🧬 Biology Tutor

🧬 Genetics

Genetics is the study of genes, heredity, and genetic variation in living organisms. It explains how traits are passed from parents to offspring and is fundamental to medicine, agriculture, and biotechnology.

Key Terms

• DNA (Deoxyribonucleic Acid) – Double-helix molecule carrying genetic information, composed of nucleotides (A, T, G, C)
• RNA (Ribonucleic Acid) – Single-stranded molecule involved in protein synthesis (mRNA, tRNA, rRNA)
• Gene – A segment of DNA that codes for a specific protein or trait
• Chromosome – Thread-like structures of DNA wrapped around histone proteins (humans have 46 = 23 pairs)
• Allele – Different versions of the same gene (e.g., brown eye allele vs blue eye allele)
• Locus – The specific location of a gene on a chromosome
• Genotype – The genetic makeup of an organism (e.g., Bb)
• Phenotype – The observable characteristics of an organism (e.g., brown eyes)

DNA Structure & Replication

• Nucleotide – Building block of DNA: phosphate group + deoxyribose sugar + nitrogenous base
• Base Pairing Rules – Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C)
• Complementary Strands – The two strands run antiparallel (5' → 3' and 3' → 5')
• Replication – DNA unzips at the replication fork; DNA polymerase adds complementary bases to create two identical copies
• Semi-Conservative – Each new DNA molecule contains one original strand and one new strand

Protein Synthesis

The process of converting genetic information into functional proteins occurs in two stages:

• Transcription (nucleus) – DNA is used as a template to create mRNA. RNA polymerase reads the template strand 3' → 5' and builds mRNA 5' → 3'. Base pairing: A→U, T→A, G→C, C→G
• Translation (ribosome) – mRNA is decoded to assemble amino acids into a polypeptide chain. tRNA brings amino acids matching each 3-base codon. Starts at AUG (methionine), ends at stop codons (UAA, UAG, UGA)

Mendel's Laws

• Law of Dominance – In a heterozygote, the dominant allele masks the recessive allele
• Law of Segregation – Each organism carries two alleles for each trait, which separate during gamete formation so each gamete carries only one allele
• Law of Independent Assortment – Genes for different traits are inherited independently (unless linked on the same chromosome)

Dominant vs Recessive

• Dominant allele (A) – Expressed when at least one copy is present (AA or Aa)
• Recessive allele (a) – Only expressed when two copies are present (aa)
• Homozygous Dominant – AA (two dominant alleles)
• Homozygous Recessive – aa (two recessive alleles)
• Heterozygous – Aa (one dominant, one recessive)
• Codominance – Both alleles are fully expressed (e.g., AB blood type)
• Incomplete Dominance – Heterozygote shows a blended phenotype (e.g., red × white → pink flowers)

Punnett Squares

A Punnett square predicts the genotype and phenotype ratios of offspring from a genetic cross.

Beyond Mendel: Complex Inheritance

• Polygenic Inheritance – Multiple genes control one trait (e.g., skin color, height)
• Sex-Linked Traits – Genes on the X chromosome (e.g., color blindness, hemophilia). Males (XY) are more affected since they have only one X
• Epistasis – One gene influences the expression of another gene
• Pleiotropy – One gene affects multiple traits (e.g., sickle cell gene affects blood, spleen, heart)
• Multiple Alleles – More than 2 alleles exist in the population (e.g., ABO blood groups: Iᴬ, Iᴮ, i)

Mutations

• Point Mutation – Change in a single base pair (substitution, insertion, deletion)
• Frameshift Mutation – Insertion or deletion shifts the reading frame of all downstream codons
• Chromosomal Mutations – Deletion, duplication, inversion, or translocation of chromosome segments
• Mutagens – Agents that cause mutations: UV radiation, chemicals, viruses

🦕 Evolution

Evolution is the change in heritable characteristics of biological populations over successive generations. It is the unifying theory of biology that explains the diversity of life on Earth over 3.8 billion years.

Darwin's Theory of Natural Selection

Natural selection is the primary mechanism of evolution. Darwin proposed four key observations:

• Overproduction – Organisms produce more offspring than can survive
• Variation – Individuals vary in their heritable traits
• Heredity – Traits can be inherited from parents to offspring
• Differential Survival – Organisms with favorable traits survive and reproduce more (survival of the fittest)

Mechanisms of Evolution

• Natural Selection – Differential survival and reproduction based on fitness
• Genetic Drift – Random changes in allele frequency, especially in small populations
  • Bottleneck Effect – Population drastically reduced by catastrophe, losing genetic diversity
  • Founder Effect – Small group colonizes new area with limited gene pool
• Gene Flow – Movement of alleles between populations through migration
• Mutation – Random changes in DNA that introduce new alleles
• Sexual Selection – Traits that increase mating success are favored (e.g., peacock tail)

Evidence for Evolution

• Fossil Record – Shows gradual changes in organisms over geological time; transitional fossils (e.g., Archaeopteryx – dinosaur to bird)
• Comparative Anatomy:
  • Homologous Structures – Same structure, different function (human arm, whale flipper, bat wing) → common ancestor
  • Analogous Structures – Different structure, same function (bird wing vs insect wing) → convergent evolution
  • Vestigial Structures – Reduced or functionless remnants (human appendix, whale pelvic bones)
• Molecular Biology – DNA and protein sequence similarities; all life shares the same genetic code
• Embryology – Similar embryonic development across vertebrates (pharyngeal slits, tails)
• Biogeography – Geographic distribution of species matches evolutionary predictions (island species)
• Direct Observation – Antibiotic resistance in bacteria, pesticide resistance in insects, Darwin's finch beak changes

Types of Natural Selection

• Directional Selection – Favors one extreme phenotype (e.g., larger body size in cold climates)
• Stabilizing Selection – Favors the average phenotype (e.g., human birth weight)
• Disruptive Selection – Favors both extreme phenotypes over intermediate (e.g., beak size in seed-cracking birds)

Speciation

Speciation is the formation of new and distinct species through evolution:

• Allopatric Speciation – Geographic barrier separates populations (e.g., river, mountain range)
• Sympatric Speciation – New species arise within the same area without physical separation (e.g., polyploidy in plants)
• Reproductive Isolation – Mechanisms that prevent interbreeding:
  • Prezygotic – Habitat, temporal, behavioral, mechanical, or gametic isolation
  • Postzygotic – Hybrid inviability, sterility, or breakdown

Types of Evolution

• Microevolution – Small changes in allele frequency within a species over generations
• Macroevolution – Large-scale changes leading to new species, genera, or higher taxa
• Convergent Evolution – Unrelated species develop similar traits in similar environments (dolphin and shark body shape)
• Divergent Evolution – Related species develop increasingly different traits (adaptive radiation of Darwin's finches)
• Coevolution – Two species evolve in response to each other (flowers and their pollinators)

🌿 Ecosystems

An ecosystem is a community of living organisms interacting with each other and their non-living environment. Ecosystems can range from a small pond to an entire rainforest or ocean.

Levels of Ecological Organization

• Organism – A single individual of a species
• Population – All individuals of one species in an area
• Community – All populations of different species in an area
• Ecosystem – Community + its abiotic environment
• Biome – A large region characterized by specific climate and organisms
• Biosphere – All ecosystems on Earth; the global ecosystem

Biotic vs Abiotic Factors

• Biotic – Living components (plants, animals, bacteria, fungi, protists)
• Abiotic – Non-living components (sunlight, water, temperature, soil, minerals, wind, pH)

Food Chains & Food Webs

• Producers (Autotrophs) – Make their own food through photosynthesis or chemosynthesis (plants, algae, cyanobacteria)
• Primary Consumers (Herbivores) – Eat producers (rabbits, deer, grasshoppers)
• Secondary Consumers (Carnivores) – Eat primary consumers (foxes, frogs, snakes)
• Tertiary Consumers (Top Predators) – Eat secondary consumers (eagles, sharks, tigers)
• Decomposers – Break down dead organic matter, returning nutrients to the soil (fungi, bacteria, earthworms)
• Detritivores – Feed on dead organic matter directly (vultures, dung beetles, woodlice)

Ecological Pyramids

• Pyramid of Energy – Always upright; shows energy decreasing at each level
• Pyramid of Numbers – Usually upright but can be inverted (one tree supporting many insects)
• Pyramid of Biomass – Shows total mass of organisms at each level

Ecological Relationships

• Mutualism (+/+) – Both species benefit (bees pollinate flowers while collecting nectar; mycorrhizae and plant roots)
• Commensalism (+/0) – One benefits, the other is unaffected (barnacles on whale skin; birds nesting in trees)
• Parasitism (+/–) – One benefits, the other is harmed (tapeworms in a host; ticks on mammals; malaria parasite)
• Predation (+/–) – Predator kills and eats prey (lion and zebra; spider and fly)
• Competition (–/–) – Two species compete for the same limited resource, both may be harmed

Biogeochemical Cycles

Essential elements cycle through ecosystems between living organisms and the environment:

• Carbon Cycle – CO₂ absorbed by plants (photosynthesis), released by respiration, decomposition, combustion of fossil fuels
• Nitrogen Cycle – N₂ fixed by bacteria → NH₃ (ammonia) → NO₃⁻ (nitrate) → absorbed by plants → returned by decomposition and denitrification
• Water Cycle – Evaporation → condensation → precipitation → collection → transpiration from plants
• Phosphorus Cycle – Released from rocks by weathering → absorbed by plants → passed through food chains → returned to soil by decomposition

Ecological Succession

• Primary Succession – Colonization of barren, lifeless land (e.g., after volcanic eruption). Pioneer species (lichens, mosses) arrive first, gradually building soil
• Secondary Succession – Recovery after a disturbance where soil remains (e.g., after a forest fire). Faster than primary succession
• Climax Community – The stable, mature community that develops at the end of succession

Major Biomes

• Tropical Rainforest – Hot, wet; highest biodiversity; near the equator
• Desert – Very dry (<25 cm rain/year); extreme temperatures; specialized organisms
• Grassland/Savanna – Dominated by grasses; moderate rainfall; large herbivores
• Temperate Forest – Four seasons; deciduous trees; moderate climate
• Taiga (Boreal Forest) – Cold, coniferous; long winters; short summers
• Tundra – Very cold; permafrost; minimal vegetation
• Aquatic – Freshwater (lakes, rivers) and marine (oceans, coral reefs)

🫀 Human Body Systems

The human body consists of 11 major organ systems that work together to maintain homeostasis and support life. Each system is composed of organs that perform specialized functions.

Major Body Systems

• Circulatory System – Heart (4 chambers), blood, blood vessels (arteries, veins, capillaries); transports O₂, nutrients, hormones, and waste. Pulmonary circulation (heart → lungs) and systemic circulation (heart → body)
• Respiratory System – Nose, trachea, bronchi, lungs, alveoli; gas exchange (O₂ in, CO₂ out) across the thin alveolar walls by diffusion. Breathing controlled by the diaphragm
• Digestive System – Mouth → esophagus → stomach → small intestine → large intestine → rectum. Accessory organs: liver (bile), pancreas (enzymes + insulin), gallbladder
• Nervous System – Brain, spinal cord (CNS) and nerves (PNS); rapid electrical signals (nerve impulses). Neurons transmit signals across synapses using neurotransmitters
• Skeletal System – 206 bones, cartilage, ligaments; provides structure, protection, movement, mineral storage, and blood cell production (bone marrow)
• Muscular System – Skeletal (voluntary), smooth (involuntary, organs), and cardiac (heart) muscle. Muscles contract by sliding filaments (actin and myosin)
• Endocrine System – Glands (pituitary, thyroid, adrenals, pancreas, gonads) secrete hormones for slow, long-lasting chemical signaling
• Immune System – White blood cells (lymphocytes, phagocytes), antibodies, lymph nodes, spleen, thymus. Innate immunity (immediate, non-specific) and adaptive immunity (specific, memory cells)
• Lymphatic System – Lymph vessels, lymph nodes, spleen; drains excess fluid, transports fats, filters pathogens
• Reproductive System – Male: testes, sperm, testosterone. Female: ovaries, eggs, estrogen, uterus. Fertilization → zygote → embryo → fetus
• Excretory/Urinary System – Kidneys filter blood to produce urine (removing urea, excess salts, water). Each kidney has ~1 million nephrons
• Integumentary System – Skin, hair, nails; protects against pathogens, UV, dehydration; regulates temperature; produces vitamin D

The Digestive Process

• Mouth – Mechanical digestion (chewing) + chemical digestion (salivary amylase breaks down starch)
• Stomach – HCl acid (pH 1.5–3.5) + pepsin breaks down proteins. Churning mixes food into chyme
• Small Intestine – Main site of digestion and absorption. Villi and microvilli increase surface area. Pancreatic enzymes (lipase, protease, amylase) and bile complete digestion
• Large Intestine – Absorbs water and remaining minerals. Houses gut bacteria that produce vitamin K and B vitamins

The Immune Response

• First Line – Physical barriers: skin, mucus membranes, tears, saliva, stomach acid
• Second Line – Non-specific immune response: phagocytes (neutrophils, macrophages), inflammation, fever, natural killer cells
• Third Line – Specific/adaptive immune response: T-cells (cell-mediated, destroy infected cells) and B-cells (humoral, produce antibodies). Memory cells provide long-term immunity

Homeostasis

Homeostasis is the ability of the body to maintain a stable internal environment despite external changes, primarily through negative feedback loops:

• Temperature regulation – Maintained at ~37°C. Too hot: vasodilation, sweating. Too cold: vasoconstriction, shivering, goosebumps. Controlled by the hypothalamus
• Blood glucose regulation – After eating: pancreas releases insulin → cells take up glucose → glycogen stored in liver. Between meals: pancreas releases glucagon → glycogen converted to glucose
• Blood pH balance – Maintained at 7.35–7.45 by buffer systems, lungs (CO₂ expiration), and kidneys (H⁺/HCO₃⁻ excretion)
• Water balance (Osmoregulation) – ADH (antidiuretic hormone) from pituitary controls water reabsorption in kidneys. Dehydrated: more ADH = concentrated urine. Hydrated: less ADH = dilute urine

🌱 Photosynthesis & Cellular Respiration

Photosynthesis and cellular respiration are the two fundamental energy-transforming processes of life. They are essentially opposite reactions that together drive the flow of energy through ecosystems.

Photosynthesis

The process by which green plants, algae, and some bacteria convert light energy into chemical energy stored as glucose.

Where It Happens

• Photosynthesis occurs in chloroplasts, which have a double membrane
• Chloroplasts contain chlorophyll a & b (green pigments) and accessory pigments (carotenoids, xanthophylls) that absorb different wavelengths of light
• Internal structure: thylakoids (disc-shaped membranes, stacked into grana) surrounded by stroma (fluid)
• Mainly in leaf mesophyll cells; CO₂ enters through stomata (pores on leaf surface)

Two Stages of Photosynthesis

• Light-Dependent Reactions
  • Occur in the thylakoid membrane
  • Chlorophyll absorbs light, exciting electrons through Photosystem II and Photosystem I
  • Photolysis: Water is split: 2H₂O → 4H⁺ + 4e⁻ + O₂ (oxygen released as byproduct)
  • Electrons pass through the electron transport chain (ETC), generating a proton gradient
  • Chemiosmosis: H⁺ ions flow through ATP synthase to produce ATP
  • Final electron acceptor: NADP⁺ + H⁺ → NADPH
  • Products: ATP, NADPH, O₂
• Light-Independent Reactions (Calvin Cycle)
  • Occur in the stroma of the chloroplast
  • Carbon Fixation: CO₂ is attached to RuBP (5-carbon) by the enzyme RuBisCO → 2 molecules of G3P (3-carbon)
  • ATP and NADPH from light reactions are used to reduce and regenerate molecules
  • After 3 turns: 1 molecule of G3P exits → used to build glucose (6 turns = 1 glucose)
  • Does NOT directly require light, but depends on ATP and NADPH from light reactions

Factors Affecting Photosynthesis

• Light intensity – More light = faster rate (up to a saturation point where all pigments are maximally excited)
• CO₂ concentration – More CO₂ = faster rate (up to a point; limited by RuBisCO capacity)
• Temperature – Optimal around 25–35°C; too hot denatures enzymes; too cold slows reactions
• Water availability – Needed as a reactant and to keep stomata open
• Wavelength of light – Red and blue light are most effective; green light is reflected

Cellular Respiration

The reverse of photosynthesis — cells break down glucose to release energy as ATP.

Stages of Aerobic Respiration

• Glycolysis (cytoplasm) – Glucose (6C) split into 2 pyruvate (3C). Net gain: 2 ATP + 2 NADH. No oxygen required
• Pyruvate Oxidation (mitochondrial matrix) – Pyruvate converted to Acetyl CoA (2C) + CO₂ + NADH
• Krebs Cycle / Citric Acid Cycle (mitochondrial matrix) – Acetyl CoA oxidized completely. Per turn: 3 NADH, 1 FADH₂, 1 ATP, 2 CO₂. Runs twice per glucose
• Electron Transport Chain & Oxidative Phosphorylation (inner mitochondrial membrane) – NADH and FADH₂ pass electrons through protein complexes. Proton gradient drives ATP synthase: ~32–34 ATP. O₂ is the final electron acceptor → H₂O

Anaerobic Respiration (Fermentation)

When oxygen is unavailable, cells use fermentation to regenerate NAD⁺ for glycolysis:

• Lactic Acid Fermentation – In animal muscles during intense exercise. Pyruvate → Lactic acid. Only 2 ATP produced
• Alcoholic Fermentation – In yeast and some bacteria. Pyruvate → Ethanol + CO₂. Used in baking and brewing

🦠 Microbiology

Microbiology is the study of microscopic organisms — bacteria, viruses, fungi, protists, and archaea. These organisms are incredibly diverse and play critical roles in ecosystems, medicine, and industry.

Bacteria

• Structure – Prokaryotic; cell wall, cell membrane, cytoplasm, ribosomes, circular DNA (nucleoid), plasmids. Some have flagella (movement), pili (attachment), and a capsule (protection)
• Shapes – Cocci (spherical), bacilli (rod-shaped), spirilla (spiral), vibrio (comma-shaped)
• Reproduction – Binary fission (asexual) — one cell splits into two identical cells. Some can exchange DNA via conjugation
• Gram Staining – Gram-positive (thick peptidoglycan wall, stains purple) vs Gram-negative (thin wall + outer membrane, stains pink)
• Roles – Decomposition, nitrogen fixation, digestion (gut flora), food production (yogurt, cheese). Only ~1% of bacteria cause disease

Viruses

• Not truly alive – Cannot reproduce independently; must hijack host cell machinery
• Structure – Genetic material (DNA or RNA) surrounded by a protein coat (capsid). Some have a lipid envelope
• Lytic Cycle – Virus attaches → injects DNA → takes over cell → replicates → cell bursts (lysis) → new viruses released
• Lysogenic Cycle – Viral DNA integrates into host genome → replicates with host → can switch to lytic cycle later
• Examples – Influenza, HIV, COVID-19 (SARS-CoV-2), Ebola, rabies, HPV
• Treatment – Antiviral drugs (e.g., tamiflu, antiretrovirals for HIV). Prevention through vaccines

Fungi

• Eukaryotic, heterotrophic organisms with cell walls made of chitin
• Structure – Made of thread-like hyphae that form a network called mycelium
• Reproduction – Sexual and asexual, often through spores
• Types – Yeasts (unicellular), molds (multicellular filaments), mushrooms (fruiting bodies)
• Roles – Decomposers (recycle nutrients), food (mushrooms, bread yeast, beer), medicine (penicillin from Penicillium), mycorrhizae (symbiotic with plant roots)
• Diseases – Athlete's foot, ringworm, candidiasis, aspergillosis

Protists

• Eukaryotic, mostly unicellular, diverse kingdom — the "catch-all" group
• Animal-like (Protozoa) – Heterotrophic, motile (e.g., Amoeba, Paramecium, Plasmodium — causes malaria)
• Plant-like (Algae) – Photosynthetic (e.g., Chlorella, diatoms, kelp). Produce ~50% of Earth's oxygen
• Fungus-like – Decomposers (e.g., slime molds, water molds)

Biotechnology Applications

• Genetic Engineering – Inserting genes into organisms (e.g., insulin-producing bacteria, Bt crops)
• PCR (Polymerase Chain Reaction) – Amplifying DNA for forensics, diagnosis, research
• CRISPR-Cas9 – Precise gene editing tool for treating genetic diseases
• Bioremediation – Using microorganisms to clean up pollution (oil spills, heavy metals)
• Fermentation – Yeast and bacteria used in food production (bread, yogurt, beer, cheese)

🔖 Classification & Taxonomy

Taxonomy is the science of naming, describing, and classifying organisms into groups based on shared characteristics. It helps us organize the incredible diversity of life — estimated at 8.7 million species.

Linnaean Classification

Taxonomic Hierarchy

Organisms are classified into increasingly specific groups:

• Domain – Broadest category (Bacteria, Archaea, Eukarya)
• Kingdom – e.g., Animalia, Plantae, Fungi, Protista
• Phylum – e.g., Chordata (animals with backbones)
• Class – e.g., Mammalia
• Order – e.g., Primates
• Family – e.g., Hominidae (great apes)
• Genus – e.g., Homo
• Species – e.g., Homo sapiens

The Three Domains

• Bacteria – Prokaryotic, unicellular, peptidoglycan cell walls, most diverse metabolically
• Archaea – Prokaryotic, unicellular, live in extreme environments (thermophiles, halophiles, methanogens). Cell membranes differ from bacteria
• Eukarya – Eukaryotic cells with membrane-bound nucleus. Includes 4 kingdoms:

The Four Kingdoms of Eukarya

• Animalia – Multicellular, heterotrophic, no cell wall, most are motile. Vertebrates and invertebrates
• Plantae – Multicellular, autotrophic (photosynthesis), cellulose cell wall. Mosses, ferns, conifers, flowering plants
• Fungi – Mostly multicellular, heterotrophic (absorptive), chitin cell wall. Mushrooms, molds, yeasts
• Protista – Mostly unicellular eukaryotes; algae, protozoa, and slime molds

Animal Classification: Vertebrates

• Fish – Aquatic, gills, scales, fins, ectothermic. ~33,000 species
• Amphibians – Moist skin, aquatic larvae, terrestrial adults, ectothermic. Frogs, salamanders
• Reptiles – Dry scaly skin, amniotic eggs, ectothermic. Snakes, lizards, turtles, crocodiles
• Birds (Aves) – Feathers, hollow bones, endothermic, hard-shelled eggs, most can fly. ~10,000 species
• Mammals – Hair/fur, mammary glands, endothermic, most give live birth. ~6,400 species

Plant Classification

• Bryophytes – Non-vascular; mosses, liverworts, hornworts. Need water for reproduction
• Ferns (Pteridophytes) – Vascular, seedless; reproduce via spores
• Gymnosperms – Vascular, seed-bearing, no flowers; conifers. Seeds in cones
• Angiosperms – Vascular, flowering plants; ~300,000 species. Seeds enclosed in fruit. Monocots vs Dicots