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

Molecular Biology of the Gene

Overview

This study guide covers the structure and function of nucleic acids, the discovery of DNA structure, mechanisms of DNA replication, transcription, translation, and the impact of mutations. Understanding these processes is fundamental to molecular genetics and cell biology.

Nucleic Acid Structure

DNA and RNA: Composition and Structure

• DNA (Deoxyribonucleic Acid): A double-stranded molecule composed of nucleotides containing deoxyribose sugar, phosphate groups, and nitrogenous bases (A, T, C, G).

• RNA (Ribonucleic Acid): Usually single-stranded, composed of nucleotides with ribose sugar and bases (A, U, C, G).

• Sugar-Phosphate Backbone: The repeating chain of sugar and phosphate groups that forms the structural framework of nucleic acids.

• Nucleotide: The basic building block of nucleic acids, consisting of a sugar, a phosphate group, and a nitrogenous base.

• Nitrogenous Bases: Adenine (A), Cytosine (C), Guanine (G), Thymine (T, in DNA), and Uracil (U, in RNA).

• Purines: Double-ringed bases (Adenine and Guanine).

• Pyrimidines: Single-ringed bases (Cytosine, Thymine, and Uracil).

• Base Pairs: A-T (or A-U in RNA) and C-G, held together by hydrogen bonds.

• 5’ and 3’ Ends: Refer to the carbon positions in the sugar; the 5’ end has a phosphate group, the 3’ end has a hydroxyl group.

• Antiparallel: The two DNA strands run in opposite directions (5’ to 3’ and 3’ to 5’).

Example: The DNA double helix consists of two antiparallel strands with complementary base pairing: A pairs with T, and C pairs with G.

Discovery of DNA Structure

Key Experiments and Scientists

• Hershey-Chase Experiment: Used bacteriophages labeled with radioactive sulfur (proteins) and phosphorus (DNA) to show that DNA is the genetic material.

• Chargaff’s Rules: The amount of A equals T, and C equals G in DNA; provided evidence for base pairing.

• Rosalind Franklin: Used X-ray crystallography to reveal the helical structure and uniform diameter of DNA.

• Watson and Crick: Built the first accurate model of DNA as a double helix based on available data.

Example: The Hershey-Chase experiment demonstrated that only DNA, not protein, entered bacterial cells and directed viral replication.

DNA Replication

Models and Mechanisms

• Semiconservative Model: Each new DNA molecule consists of one old and one new strand (supported by the Meselson-Stahl experiment).

• Conservative Model: The parental DNA remains intact, and a completely new molecule is synthesized.

• Dispersive Model: Parental and new DNA are interspersed in both strands.

Key Enzymes and Steps

• Origin of Replication: Specific sequence where replication begins.

• Replication Bubble and Fork: The area where DNA unwinds and replication occurs.

• Helicase: Unwinds the DNA double helix.

• Single-Stranded Binding Proteins: Stabilize unwound DNA.

• Topoisomerase: Relieves tension ahead of the replication fork.

• Primase: Synthesizes RNA primers to initiate DNA synthesis.

• DNA Polymerase III: Extends new DNA from the primer.

• DNA Polymerase I: Replaces RNA primers with DNA.

• DNA Ligase: Joins Okazaki fragments on the lagging strand.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork in Okazaki fragments.

Example: The Meselson-Stahl experiment used isotopes of nitrogen to show that DNA replication is semiconservative.

Transcription

From DNA to RNA

• RNA Polymerase: Enzyme that synthesizes RNA from a DNA template.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Terminator Sequence: Signals the end of transcription.

• mRNA (Messenger RNA): Carries genetic information from DNA to ribosomes.

RNA Processing (Eukaryotes)

• 5’ Cap: Modified guanine nucleotide added to the 5’ end for stability and ribosome recognition.

• Poly-A Tail: String of adenine nucleotides added to the 3’ end for stability.

• Introns: Non-coding sequences removed from pre-mRNA.

• Exons: Coding sequences that remain in mature mRNA.

• Spliceosome: Complex that removes introns and joins exons.

• Alternative Splicing: Allows a single gene to code for multiple proteins by varying exon combinations.

Example: Mature mRNA contains only exons, a 5’ cap, and a poly-A tail, ready for translation.

Translation

From mRNA to Protein

• Codon: Three-nucleotide sequence on mRNA that specifies an amino acid.

• tRNA (Transfer RNA): Brings amino acids to the ribosome; contains an anticodon complementary to the mRNA codon.

• Wobble Pairing: Flexibility in base pairing at the third codon position, allowing redundancy in the genetic code.

• Genetic Code: Universal set of codon-amino acid assignments; degenerate (redundant) because multiple codons can specify the same amino acid.

• tRNA Synthetase: Enzyme that attaches the correct amino acid to its tRNA.

• Ribosome: Composed of large and small subunits; site of protein synthesis.

• A Site: Accepts incoming tRNA with amino acid.

• P Site: Holds tRNA with growing polypeptide chain.

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

• Stop Codons: UAA, UAG, UGA; signal the end of translation.

Example: The genetic code table is used to determine which codon specifies which amino acid during translation.

Mutations

Types and Effects

• Nucleotide-Pair Substitution (Point Mutation): Replacement of one base pair with another.

• Silent Mutation: Alters a codon but does not change the amino acid.

• Missense Mutation: Changes one amino acid to another.

• Nonsense Mutation: Changes a codon to a stop codon, truncating the protein.

• Nucleotide-Pair Insertion or Deletion: Addition or loss of nucleotide pairs.

• Frameshift Mutation: Insertion or deletion not in multiples of three, altering the reading frame and usually resulting in a nonfunctional protein.

Example: A frameshift mutation early in a gene is likely to disrupt protein function more than a silent mutation or a mutation in an intron.

Comparisons and Tables

DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strandedness	Double-stranded	Single-stranded
Function	Genetic storage	Information transfer, catalysis

Types of Mutations and Their Effects

Mutation Type	Effect	Likelihood of Disruption
Silent	No amino acid change	Least
Missense	One amino acid changed	Variable
Nonsense	Premature stop codon	High
Frameshift	Reading frame altered	Very high

Key Equations and Concepts

• Base Pairing Rule: $A = T$ and $C = G$ (Chargaff's rules)

• Directionality: DNA and RNA are synthesized in the 5’ to 3’ direction.

Additional Info

• Degeneracy of the Genetic Code: Multiple codons can code for the same amino acid, providing a buffer against some mutations.

• Complementarity: The property that allows specific base pairing, essential for replication and transcription fidelity.