DNA as Genetic Material — Revision Notes

⚡ 30-Second Revision

Griffith (1928): — Transforming Principle (S-strain $ightarrow$ R-strain virulence transfer). Organism: *Streptococcus pneumoniae*.
Avery, MacLeod, McCarty (1944): — DNA is the transforming principle. Used DNase to stop transformation.
Hershey-Chase (1952): — DNA is genetic material. Used bacteriophages, $^{32}$P (DNA) and $^{35}$S (Protein). Only $^{32}$P entered bacteria.
Properties of Genetic Material: — Replication, Stability, Mutation, Expression.
DNA vs. RNA Stability: — DNA more stable due to deoxyribose (no 2'-OH), thymine, double-stranded structure. RNA less stable (ribose with 2'-OH, uracil, single-stranded).
RNA as Genetic Material: — In some viruses (e.g., TMV, HIV).

2-Minute Revision

The journey to establish DNA as the genetic material involved several key experiments. Frederick Griffith (1928) discovered the 'transforming principle' in *Streptococcus pneumoniae*, where heat-killed virulent bacteria could transfer a trait to non-virulent ones.

This hinted at a transferable genetic substance. Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944) identified this principle as DNA by showing that only DNase treatment prevented transformation, thus eliminating proteins and RNA.

The definitive proof came from Alfred Hershey and Martha Chase (1952), who used bacteriophages with radioactively labeled DNA ($^{32}$P) and protein ($^{35}$S). They demonstrated that only DNA entered the bacterial cells to direct viral replication, confirming DNA as the genetic material.

For a molecule to be genetic material, it must replicate, be stable, allow for mutation, and be able to express itself. DNA's deoxyribose sugar (lacking a 2'-OH group), thymine base, and double-stranded structure contribute to its superior stability over RNA, making it ideal for long-term genetic information storage, though RNA serves as genetic material in some viruses.

5-Minute Revision

The understanding of DNA as the genetic material is a cornerstone of modern biology, built upon a series of elegant experiments. Initially, proteins were considered strong candidates due to their complexity.

However, the evidence shifted towards DNA. Frederick Griffith's 1928 experiment with *Streptococcus pneumoniae* showed that a 'transforming principle' from heat-killed virulent S-strain bacteria could convert live non-virulent R-strain bacteria into virulent S-strain.

This phenomenon, bacterial transformation, indicated the transfer of hereditary information, though the chemical nature of the principle was unknown.

In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty meticulously purified components from the S-strain and tested their ability to transform R-strain. They found that only the DNA fraction caused transformation, and treating the extract with DNase (an enzyme that degrades DNA) abolished the transforming ability. This strongly implicated DNA. However, some skepticism persisted.

The conclusive evidence arrived with Alfred Hershey and Martha Chase's 1952 experiment using bacteriophages. They differentially labeled phage DNA with radioactive phosphorus ($^{32}$P) and phage proteins with radioactive sulfur ($^{35}$S).

After allowing the labeled phages to infect *E. coli*, they found that only $^{32}$P (DNA) entered the bacterial cells, while $^{35}$S (protein) remained outside. The infected bacteria then produced new phages containing $^{32}$P, proving that DNA was the genetic material responsible for directing viral synthesis.

For any molecule to serve as genetic material, it must fulfill four criteria: it must be able to replicate accurately, be chemically and structurally stable, allow for slow mutations for evolution, and be able to express its information.

DNA excels in these properties, particularly its stability, due to its deoxyribose sugar (lacking a reactive 2'-OH group), the presence of thymine instead of uracil, and its robust double-stranded helical structure, which also facilitates repair mechanisms.

While RNA acts as genetic material in some viruses, DNA is the universal genetic blueprint for most life forms.

Prelims Revision Notes

DNA as Genetic Material: NEET Quick Recall

I. Historical Experiments & Conclusions:

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Griffith's Experiment (1928):

* Organism: *Streptococcus pneumoniae* (pneumococcus). * Strains: S-strain (virulent, encapsulated), R-strain (non-virulent, non-encapsulated). * Observation: Heat-killed S-strain + live R-strain $ightarrow$ mouse dies, live S-strain recovered. * Conclusion: Discovered 'Transforming Principle' – some substance transferred genetic information, causing transformation. Nature unknown.

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Avery, MacLeod, McCarty Experiment (1944):

* Objective: Identify chemical nature of Griffith's transforming principle. * Method: Treated heat-killed S-strain extract with enzymes. * Key Finding: Transformation *prevented* only when extract treated with DNase (digests DNA). * Conclusion: DNA is the transforming principle.

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Hershey-Chase Experiment (1952):

* Objective: Definitive proof of DNA as genetic material. * Organism: Bacteriophage (T2) and *E. coli*. * Labeling: * DNA labeled with radioactive **$^{32}$P** (DNA contains P, not S).

* Proteins labeled with radioactive **$^{35}$S** (Proteins contain S, not P). * Method: Infection, blending (to detach phage coats), centrifugation (to separate bacteria from phages). * Observation: $^{32}$P (DNA) found inside bacterial cells and progeny phages; $^{35}$S (protein) remained outside.

* Conclusion: DNA, not protein, is the genetic material that enters the host cell and directs synthesis of new viruses.

II. Properties of Genetic Material:

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Replication: — Must be able to make copies of itself.
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Stability: — Chemically and structurally stable (not easily degradable).
3
Mutation: — Must allow for slow changes (mutations) for evolution.
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Expression: — Must be able to express itself as Mendelian characters (direct protein synthesis).

III. DNA vs. RNA as Genetic Material:

DNA:

* Sugar: Deoxyribose (lacks 2'-OH group $ightarrow$ less reactive, more stable). * Base: Thymine (T) instead of Uracil (U) $ightarrow$ better repair mechanisms. * Structure: Double-stranded $ightarrow$ more stable, template for repair. * Primary Role: Long-term storage of genetic information in most organisms.

RNA:

* Sugar: Ribose (has 2'-OH group $ightarrow$ more reactive, less stable). * Base: Uracil (U) instead of Thymine (T). * Structure: Usually single-stranded $ightarrow$ less stable. * Primary Role: Gene expression (mRNA, tRNA, rRNA), genetic material in some viruses (e.g., retroviruses, TMV). * Additional Property: Can act as a catalyst (ribozyme).

Vyyuha Quick Recall

Great Apes Have Properties Suitable Mostly Everywhere.

Griffith (Transforming Principle)
Avery, MacLeod, McCarty (DNA is the principle)
Hershey-Chase (DNA is genetic material)
Properties (Replication)
Stability
Mutation
Expression