Molecular Basis of Inheritance

Inborn Errors of Metabolism and the Role of Genes

Early studies in genetics revealed that inherited genes can affect biochemical pathways, leading to metabolic disorders. Archibald Garrod proposed the concept of "inborn errors of metabolism," connecting genetic inheritance to biochemical function.

• Inborn Errors of Metabolism: Heritable genetic disorders caused by the absence of functional enzymes.

• Example: Alkaptonuria – caused by accumulation of homogentisic acid due to a missing enzyme in the breakdown of phenylalanine, resulting in dark urine.

• Genes and Enzymes: Genes determine which enzymes are produced, influencing cellular chemistry.

One Gene-One Enzyme Hypothesis

Beadle and Tatum used Neurospora (bread mold) to show that mutations in genes affect specific biochemical reactions, leading to the "one gene-one enzyme" hypothesis.

• Experimental Approach: Mutate mold strains and identify those unable to synthesize certain amino acids.

• Pathway Analysis: By providing intermediates, they determined which step was defective.

• Refinement: Not all genes code for enzymes; some code for structural proteins or RNAs.

• Example: One gene can yield multiple products through alternative splicing.

The Molecular Basis of Transformation

Griffith's Transformation Experiment

Griffith demonstrated that material from dead bacteria could transform living bacteria, conferring new heritable traits.

• Smooth (S) Strain: Encapsulated, virulent, causes pneumonia.

• Rough (R) Strain: Non-encapsulated, non-virulent.

• Key Finding: Live R cells mixed with heat-killed S cells caused death, indicating transformation.

Avery, MacLeod, and McCarty

Identified DNA as the transforming material by selectively destroying RNA, DNA, or proteins and observing transformation.

• Destroy DNA: No transformation occurred.

• Destroy RNA or Protein: Transformation still occurred.

• Conclusion: DNA is the genetic material responsible for transformation.

Hershey and Chase Experiment

Used bacteriophages to show that DNA, not protein, carries genetic information.

• Radioactive Labeling: DNA labeled with 32P, protein with 35S.

• Result: Only DNA entered bacteria and directed new virus formation.

Structure and Function of DNA

DNA Composition and Structure

DNA is composed of nucleotides, each containing a deoxyribose sugar, phosphate group, and nitrogenous base.

• Pyrimidines: Single-ring bases (cytosine, thymine, uracil).

• Purines: Double-ring bases (adenine, guanine).

• Chargaff's Rule: The amount of adenine equals thymine, and cytosine equals guanine.

• Double Helix: Discovered by Rosalind Franklin (X-ray crystallography); Watson and Crick proposed the model.

• Base Pairing: A-T pairs via 2 hydrogen bonds; C-G pairs via 3 hydrogen bonds.

• Antiparallel Orientation: Two strands run in opposite directions.

DNA Replication

DNA replication is semiconservative, producing two identical double helices, each with one original and one new strand.

• Initiator Protein: Recruits helicase to unwind DNA at origins of replication.

• Helicase: Unwinds the double helix.

• Single-Stranded Binding Proteins: Prevent re-annealing.

• Topoisomerase: Relieves twisting strain.

• Primase: Adds RNA primer.

• DNA Polymerase: Synthesizes new DNA in 5' to 3' direction.

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized in Okazaki fragments, joined by DNA ligase.

• Telomerase: Solves replication problem at chromosome ends (active in some cells).

Key Equation:

Transcription and RNA Processing

Transcription: DNA to RNA

Transcription is the synthesis of RNA using DNA as a template, catalyzed by RNA polymerase.

• Template Strand: DNA strand read by RNA polymerase.

• Non-Template Strand: Sequence matches RNA transcript (except T replaced by U).

• Promoter: DNA sequence where RNA polymerase binds.

• TATA Box: Common promoter element, easier to unwind due to A-T bonds.

• Direction: RNA synthesized 5' to 3'.

RNA Processing in Eukaryotes

Pre-mRNA undergoes modifications before becoming mature mRNA.

• 5' Cap: Modified guanine added to 5' end for protection and transport.

• Poly-A Tail: 50-250 adenines added to 3' end for stability and signaling.

• Splicing: Removal of introns and joining of exons by spliceosomes.

• Ribozymes: RNA molecules acting as catalysts in splicing.

Translation: RNA to Protein

Genetic Code and Translation

The genetic code is read in groups of three nucleotides (codons), each specifying an amino acid.

• Codons: 64 possible codons; 61 code for amino acids, 3 are stop codons.

• Start Codon: AUG (methionine) initiates translation.

• Degeneracy: Multiple codons can specify the same amino acid.

• Wobble: Flexibility at the third codon position allows one tRNA to recognize multiple codons.

tRNA and Ribosomes

tRNAs are adaptor molecules that match codons in mRNA to their corresponding amino acids. Ribosomes are the cellular machinery for protein synthesis.

• tRNA Structure: Anticodon loop pairs with mRNA codon; 3' end attaches amino acid.

• Aminoacyl-tRNA Synthetase: Enzyme that attaches correct amino acid to tRNA using ATP.

• Ribosome Sites: A (aminoacyl), P (peptidyl), E (exit).

Translation Steps

• Initiation: Small ribosomal subunit binds mRNA, identifies start codon, initiator tRNA binds, large subunit joins.

• Elongation: tRNA enters A site, peptide bond forms, ribosome moves forward, tRNA shifts from A to P to E site.

• Termination: Stop codon enters A site, release factor binds, polypeptide released, ribosome dissociates.

• Polyribosome: Multiple ribosomes translate a single mRNA simultaneously for efficient protein production.

Key Equation:

Regulation of Gene Expression: Prokaryote Operons

Operon Structure and Function

Operons are clusters of genes under control of a single promoter, transcribed together as one mRNA.

• Promoter: Site where RNA polymerase binds.

• Operator: DNA region where repressor binds.

• Regulatory Gene: Encodes repressor protein.

Lac Operon (Inducible)

The lac operon enables E. coli to metabolize lactose only when it is present.

• Genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase).

• Regulation: Lac repressor binds operator when lactose is absent, blocking transcription.

• Inducer: Allolactose binds repressor when lactose is present, inactivating it and allowing transcription.

Trp Operon (Repressible)

The trp operon controls synthesis of tryptophan, repressing gene expression when tryptophan is abundant.

• Genes: trpE, trpD, trpC, trpB, trpA.

• Regulation: Tryptophan acts as a corepressor, activating the repressor to block transcription when present.

• Feedback Inhibition: Tryptophan inhibits both enzyme activity and gene expression.

Comparison Table: Inducible vs. Repressible Operons

Protein Structure and Function

Levels of Protein Structure

Proteins are polymers of amino acids, folded into specific structures that determine their function.

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Alpha-helix and beta-pleated sheet, stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape due to interactions among side chains.

• Quaternary Structure: Multiple polypeptide chains interacting.

Peptide Bond Formation

• Dehydration Synthesis: Carboxyl group of one amino acid joins amino group of another, releasing water.

• Polypeptide: Has amino (N) and carboxyl (C) ends.

Mutations and Their Effects

Types of Mutations

Mutations are heritable changes in genetic material, affecting gene function and protein structure.

• Point Mutations: Single nucleotide changes (base substitution).

• Frameshift Mutations: Insertions or deletions altering reading frame.

• Large-Scale Mutations: Affect chromosome structure (e.g., trinucleotide expansion, non-disjunction).

• Somatic Mutations: Affect cells within an organism, not passed to offspring.

• Germline Mutations: Passed to offspring.

• Polymorphisms: Common variants (>1% frequency), not considered mutations.

Causes of Mutations

• Spontaneous: Errors during DNA replication.

• Induced: Caused by physical (X-rays, UV) or chemical agents.

Cell Cycle Regulation and Cancer

Cell Cycle Control Genes

Cell cycle progression is regulated by genes that provide stop and go signals. Disruption can lead to cancer.

• Proto-oncogenes: Promote cell division; mutations can cause uncontrolled division (oncogenes).

• Tumor Suppressor Genes: Prevent cell division; mutations inactivate their function.

• Dominant Mutations: One mutated allele is sufficient for oncogenic effect.

• Recessive Mutations: Both alleles must be mutated to lose tumor suppressor function.

Sporadic vs. Familial Cancer

• Sporadic: Accumulation of random mutations; most cancers are sporadic.

• Familial: Inherited mutations in tumor suppressor genes increase risk.

Example: Inheriting one defective tumor suppressor gene increases likelihood of developing cancer compared to acquiring two random mutations.

Additional info: Some explanations and context were expanded for clarity and completeness, including definitions, examples, and comparisons.