DNA and RNA — Revision Notes

⚡ 30-Second Revision

DNA — Deoxyribonucleic Acid. Genetic material. Double helix.
RNA — Ribonucleic Acid. Gene expression. Single-stranded.
Nucleotide — Base + Sugar + Phosphate.
Nucleoside — Base + Sugar.
DNA Sugar — Deoxyribose (no -OH at 2').
RNA Sugar — Ribose (with -OH at 2').
DNA Bases — A, G, C, T.
RNA Bases — A, G, C, U.
Base Pairing (DNA) — A=T (2 H-bonds), G≡C (3 H-bonds).
Backbone Bond — Phosphodiester bond (5'-phosphate to 3'-OH).
Base-Sugar Bond — $\beta$-N-glycosidic bond.
Chargaff's Rules — In dsDNA, $A=T$, $G=C$, so $A+G = T+C$.
Stability — DNA > RNA (due to deoxyribose and double helix).

2-Minute Revision

DNA and RNA are nucleic acids, polymers of nucleotides. Each nucleotide has a nitrogenous base, a pentose sugar, and a phosphate group. DNA uses deoxyribose sugar and bases A, G, C, T, forming a stable double helix with antiparallel strands linked by A-T (2 H-bonds) and G-C (3 H-bonds) pairing.

Its primary role is genetic information storage. RNA uses ribose sugar and bases A, G, C, U, and is typically single-stranded, playing diverse roles in gene expression (mRNA, tRNA, rRNA). The 2'-OH group in ribose makes RNA less stable than DNA.

Phosphodiester bonds form the sugar-phosphate backbone, providing directionality (5' to 3'). Chargaff's rules state that in dsDNA, A=T and G=C.

5-Minute Revision

Nucleic acids, DNA and RNA, are essential biopolymers. Their monomeric units are nucleotides, each consisting of a nitrogenous base, a pentose sugar, and a phosphate group. A nucleoside is just the base and sugar.

The sugar in DNA is deoxyribose, lacking a hydroxyl group at the 2' carbon, which contributes to its stability. RNA contains ribose, with a 2'-hydroxyl group, making it more susceptible to hydrolysis.

DNA's bases are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). RNA replaces Thymine with Uracil (U).

DNA typically forms a double helix, with two antiparallel polynucleotide strands. These strands are held together by specific hydrogen bonds between complementary bases: A pairs with T (two H-bonds), and G pairs with C (three H-bonds).

This specific pairing is crucial for genetic fidelity and is summarized by Chargaff's rules ($A=T, G=C$). The sugar-phosphate backbone of each strand is formed by strong phosphodiester bonds linking the 5'-phosphate of one nucleotide to the 3'-hydroxyl of the next, giving the strand 5' to 3' directionality.

RNA is generally single-stranded but can fold into complex structures. It exists in various functional forms: mRNA carries genetic messages, tRNA transports amino acids during protein synthesis, and rRNA is a structural and catalytic component of ribosomes. The chemical differences (sugar, bases, strandedness) between DNA and RNA dictate their distinct roles: DNA for stable genetic storage, and RNA for dynamic gene expression and regulation.

Prelims Revision Notes

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Nucleic Acids — DNA and RNA are polymers of nucleotides.
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Nucleotide Structure — Base + Pentose Sugar + Phosphate group.
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Nucleoside Structure — Base + Pentose Sugar (no phosphate).
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Pentose Sugars

* Deoxyribose (DNA): Lacks -OH group at 2' carbon. More stable. * Ribose (RNA): Has -OH group at 2' carbon. Less stable, more reactive.

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Nitrogenous Bases

* Purines: Adenine (A), Guanine (G) (double-ring). * Pyrimidines: Cytosine (C), Thymine (T), Uracil (U) (single-ring). * DNA Bases: A, G, C, T. * RNA Bases: A, G, C, U (Uracil replaces Thymine).

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Bonds in Nucleic Acids

* **$\beta$-N-glycosidic bond**: Links nitrogenous base to 1' carbon of pentose sugar. * Phosphoester bond: Links phosphate group to 5' carbon of pentose sugar. * Phosphodiester bond: Links 5'-phosphate of one nucleotide to 3'-OH of another, forming the sugar-phosphate backbone. Gives 5' to 3' directionality. * Hydrogen bonds: Non-covalent bonds between complementary bases in dsDNA. * A=T (2 H-bonds) * G≡C (3 H-bonds)

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DNA Structure

* Double helix, antiparallel strands (one 5'->3', other 3'->5'). * Chargaff's Rules: In dsDNA, $A=T$ and $G=C$. Therefore, $A+G = T+C$. * Stores genetic information.

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RNA Structure & Types

* Typically single-stranded, can fold into complex 3D structures. * mRNA (messenger RNA): Carries genetic code from DNA to ribosomes. * tRNA (transfer RNA): Carries specific amino acids to ribosomes during protein synthesis. * rRNA (ribosomal RNA): Component of ribosomes; some have catalytic activity (ribozymes).

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Stability — DNA is more stable than RNA due to deoxyribose (no 2'-OH) and its double-helical structure.

Vyyuha Quick Recall

To remember the bases for DNA vs. RNA: DNA has T, RNA has U. (Thymine vs. Uracil). And for the sugars: DNA is 'De-Oxygenated' at 2' (Deoxyribose lacks 2'-OH).