Nucleic Acids — Revision Notes

⚡ 30-Second Revision

Monomer — Nucleotide
Nucleotide = Sugar + Base + Phosphate
Nucleoside = Sugar + Base
DNA Sugar — Deoxyribose (no -OH at 2')
RNA Sugar — Ribose (with -OH at 2')
DNA Bases — A, T, G, C
RNA Bases — A, U, G, C (Uracil replaces Thymine)
Base Pairing (DNA) — A=T (2 H-bonds), G≡C (3 H-bonds)
Chargaff's Rules — In dsDNA, A=T, G=C; A+G = T+C
Bonds — N-glycosidic (Base-Sugar), Phosphodiester (Sugar-Phosphate backbone), Hydrogen (between base pairs)
DNA Structure — Double helix, antiparallel strands (5' to 3', 3' to 5')
RNA Types — mRNA (messenger), tRNA (transfer), rRNA (ribosomal)

2-Minute Revision

Nucleic acids, DNA and RNA, are the genetic material and key players in gene expression. Their fundamental unit is the nucleotide, comprising a pentose sugar, a nitrogenous base, and a phosphate group.

DNA contains deoxyribose sugar and bases A, T, G, C, forming a stable double helix with antiparallel strands linked by A-T (2 H-bonds) and G-C (3 H-bonds) pairing. RNA has ribose sugar and bases A, U, G, C, and is typically single-stranded, making it less stable.

Nucleotides are joined by phosphodiester bonds to form polynucleotide chains. Key RNA types include mRNA (carries genetic code), tRNA (transports amino acids), and rRNA (ribosomal component). Remember Chargaff's rules for DNA: A=T and G=C.

The N-glycosidic bond connects the base to the sugar. These structural and functional distinctions are crucial for NEET.

5-Minute Revision

Begin your revision by clearly differentiating between DNA and RNA. DNA, the genetic blueprint, uses deoxyribose sugar and bases Adenine, Guanine, Cytosine, and Thymine. It forms a stable double helix with two antiparallel strands, where A pairs with T via two hydrogen bonds, and G pairs with C via three hydrogen bonds.

This complementary pairing is summarized by Chargaff's rules (A=T, G=C). RNA, involved in gene expression, uses ribose sugar and bases Adenine, Guanine, Cytosine, and Uracil (replacing Thymine). It's generally single-stranded and less stable due to the 2'-OH group in ribose.

Next, focus on the building blocks: a nucleotide is a nucleoside (sugar + base via N-glycosidic bond) plus a phosphate group. These nucleotides polymerize through phosphodiester bonds, forming the sugar-phosphate backbone of the nucleic acid chain, which has a distinct 5' to 3' polarity.

Understand the specific roles of the three major RNA types: mRNA carries the genetic message from DNA to ribosomes, tRNA acts as an adaptor molecule bringing amino acids to the ribosome, and rRNA is a structural and catalytic component of the ribosome itself.

Pay attention to the types of bonds: N-glycosidic (base-sugar), phosphodiester (sugar-phosphate backbone), and hydrogen bonds (between base pairs). A common mistake is confusing the number of hydrogen bonds in A-T vs.

G-C pairs. Practice applying Chargaff's rules to calculate base percentages in DNA. For example, if a DNA sample has 25% Adenine, then it must also have 25% Thymine. This leaves 50% for Guanine and Cytosine combined, so each would be 25%.

Prelims Revision Notes

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Nucleic Acid Types — DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid).
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Monomer Unit — Nucleotide.
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Nucleotide Composition — Pentose sugar + Nitrogenous base + Phosphate group.
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Nucleoside Composition — Pentose sugar + Nitrogenous base.
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Pentose Sugars

* DNA: Deoxyribose (lacks -OH at 2' carbon). * RNA: Ribose (has -OH at 2' carbon).

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Nitrogenous Bases

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

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Bonds

* N-glycosidic bond: Links nitrogenous base to C1' of pentose sugar. * Phosphodiester bond: Links 5'-phosphate of one nucleotide to 3'-OH of adjacent nucleotide, forming sugar-phosphate backbone. * Hydrogen bonds: Between complementary base pairs (A-T/U, G-C).

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DNA Structure

* Double helix, right-handed. * Two polynucleotide strands, antiparallel (one 5'->3', other 3'->5'). * Complementary base pairing: A pairs with T (2 H-bonds); G pairs with C (3 H-bonds). * Chargaff's Rules: In dsDNA, A=T and G=C. Therefore, (A+G) = (T+C).

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RNA Structure

* Typically single-stranded. * Can fold into complex 3D structures (e.g., tRNA).

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Functions of RNA Types

* 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): Structural and catalytic component of ribosomes.

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

Vyyuha Quick Recall

To remember the purines and pyrimidines:

'Pure As Gold': Purines are Adenine and Guanine.

'CUT the Py': Cytosine, Uracil, Thymine are Pyrimidines.