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

## Slide 11:

- 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)

## Slide 12:

- 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

## Slide 13:

- 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

## Slide 14:

- 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

## Slide 15:

- 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

## Slide 16:

- 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

## Slide 17:

- 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

## Slide 18:

- 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

## Slide 19:

- 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

## Slide 20:

- 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
## Slide 21:

- 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

## Slide 22:

- 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

## Slide 23:

- 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

## Slide 24:

- 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

## Slide 25:

- 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

## Slide 26:

- 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

## Slide 27:

- 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

## Slide 28:

- 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

## Slide 29:

- 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

## Slide 30:

- 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

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