Structure of DNA and RNA — Revision Notes

⚡ 30-Second Revision

DNA: — Deoxyribose sugar, A, G, C, T bases. Double-stranded, antiparallel helix. A=T (2 H-bonds), G≡C (3 H-bonds). Stable, stores genetic info.
RNA: — Ribose sugar, A, G, C, U bases. Single-stranded (can fold). A=U (2 H-bonds), G≡C (3 H-bonds). Less stable, involved in gene expression.
Nucleotide: — Sugar + Phosphate + Base.
Nucleoside: — Sugar + Base.
Bonds: — Phosphodiester (backbone), Hydrogen (inter-strand), Glycosidic (sugar-base).
Chargaff's Rule: — In dsDNA, $A=T$, $G=C$, so $A+G = C+T$.

2-Minute Revision

DNA and RNA are nucleic acids, polymers of nucleotides. Each nucleotide has a pentose sugar, a phosphate, and a nitrogenous base. DNA uses deoxyribose sugar, Thymine (T), and is typically a stable double helix with two antiparallel strands.

Adenine (A) pairs with T via two hydrogen bonds, and Guanine (G) pairs with Cytosine (C) via three hydrogen bonds. This complementary pairing is explained by Chargaff's rules ($A=T, G=C$). DNA's primary role is genetic information storage.

RNA uses ribose sugar, Uracil (U) instead of T, and is generally single-stranded, making it less stable but more versatile. The 2'-OH group in ribose contributes to RNA's reactivity. RNA exists as mRNA (messenger), tRNA (transfer), and rRNA (ribosomal), each crucial for protein synthesis and gene regulation.

Phosphodiester bonds form the sugar-phosphate backbone in both.

5-Minute Revision

Let's quickly recap the essential structures of DNA and RNA for NEET. Both are polynucleotides, meaning long chains of repeating nucleotide units. A nucleotide is the fundamental monomer, comprising three parts: a pentose sugar, a phosphate group, and a nitrogenous base. If the phosphate is absent, it's a nucleoside.

DNA (Deoxyribonucleic Acid):

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Sugar: — Deoxyribose (lacks -OH at 2' carbon).
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Bases: — Adenine (A), Guanine (G) - Purines; Cytosine (C), Thymine (T) - Pyrimidines.
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Structure: — Double helix, two polynucleotide strands coiled around a central axis.
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Strands: — Antiparallel (one 5' to 3', other 3' to 5').
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Bonding: — Sugar-phosphate backbone linked by phosphodiester bonds. Complementary bases linked by hydrogen bonds: A=T (2 H-bonds), G≡C (3 H-bonds).
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Chargaff's Rules: — In dsDNA, $A=T$ and $G=C$. Thus, $A+G = C+T$ (purines = pyrimidines).
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Stability: — Highly stable due to deoxyribose and double-stranded nature, ideal for long-term genetic storage.

RNA (Ribonucleic Acid):

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Sugar: — Ribose (has -OH at 2' carbon).
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Bases: — Adenine (A), Guanine (G) - Purines; Cytosine (C), Uracil (U) - Pyrimidines.
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Structure: — Generally single-stranded, but can fold into complex 3D structures (e.g., tRNA cloverleaf).
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Bonding: — Sugar-phosphate backbone by phosphodiester bonds. Intramolecular H-bonds can form (A=U, G≡C) in folded regions.
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Stability: — Less stable and more reactive than DNA due to the 2'-OH group on ribose.
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Types & Functions:

* mRNA: Carries genetic code from DNA to ribosomes. * tRNA: Carries specific amino acids to ribosomes during protein synthesis. * rRNA: Structural and catalytic component of ribosomes.

Key Takeaways for NEET: Memorize the specific differences in sugar and bases, the number of hydrogen bonds, the antiparallel nature of DNA, and the distinct functions of different RNA types. Practice Chargaff's rule problems diligently.

Prelims Revision Notes

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Nucleic Acids: — DNA and RNA are polymers of nucleotides.
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Nucleotide Structure: — Each nucleotide has 3 components:

* Pentose Sugar: Deoxyribose (DNA) or Ribose (RNA). * Phosphate Group: Attached to 5' carbon of sugar. * Nitrogenous Base: Purines (Adenine, Guanine) or Pyrimidines (Cytosine, Thymine/Uracil).

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Nucleoside: — Sugar + Base (no phosphate).
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DNA Structure (Watson-Crick Model):

* Double Helix: Two polynucleotide strands coiled. * Antiparallel: One strand 5'->3', other 3'->5'. * Backbone: Sugar-phosphate, linked by phosphodiester bonds (between 3'-OH and 5'-phosphate).

* Bases: Project inwards, perpendicular to helix axis. * Complementary Base Pairing: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds). * Dimensions: 3.4 nm per turn, 10 base pairs per turn, 2 nm diameter.

* Chargaff's Rules: In dsDNA, $[A]=[T]$, $[G]=[C]$, and $[A]+[G] = [C]+[T]$. * Stability: More stable due to deoxyribose (no 2'-OH) and double-stranded nature.

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RNA Structure:

* Single-stranded: Generally, but can fold into complex 3D structures. * Sugar: Ribose (has 2'-OH group, making it more reactive). * Bases: A, G, C, U (Uracil replaces Thymine). * Base Pairing (intramolecular): A-U, G-C.

* Types and Functions: * mRNA: Messenger RNA, carries genetic code. * tRNA: Transfer RNA, carries specific amino acids. * rRNA: Ribosomal RNA, structural and catalytic component of ribosomes.

* Stability: Less stable and more reactive than DNA.

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Key Bonds:

* Glycosidic bond: Between sugar and base (at 1' carbon). * Phosphoester bond: Between sugar and phosphate (at 5' carbon). * Phosphodiester bond: Links nucleotides (3'-OH to 5'-phosphate). * Hydrogen bonds: Between complementary bases (A-T/U, G-C).

Vyyuha Quick Recall

DNA vs. RNA - 'DR. T.U.G.S.'

DNA: Deoxyribose, Thymine, Double strand, Stable. RNA: Ribose, Uracil, Single strand, Less stable.

Thymine in DNA, Uracil in RNA. Guanine and Cytosine are common to both. Sugar difference (Deoxyribose vs. Ribose) is key to stability.