DNA Structure and Function

Structure of DNA

DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. Its structure is essential for its function in storing and transmitting genetic information.

• Nucleotide Subunits: DNA is composed of repeating units called nucleotides, each consisting of a deoxyribose sugar, a phosphate group, and a nitrogen-containing base.

• Sugar-Phosphate Backbone: The backbone of DNA is formed by covalent bonds between the sugar of one nucleotide and the phosphate of the next.

• Nitrogenous Bases: Four types: adenine (A), guanine (G), thymine (T), and cytosine (C).

Double Helix and Base Pairing

• DNA consists of two strands that coil to form a double helix.

• The strands are complementary: A pairs with T, and G pairs with C.

• Base pairs are held together by hydrogen bonds (A=T has 2 bonds, G≡C has 3 bonds).

DNA Replication

DNA replication ensures genetic information is accurately passed to new cells.

• Replication begins at origins of replication, forming replication bubbles.

• The two parent strands separate and serve as templates for new strands.

• Each new DNA molecule consists of one old (parent) strand and one new strand (semiconservative replication).

From DNA to Traits: The Role of Proteins

• DNA dictates the synthesis of proteins, which determine an organism's traits.

RNA Structure and Types

• RNA (ribonucleic acid) is usually single-stranded.

• Contains ribose sugar and uracil (U) instead of thymine.

• Three main types:

  • mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.

  • tRNA (transfer RNA): Brings amino acids to ribosomes during translation.

  • rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Gene Expression: Transcription and Translation

• Transcription: Synthesis of mRNA from DNA template; occurs in the nucleus.

  • DNA → mRNA

• Translation: Synthesis of polypeptide (protein) from mRNA; occurs on ribosomes in the cytoplasm.

  • mRNA → Protein

• Central Dogma: Information flows from DNA to RNA to protein.

Mutations

• Mutation: Any change in the genetic material of a cell.

• Point Mutation: Change in a single base pair; can alter protein product.

• Frameshift Mutation: Insertion or deletion of bases that shifts the reading frame, often resulting in nonfunctional proteins.

DNA Technology and Genetic Engineering

Key Terms

• Plasmid: Small, circular DNA molecule in bacteria, used as a vector in genetic engineering.

• Genetically Modified Organism (GMO): Organism with artificially altered DNA.

• Transgenic Organism: Organism containing genes from another species.

• Restriction Enzyme: Enzyme that cuts DNA at specific sequences.

• PCR (Polymerase Chain Reaction): Technique to amplify DNA sequences.

• Stem Cell: Undifferentiated cell capable of giving rise to various cell types.

• Gene Therapy: Treating disease by introducing, removing, or altering genetic material.

• Therapeutic Cloning: Producing embryonic stem cells for medical use.

Genetic Engineering Procedures

1. Identify and isolate the gene of interest.

2. Insert gene into a plasmid vector using restriction enzymes.

3. Introduce recombinant plasmid into host organism (e.g., bacteria).

4. Host expresses the gene, producing the desired protein or trait.

Applications of Genetic Engineering

• Agriculture: Herbicide-resistant crops, higher yields.

• Medicine: Production of vaccines, gene therapy for genetic diseases.

DNA Fingerprinting

• Technique to identify individuals based on unique DNA patterns.

• Used in forensic science, paternity testing, and biodiversity studies.

Evolution and Natural Selection

Darwin's Observations and Theory

• Darwin observed species' geographic distribution (e.g., Galápagos finches) and adaptation to environments.

• Adaptation: Trait that increases survival and reproduction.

• Two main ideas from The Origin of Species:

  • Evolution explains life's unity and diversity.

  • Natural selection is the mechanism of adaptive evolution.

Natural Selection

• Organisms vary in traits; some variations are heritable.

• Organisms compete for resources; not all survive and reproduce.

• Individuals with advantageous traits (higher Darwinian fitness) are more likely to reproduce.

• Over generations, favorable traits become more common.

Natural vs. Artificial Selection

• Natural Selection: Environment selects for traits that increase fitness.

• Artificial Selection: Humans select for desirable traits (e.g., dog breeding).

Population Evolution

• Populations, not individuals, evolve over time.

• Only heritable traits are subject to natural selection.

• Descent with Modification: All organisms share a common ancestor; adaptations arise as species spread into new habitats.

Evidence for Evolution

• Biogeographical: Distribution of species.

• Embryological: Similarities in early development.

• Fossil Record: Transitional forms.

• DNA/Biochemical: Genetic similarities.

Homology vs. Analogy

• Homologous Structures: Similar due to shared ancestry (e.g., vertebrate limbs).

• Analogous Structures: Similar due to convergent evolution, not ancestry (e.g., wings of birds and insects).

Speciation and Genetic Drift

• Speciation: Formation of new species.

• Founder Effect: Genetic drift in a small, isolated population.

• Genetic Drift: Random changes in allele frequencies.

• New species can form via geographic isolation or other mechanisms (chromosomal changes, habitat differentiation, sexual selection).

• Adaptive Radiation: Diversification of a common ancestor into many species adapted to different environments.

Ecology: Communities and Ecosystems

Communities and Ecosystems

• Community: Populations of different species interacting in an environment.

• Ecosystem: Community plus the physical environment.

Ecological Succession

• Primary Succession: Occurs in lifeless areas (e.g., after lava flow).

• Secondary Succession: Occurs where a community previously existed (e.g., after fire).

Species Interactions

• Predation: One species (predator) eats another (prey).

• Competition: Species compete for resources.

• Parasitism: One species benefits, the other is harmed.

• Commensalism: One benefits, the other is unaffected.

• Mutualism: Both species benefit.

Symbiosis

• Close relationship between two species; includes mutualism, commensalism, and parasitism.

• Examples:

  • Mutualism: Bees and flowers

  • Commensalism: Barnacles on whales

  • Parasitism: Tapeworms in mammals

Ecological Niche and Resource Partitioning

• Ecological Niche: Species' role, habitat, and interactions.

• Resource Partitioning: Reduces competition by dividing resources.

Biosphere and Energy Flow

• Biosphere: All ecosystems on Earth; energy source is the sun.

• Autotrophs: Produce their own food (e.g., plants).

• Heterotrophs: Consume other organisms.

• Producers, Consumers, Decomposers: Producers make food, consumers eat other organisms, decomposers break down dead matter.

Energy Flow and Trophic Levels

• Energy flows through food chains and food webs.

• Four main trophic levels:

  • Producers (plants)

  • Primary consumers (herbivores)

  • Secondary consumers (carnivores)

  • Tertiary consumers (top predators)

• Energy is lost as heat at each transfer (about 90% lost per level).

Detritus Feeders

• Organisms that consume dead organic matter (e.g., earthworms, fungi).

Biogeochemical Cycles

• Carbon Cycle: Movement of carbon through atmosphere, organisms, and Earth.

• Nitrogen Cycle: Conversion of nitrogen between atmospheric, organic, and inorganic forms.

• Phosphorus Cycle: Movement of phosphorus through rocks, water, soil, and organisms.

Major Ecosystems

• Oceans, lakes, ponds, rivers, streams, coral reefs, marshes, tundra, taiga, temperate forest, tropical rain forest, temperate grassland, tropical grassland, desert

• Tropical rain forest is the most diverse ecosystem.

Human Impact on Biodiversity

• Human activities (deforestation, pollution, climate change) reduce biodiversity.