Genetics and Evolution

Molecular Basis of Inheritance - Eukaryotic Chromosomes

  • Eukaryotic chromosomes
    • Found in the nucleus of eukaryotic cells
    • Composed of DNA and proteins
    • Carry genetic information
  • Structure of eukaryotic chromosomes
    • Consists of chromatin fibers
    • Chromatin fibers are made up of DNA and proteins
    • Can be condensed or decondensed
    • Different levels of organization
  • Levels of organization
    • DNA double helix
    • Nucleosomes
    • Chromatin fibers
    • Loops
    • Coils
    • Supercoils
  • DNA double helix
    • Double-stranded structure
    • Composed of nucleotides
    • Base pairing: A-T, G-C
    • Antiparallel strands
    • Backbone made up of sugar and phosphate molecules
  • Nucleosomes
    • Basic unit of DNA packaging
    • Consists of a core particle and linker DNA
    • Core particle composed of histone proteins
    • DNA wraps around the core particle
  • Chromatin fibers
    • Nucleosomes packed together
    • Further coiled and condensed
    • Forms chromatin thread
  • Loops
    • Chromatin fibers form loops
    • Looping promotes gene regulation
    • Interaction of different regulatory regions
  • Coils
    • Further coiling of looped chromatin fibers
    • Compact structure of chromosomes
    • Essential for proper chromosome segregation
  • Supercoils
    • Additional twisting of coiled chromatin fibers
    • Helps maintain chromosome stability
    • Ensures proper DNA packaging
  • Chromosome territories
    • Each chromosome occupies a specific region in the nucleus
    • Topological organization of chromosomes
    • Chromosome territories help regulate gene expression

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  • Chromosome abnormalities
    • Structural abnormalities
      • Deletion: Loss of a portion of a chromosome
      • Duplication: Presence of extra copies of a chromosome segment
      • Inversion: Reversal of a chromosome segment orientation
      • Translocation: Exchange of chromosome segments between non-homologous chromosomes
    • Numerical abnormalities
      • Aneuploidy: Extra or missing chromosomes
        • Trisomy: Presence of an extra chromosome (e.g., Down syndrome)
        • Monosomy: Absence of a chromosome (e.g., Turner syndrome)
      • Polyploidy: Extra sets of chromosomes (e.g., tetraploidy, triploidy)

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  • Chromosome disorders
    • Down syndrome (Trisomy 21)
      • Extra copy of chromosome 21
      • Intellectual disability, characteristic facial features, and developmental delays
      • Occurs in approximately 1 in 800 births
      • Risk increases with maternal age
    • Turner syndrome (Monosomy X)
      • Missing or partially missing X chromosome in females
      • Short stature, infertility, and webbed neck
      • Occurs in approximately 1 in 2,500 female births
    • Klinefelter syndrome (XXY)
      • Extra X chromosome in males
      • Infertility, reduced testosterone levels, and developmental delays
      • Occurs in approximately 1 in 500 male births

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  • Regulation of gene expression
    • Gene expression can be regulated at multiple steps:
      • Transcriptional regulation: Control of mRNA synthesis
      • Post-transcriptional regulation: Processing, transport, and stability of mRNA
      • Translational regulation: Control of protein synthesis from mRNA
      • Post-translational regulation: Modifications and degradation of proteins

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  • Transcriptional regulation
    • Regulatory regions: Promoters and enhancers
    • Transcription factors:
      • Proteins that bind to specific DNA sequences
      • Activators enhance gene expression
      • Repressors inhibit gene expression
    • Chromatin remodeling:
      • Covalent modifications of histones and DNA
      • Affects accessibility of DNA to transcription factors

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  • Post-transcriptional regulation
    • RNA processing:
      • Splicing: Removal of introns and joining of exons
      • Alternative splicing: Variable exon usage
      • RNA editing: Changes in nucleotide sequence
    • RNA transport: Movement of mRNA from nucleus to cytoplasm
    • mRNA stability: Degradation or stabilization of mRNA molecules

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  • Translational regulation
    • Initiation factors: Proteins required for translation initiation
      • Can be regulated by various factors (e.g., signaling pathways, RNA-binding proteins)
    • microRNAs (miRNAs):
      • Small RNA molecules that bind to mRNA
      • Inhibit translation or promote mRNA degradation

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  • Post-translational regulation
    • Protein modifications:
      • Phosphorylation: Addition of a phosphate group
      • Acetylation: Addition of acetyl groups
      • Methylation: Addition of methyl groups
      • Ubiquitination: Tagging for degradation
    • Protein degradation:
      • Proteasomes: Complexes that degrade proteins
      • Ubiquitin-proteasome system

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  • Epigenetics
    • Heritable changes in gene expression without alterations in DNA sequence
    • Influenced by environmental factors and development
    • Mechanisms:
      • DNA methylation: Addition of a methyl group to DNA
      • Histone modifications: Covalent modifications of histone proteins
      • Non-coding RNAs: Influence gene expression at multiple levels

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  • Genetic and environmental factors
    • Interplay between genes and environment
    • Genetic predisposition: Increased susceptibility to certain conditions
    • Environmental factors can modify gene expression and influence phenotype
    • Examples:
      • Gene-environment interactions in cancer development
      • Nutritional influences on gene expression and health outcomes

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  • Evolutionary significance of genetic variation
    • Genetic variation is the raw material for evolution
    • Populations with greater genetic variation have higher adaptive potential
    • Sources of genetic variation:
      • Mutations: Changes in DNA sequence
      • Sexual reproduction: Genetic recombination
    • Natural selection acts on genetic variation to drive evolution

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  • Hardy-Weinberg Principle
    • Describes genetic equilibrium in idealized populations
    • The equation: p^2 + 2pq + q^2 = 1
    • p and q represent the frequencies of alleles in a population
    • p^2 represents the frequency of the homozygous dominant genotype
    • q^2 represents the frequency of the homozygous recessive genotype
    • 2pq represents the frequency of the heterozygous genotype

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  • Genetic drift
    • Random changes in allele frequencies in a population
    • More pronounced in small populations
    • Founder effect: When a small group of individuals establish a new population
      • Example: The Amish population in the United States
    • Bottleneck effect: Sharp reduction in population size
      • Example: The cheetah population

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  • Gene flow
    • Movement of alleles between populations
    • Increases genetic variation within a population
    • Can counteract genetic drift and maintain genetic diversity
    • Example: Migration of individuals between different regions

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  • Natural selection
    • Process by which individuals with certain traits have higher reproductive success
    • Acts on genetic variation already present in a population
    • Types of natural selection:
      • Stabilizing selection: Selects against extreme phenotypes
      • Directional selection: Shifts the average phenotype toward one extreme
      • Disruptive selection: Selects for extreme phenotypes and against intermediate phenotypes

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  • Sexual selection
    • Differential reproductive success based on traits related to mating
    • Intra-sexual selection: Competition between members of the same sex for access to mates
      • Example: Male-male competition in deer species
    • Inter-sexual selection: Mate choice based on traits preferred by the opposite sex
      • Example: Peacock’s tail in peafowls

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  • Speciation
    • Process by which new species arise
    • Two major modes of speciation:
      • Allopatric speciation: Geographic barriers separate populations
      • Sympatric speciation: New species form within the same geographic area
    • Example: Darwin’s finches in the Galapagos Islands

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  • Molecular clock
    • Uses the rate of genetic mutations to estimate the time of divergence between species
    • Assumes a constant rate of mutation accumulation over time
    • Can provide insights into evolutionary relationships and the timing of evolutionary events
    • Example: Estimating the divergence between humans and other primates

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  • Phylogenetics
    • Study of evolutionary relationships among species
    • Uses phylogenetic trees or cladograms to represent relationships
    • Based on shared characteristics or genetic information
    • Example: Constructing a phylogenetic tree for different bird species

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  • Human evolution
    • Evolutionary history of the Homo genus
    • Key milestones and species:
      • Ardipithecus: Early hominins, adapted to both arboreal and bipedal locomotion
      • Australopithecus: Bipedal hominins, predecessors to the Homo genus
      • Homo habilis: The “handy man,” earliest known tool-user
      • Homo erectus: First hominins to have a wide geographic distribution
      • Homo neanderthalensis: Close relatives to modern humans, went extinct
      • Homo sapiens: Modern humans

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  • Applications of genetics and evolution
    • Forensic genetics: DNA profiling for identification
    • Medical genetics: Diagnosis and treatment of genetic disorders
    • Agricultural genetics: Improving crop yields and livestock breeding
    • Conservation genetics: Protecting endangered species and preserving genetic diversity
    • Evolutionary medicine: Studying how evolution influences human health