Nucleic Acids & Hormones
Module 3 | CBSE Class 12 Chemistry | Biomolecules
1. Introduction to Nucleic Acids
Every generation of each and every species resembles its ancestors in many ways. The transmission of these inherent characters from one generation to the next is called heredity. The particles in the nucleus of the cell, responsible for heredity, are called chromosomes, which are made up of proteins and another type of biomolecules called Nucleic Acids.
Nucleic acids are also called polynucleotides since they are long-chain polymers of nucleotides. There are two types of nucleic acids: DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid).
2. Chemical Composition of Nucleic Acids
Complete hydrolysis of DNA (or RNA) yields three main components: a pentose sugar, phosphoric acid, and nitrogen-containing heterocyclic compounds (called bases).
2.1 Pentose Sugars and Nitrogenous Bases
In RNA, the sugar is β-D-ribose.
In DNA, the sugar is β-D-2-deoxyribose. (It is called "2-deoxy" because it lacks an oxygen atom at the 2' carbon position compared to ribose).
2. The Nitrogenous Bases: There are two classes of nitrogenous bases:
- Purines (Double-ring structure): Adenine (A) and Guanine (G). These are present in both DNA and RNA.
- Pyrimidines (Single-ring structure): Cytosine (C), Thymine (T), and Uracil (U).
DNA contains Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
RNA contains Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).
(Thymine is exclusively found in DNA, while Uracil is exclusively found in RNA).
3. Structure of Nucleic Acids
3.1 Nucleosides vs. Nucleotides
The attachment of a base to the pentose sugar forms a Nucleoside. The base is attached at the 1' position of the sugar through an N-glycosidic linkage.
When phosphoric acid is esterified to the 5' position of the sugar moiety of a nucleoside, we get a Nucleotide. (The bond is a phosphoester linkage).
3.2 The Phosphodiester Linkage
Nucleotides are joined together to form a polynucleotide chain (nucleic acid). The linkage between individual nucleotides is highly specific.
Because of this specific linking, one end of the chain has a free phosphate group at the 5' position (called the 5' end), and the other end has a free hydroxyl group at the 3' position (called the 3' end). This gives the nucleic acid chain directionality (5' → 3').
4. The Double Helix Structure of DNA
In 1953, James Watson and Francis Crick proposed the famous Double Helix model for the secondary structure of DNA. It consists of two polynucleotide chains wrapped around each other in a right-handed helix.
4.1 Complementary Base Pairing Rules
The two strands are anti-parallel (one runs 5'→3' and the other 3'→5') and are held together by Hydrogen bonds formed between specific pairs of bases on opposite strands.
A Purine always pairs with a specific Pyrimidine to maintain a uniform distance between the two strands.
- Adenine (A) pairs with Thymine (T) via two (2) hydrogen bonds.
- Cytosine (C) pairs with Guanine (G) via three (3) hydrogen bonds.
Because of this specific base pairing, the two strands of DNA are complementary to each other. If the sequence of bases on one strand is known, the sequence on the other strand can be accurately predicted.
5. RNA and its Types
Unlike DNA, RNA is typically a single-stranded molecule. However, it can fold back on itself to form localized double helix structures (like hairpin loops).
There are three major types of RNA, which perform different functions in the synthesis of proteins:
- Messenger RNA (mRNA): Carries the genetic information (code) from DNA in the nucleus to the ribosomes in the cytoplasm for protein synthesis.
- Ribosomal RNA (rRNA): A major structural component of ribosomes, where protein synthesis occurs.
- Transfer RNA (tRNA): Acts as the "adapter" molecule. It brings specific amino acids to the ribosome according to the sequence dictated by mRNA.
6. Biological Functions of Nucleic Acids
- Replication: DNA is the chemical basis of heredity and has the unique property of self-duplication during cell division. The two strands separate, and each acts as a template for synthesizing a new complementary strand, ensuring identical copies of DNA are passed to daughter cells.
- Protein Synthesis: The genetic information encoded in DNA is translated into the specific amino acid sequences of proteins. DNA synthesizes mRNA (Transcription), and mRNA directs the assembly of amino acids (Translation). "DNA makes RNA makes Protein".
7. Hormones (Chemical Messengers)
Hormones are molecules that act as intercellular messengers. These are produced by endocrine glands in the body and are poured directly into the blood stream which transports them to the site of action.
Classification based on chemical nature:
- Steroid Hormones: Derivatives of cholesterol. Examples: Estrogens and Androgens (sex hormones), Cortisone (from adrenal cortex).
- Polypeptide Hormones: Examples: Insulin (produced by pancreas, lowers blood glucose), Glucagon (raises blood glucose), Endorphins.
- Amino Acid Derivatives: Examples: Epinephrine / Adrenaline (prepares body for 'fight or flight'), Thyroxine (produced by thyroid gland, contains Iodine, regulates metabolism).
8. NCERT Solved Examples (Step-by-Step)
NCERT Example 14.5: Write the important structural and functional differences between DNA and RNA.
Structural Differences:
1. Sugar: DNA contains β-D-2-deoxyribose. RNA contains β-D-ribose.
2. Bases: DNA contains Thymine (T) alongside A, C, and G. RNA contains Uracil (U) instead of Thymine.
3. Strands: DNA has a double-stranded helical structure. RNA is generally single-stranded.
Functional Differences:
1. DNA has the unique property of self-replication. RNA does not replicate itself (usually synthesized from DNA).
2. DNA controls the transmission of hereditary effects. RNA strictly controls the synthesis of proteins.
9. Previous Year Questions (PYQs) & Exhaustive Question Bank
Part A: Conceptual (1-2 Marks)
Q1. What is the type of linkage holding together the monomers in a DNA molecule?
Q2. Why are the two strands of DNA not identical, but are complementary to each other?
Part B: Assertion-Reason Type (1 Mark)
Q3. Assertion (A): DNA is the chemical basis of heredity.
Reason (R): The two strands of DNA are complementary to each other.
DNA is the chemical basis of heredity because of its unique property of self-replication during cell division, allowing identical copies of genetic information to be passed to daughter cells. While the complementarity of strands is the mechanism that allows this replication to happen flawlessly, it is the act of replication itself that defines its role in heredity.
Part C: Differences & Structure (3 Marks)
Q4. Differentiate between the following:
(a) Nucleoside and Nucleotide
(b) DNA and RNA (on the basis of nitrogenous bases)
(a) A Nucleoside is a two-component molecule consisting of a nitrogenous base attached to a pentose sugar (at the 1' position). A Nucleotide is a three-component molecule consisting of a nitrogenous base, a pentose sugar, and a phosphoric acid group (attached at the 5' position of the sugar). Thus, Nucleotide = Nucleoside + Phosphate.
(b) In DNA, the four nitrogenous bases are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). In RNA, the four bases are Adenine (A), Guanine (G), Cytosine (C), and Uracil (U). (RNA lacks Thymine).
Q5. Name the hormone containing iodine. What disease is caused by its deficiency?
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