Nucleic Acids

Concept:
The genetic code refers to the set of rules by which the sequence of nucleotides in DNA or RNA determines the sequence of amino acids in a protein. Each amino acid is encoded by a specific sequence of nucleotides known as a codon. A key feature of the genetic code is that it is a triplet code, meaning that three nucleotides together specify one amino acid. The discovery of this triplet nature involved both genetic experiments and biochemical studies carried out by several scientists.
Step 1: Evidence provided by Francis Crick.
In 1961, Francis Crick and his colleagues conducted experiments on the T4 bacteriophage. They introduced insertion and deletion mutations into the DNA sequence and observed their effect on protein synthesis. They found that:

• When one or two nucleotides were inserted or deleted, the reading frame of the genetic message was disturbed.
• When three nucleotides were inserted or deleted, the original reading frame was restored.

This experiment clearly demonstrated that the genetic message is read in groups of three nucleotides, providing the first strong evidence that the genetic code is a triplet code.
Step 2: Contributions of other scientists.

• Marshall Nirenberg and Heinrich Matthaei performed biochemical experiments using synthetic RNA sequences (such as poly-U). They showed that the codon UUU codes for the amino acid phenylalanine. This experiment helped begin the process of deciphering the genetic code.
• Har Gobind Khorana and Severo Ochoa later developed advanced techniques using synthetic RNAs to determine many other codon–amino acid relationships, ultimately helping to complete the genetic code table.

Step 3: Identifying the correct answer.
Although several scientists contributed to decoding the genetic code, the first evidence that the code is triplet in nature was provided by Francis Crick through mutation experiments. Final Answer:

Concept: In genetic engineering, special enzymes are used to manipulate DNA. Restriction enzymes are known as molecular scissors because they cut DNA at specific recognition sequences. These enzymes were discovered in bacteria and are used by them as a defense mechanism against invading viral DNA.

Step 1: Understand the role of restriction enzymes. Restriction enzymes recognize specific short DNA sequences called recognition sites. They cut the DNA at these sites, producing fragments.

Step 2: Example of restriction enzyme action. For example, the enzyme EcoRI recognizes the sequence: and cuts between specific bases, producing sticky ends.

Step 3: Role in recombinant DNA technology. Restriction enzymes are widely used in:
• Gene cloning
• Recombinant DNA technology
• Genetic engineering
• DNA analysis Because they precisely cut DNA, they are called "molecular scissors".

Concept: DNA (Deoxyribonucleic Acid) is the hereditary material responsible for storing and transmitting genetic information in living organisms. The discovery of DNA laid the foundation for modern genetics and molecular biology.

Step 1: Historical discovery.
DNA was first discovered in 1869 by the Swiss scientist Friedrich Miescher. While studying the nuclei of white blood cells obtained from pus on surgical bandages, he isolated a new phosphorus-rich substance.

Step 2: Naming of the substance.
Miescher called this substance “nuclein” because it was found in the cell nucleus. Later research identified this material as DNA.

Step 3: Later contributions.
Many scientists contributed to understanding DNA:
• Watson and Crick proposed the double helix structure.
• Rosalind Franklin provided crucial X-ray diffraction data. However, the initial discovery of DNA itself is credited to Friedrich Miescher.

Step 4: Conclusion.
Thus, the scientist credited with the discovery of DNA is F. Miescher.

Step 1: Understanding the Concept:
The genetic code is the set of rules by which the sequence of nucleotides in DNA or RNA is translated into the sequence of amino acids in proteins.
A triplet code means that a sequence of three nucleotides specifies a single amino acid.

Step 2: Key Formula or Approach:
Recall the historical timeline of genetic code discoveries and specifically who provided the first direct experimental proof of its triplet nature.

Step 3: Detailed Explanation:
George Gamow theoretically proposed the triplet code.
Francis Crick and his colleagues provided the first experimental evidence for the triplet nature of the genetic code in 1961 using frameshift mutations in the T4 bacteriophage.
Nirenberg and Matthaei were the first to decipher a specific codon (UUU for Phenylalanine) using a cell-free system.
Dr. H. G. Khorana developed chemical methods to synthesize RNA molecules to further decode the dictionary.

Step 4: Final Answer:
Since Crick provided the first experimental evidence, option (A) is correct.