Molecular Biology of the Gene: Structure, Function, and Expression

Chapter 10: Molecular Biology of the Gene

Introduction: Viruses and the Genetic Material

Understanding the molecular basis of heredity is essential for combating viral diseases such as measles. The study of DNA and RNA, the molecules of heredity, provides the foundation for modern genetics and molecular biology.

The Structure of the Genetic Material

Experiments Demonstrating DNA as Genetic Material

Early 20th-century experiments sought to identify the molecule responsible for inheritance. Frederick Griffith's transformation experiment and subsequent work by Avery, McCarty, and MacLeod demonstrated that DNA is the hereditary material.

• Griffith's Experiment: Showed that non-pathogenic bacteria could be transformed into pathogenic forms by exposure to heat-killed pathogenic bacteria.

• Avery, McCarty, and MacLeod: Identified DNA as the transforming substance by using enzymes to selectively degrade proteins, RNA, or DNA.

Enzyme Added	Result in Mice
Protease	Mice Die
RNase	Mice Die
DNase	Mice Live

Interpretation: Only DNase (which destroys DNA) prevented transformation, indicating DNA is the genetic material.

The Hershey-Chase Experiment

Hershey and Chase used bacteriophages to confirm that DNA, not protein, is the genetic material injected into bacteria to direct viral replication.

• Phage T2: Consists of DNA and protein; only DNA enters the bacterial cell during infection.

DNA and RNA Structure

Polymers of Nucleotides

DNA and RNA are nucleic acids composed of long chains of nucleotides. Each nucleotide consists of a nitrogenous base, a five-carbon sugar, and a phosphate group.

• DNA Bases: Adenine (A), Cytosine (C), Thymine (T), Guanine (G)

• RNA Bases: Adenine (A), Cytosine (C), Uracil (U), Guanine (G)

• Sugar: Deoxyribose in DNA, ribose in RNA

Double Helix Structure

Watson and Crick determined that DNA is a double helix, with two antiparallel strands held together by hydrogen bonds between complementary bases (A-T, G-C).

• Base Pairing: A pairs with T, G pairs with C

• Genetic Information: Encoded in the sequence of nucleotides

DNA Replication

Semiconservative Model

DNA replication involves the separation of strands and synthesis of complementary strands, resulting in two DNA molecules, each with one old and one new strand.

• Enzymes: DNA polymerase synthesizes new DNA; DNA ligase joins fragments on the lagging strand.

The Flow of Genetic Information: DNA → RNA → Protein

Gene Expression

Genes control phenotypic traits by directing the synthesis of proteins through two main processes: transcription and translation.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a polypeptide using the information in mRNA.

The Genetic Code

The genetic code consists of codons—triplets of nucleotides in mRNA—that specify amino acids. The code is nearly universal among organisms.

• Codon: Three-base sequence in mRNA that codes for an amino acid.

• Start Codon: AUG (methionine) signals the start of translation.

• Stop Codons: UAA, UAG, UGA signal termination of translation.

Transcription and RNA Processing

Transcription

RNA polymerase binds to a promoter sequence and synthesizes RNA until it reaches a terminator sequence.

• Promoter: DNA sequence where RNA polymerase initiates transcription.

• Terminator: Sequence signaling the end of a gene.

RNA Processing in Eukaryotes

Before mRNA leaves the nucleus, it undergoes processing:

• Introns are removed, exons are spliced together.

• A 5' cap and a 3' poly-A tail are added for stability and export.

Translation and Protein Synthesis

tRNA and Ribosomes

tRNA molecules match amino acids to codons in mRNA via their anticodon regions. Ribosomes facilitate the coupling of tRNA anticodons with mRNA codons and catalyze peptide bond formation.

Stages of Translation

• Initiation: Assembly of the translation machinery at the start codon.

• Elongation: Addition of amino acids to the growing polypeptide chain.

• Termination: Release of the completed polypeptide when a stop codon is reached.

Mutations and Their Effects

Types of Mutations

Mutations are changes in the DNA sequence that can affect gene function.

• Substitution: One base is replaced by another; may result in silent, missense, or nonsense mutations.

• Insertion/Deletion: Addition or loss of nucleotides; may cause frameshift mutations, altering downstream amino acid sequence.

Mutations can arise spontaneously or be induced by mutagens (physical or chemical agents).

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded	Single-stranded
Function	Genetic information storage	Protein synthesis, gene regulation

Key Equations

• Base Pairing Rule: $A = T$, $G = C$

• Central Dogma: $DNA \rightarrow RNA \rightarrow Protein$

Additional info: This summary integrates foundational experiments, molecular structures, and the flow of genetic information, providing a comprehensive overview suitable for exam preparation in a college-level biology course.