Topic 1.6 Notes – Nucleic Acids

What Nucleic Acids Are

Nucleic acids are macromolecules made of repeating subunits called nucleotides. Their core job is to store, transmit, and express genetic information.

There are two types:

• DNA (deoxyribonucleic acid)
  • Long-term information storage
  • Found in chromosomes
• RNA (ribonucleic acid)
  • Carries and helps use that information
  • Central in protein synthesis

A gene is a specific segment of DNA with a particular nucleotide sequence that codes for a functional product, usually a protein.

The key idea for this whole topic:

The order of nucleotides is the biological instruction manual of the cell.

Change the sequence → change the instructions → change the protein → change the trait.

The Structure of a Nucleotide

All nucleic acids are built from nucleotides, and each nucleotide has three parts.

1. Five-Carbon Sugar

• DNA → deoxyribose
• RNA → ribose
• Carbons are numbered 1′ to 5′
  • 3′ carbon has a hydroxyl group (-OH)
  • 5′ carbon attaches to a phosphate group

You don’t need to memorize the full chemical structure for AP, but you must understand the 3′ and 5′ positions because they determine strand direction.

2. Phosphate Group

• Links nucleotides together via covalent phosphodiester bonds
• Forms part of the sugar-phosphate backbone

These covalent bonds are strong. They hold each strand together.

3. Nitrogenous Base

There are five total bases:

• Adenine (A)
• Guanine (G)
• Cytosine (C)
• Thymine (T) - DNA only
• Uracil (U) - RNA only

Two categories:

• Purines (double ring): A, G
• Pyrimidines (single ring): C, T, U

A purine always pairs with a pyrimidine. That keeps the width of DNA consistent.

DNA and RNA Structure

Linear Sequence and Directionality

Nucleotides form a linear chain, but it’s not random. The strand has direction.

• 5′ end → free phosphate
• 3′ end → free -OH

New nucleotides are always added to the 3′ end during synthesis.
So strands grow 5′ → 3′.

Here’s what that orientation looks like at the level of a single nucleotide and its sugar carbons:

If you see a question about replication or transcription direction, this is what it’s testing. Always identify the 5′ and 3′ ends first.

DNA Structure

DNA is:

• Double-stranded
• Twisted into a double helix
• Antiparallel

Antiparallel means:

• One strand runs 5′ → 3′
• The other runs 3′ → 5′

Base pairing follows specific rules (Chargaff’s rule):

• A pairs with T (2 hydrogen bonds)
• G pairs with C (3 hydrogen bonds)

Hydrogen bonds hold the two strands together.
Covalent bonds hold each strand together.

Here’s the overall structure, including the antiparallel 5′ and 3′ ends and complementary base pairing:

Consequences you must know:

• %A = %T
• %G = %C
• The sequence of one strand determines the other.

If an MCQ gives base percentages, use those equalities.

RNA Structure

RNA is usually:

• Single-stranded
• Has a sugar-phosphate backbone
• Uses uracil instead of thymine

Base pairing:

• A pairs with U
• G pairs with C

Three major types:

• mRNA - carries instructions from DNA
• tRNA - brings amino acids
• rRNA - structural part of ribosomes

Even though RNA is single-stranded, it can fold and base-pair with itself.

Structural Differences Between DNA and RNA

DNA is more chemically stable, which makes it ideal for long-term storage.
RNA is more reactive and flexible, which makes it useful for temporary messages and functional roles.

How Nucleic Acids Store and Transmit Information

Everything comes back to sequence.

Information Storage

• The sequence of bases encodes instructions.
• Groups of three bases (codons, later topic) specify amino acids.
• Changing the base sequence can change the protein’s structure and function.

This is where mutations matter. A single base change can alter a trait.

Complementary Base Pairing Preserves Information

During DNA replication:

1. The two strands separate.
2. Each serves as a template.
3. Complementary base pairing ensures accurate copying.

Because A only pairs with T and G only with C, the sequence is reliably preserved.

When the AP gives you an experiment about replication errors, think about how base pairing maintains fidelity.

Flow of Genetic Information

The overall direction of information flow is:

DNA → RNA → Protein

• DNA stores the information.
• RNA carries and helps interpret it.
• Proteins carry out most cellular functions.

The structure of nucleic acids makes this flow possible.