Author: Admin

📌Definition Table

Term	Definition
LUCA	Last Universal Common Ancestor; the most recent organism from which all life on Earth descended.
Phylogenetic Tree	Diagram showing evolutionary relationships among species based on genetic and morphological data.
Molecular Clock	Method of estimating evolutionary time by measuring genetic mutations over time.
Hydrothermal Vent Hypothesis	Theory that life originated in deep-sea vents rich in minerals and chemical energy.
Radiometric Dating	Technique for determining the age of rocks or fossils by measuring isotopic ratios.

📌Introduction

The theory that all living organisms share a common origin is supported by multiple lines of evidence, including biochemical similarities, fossil records, and genetic data. The Last Universal Common Ancestor (LUCA) is thought to have existed about 4 billion years ago, giving rise to the three major domains of life — Bacteria, Archaea, and Eukarya. Fossils, molecular clocks, and the discovery of ancient microbial structures in hydrothermal vents provide strong evidence for the evolutionary timeline. By comparing DNA sequences, amino acid structures, and metabolic pathways, scientists can reconstruct the evolutionary tree of life and trace the shared heritage of all organisms.

📌 Evidence for a Common Ancestor

• All organisms share the same DNA bases (A, T/U, G, C) and a universal genetic code.
• The same set of amino acids is used to build proteins in all known life forms.
• Many genes are shared between Archaea and Bacteria, indicating inheritance from LUCA.
• Structural similarities, such as the vertebrate forelimb bone pattern, show morphological evidence of common descent.
• LUCA likely existed around 4 billion years ago and occupied the base of the phylogenetic tree of life.
• Other life forms existing alongside LUCA may have gone extinct due to competition.

🧠 Examiner Tip: Use both molecular and morphological evidence when explaining common ancestry — this strengthens your argument in long-answer questions.

📌 Evolutionary Timescale

• Fossil evidence provides chronological snapshots of past life forms.
• Carbon-14 dating works for relatively recent samples (up to ~60,000 years old).
• Radiometric dating (e.g., uranium-lead dating) measures isotopic decay in older rocks.
• Genome analysis allows estimation of divergence times based on mutation rates.
• The molecular clock assumes mutations occur at a relatively constant rate.
• These methods together allow scientists to approximate when major evolutionary events occurred.

🧬 IA Tips & Guidance: For an IA, consider using simulated mutation rate data to model how molecular clocks estimate divergence times between species.

📌 Hydrothermal Vent Hypothesis

• Suggests that LUCA evolved near deep-sea hydrothermal vents.
• Vents provide chemical energy for chemosynthesis, supporting life without sunlight.
• Fossilised haematite tubes in Canada (3.77–4 billion years old) resemble modern vent microbes.
• LUCA may have been an autotrophic extremophile, using CO₂ and H₂ for energy.
• Likely survived high temperatures and lacked oxygen-based metabolism (anaerobic).
• Genetic analysis of modern vent organisms supports a common ancestry with LUCA.

🌐 EE Focus: An EE could explore the biochemical similarities between modern vent extremophiles and the inferred properties of LUCA.

📌 Characteristics of LUCA

• Anaerobic metabolism, using hydrogen as an energy source.
• Ability to convert CO₂ into glucose (autotrophy).
• Nitrogen fixation into ammonia for amino acid synthesis.
• Tolerance for high temperatures (thermophilic).
• Reliance on iron and other minerals abundant in hydrothermal vent environments.
• Basic biochemical pathways similar to those in modern microbes.

❤️ CAS Link: A CAS project could involve developing an interactive timeline of life on Earth, integrating fossil, genetic, and geological evidence.

📌 Challenges in Studying Early Evolution

• Ancient cells did not fossilise, so direct evidence is rare.
• Geological activity has destroyed much of Earth’s earliest rock record.
• Dating methods have margins of error, especially for the oldest samples.
• Molecular clocks require accurate mutation rate assumptions, which may vary between species.
• Fossil and molecular evidence must be interpreted together for reliability.
• Multiple competing hypotheses for life’s origin remain under investigation.

🔍 TOK Perspective: The search for LUCA illustrates the interdisciplinary nature of scientific knowledge — biology, geology, and chemistry must all contribute to construct plausible historical reconstructions, highlighting the role of collaboration in science.

📝 Paper 2:
Expect questions asking you to explain molecular evidence for common ancestry, interpret a phylogenetic tree, or evaluate the hydrothermal vent hypothesis using fossil and genetic data.

📌Definition Table

Term	Definition
Protocell	A primitive, membrane-bound droplet capable of basic metabolic processes but lacking fully evolved genetic systems.
RNA World Hypothesis	Theory that early life used RNA as both genetic material and catalyst before DNA and proteins evolved.
Vesicle	Small membrane-bound compartment formed spontaneously from amphipathic molecules.
Amphipathic Molecule	A molecule with both hydrophilic and hydrophobic regions, enabling membrane formation.
Metabolism-First Theory	Hypothesis that self-sustaining chemical reaction networks arose before genetic material.
Gene-First Theory	Hypothesis that self-replicating nucleic acids appeared before metabolic systems and membranes.

📌Introduction

The origin of the first cells marks a pivotal transition from non-living chemistry to biological systems. Early Earth provided an environment where organic molecules could assemble into protocells—structures capable of growth, division, and primitive heredity. These protocells eventually evolved into true cells with DNA genomes, protein enzymes, and phospholipid membranes, setting the stage for all modern life.

📌 Key Stages in the Origin of Cells

• Abiotic synthesis of small organic molecules under early Earth conditions, facilitated by energy sources like UV light, lightning, and volcanic heat.
• Polymerisation of monomers into macromolecules such as proteins and nucleic acids, often aided by mineral catalysts.
• Emergence of self-replicating RNA molecules that could store information and catalyse reactions.
• Spontaneous formation of fatty-acid membranes enclosing a microenvironment distinct from the surroundings.
• Evolution of metabolic networks within protocells, allowing sustained energy use and molecular turnover.
• Transition to DNA as the primary genetic material due to its stability and to proteins for catalysis due to their versatility.

🧠 Examiner Tip:
Be able to state the four key requirements for life’s origin: abiotic synthesis of organic molecules, polymerisation into macromolecules, self-replication, and membrane formation.

📌 Theories for the Origin of Cells

• Protocell-first: Membrane-bound droplets capable of metabolism arose before genetic material; genetic systems evolved later.
• Gene-first: Self-replicating RNA molecules emerged first, followed by membranes and metabolism.
• Metabolism-first: Life began as self-sustaining chemical cycles that later incorporated genetic material.
• All three require energy sources such as UV light, hydrothermal heat, or chemical gradients.
• Each theory attempts to explain how self-replication, metabolism, and compartmentalisation originated.
• None can yet be proven, but they can be partially tested with lab simulations.

🧬 IA Tips & Guidance: If modelling protocell formation, focus on controlling pH, lipid composition, and salt concentration to replicate realistic prebiotic environments.

📌 Membrane Formation (Vesicles)

• Fatty acids naturally form monolayers and bilayers in water due to their amphipathic nature.
• Bilayers can spontaneously curve into vesicles, creating enclosed compartments.
• Early vesicles likely trapped RNA, proteins, or metabolic molecules inside.
• Over time, fatty acids may have been replaced by phospholipids for greater stability.
• Compartmentalisation allowed different reactions to occur without interference.
• This was a critical evolutionary step toward complex cells.

🌐 EE Focus: A possible EE could compare stability and permeability of fatty acid vs phospholipid vesicles under simulated hydrothermal vent conditions.

📌 RNA as First Genetic Material

• RNA can both store genetic information and catalyse chemical reactions (ribozymes).
• It can form spontaneously from nucleotides under certain conditions.
• Ribozymes in modern cells support the idea that RNA had catalytic roles before proteins.
• DNA likely replaced RNA as the main genetic material due to greater stability.
• Proteins replaced RNA’s catalytic functions due to greater efficiency.
• Evidence: ribose synthesis from methanal in prebiotic simulations; deoxyribose production from ribose in cells.

❤️ CAS Link: A CAS project could involve creating interactive models or animations of the RNA world hypothesis for a school science fair.

📌 Challenges in Testing Theories

• Conditions on early Earth cannot be replicated exactly in the lab.
• No direct fossil evidence of the first cells exists.
• Scientists test individual processes (e.g., monomer synthesis, vesicle formation) rather than the entire origin event.
• Results must be pieced together from chemistry, molecular biology, and geology.
• Theories remain provisional but become stronger with interdisciplinary evidence.
• Understanding cell origin helps frame research into life beyond Earth.

🔍 TOK Perspective: This topic shows the difficulty of testing historical scientific theories — we rely on indirect evidence and models, so knowledge claims are often supported by multiple smaller experiments rather than a single definitive proof.

📝 Paper 2:
Be able to compare and contrast the protocell-first, gene-first, and metabolism-first theories. Include key experimental evidence like vesicle self-assembly and RNA catalytic activity, and discuss why testing origin-of-life hypotheses is challenging.

📌Definition Table

Term	Definition
Primordial Soup Hypothesis	Theory by Oparin and Haldane suggesting that organic molecules formed in Earth’s early oceans from simple inorganic compounds.
Greenhouse Effect	Warming of a planet’s surface due to atmospheric gases trapping infrared radiation.
UV Radiation	High-energy electromagnetic radiation from the Sun, which can break chemical bonds and cause DNA damage.
Organic Molecule	Carbon-based compound found in living organisms, including amino acids, sugars, nucleotides, and fatty acids.
Polymer	A large molecule composed of repeating subunits (monomers).
Miller–Urey Experiment	1953 laboratory experiment simulating early Earth’s conditions to test the formation of organic molecules from inorganic gases.

📌Introduction

The formation of carbon compounds on early Earth was a critical step in the origin of life. Conditions on the young planet were harsh, with a hot atmosphere rich in methane and carbon dioxide, no ozone layer to block harmful UV radiation, and frequent lightning storms. These extreme conditions may have driven chemical reactions that produced organic molecules — the building blocks of life — from simple inorganic compounds.

📌 Conditions on Early Earth

• Higher atmospheric temperatures due to large amounts of CO₂ and CH₄ trapping heat via the greenhouse effect.
• Absence of free oxygen prevented the formation of an ozone layer, allowing intense UV radiation to reach the surface.
• UV light and heat could catalyse the formation of simple organic compounds such as amino acids, sugars, nucleotides, and fatty acids.
• Energy sources included UV radiation, volcanic heat, and lightning discharges.
• The ‘primordial soup’ hypothesis proposed that oceans were a warm, nutrient-rich mixture of organic molecules.
• Over time, these molecules could polymerise into more complex structures like proteins and nucleic acids.

🧠 Examiner Tip:
You should be able to explain why early Earth’s lack of oxygen both hindered life (due to UV damage) and promoted organic synthesis (as oxygen would have oxidised organic molecules).

📌 Evidence – The Miller–Urey Experiment

• Recreated early Earth’s atmosphere with water vapour, methane, hydrogen, and ammonia.
• Boiled water to simulate evaporation from early oceans.
• Added electrical sparks to mimic lightning as an energy source.
• Cooling system condensed the vapour, simulating rainfall.
• After one week, the apparatus contained amino acids and other organic molecules.
• Demonstrated that abiotic synthesis of life’s building blocks was possible under certain conditions.

🧬 IA Tips & Guidance:
Link this topic to practical work by designing a safe simulation of atmospheric chemistry, or by analysing chemical pathways for abiotic synthesis using molecular modelling software.

📌 Limitations of the Miller–Urey Experiment

• Early atmosphere may have contained less methane than assumed.
• Used electrical discharge rather than UV light, which may have played a larger role in reality.
• Amino acids tend to remain as monomers in watery environments, challenging polymerisation under such conditions.
• Failed to produce nucleotides, requiring alternative synthesis pathways.
• More recent experiments show additional energy sources (e.g., hydrothermal vents) could be important.
• Highlights the evolving nature of scientific understanding about early Earth.

🔍 TOK Perspective:
This experiment illustrates how scientific models are influenced by assumptions — changes in our understanding of early Earth’s atmosphere lead to new interpretations of Miller–Urey’s results. It also raises epistemological questions about reconstructing events from billions of years ago with limited evidence.

📝 Paper 2:
Be prepared to describe the Miller–Urey experiment, evaluate its limitations, and link early Earth conditions to the possible chemical origins of life.

📌Definition Table

Term	Definition
Hershey–Chase Experiment	1952 study proving that DNA, not protein, is the hereditary material by tracking radioactive isotopes in bacteriophages.
Radioisotope Labelling	Technique using radioactive elements to trace molecules in biological systems.
Chargaff’s Rules	Observations that in DNA, A = T and C = G, indicating base pairing regularity.
Tetranucleotide Hypothesis	An early, incorrect theory proposing DNA was composed of repeating units of four nucleotides.
Double Helix Model	3D structure of DNA proposed by Watson & Crick in 1953, showing antiparallel strands and complementary base pairing.
Complementary Base Pairing	Specific pairing of A–T and C–G in DNA through hydrogen bonds.

📌Introduction

The journey to understanding the structure and function of nucleic acids was shaped by decades of experimentation and technological progress. From disproving early misconceptions like the tetranucleotide hypothesis to proving DNA’s role in heredity, historical breakthroughs have laid the foundation for modern molecular biology. Key contributions from Hershey & Chase, Chargaff, and Watson & Crick revolutionised our understanding of genetic material.

📌 Hershey & Chase Experiment (1952)

• Used bacteriophages (viruses infecting bacteria) containing DNA and a protein coat to determine the molecule of heredity.
• DNA was labelled with radioactive phosphorus (^32P), while protein was labelled with radioactive sulfur (^35S).
• After infecting E. coli, the viral coats were removed using a blender, and bacteria were separated from viral debris via centrifugation.
• Only the bacteria infected with ^32P-labelled viruses became radioactive, indicating DNA entered the host cells.
• Demonstrated unequivocally that DNA, not protein, is the genetic material.
• Showed the importance of radioisotopes as research tools in molecular biology.

🧠 Examiner Tip:
Be able to explain the experimental setup and outcome of the Hershey–Chase experiment, and why phosphorus and sulfur were specifically chosen as markers.

📌 Chargaff’s Data (1940s)

• Erwin Chargaff analysed DNA composition in multiple species and found that A = T and C = G in molar ratios.
• This indicated purine–pyrimidine pairing, providing key evidence for base complementarity.
• Falsified the tetranucleotide hypothesis proposed by Phoebus Levene, which suggested DNA was repetitive and lacked variation.
• Demonstrated species-specific variation in base composition, showing that DNA could store complex genetic information.
• Provided essential input for Watson and Crick’s double helix model.
• Illustrated the role of falsification in scientific progress (Karl Popper’s philosophy).

🧬 IA Tips & Guidance:
This topic links well to modelling investigations — you could use software or kits to recreate the Hershey–Chase experiment or compare base compositions across species using genome databases.

📌 Watson & Crick’s Double Helix Model (1953)

• Proposed DNA’s 3D structure as a right-handed double helix with two antiparallel strands.
• Built on Rosalind Franklin’s X-ray crystallography images, which revealed DNA’s helical nature and uniform width.
• Explained how complementary base pairing allows accurate replication of genetic material.
• Showed that A–T pairs form two hydrogen bonds, while C–G pairs form three, maintaining consistent helix width.
• Directionality (5′ to 3′ and 3′ to 5′ strands) is crucial for replication and transcription processes.
• Revolutionised molecular biology, influencing genetics, medicine, and biotechnology.

🌐 EE Focus:
An Extended Essay could investigate how technological advancements (e.g., X-ray crystallography, radioisotope tracing) enabled breakthroughs in nucleic acid research, linking NOS (Nature of Science) concepts with experimental evidence.

📌 Technological Advances Enabling Discoveries

• X-ray diffraction imaging (Franklin & Wilkins) revealed DNA’s repeating helical structure.
• Availability of radioisotopes post–World War II enabled molecular tracing in experiments like Hershey–Chase.
• Improved biochemical analysis allowed accurate measurement of nucleotide ratios.
• Model-building techniques (Watson & Crick) helped visualise molecular interactions.
• Bioinformatics tools now enable DNA structure analysis at atomic resolution.
• Advances in microscopy and molecular modelling have extended these foundational findings into applied science.

❤️ CAS Link:
Create a museum-style exhibit or interactive timeline showcasing major milestones in DNA discovery, including models, photos, and simplified explanations for public education.

📌 Impact on Modern Biology

• Confirmed DNA as the universal genetic material, transforming genetics from a descriptive to a molecular science.
• Led to the development of recombinant DNA technology, PCR, genome sequencing, and synthetic biology.
• Enabled gene cloning, forensic DNA analysis, and targeted therapies.
• Provided evidence supporting the theory of evolution through shared genetic mechanisms.
• Inspired the Human Genome Project and personalised medicine.
• Continues to inform research into epigenetics and gene regulation.

🔍 TOK Perspective:
The history of DNA discovery illustrates the collaborative, sometimes competitive, nature of science. It also highlights the importance of falsifiability, technological tools, and interdisciplinary approaches in advancing knowledge.

📝 Paper 2:
Expect questions that require you to describe the Hershey–Chase experiment, explain Chargaff’s rules, or apply knowledge of historical discoveries to modern genetic techniques.

📌Definition Table

Term	Definition
Genetic Code	The set of rules by which the sequence of nucleotide bases in DNA or mRNA is translated into amino acid sequences in proteins.
Codon	A triplet of nucleotide bases in mRNA that codes for a specific amino acid or a stop signal during translation.
Coding Strand	The DNA strand whose base sequence matches the mRNA (except T is replaced by U).
Anticodon	A triplet of bases on tRNA complementary to a codon in mRNA.
Universal Code	The principle that the same codons specify the same amino acids in almost all organisms.
Conserved Sequence	DNA or protein sequences that have remained largely unchanged during evolution due to their critical biological functions.

📌Introduction

The genetic code is the molecular language that translates DNA sequences into functional proteins. Encoded as triplets of nitrogenous bases, it ensures that genetic information is expressed with precision, allowing the synthesis of specific amino acid chains. This universal system applies to nearly all known life forms, enabling genetic information to be read, interpreted, and transferred across species.

📌 The Genetic Code: Structure and Reading

• DNA contains sequences of four bases (A, T, C, G) that encode genetic information.
• One strand of the DNA double helix (the coding strand) holds the sequence used during transcription.
• Each codon consists of three bases, which collectively specify one amino acid or a stop signal.
• With 64 possible codons and 20 amino acids, the genetic code is redundant (multiple codons can specify the same amino acid).
• The base sequence in genes determines the primary structure of proteins, which in turn defines their 3D structure and function.
• Translation of the code is directional, proceeding from the 5′ to 3′ end of mRNA.

🧠 Examiner Tip:
Be able to identify the coding and template strands of DNA in a diagram, transcribe DNA into mRNA, and then translate mRNA codons into amino acids using a codon table.

📌 Conservation and Universality of the Genetic Code

• The genetic code is nearly universal across all organisms, from bacteria to humans, with only minor exceptions in some mitochondrial DNA and rare species.
• The same codon specifies the same amino acid in almost all living systems.
• This universality allows genes from one species to be expressed in another, forming the basis for genetic engineering and recombinant DNA technology.
• Evidence of a shared common ancestor is supported by the conservation of coding sequences across diverse taxa.
• Many coding sequences have remained unchanged for millions of years, especially those involved in core processes like transcription and translation.
• Highly conserved sequences include histone proteins and rRNA genes, indicating their fundamental biological roles.

🧬 IA Tips & Guidance:
You could design an investigation into base composition and codon usage in different organisms using online genetic databases, linking it to bioinformatics and evolutionary biology.

📌 Mutations and Genetic Variation

• Mutations are changes in the base sequence of DNA that can alter codons, potentially changing amino acids in proteins.
• Some mutations are silent (no change in amino acid), while others may alter protein structure or function.
• Beneficial mutations can drive adaptation, while harmful ones can cause genetic disorders.
• Mutations in non-coding sequences may affect gene regulation rather than protein structure.
• Mutations in highly conserved regions are more likely to be deleterious, as these regions are critical to survival.
• Mutagenic factors include radiation, chemical exposure, and errors during DNA replication.

🌐 EE Focus:
A student could explore the effect of codon bias on gene expression efficiency in different organisms, connecting molecular biology to biotechnology applications.

📌 From Gene to Protein

• Transcription copies the coding sequence of DNA into mRNA using complementary base pairing.
• mRNA leaves the nucleus and binds to ribosomes in the cytoplasm.
• tRNA molecules with complementary anticodons deliver amino acids to the ribosome.
• Ribosomes catalyse peptide bond formation between amino acids, producing a polypeptide chain.
• The sequence of amino acids determines the protein’s folding and function.
• Errors in transcription or translation can lead to dysfunctional proteins, affecting the organism’s phenotype.

❤️ CAS Link:
Developing an educational workshop to teach younger students how DNA sequences translate into proteins using 3D models or interactive software.

📌 Importance in Biotechnology

• Understanding the genetic code allows scientists to modify organisms through genetic engineering.
• It is essential for producing recombinant proteins like insulin, growth hormones, and vaccines.
• Codon optimization is used in synthetic biology to enhance gene expression in host organisms.
• CRISPR-Cas9 genome editing relies on precise targeting of DNA sequences.
• DNA barcoding uses conserved gene sequences to identify species.
• Gene therapy depends on accurate delivery and expression of functional genes.

🔍 TOK Perspective:
The universality of the genetic code challenges traditional definitions of species boundaries and prompts ethical debates on genetic modification. It also illustrates how shared biological systems provide strong evidence for evolutionary theory.

📝 Paper 2:
Be prepared to answer questions requiring codon table interpretation, prediction of amino acid sequences, and explanation of why the genetic code is universal yet redundant.

📌Definition Table

Term	Definition
Nucleic Acid	Large biomolecules (DNA & RNA) composed of nucleotide monomers that store and transmit genetic information.
Nucleotide	Basic unit of nucleic acids consisting of a pentose sugar, a phosphate group, and a nitrogenous base.
Nitrogenous Base	Organic molecule with nitrogen atoms that forms base pairs in nucleic acids (purines & pyrimidines).
Purines	Double-ring nitrogenous bases (Adenine & Guanine).
Pyrimidines	Single-ring nitrogenous bases (Cytosine, Thymine in DNA, Uracil in RNA).
Sugar-Phosphate Backbone	Repeating structure of alternating sugar and phosphate groups linked by covalent bonds in nucleic acids.

📌Introduction

Nucleic acids, DNA and RNA, are the fundamental molecules that store, transmit, and express genetic information in living organisms. They are polymers of nucleotides, joined via covalent phosphodiester bonds to form long chains. DNA is the primary genetic material in most organisms, whereas RNA plays a central role in protein synthesis and, in some viruses, acts as the genetic material itself. The structure of these molecules is key to their ability to store vast amounts of information, maintain stability, and allow for precise replication.

📌 DNA and RNA as Genetic Material

• DNA contains the genetic blueprint for all cellular activities, stored as sequences of nitrogenous bases arranged along two antiparallel strands in a double helix.
• Found mainly in the nucleus of eukaryotic cells, DNA is also present in mitochondria and chloroplasts, and in the cytoplasm of prokaryotes.
• RNA exists in several forms — mRNA, tRNA, and rRNA — each with specific roles in transcription and translation.
• Certain viruses, such as SARS-CoV-2 and influenza, use RNA as their genetic material, providing exceptions to DNA-based heredity.
• Viruses lack cellular structures and cannot self-replicate, depending on host cells to reproduce.
• The universality of DNA as genetic material across life forms suggests a common evolutionary origin.

🧠 Examiner Tip:
Know how to differentiate DNA and RNA by sugar type, nitrogenous base composition, and strand structure. You should also be able to identify and label DNA diagrams, including sugar-phosphate backbones, complementary base pairs, and hydrogen bonds.

📌 Nucleotide Components

• Both DNA and RNA are polymers made of nucleotides, each consisting of a pentose sugar, a phosphate group, and a nitrogenous base.
• The nitrogenous bases are grouped into purines (A, G) and pyrimidines (C, T in DNA, U in RNA).
• The phosphate group is acidic and negatively charged, covalently bonded to the sugar.
• In DNA, the sugar is deoxyribose; in RNA, it is ribose, differing by one oxygen atom at the 2′ carbon.
• Bases attach to the sugar at the 1′ carbon; phosphate groups attach at the 5′ carbon.
• Diagrams in IB exams can use simple shapes (pentagon for sugar, circle for phosphate, rectangle for base).

🧬 IA Tips & Guidance:
You can link this topic to experimental skills by modelling nucleotides using physical or digital tools, investigating DNA extraction methods, or measuring DNA concentrations in biological samples.

📌 Linking Nucleotides into Strands

• Nucleotides join via condensation reactions, forming phosphodiester bonds between the phosphate of one nucleotide and the 3′ OH group of another.
• This forms the sugar-phosphate backbone, with nitrogenous bases projecting inward.
• DNA is double-stranded; RNA is usually single-stranded.
• Base sequences vary infinitely, enabling the storage of diverse genetic information.
• The 5′ end of a strand contains a phosphate group; the 3′ end has a free OH group.
• The order of bases on one strand determines the complementary strand sequence.

🌐 EE Focus:
Students could investigate nucleotide composition in different species or use bioinformatics tools to compare conserved genetic sequences across organisms to study evolutionary relationships.

📌 RNA Structure

• RNA molecules are shorter than DNA, typically ranging from hundreds to a few thousand nucleotides.
• They are usually single-stranded and contain ribose instead of deoxyribose.
• RNA nucleotides use uracil (U) instead of thymine (T).
• Three main types: mRNA (carries genetic information from DNA), tRNA (transfers amino acids during protein synthesis), and rRNA (forms part of ribosomes).
• Phosphodiester bonds link nucleotides in RNA through condensation reactions.
• Orientation is important: RNA is synthesized in the 5′ to 3′ direction.

❤️ CAS Link:
A CAS project could involve creating interactive models or educational resources to teach younger students about DNA and RNA structure.

📌 DNA structure

• DNA consists of two antiparallel strands in a right-handed double helix.
• Each strand is made of a sugar-phosphate backbone with bases pointing inward.
• Complementary base pairing rules: A–T (two hydrogen bonds), C–G (three hydrogen bonds).
• The antiparallel arrangement ensures proper base pairing and replication fidelity.
• DNA’s double-helix model was elucidated by Watson and Crick in 1953.
• Stability is enhanced by hydrogen bonding and base stacking interactions

🔍 TOK Perspective:
The discovery of DNA’s structure reflects the collaborative and iterative nature of scientific progress, where models are refined based on experimental evidence and technological advances (e.g., X-ray crystallography).

📝 Paper 2:
Be prepared to draw and annotate DNA and RNA diagrams, explain differences in their structure, and apply complementary base pairing rules to solve genetic code problems.

📌Definition Table

Term	Definition
Fossil Record	Preserved remains or traces of organisms that provide chronological evidence of life’s history.
Transitional Fossil	Fossil showing intermediate traits between ancestral and descendant groups.
Comparative Anatomy	Study of similarities and differences in organism structures to infer evolutionary relationships.
Homologous Structures	Anatomical features derived from a common ancestor but adapted for different functions.
Analogous Structures	Features with similar functions but different evolutionary origins.
Molecular Evidence	DNA, RNA, and protein sequence comparisons revealing genetic relationships.
Biogeography	Study of species distribution patterns across geographical locations.

📌Introduction

Multiple lines of evidence, from fossils to molecular comparisons, demonstrate that all life shares a common origin and has diversified over time. These independent but converging data sets provide a robust framework for understanding how life evolved on Earth.

📌 Fossil Record

• Older strata contain simpler organisms, younger strata contain more complex forms.
• Transitional fossils (e.g., Archaeopteryx) link major groups.
• Provides chronological evidence of change and extinction events.
• Reveals adaptive radiations after mass extinctions.

🧠 Examiner Tip: Always note that the fossil record is incomplete due to preservation bias and erosion.

📌 Comparative Anatomy

• Homologous structures (e.g., vertebrate forelimbs) show divergent evolution.
• Analogous structures (e.g., bird vs. insect wings) show convergent evolution.
• Vestigial structures (e.g., whale pelvis, human appendix) indicate evolutionary remnants.
• Supports inference of common ancestry or similar selective pressures.

🧬 IA Tips & Guidance: Measure and compare homologous bone lengths across species to model evolutionary relationships.

📌 Molecular Evidence

• DNA sequence similarities indicate relatedness (more similarities → closer relation).
• Protein sequences (e.g., cytochrome c) act as molecular clocks.
• Highly conserved genes across taxa (e.g., HOX genes) show shared ancestry.

🌐 EE Focus: Compare molecular clock results with fossil record dates to test evolutionary timelines.

📌 Embryological Evidence

• Early embryonic stages of vertebrates share features like gill slits and tails.
• Similar developmental pathways suggest common ancestry.
• Differences arise in later development due to divergent evolution.

❤️ CAS Link: Create an educational poster showing vertebrate embryonic similarities and differences.

📌Biogeography

• Species distribution patterns reflect evolutionary history.
• Island species (e.g., Darwin’s finches) show adaptive radiation from common ancestors.
• Continental drift explains related fossils on now-distant continents.

📌 Observed Evolution in Real Time

• Examples: antibiotic resistance in bacteria, pesticide resistance in insects.
• Documented allele frequency changes over generations.
• Shows that evolution is ongoing and observable.

🔍 TOK Perspective: Evidence is interpreted through current scientific models; interpretations may change with new data.

📝 Paper 2: Data Response Tips: Be ready to interpret fossil diagrams, anatomical comparisons, DNA sequence tables, or geographic distribution maps.

📌Definition Table

Term	Definition
Prokaryotic Cell	Simple cell without a nucleus or membrane-bound organelles.
Eukaryotic Cell	Cell with a nucleus and membrane-bound organelles.
Endosymbiotic Theory	Theory that eukaryotic organelles originated from engulfed prokaryotes.
Cyanobacteria	Photosynthetic bacteria that released oxygen into early Earth’s atmosphere.
Great Oxidation Event	Period when atmospheric oxygen levels rose sharply (~2.4 bya).

📌Introduction

Cellular evolution marks the transition from simple prokaryotes to complex eukaryotes. This shift was driven by environmental change, metabolic innovation, and symbiosis, ultimately enabling multicellular life.

📌 Early Prokaryotic Cells

• First life forms (~3.5 bya), anaerobic and simple in structure.
• Contained circular DNA and 70S ribosomes.
• Relied on fermentation or anaerobic respiration.
• Thrived in extreme environments.
• Played major roles in nutrient cycling.
• Formed the basis for all later life forms.

🧠 Examiner Tip: Mention ribosome type and DNA form when distinguishing prokaryotes from eukaryotes.

📌 Rise of Photosynthetic Prokaryotes

• Cyanobacteria evolved oxygenic photosynthesis.
• Released oxygen into oceans and atmosphere.
• Triggered Great Oxidation Event.
• Oxygen allowed aerobic respiration to evolve.
• Caused extinction of many anaerobic organisms.
• Set stage for complex life.

🧬 IA Tips & Guidance: Compare oxygen production in modern cyanobacteria under different light intensities.

📌 Endosymbiotic Theory

• Large prokaryotes engulfed smaller aerobic/phototrophic ones.
• Engulfed cells survived, forming symbiotic relationships.
• Mitochondria from aerobic bacteria; chloroplasts from cyanobacteria.
• Both have double membranes and circular DNA.
• Reproduce by binary fission.
• Provide strong evidence for symbiotic origins of eukaryotes.

🌐 EE Focus: Sequence mitochondrial DNA to compare with bacterial genomes.

📌 Transition to Eukaryotic Cells

• Development of nucleus and internal membranes.
• Cytoskeleton allowed structural support and movement.
• Compartmentalization improved efficiency.
• Endomembrane system evolved for transport and synthesis.
• Enabled greater cell size and complexity.
• Formed basis for multicellular life.

❤️ CAS Link: Create a classroom model showing prokaryote-to-eukaryote transition.

📌 Multicellularity and Specialization

• Evolved independently in plants, fungi, and animals.
• Cell adhesion proteins enabled tissue formation.
• Specialization increased efficiency and survival.
• Communication systems developed between cells.
• Allowed development of organs and body systems.
• Increased ecological diversity.

🔍 TOK Perspective: Endosymbiotic theory shows how scientific models evolve with new genetic evidence.

📝 Paper 2: Data Response Tips: Expect diagrams comparing prokaryotic and eukaryotic structures and identifying evolutionary evidence.

📌Definition Table

Term	Definition
Abiogenesis	The origin of life from non-living matter under early Earth conditions.
Organic Molecule	Carbon-containing compound, typically associated with life.
Monomer	Small organic molecule that can join to form polymers.
Polymer	Large molecule made of repeating monomer units.
Catalysis	Process that speeds up chemical reactions without being consumed.

📌Introduction

The early Earth provided conditions that allowed simple carbon compounds to form spontaneously. These molecules became the building blocks for life, setting the stage for the evolution of complex biological systems.

📌 Carbon’s Bonding Properties

• Carbon forms four covalent bonds → allows complex molecules.
• Can bond with C, H, O, N, P, S to form diverse structures.
• Forms chains, branched molecules, and rings.
• Double and triple bonds add variability.
• Backbone for carbohydrates, lipids, proteins, nucleic acids.
• Versatility makes carbon unique among elements.

🧠 Examiner Tip: Always mention carbon’s tetravalency when explaining why it is suited for life.

📌 Early Earth Conditions

• Atmosphere rich in CH₄, NH₃, H₂, and H₂O vapour.
• Frequent volcanic activity and lightning.
• High UV radiation due to lack of ozone.
• Oceans acted as reaction sites (“primordial soup”).
• No oxygen — reducing environment promoted synthesis.

🧬 IA Tips & Guidance: Model chemical synthesis in a closed system under varied gas compositions.

📌 Miller–Urey Experiment (1953)

• Simulated early Earth conditions in a closed apparatus.
• Gases: methane, ammonia, hydrogen, water vapour.
• Electric sparks simulated lightning.
• After a week, amino acids and other organics formed.
• Proved abiotic synthesis of organics was possible.
• Demonstrated importance of environment in chemical evolution.

🌐 EE Focus: Compare organic yields under varying atmospheric gas compositions.

📌 Formation of Monomers

• Simple molecules formed via atmospheric reactions.
• Amino acids from carbon, hydrogen, oxygen, nitrogen.
• Nucleotides from sugar, phosphate, nitrogenous base.
• Fatty acids from carbon chains.
• Monomers accumulated in oceans.
• Could be catalyzed by mineral surfaces.

❤️ CAS Link: Create a science fair model demonstrating abiotic amino acid formation.

📌 Polymerization

• Monomers joined via condensation reactions.
• Removal of water forms covalent bonds.
• Possible catalysts: clay minerals, metal ions.
• Produces proteins, nucleic acids, polysaccharides.
• Requires concentration of monomers (e.g., tidal pools).
• Step toward formation of functional macromolecules.

📌 Role of Catalysts in Early Chemistry

• Minerals and metal sulfides catalyzed polymerization.
• UV light could drive photochemical reactions.
• Hydrothermal vents supplied heat and minerals.
• Surfaces provided sites for molecular assembly.
• Catalysis increased reaction efficiency.

🔍 TOK Perspective: Scientific reconstructions of early Earth are models — their validity depends on available evidence and assumptions.

📝 Paper 2: Data Response Tips: Expect diagrams of experimental setups, identification of gases used, or interpretation of prebiotic synthesis data.

📌Definition Table

Term	Definition
Transformation	Process where genetic material is transferred from one organism to another, changing its phenotype.
X-ray Crystallography	Technique that uses X-ray diffraction to determine molecular structure.
Semiconservative Replication	Each new DNA molecule contains one original and one new strand.
Model Building	Using physical or computer-based structures to hypothesize biological mechanisms.
Radioisotope Labeling	Using radioactive isotopes to trace molecules in experiments.

📌Introduction

The discovery of DNA’s role and structure was shaped by a series of groundbreaking experiments. From proving DNA as the genetic material to revealing its double-helical structure, these advances laid the foundation of modern molecular biology.

📌 Griffith’s Transformation Experiment (1928)

• Used Streptococcus pneumoniae in mice.
• Smooth (S) strain = virulent; Rough (R) strain = non-virulent.
• Heat-killed S strain + live R strain → mice died.
• Concluded a “transforming principle” transferred virulence.
• Did not identify the molecule responsible.
• First evidence of genetic material transfer.

🧠 Examiner Tip: Always state the organism and strains used when describing historical experiments.

📌 Avery, MacLeod, and McCarty (1944)

• Purified different biomolecules from S strain bacteria.
• Only DNA fraction transformed R strain into virulent form.
• Used DNase, RNase, and protease to confirm DNA’s role.
• Proved DNA was the genetic material in bacteria.
• Paved the way for acceptance of DNA’s central role.
• Still faced skepticism at the time.

🧬 IA Tips & Guidance: Use bacterial transformation to demonstrate uptake of plasmid DNA with visible traits (e.g., fluorescence).

📌 Hershey–Chase Experiment (1952)

• Used bacteriophages labeled with ³²P (DNA) and ³⁵S (protein).
• Only radioactive phosphorus entered bacterial cells.
• Showed DNA, not protein, is the genetic material.
• Used blender and centrifugation to separate phage coats from cells.
• Confirmed Avery’s conclusion in a viral system.

🌐 EE Focus: Compare modern viral genome tracing techniques with Hershey–Chase’s approach.

📌 Chargaff’s Rules (1950)

• Analyzed base composition of DNA from various species.
• Found A = T and G = C in molar ratios.
• Ratios vary between species but are constant within one species.
• Provided evidence for complementary base pairing.
• Suggested a structural relationship between bases.

❤️ CAS Link: Create an educational poster showing how Chargaff’s findings led to the base-pairing model.

📌 Franklin and Wilkins (1951–1953)

• Used X-ray diffraction to study DNA fibers.
• Franklin’s Photo 51 showed helical structure with uniform diameter.
• Data indicated two strands and bases stacked inside.
• Helped establish dimensions of the helix.
• Wilkins collaborated with Watson and Crick indirectly.

📌 Watson and Crick (1953)

• Built 3D models based on Franklin’s data and Chargaff’s rules.
• Proposed antiparallel double helix with complementary base pairing.
• Explained DNA replication mechanism via strand separation.
• Structure supported semiconservative replication model.
• Their model unified multiple lines of evidence.

🔍 TOK Perspective: Debate exists over ethical issues of credit in scientific discoveries, highlighting the role of collaboration and competition.

📝 Paper 2: Data Response Tips: Be prepared to interpret historical experiment setups, isotope labeling results, or X-ray diffraction patterns.