The search for the genetic material in organisms has a long history. Initially, some scientists believed that protein was the hereditary substance. However, later experiments demonstrated that DNA is the true carrier of genetic information, responsible for inheriting traits. Hershey-Chase’s experiment clearly demonstrated it, but the search for the hereditary material continued.
Studies revealed that in some viruses, such as Tobacco mosaic virus, Mosaic virus, and QB bacteriophage, RNA can replace DNA. Why is this the case? “How do genetic materials differ in terms of their properties? How do DNA and RNA vary in structure and function? Let’s explore the answers to these questions.
Properties of an Ideal Genetic Material:
A molecule acting as genetic material must fulfill the following criteria:
- Replication – It should be able to generate its own replica.
- Stability – It should be chemically and structurally stable.
- Mutation – It should provide scope for slow changes (mutations), necessary for evolution.
- Expression – It should be able to express itself in the form of Mendelian characters.
Structure of DNA vs RNA
The distinction between DNA and RNA elucidates why DNA functions as the genetic material rather than RNA. Stability is a key factor in maintaining continuity. DNA exhibits this feature, as shown in Griffith’s experiment. The heat-killed S strain bacteria retained their virulent properties when the proper conditions were provided. Due to the presence of the 2`-OH group, RNA molecules are more reactive compared to DNA, making DNA structurally more stable and less chemically reactive.
The malleable structure of RNA, coupled with its instability, contributes to faster mutation rates. Consequently, viruses with RNA as their genetic makeup evolve more rapidly. RNA can independently perform protein synthesis, unlike DNA, which requires RNA as an intermediary.
Why DNA Fulfills These Criteria Better Than RNA to be the Genetic Material:
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Chemical and Structural Stability:
- DNA has deoxyribose sugar, which lacks a hydroxyl (-OH) group at the 2′ position. This makes DNA less reactive and more stable.
- RNA has ribose sugar with an extra –OH group at the 2′ position, making it more reactive and easily degradable.
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Heat Stability and Griffith’s Experiment:
- In Griffith’s experiment, the transforming principle (now known as DNA) remained intact even after heat treatment, proving its heat stability.
- This is because DNA strands are complementary and can re-anneal under proper conditions after being separated by heat.
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Lack of Catalytic Activity in DNA:
- RNA can act as a ribozyme (catalytic RNA), which makes it more reactive and less stable.
- DNA does not show catalytic properties, adding to its structural and functional stability.
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Better Suitability for Long-Term Storage:
- DNA is double-stranded, allowing proofreading and repair mechanisms, and protecting the genetic code.
- RNA is single-stranded and lacks such repair systems, making it less suitable for permanent information storage.
Conclusion:
The success of DNA as the dominant genetic material stems from its superior ability to meet replication, stability, mutation, and expression requirements compared to RNA. The double-stranded nature, absence of reactive hydroxyl groups, and proofreading capabilities render it exceptionally stable and optimal for long-term genetic information preservation. Unlike RNA, which is more reactive and susceptible to degradation, DNA exhibits greater stability, helping prevent mutations and preserve accurate genetic information. Hence, DNA’s structural stability and functional effectiveness make it the dominant genetic material in nearly all living organisms.