Lehman and colleagues (1958) in PNAS.
Try it with sample numbers (not from the paper). A template DNA is 30% adenine.
In double-stranded DNA, A = T, so T = 30%.
A + T = 60%, so G + C = 40%.
G = C, so each is 20%.
If the enzyme copies by base pairing, the new DNA should also read A 30%, T 30%, G 20%, C 20%. That is the pattern Kornberg saw.
A common slip is to assume all four bases are 25%. Chargaff's rule says A = T and G = C, not that all four are equal.
Why is the textbook rate about 2000 base pairs per second?
NCERT Class 12 Biology, Chapter 5 says *E. coli*, with 4.6 × 10⁶ bp, finishes replication in 18 minutes, so polymerisation must run at about 2000 bp per second. Check it:
18 minutes = 1080 s.
4.6 × 10⁶ ÷ 1080 ≈ 4260 bp per second for the whole chromosome.
Replication of the *E. coli* chromosome is bidirectional (Prescott and Kuempel, 1972): two forks each copy about half. 4260 ÷ 2 ≈ 2130 bp per second per fork, which is the "approximately 2000" in the book.
If you got 4260 and stopped, you forgot the second fork.
What came next?
In 1967, Mehran Goulian, Kornberg and Robert Sinsheimer reported in PNAS the enzymatic synthesis of infectious DNA of the virus φX174: DNA made in a test tube that could still infect cells. Kornberg's Nobel biography adds that the enzymes of DNA metabolism found in his and other labs became the basis of recombinant DNA. For the experiment that showed how the two strands are shared between daughter molecules, read our post on Meselson and Stahl, and for the exam view of the whole process see DNA replication and transcription compared.
What can a student take from Kornberg's way of working?
Purify before you conclude. Kornberg's progress came from cleaning up the enzyme, not from guessing about crude extracts. In revision, isolate one idea at a time.
Ask what each ingredient is for. Leaving out one nucleotide, or the template, told him how the enzyme works. Do the same with formulas: drop a term and see what breaks.
Check the numbers against the theory. Base ratios and fork rates are quick arithmetic checks you can do in an exam.
If the steps of replication still feel like a list of names, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise templates, primers and polymerases one idea at a time.
Next step: open NCERT Chapter 5, find the 2000 bp per second line, and redo the calculation with two forks before you read on.
