The Molecular Basis of Inheritance & Gene Expression: From Gene to Protein

The Molecular Basis of Inheritance

Key Contributors to DNA Discovery

• Rosalind Franklin: Used X-ray crystallography to reveal the helical structure of DNA, providing critical evidence for the double helix model.

• James Watson & Francis Crick: Built the first accurate model of DNA's double helix structure, integrating Franklin's data and Chargaff's rules.

Evidence That DNA Is the Genetic Material

• Griffith's Transformation Experiment: Demonstrated that a "transforming principle" from dead bacteria could genetically alter living bacteria.

• Avery, MacLeod, and McCarty: Identified DNA as the "transforming principle" responsible for heredity.

• Hershey-Chase Experiment: Showed that DNA, not protein, is the genetic material in bacteriophages by using radioactive labeling.

Nucleic Acids: Types and Functions

• DNA (Deoxyribonucleic Acid): Stores genetic information; double-stranded.

• RNA (Ribonucleic Acid): Involved in protein synthesis; usually single-stranded.

Structural Differences: DNA vs. RNA

• Sugar: DNA contains deoxyribose; RNA contains ribose.

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

• Strandedness: DNA is double-stranded; RNA is single-stranded.

Formation of Nucleic Acid Strands

• Condensation (Dehydration) Synthesis: Nucleotides are joined by phosphodiester bonds, releasing water.

• Directionality: Strands have a 5' (phosphate) end and a 3' (hydroxyl) end.

DNA Double Helix Structure

• Antiparallel Strands: Two strands run in opposite directions (5' to 3' and 3' to 5').

• Complementary Base Pairing: A pairs with T, C pairs with G via hydrogen bonds.

• Double Helix: Twisted ladder structure with sugar-phosphate backbones on the outside and bases on the inside.

DNA Replication

• Semiconservative Replication: Each new DNA molecule consists of one old strand and one new strand.

• Origin of Replication: Specific sequence where replication begins, forming a "bubble" with two replication forks.

• Bidirectional Replication: Replication proceeds in both directions from the origin.

Key Enzymes and Proteins in DNA Replication

• Helicase: Unwinds the DNA double helix.

• Single-Strand Binding Proteins: Stabilize unwound DNA.

• Topoisomerase: Relieves supercoiling ahead of the replication fork.

• Primase: Synthesizes short RNA primers.

• DNA Polymerase III: Adds nucleotides to the 3' end of the primer, synthesizing new DNA.

• DNA Polymerase I: Replaces RNA primers with DNA nucleotides.

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

Leading vs. Lagging Strand Synthesis

• Leading Strand: Synthesized continuously toward the replication fork.

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

• Okazaki Fragments: Short DNA segments on the lagging strand, later joined by DNA ligase.

Replication Challenges and Solutions

• RNA Primer Removal: DNA polymerase I replaces RNA primers with DNA, except at the 5' end of the leading strand, leading to telomere shortening.

• Telomeres: Repetitive DNA sequences at chromosome ends that protect genes from erosion.

• Telomerase: Enzyme that extends telomeres in germ cells, preventing loss of genetic information.

DNA Packaging in Cells

• Supercoiling (Prokaryotes): DNA is tightly coiled in the nucleoid region.

• Chromatin (Eukaryotes): DNA is wrapped around histone proteins, forming nucleosomes ("beads on a string").

• 10 nm Fiber: Nucleosome chain.

• 30 nm Fiber: Further coiling of nucleosomes.

• Euchromatin: Loosely packed, transcriptionally active.

• Heterochromatin: Densely packed, transcriptionally inactive.

Key Terms Table: DNA Structure and Replication

Term	Definition
Origin of Replication	Specific DNA sequence where replication begins
Replication Fork	Y-shaped region where DNA is being unwound and replicated
Primer	Short RNA sequence that provides a starting point for DNA synthesis
Okazaki Fragment	Short DNA fragment synthesized on the lagging strand
Telomere	Repetitive DNA at chromosome ends, protecting coding regions
Histone	Protein around which DNA winds to form nucleosomes

Gene Expression: From Gene to Protein

Central Dogma of Molecular Biology

• Central Dogma: Information flows from DNA → RNA → Protein.

• Gene Expression: The process by which genetic information directs protein synthesis.

Genes and Proteins

• Gene: A region of DNA that encodes a functional product (usually a protein).

• Protein Structure: Composed of amino acids; enzymes are proteins that catalyze biochemical reactions.

• Beadle and Tatum: Proposed "one gene, one enzyme" hypothesis, linking genes to enzyme function.

Transcription: DNA to RNA

• Location: Nucleus (eukaryotes).

• Steps:

  1. Initiation: RNA polymerase binds to promoter (with help from transcription factors); TATA box is a common promoter element.

  2. Elongation: RNA polymerase synthesizes RNA in the 5' → 3' direction, reading the DNA template strand 3' → 5'.

  3. Termination: RNA polymerase stops at the terminator sequence, releasing the primary transcript.

• Primary Transcript: The initial RNA copy, also called pre-mRNA in eukaryotes.

RNA Processing (Eukaryotes)

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

• Poly-A Tail: String of adenine nucleotides added to the 3' end for stability and export from the nucleus.

• Splicing: Removal of non-coding introns and joining of coding exons by spliceosomes (complexes of snRNPs and proteins).

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

Translation: RNA to Protein

• Location: Cytoplasm at ribosomes.

• Types of RNA Involved: Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomal RNA (rRNA).

• Steps:

  1. Initiation: Small ribosomal subunit binds mRNA at the start codon (AUG); initiator tRNA binds; large subunit assembles.

  2. Elongation: tRNAs bring amino acids to the ribosome; peptide bonds form; ribosome moves along mRNA (A → P → E sites).

  3. Termination: Stop codon is reached; release factor binds; polypeptide is released.

• Genetic Code: Triplet codons in mRNA specify amino acids; code is redundant but not ambiguous.

• Wobble: Flexibility in base pairing at the third codon position allows some tRNAs to recognize multiple codons.

Gene Expression Summary Table

Process	Location	Main Enzyme/Structure	Product
Transcription	Nucleus	RNA Polymerase	Pre-mRNA
RNA Processing	Nucleus	Spliceosome, enzymes	Mature mRNA
Translation	Cytoplasm	Ribosome	Polypeptide

Mutations and Their Effects

• Point Mutation: Change in a single nucleotide pair (substitution, insertion, or deletion).

• Silent Mutation: No change in amino acid sequence.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Creates a premature stop codon.

• Frameshift Mutation: Insertion or deletion shifts the reading frame, altering downstream amino acids.

• Mutagen: Physical or chemical agent that increases mutation rate.

Summary of Gene Expression Steps

1. Transcription (DNA → pre-mRNA)

2. RNA Processing (pre-mRNA → mature mRNA)

3. Export of mRNA from nucleus to cytoplasm

4. Translation (mRNA → polypeptide)

5. Polypeptide folding and modification into functional protein

Key Terms Table: Gene Expression

Term	Definition
Codon	Three-nucleotide sequence in mRNA specifying an amino acid
Anticodon	Three-nucleotide sequence in tRNA complementary to mRNA codon
Promoter	DNA sequence where RNA polymerase binds to initiate transcription
Exon	Coding region of a gene
Intron	Non-coding region removed during RNA processing
Polyribosome	Multiple ribosomes translating a single mRNA simultaneously

Equations and Diagrams

• Phosphodiester Bond Formation:

• $\text{Nucleotide}_1\text{-}3'\text{OH} + \text{Nucleotide}_2\text{-}5'\text{P} \rightarrow \text{Nucleotide}_1\text{-}3'\text{O}-\text{P}-\text{O}-5'\text{Nucleotide}_2 + \text{H}_2\text{O}$

• Central Dogma:

• $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Additional info: This guide expands on the provided objectives and vocabulary, offering definitions, explanations, and tables for clarity. For exam preparation, focus on understanding the flow of genetic information, the roles of key enzymes, and the impact of mutations on gene expression.