Nucleic Acids

🧬 Nucleic Acids: The Information Molecules

Nucleic acids store, transmit, and express the genetic information of every living organism. The two types are:

• DNA (deoxyribonucleic acid) — the long-term genetic blueprint
• RNA (ribonucleic acid) — the working copy used to make proteins

Element fingerprint

Nucleic acids contain C, H, O, N, and P. The presence of phosphorus distinguishes nucleic acids from carbohydrates and most lipids.

Where you'll meet nucleic acids

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The Monomer: Nucleotide

A nucleotide has three parts:

1. Pentose sugar (5-carbon ring) — ribose in RNA, deoxyribose in DNA
2. Phosphate group — gives the molecule its acidic character
3. Nitrogenous base — purine (A, G) or pyrimidine (C, T, U)

DNA vs RNA: side-by-side

Pairing rules (Chargaff)

In DNA: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds).

In RNA: A pairs with U instead.

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Nucleic Acids: DNA & RNA

Nucleotide Structure

Each nucleotide has three components:

1. 5-carbon sugar (ribose in RNA, deoxyribose in DNA)
2. Phosphate group ($PO_4^{3-}$)
3. Nitrogenous base

DNA vs. RNA

Base Pairing Rules

• DNA: A=T (2 H-bonds), G≡C (3 H-bonds)
• RNA: A=U, G≡C

Chargaff's Rules

In any DNA molecule: %A = %T and %G = %C

If a DNA sample is 30% adenine, then:

• %T = 30%, %G = 20%, %C = 20%

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Phosphodiester Bonds & Directionality

Nucleotides are linked by phosphodiester bonds between the 3' carbon of one sugar and the 5' carbon of the next.

• DNA strands run antiparallel (5'→3' and 3'→5')
• New nucleotides are always added to the 3' end
• The sugar-phosphate backbone is on the outside
• The bases are on the inside, forming complementary pairs

This directionality is crucial for replication and transcription.

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Functions Beyond Information Storage

The central dogma

$DNA \xrightarrow{\text{transcription}} mRNA \xrightarrow{\text{translation}} \text{Protein}$

DNA is the archive. mRNA is the photocopy sent to the workshop. Protein is the product.

Why double-stranded matters

The double helix is antiparallel (5′→3′ on one strand opposite 3′→5′ on the other) and complementary — each strand can serve as a template for an exact copy. This is what makes DNA replication possible.

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Nucleotides as Energy Currency

ATP (adenosine triphosphate) is a nucleotide with three phosphate groups. The bonds between phosphates are high-energy — when one is broken, energy is released to drive cellular work.

$ATP \rightarrow ADP + P_i + \text{energy}$

This single reaction powers nearly everything cells do — muscle contraction, active transport, biosynthesis, signaling.

Other nucleotide-based cofactors

• $NAD^{+}$ / NADH — electron carrier in respiration
• FAD / $FADH_{2}$ — electron carrier in respiration
• NADP${}^{+}$ / NADPH — electron carrier in photosynthesis and biosynthesis
• cAMP — second messenger in cell signaling

So nucleotides do far more than store information — they are also the cell's energy and signaling currency.

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🎯 AP Review: Nucleic Acids

Must-know synthesis points

1. Nucleotides = sugar + phosphate + nitrogenous base.
2. DNA vs RNA — DNA: deoxyribose, T, double-stranded, stable. RNA: ribose, U, usually single-stranded.
3. Chargaff's pairing — A–T (or A–U), G–C; G–C bonds are stronger (3 H-bonds).
4. Antiparallel orientation enables semiconservative replication and accurate copying.
5. Central dogma — DNA → mRNA → protein (transcription → translation).
6. Nucleotides also serve as energy carriers — ATP, $NAD^{+}$, FAD, cAMP all share a nucleotide backbone.

Common AP traps

• mRNA is the photocopy, not the original. The DNA stays in the nucleus (in eukaryotes).
• A nucleotide is the monomer; nucleic acid is the polymer.
• Both DNA and RNA contain phosphate groups — phosphate isn't unique to one.

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Workshop Problem 📐

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