DOSSIER: Molecular Basis of Inheritance, NCERT LINE BY LINE

The Atomic History of DNA-A Chronological Dossier

The Atomic History of DNA | BSI Style

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DNA HISTORY – A Chronological Dossier
DISCOVERY 1869

Friedrich Miescher

Identified DNA as an acidic substance present in the nucleus and named it ‘Nuclein’. However, isolating such a long polymer intact was technically limited at the time.

BIOCHEMISTRY 1889

Richard Altmann

A student of Miescher, Altmann purified ‘nuclein’ further, removing associated proteins. Recognizing its acidic properties, he coined the modern term “nucleic acid”.

THE BASES Late 1800s

Albrecht Kossel

Determined the chemical structure of the nitrogenous bases found in nucleic acids. He identified adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U).

THE NUCLEOTIDE 1920s

Phoebus Levene

Discovered the three major components of a single nucleotide unit: phosphate, pentose sugar (deoxyribose in DNA), and a nitrogenous base. He proposed that nucleic acids are polymers of these units.

TRANSFORMATION 1928

Frederick Griffith

Experimented with Streptococcus pneumoniae. He found that heat-killed virulent ‘S’ strain bacteria could somehow transform non-virulent ‘R’ strain bacteria into killers.

He called this the “Transforming Principle”.

BIOCHEMISTRY 1944

Avery, MacLeod & McCarty

They purified biochemicals from heat-killed S cells. Digestion with proteases and RNases did not stop transformation, but digestion with DNase did.

VERDICT: DNA is the hereditary material.

THE RULES 1950s

Erwin Chargaff

Observed that for double-stranded DNA, the ratios between Adenine and Thymine, and Guanine and Cytosine, are constant and equal to one.

THE PROOF 1952

Hershey & Chase

Worked with bacteriophages using radioactive Phosphorus (P32 for DNA) and Sulfur (S35 for protein). They proved that only DNA enters the bacteria upon infection.

This provided unequivocal proof that DNA is the genetic material.

X-RAY DATA 1953

Franklin & Wilkins

Maurice Wilkins and Rosalind Franklin produced critical X-ray diffraction data of DNA at King’s College. This visual data was the key to unlocking the helix structure.

THE DOUBLE HELIX 1953

Watson & Crick

Based on X-ray data and Chargaff’s rules, they proposed the famous Double Helix model.

Salient Features of the Model:

  • DNA is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside.
  • The two chains have anti-parallel polarity. (If one chain has polarity 5’→3′, the other has 3’→5′).
  • The bases in two strands are paired through hydrogen bonds (H-bonds) forming base pairs (bp).
  • Adenine forms two hydrogen bonds with Thymine (A=T) from opposite strand and vice-versa.
  • Guanine is bonded with Cytosine with three H-bonds (G≡C).
  • Consequently, a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two chains.
  • The two chains are coiled in a right-handed fashion.
  • The pitch of the helix is 3.4 nm.
  • There are roughly 10 bp in each turn. Therefore, the distance between base pairs in a helix is approximately 0.34 nm.
  • The plane of one base pair stacks over the other in the double helix, conferring stability to the structure.
PROKARYOTIC PROOF 1958

Meselson & Stahl

Used heavy Nitrogen isotope (N15) and CsCl density gradient centrifugation in E. coli to prove experimentally that DNA replicates semi-conservatively in prokaryotes.

EUKARYOTIC PROOF 1958

J. Herbert Taylor

Complementing Meselson & Stahl’s work on bacteria, Taylor used radioactive thymidine (H3) on the root tips of Vicia faba (broad bean).

Using autoradiography, he proved that DNA replication is also semi-conservative in eukaryotes.

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