Nucleic Acids — Revision Notes

⚡ 30-Second Revision

Nucleotide: — Base + Sugar + Phosphate
Nucleoside: — Base + Sugar
DNA Sugar: — Deoxyribose (no 2'-OH)
RNA Sugar: — Ribose (with 2'-OH)
DNA Bases: — A, G, C, T
RNA Bases: — A, G, C, U
Base-Sugar Bond: — N-glycosidic bond
Sugar-Phosphate Bond: — Ester bond
Nucleotide-Nucleotide Bond: — Phosphodiester bond (3'-5' linkage)
DNA Structure: — Double helix, antiparallel strands
Base Pairing (DNA): — $A=T$ (2 H-bonds), $G equiv C$ (3 H-bonds)
Chargaff's Rules (dsDNA): — $A=T$, $G=C$, $(A+G)=(T+C)$
DNA Stability: — Higher (due to deoxyribose, double helix)
RNA Stability: — Lower (due to ribose 2'-OH, usually single-stranded)
Types of RNA: — mRNA, tRNA, rRNA (main functional types)

2-Minute Revision

Nucleic acids, DNA and RNA, are the carriers of genetic information. Their basic building blocks are nucleotides, which comprise a nitrogenous base, a pentose sugar, and a phosphate group. Nucleosides are just the base and sugar.

DNA contains deoxyribose sugar and bases A, G, C, T, forming a stable double helix with A pairing with T (2 H-bonds) and G pairing with C (3 H-bonds). This complementary pairing is summarized by Chargaff's rules ($A=T, G=C$).

RNA contains ribose sugar and bases A, G, C, U (Uracil replaces Thymine), and is typically single-stranded, making it less stable due to the reactive 2'-OH group in ribose. Nucleotides are linked by strong phosphodiester bonds to form the sugar-phosphate backbone.

Remember the distinct roles: DNA stores information, while RNA (mRNA, tRNA, rRNA) expresses it through protein synthesis. Focus on structural differences and their functional implications for NEET.

5-Minute Revision

Nucleic acids are biopolymers critical for life, primarily DNA and RNA. They are polymers of nucleotides. A nucleotide has three parts: a nitrogenous base, a pentose sugar, and a phosphate group. A nucleoside is just the base and sugar.

The base links to the C1' of the sugar via an N-glycosidic bond, and the phosphate links to the C5' of the sugar via an ester bond. Nucleotides polymerize through phosphodiester bonds (between 3'-OH of one sugar and 5'-phosphate of another) to form the sugar-phosphate backbone.

DNA (Deoxyribonucleic Acid):

Sugar: — Deoxyribose (lacks -OH at C2').
Bases: — Adenine (A), Guanine (G), Cytosine (C), Thymine (T).
Structure: — Typically a double helix, two antiparallel strands.
Bonding: — Strands held by hydrogen bonds: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds).
Chargaff's Rules: — In dsDNA, $A=T$ and $G=C$, thus $(A+G)=(T+C)$.
Stability: — Highly stable, due to deoxyribose (no 2'-OH) and double-stranded structure.

RNA (Ribonucleic Acid):

Sugar: — Ribose (has -OH at C2').
Bases: — Adenine (A), Guanine (G), Cytosine (C), Uracil (U).
Structure: — Usually single-stranded, can fold into complex 3D shapes.
Stability: — Less stable than DNA, primarily due to the reactive 2'-OH group in ribose, making it prone to hydrolysis.
Types: — mRNA (messenger), tRNA (transfer), rRNA (ribosomal) – all crucial for protein synthesis.

Key Differences to Remember:

Sugar: — Deoxyribose (DNA) vs. Ribose (RNA)
Base: — Thymine (DNA) vs. Uracil (RNA)
Strandedness: — Double (DNA) vs. Single (RNA)
Stability: — More (DNA) vs. Less (RNA)

Example: If a DNA sample has 25% Adenine, then T = 25%. Total A+T = 50%. Remaining G+C = 50%. So, G = 25% and C = 25%.

Prelims Revision Notes

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Nucleotide vs. Nucleoside: — A nucleotide is (Base + Sugar + Phosphate). A nucleoside is (Base + Sugar). Remember the 'P' for phosphate in 'nucleotide'.
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Components of a Nucleotide:

* Nitrogenous Base: Purines (Adenine, Guanine - double ring) and Pyrimidines (Cytosine, Thymine, Uracil - single ring). Remember 'CUT' for pyrimidines. 'AG' for purines. * Pentose Sugar: Deoxyribose in DNA (lacks -OH at C2'). Ribose in RNA (has -OH at C2'). The 2'-OH makes RNA more reactive and less stable. * Phosphate Group: Attached to C5' of the sugar.

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Bonds:

* N-glycosidic bond: Links base to C1' of sugar. * Ester bond: Links phosphate to C5' of sugar. * Phosphodiester bond: Links 3'-OH of one sugar to 5'-phosphate of another, forming the backbone. This is a strong covalent bond. * Hydrogen bonds: Non-covalent, hold complementary bases together in double helix (A=T, G≡C).

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DNA Structure: — Double helix, antiparallel strands (one 5'->3', other 3'->5'). Diameter $20,\text{Å}$. One turn is $3.4,\text{nm}$ with 10 base pairs.
2
Chargaff's Rules (for double-stranded DNA):

* Amount of Adenine (A) = Amount of Thymine (T) * Amount of Guanine (G) = Amount of Cytosine (C) * Total Purines (A+G) = Total Pyrimidines (T+C) * The ratio $(A+T)/(G+C)$ is constant for a species but varies between species.

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RNA Types and Functions:

* mRNA (messenger RNA): Carries genetic code from DNA to ribosome. * tRNA (transfer RNA): Carries specific amino acids to ribosome during protein synthesis. * rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

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Key Differences (DNA vs. RNA):

* Sugar: Deoxyribose vs. Ribose * Base: Thymine vs. Uracil * Strandedness: Double vs. Single (mostly) * Stability: More stable vs. Less stable (due to 2'-OH) * Primary Function: Genetic storage vs. Genetic expression.

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Nomenclature: — Be careful: Adenine (base), Adenosine (nucleoside), Adenosine Monophosphate (nucleotide). Deoxyadenosine (DNA nucleoside), Deoxyadenosine Monophosphate (DNA nucleotide).

Vyyuha Quick Recall

To remember the components of a nucleotide: Be Sure to Phosphate! (Base, Sugar, Phosphate). For DNA bases: All Tigers Grow Claws (A-T, G-C). For RNA, just remember 'U' replaces 'T'.