Comprehensive Study Notes: Evolution, Epigenetics, Quantitative Genetics, and Gene Regulation

Evolution & Epigenetics

Darwin’s Theory of Evolution and Natural Selection

Darwin proposed that evolution occurs through natural selection, where organisms with advantageous traits survive and reproduce more successfully, passing these traits to future generations.

• Population Growth: Populations grow faster than resources allow, leading to competition.

• Variation: Individuals vary in their inherited traits.

• Survival and Reproduction: Individuals with beneficial traits are more likely to survive and reproduce.

Over time, beneficial traits become more common in the population.

Domestication vs. Natural Selection

• Artificial Selection: Humans select traits to breed (e.g., dogs, crops), resulting in rapid changes.

• Natural Selection: The environment selects traits that enhance survival and reproduction.

Both processes lead to changes in populations, but artificial selection is directed by humans, while natural selection is driven by environmental pressures.

Evidence for Evolution

• Homologous Structures: Similar structures in different species indicate common ancestry.

• Analogous Structures: Similar functions but different evolutionary origins.

• Fossil Record: Shows changes in organisms over time.

• Embryology: Similar embryonic development in related species.

• Biogeography: Geographic distribution of species supports evolutionary theory.

Epigenetics: Definition and Mechanisms

Epigenetics refers to changes in gene expression that do not involve alterations to the DNA sequence. These changes can be influenced by environmental factors and can be heritable.

• DNA Methylation: Addition of methyl groups to DNA, typically silencing gene expression.

• Histone Modification: Acetylation (opens DNA, increases expression) and deacetylation (closes DNA, decreases expression).

Epigenetics and the Environment

• Twin Studies: Identical twins can have different epigenetic patterns due to environmental influences.

• Famine Studies: Prenatal exposure to famine can lead to epigenetic changes affecting health.

Epigenetics in Cancer

• Hypermethylation: Can silence tumor suppressor genes.

• Hypomethylation: Can activate oncogenes.

Both excessive and insufficient DNA methylation are associated with cancer development.

Dopamine DREADDs and Mice

DISC1 Gene and Schizophrenia

• DISC1: Disrupted in schizophrenia-1; mutations affect neuron development and signaling.

• Heritability: Schizophrenia shows high heritability, with genetic and environmental contributions.

VTA-Insular Cortex Pathway

• Key Hypothesis: Dopamine signaling from the VTA to the insular cortex causes impaired reality testing in schizophrenia.

• Experimental Evidence: Manipulating dopamine signaling in mice affects reality testing and behavior.

Quantitative Genetics (Chapter 20)

Polygenic Traits and Quantitative Inheritance

Quantitative traits are influenced by many genes (polygenic) and show continuous variation (e.g., height, weight).

• Additive Alleles: Each allele adds to the effect of the trait.

• Non-Additive Alleles: Do not contribute to the trait’s effect.

Heritability

• Definition: Proportion of phenotypic variation in a population due to genetic differences.

• Formula: $h^2 = \frac{V_G}{V_P}$, where $V_G$ is genetic variance and $V_P$ is total phenotypic variance.

Quantitative Trait Loci (QTL)

• QTL: Genomic regions that contribute to variation in quantitative traits.

• Mapping: QTL mapping identifies which chromosome regions are associated with traits.

Twin Studies

• Monozygotic Twins: Identical, share 100% of genes.

• Dizygotic Twins: Fraternal, share 50% of genes.

• Concordance Rate: Probability that both twins show a trait if one does.

• Heritability Formula (Twins): $h^2 = 2(r_{MZ} - r_{DZ})$

Gene Regulation (Ch. 15 & 16)

Gene Expression Control

• All cells contain the same DNA, but gene expression varies by cell type.

• Gene regulation occurs at multiple levels: chromatin structure, transcription, RNA processing, translation, and post-translational modification.

Chromatin Structure and DNA Packaging

• Histones: Proteins that package DNA into nucleosomes.

• Acetylation: Opens chromatin, increases gene expression.

• Methylation: Usually silences genes.

Promoters, Enhancers, and Silencers

• Promoters: DNA sequences where RNA polymerase binds to initiate transcription.

• Enhancers: Increase transcription; can be distant from the gene.

• Silencers: Decrease transcription.

Alternative Splicing

• Allows one gene to produce multiple mRNA variants and proteins.

DNA Methylation

• Addition of methyl groups (usually to CpG islands) silences genes.

Gene Therapy and Genome Editing

Gene Therapy

• Ex Vivo: Cells are modified outside the body and returned to the patient.

• In Vivo: Genes are delivered directly into the patient’s body.

• Viral Vectors: Used to deliver genes; can integrate into the genome or remain episomal.

Genome Editing with CRISPR

• CRISPR-Cas9 allows targeted editing of DNA at specific sites.

• Potential for off-target effects; ethical considerations are important.

Cumulative Material for the Exam

Mitosis and Meiosis

• Mitosis: Produces genetically identical diploid cells for growth and repair.

• Meiosis: Produces haploid gametes, introduces genetic variation through crossing over and independent assortment.

Chromosome Structure and Function

• DNA Structure: Double helix, antiparallel strands, base pairing (A-T, G-C).

• Chromosome Mutations: Changes in number or structure can affect gene expression and phenotype.

Mendelian Genetics and Inheritance

• Alleles: Dominant and recessive forms of genes.

• Genotype vs. Phenotype: Genetic makeup vs. observable traits.

• Inheritance Patterns: Autosomal, sex-linked, and mitochondrial inheritance.

Genetic Code and Protein Synthesis

• Transcription: DNA to RNA.

• Translation: RNA to protein; ribosomes read codons and assemble amino acids.

• Start Codon: AUG

• Stop Codons: UAA, UAG, UGA

• Degeneracy: Multiple codons can code for the same amino acid.

Mutations

• Nonsense Mutation: Introduces a premature stop codon, truncating the protein.

• Missense Mutation: Substitutes one amino acid for another.

• Silent Mutation: No change in the protein sequence.