DNA & RNA Structure: Foundations of Molecular Biology

Topic 1: DNA & RNA Structure

Course Overview

This section introduces the fundamental questions and concepts in molecular biology, focusing on the structure and function of DNA, RNA, and proteins. The major emphasis is on gene expression and its role in cellular function and human disease.

• Key Questions:

  • What are the structures and functions of DNA, RNA, and protein?

  • How are DNA, RNA, and protein produced?

  • What methods are available for studying and manipulating these molecules?

  • How does a cell control and modify its properties using these molecules?

  • How can molecular biology be used to understand, diagnose, and treat human diseases?

Learning Objectives

Students should be able to:

• Define key terms: nucleic acids, base, sugar, phosphate, nucleotide vs. nucleoside, polymerization, 5' and 3' ends, base-pairing, sense vs. anti-sense.

• List and identify the three components of nucleic acids and their arrangement in a nucleotide.

• Differentiate between the chemical structures of DNA and RNA (bases used, sugars used, strandedness).

• Diagram how a nucleotide is added to an existing nucleic acid chain.

• Explain the Watson and Crick model of DNA structure.

• Label the ends (5' vs. 3') and strands (sense vs. antisense) of nucleic acids according to conventional rules.

Big Picture: Nucleic Acids as Genetic Material

Overview of Nucleic Acid Structure

DNA and RNA are polymers made of building blocks called nucleotides. Each nucleotide consists of three components: a nitrogenous base, a five-carbon sugar, and a phosphate group.

• Nucleotides encode genetic information; the sequence of bases is critical for function.

• Sugars and phosphates form the backbone of the molecule.

• Base-pairing holds two strands together and allows for easy separation during replication and transcription.

• DNA and RNA have subtle differences in composition and structure but are both polymerized in one direction: the 3'-OH group is essential for chain elongation.

Nucleotide Structure

A nucleotide is composed of:

• Base: Nitrogenous base (Adenine, Thymine, Cytosine, Guanine, Uracil)

• Sugar: Deoxyribose (DNA) or Ribose (RNA)

• Phosphate group: One or more phosphate groups attached to the 5' carbon of the sugar

Nucleoside = Base + Sugar Nucleotide = Base + Sugar + Phosphate

Types of Nitrogenous Bases

• Purines: Adenine (A), Guanine (G)

• Pyrimidines: Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only)

• Mnemonic: "Angels and God are PURE" (A and G are purines)

DNA vs. RNA: Chemical Differences

• DNA: Deoxyribose sugar (lacks 2' OH group), uses Thymine (T), usually double-stranded

• RNA: Ribose sugar (has 2' OH group), uses Uracil (U), usually single-stranded

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Base	Thymine (T)	Uracil (U)
Strandedness	Double-stranded	Single-stranded

Polymerization of Nucleic Acids

Nucleic acids are synthesized by adding nucleotides to the 3' end of the existing chain. The process is called polymerization and involves the formation of phosphodiester bonds.

• Free nucleotides have three phosphate groups; during polymerization, two are released as pyrophosphate.

• Polymerization only occurs at the 3' end of the molecule.

Polymerization Reaction:

$\text{(Nucleotide)}_{n} + \text{dNTP} \rightarrow \text{(Nucleotide)}_{n+1} + \text{PP}_i$

Watson and Crick Model of DNA Structure

The Watson and Crick model describes DNA as a double helix with two antiparallel strands held together by specific base-pairing.

• Base-pairing: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds)

• Antiparallel orientation: One strand runs 5' to 3', the other 3' to 5'

• Major and minor grooves: Structural features that affect protein binding and DNA function

Base Pair	Number of Hydrogen Bonds
A-T	2
G-C	3

Labeling DNA Strands: 5' and 3' Ends, Sense and Antisense

DNA strands are conventionally labeled with a 5' end (free phosphate group) and a 3' end (free hydroxyl group). Strands are always antiparallel.

• Sense strand: The DNA strand with the same sequence as the resulting RNA (except T is replaced by U)

• Antisense strand: The template strand used for RNA synthesis

Strand	Direction	Function
Sense	5' to 3'	Matches RNA sequence
Antisense	3' to 5'	Template for transcription

Denaturation and Hybridization

Double-stranded DNA can be separated into single strands by heat, high pH, chemicals, or proteins. This process is called denaturation or "melting" DNA. Hybridization refers to the re-annealing of complementary strands.

• High G:C content makes DNA harder to denature due to more hydrogen bonds.

• Base-pairing is the basis for many molecular biology techniques, such as PCR and DNA probes.

Applications in Human Health

Understanding nucleic acid structure and base-pairing enables the detection and diagnosis of human diseases, identification of pathogens, and forensic analysis.

• Use of antisense probes to detect specific DNA sequences

• Polymerase Chain Reaction (PCR) for amplifying DNA

• Detection of unusual nucleic acid structures in pathogens

Summary Table: Key Terms

Term	Definition
Nucleic Acid	Polymer of nucleotides (DNA or RNA)
Nucleotide	Base + Sugar + Phosphate
Nucleoside	Base + Sugar
Polymerization	Process of linking nucleotides into a chain
5' End	End with free phosphate group
3' End	End with free hydroxyl group
Sense Strand	DNA strand with same sequence as RNA
Antisense Strand	Template strand for RNA synthesis

Additional info: The notes infer some context about the importance of base-pairing in molecular biology techniques and human health applications, as well as the conventions for labeling DNA strands and the significance of major/minor grooves in DNA structure.