DEOXYRIBONUCLEIC ACID (DNA)
1. Context
Seventy years ago, two male scientists, Francis Crick, and James Watson, proclaimed they had discovered the secret of life: The structure of DNA. Since then, history has acknowledged how Rosalind Franklin was sidelined. But new archive evidence has cast doubt on the widely accepted narrative that Franklin collected an all-important image but didn’t appreciate the meaning of what she was looking at.
2. Deoxyribonucleic Acid (DNA)
- Deoxyribonucleic acid (abbreviated DNA) is the molecule that carries genetic information for the development and functioning of an organism.
- DNA is made of two linked strands that wind around each other to resemble a twisted ladder a shape known as a double helix.
- Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups.
- Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G), or thymine (T).
- The two strands are connected by chemical bonds between the bases: adenine bonds with thymine, and cytosine bonds with guanine.
- The sequence of the bases along DNA’s backbone encodes biological information, such as the instructions for making a protein or RNA molecule.
Image Source: National Human Genome Research Institute
3. DNA Structure and Function
- DNA is the information molecule. It stores instructions for making other large molecules, called proteins.
- These instructions are stored inside each of your cells, distributed among 46 long structures called chromosomes.
- These chromosomes are made up of thousands of shorter segments of DNA, called genes. Each gene stores the directions for making protein fragments, whole proteins, or multiple specific proteins.
- DNA is well-suited to perform this biological function because of its molecular structure, and because of the development of a series of high-performance enzymes that are fine-tuned to interact with this molecular structure in specific ways.
- The match between DNA structure and the activities of these enzymes is so effective and well-refined that DNA has become, over evolutionary time, the universal information-storage molecule for all forms of life.
- Nature has yet to find a better solution than DNA for storing, expressing, and passing along instructions for making proteins.
3.1 Molecular structure of DNA
- In order to understand the biological function of DNA, you first need to understand its molecular structure.
- This requires learning the vocabulary for talking about the building blocks of DNA, and how these building blocks are assembled to make DNA molecules.
3.2 DNA Molecules are Polymers
- Polymers are large molecules that are built up by repeatedly linking together smaller molecules, called monomers.
- Think of how a freight train is built by linking lots of individual boxcars together, or how this sentence is built by sticking together a specific sequence of individual letters (plus spaces and punctuation).
- In all three cases, the large structure of a train, a sentence, and a DNA molecule is composed of smaller structures that are linked together in non-random sequences boxcars, letters, and, in the biological case, DNA monomers.
3.3 DNA Monomers are called Nucleotides
- Just like the sentence “polymer” is composed of the letter “monomers,” a DNA polymer is composed of monomers called nucleotides.
- A molecule of DNA is a bunch of nucleotide monomers, joined one after another into a very long chain.
4. Four Nucleotide Monomers
- The English language has a 26-letter alphabet. In contrast, the DNA “alphabet” has only four “letters,” the four nucleotide monomers.
- They have short and easy-to-remember names: A, C, T, G. Each nucleotide monomer is built from three simple molecular parts: a sugar, a phosphate group, and a nucleobase. (Don’t confuse this use of “base” with the other one, which refers to a molecule that raises the pH of a solution; they’re two different things.)
5. The sugar and acid in all four monomers are the same
- All four nucleotides (A, T, G, and C) are made by sticking a phosphate group and a nucleobase to a sugar.
- The sugar in all four nucleotides is called deoxyribose. It’s a cyclical molecule most of its atoms are arranged in a ring structure.
- The ring contains one oxygen and four carbons. A fifth carbon atom is attached to the fourth carbon of the ring.
- Deoxyribose also contains a hydroxyl group (-OH) attached to the third carbon in the ring.
6. Four Nucleotide Monomers are distinguished by their bases
Each type of nucleotide has a different nucleobase stuck to its deoxyribose sugar.
- A nucleotide contains adenine
- A nucleotide contains thymine
- G nucleotide contains guanine
- C nucleotide contains cytosine
All four of these nucleobases are relatively complex molecules, with the unifying feature that they all tend to have multiple nitrogen atoms in their structures. For this reason, nucleobases are often also called nitrogenous bases.
7. DNA Fingerprinting
- It is known that every individual has unique fingerprints. These occur at the tips of the fingers and have been used for identification for a long time but these can be altered by surgery.
- A sequence of bases on DNA is also unique for a person and information regarding this is called DNA fingerprinting. It is the same for every cell and cannot be altered by any known treatment.
- DNA fingerprinting is now used (i) in forensic laboratories for the identification of criminals. (ii) to determine the paternity of an individual. (iii) to identify the dead bodies in any accident by comparing the DNAs of parents or children. (iv) to identify racial groups to rewrite biological evolution.
8. Recombinant DNA
- Recombinant DNA (rDNA) molecules are DNA molecules formed by laboratory methods of genetic recombination (such as molecular cloning) to bring together genetic material from multiple sources, creating sequences that would not otherwise be found in the genome.
- Recombinant DNA is possible because DNA molecules from all organisms share the same chemical structure. They differ only in the nucleotide sequence within that identical overall structure.
- In most cases, organisms containing recombinant DNA have apparently normal phenotypes. That is, their appearance, behavior, and metabolism are usually unchanged.
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For Prelims: Deoxyribonucleic acid (DNA), adenine (A), cytosine (C), guanine (G), or thymine (T), RNA molecule, Polymers, Nucleotide, Nucleotide Monomers, DNA Fingerprinting and Recombinant DNA (rDNA).
For Mains: 1. What is Deoxyribonucleic acid (DNA)? Discuss the structure and function of the Deoxyribonucleic acid (DNA) and explain how it is different from RNA.(250 Words)
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Previous year Question
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1. Recombinant DNA technology (Genetic Engineering) allows genes to be transferred (UPSC 2013)
1. across different species of plants
2. from animals to plants
3. from microorganisms to higher organisms
Select the correct answer using the codes given below.
A. 1 only
B. 2 and 3 only
C. 1 and 3 only
D. 1, 2 and 3
Answer: D
2. With reference to the recent developments in science, which one of the following statements is not correct? (UPSC 2019)
A. Functional chromosomes can be created by joining segments of DNA taken from cells of different species.
B. Pieces of artificial functional DNA can be created in laboratories.
C. A piece of DNA taken out from an animal cell can be made to replicate outside a living cell in a laboratory.
D. Cells taken out from plants and animals can be made to undergo cell division in laboratory Petri dishes.
Answer: A
3. Consider the following statements: (UPSC 2022)
DNA Barcoding can be a tool to:
1. assess the age of a plant or animal.
2. distinguish among species that look alike.
3. identify undesirable animal or plant materials in processed foods.
Which of the statements given above is/are correct?
A. 1 only
B. 3 only
C. 1 and 2
D. 2 and 3
Answer: D
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Source: Down to Earth
