9/2/25 bio notes
Polymerization in Early Biochemistry
Polymers were formed in the second stage of the reaction, described as oligoglycine, illustrating that polymers could have been built in prebiotic settings.
Polymers are the most important biological molecules today: proteins (amino acids), nucleic acids (RNA, DNA), carbohydrates (glucose polymers like starch), and lipids (fatty acids attached to glycerol).
Forming polymers is a key step in the origin of life and is shown as an example of how complexity could arise under conditions similar to land-based volcanic environments.
An early experiment-like scenario: land volcanic processes and intertidal pools could yield amino acids, nucleotides, and lipids, enabling polymer formation and eventual cell-like systems.
2014 Cambridge study (non-enzymatic metabolism): metabolic reactions of core cellular metabolism were observed without enzymes using cyanide, iron, and sodium to generate reactions resembling parts of glycolysis and, to some extent, the citric acid cycle. This suggests primitive biochemistry could start assembling more complex molecules needed for cells.
The overarching idea: primitive metabolism and polymer formation could precede fully functional cells, gradually building blocks that later supported more complex processes.
RNA World, Replication, and the Origin of Life
What is the most characteristic living condition? The ability to replicate itself. The molecule that makes a copy is DNA, which separates its two strands to copy and then RNA reads the nucleotides to make proteins.
The chicken-and-egg problem: which came first, DNA or RNA? Most researchers today believe RNA was the first replicative molecule of one kind or another because RNA is simpler (single-stranded) and can act without as many enzymes.
Evidence for RNA world:
Altman and Check discovered an RNA molecule with catalytic activity (a ribozyme). This shows RNA can have enzyme-like catalytic functions.
Catalytic activity means the ability to form or break covalent bonds between molecules.
In cells today, enzymes (proteins) catalyze reactions, but ribozymes show RNA can also catalyze reactions, supporting an RNA-based early biochemistry.
What ribozymes do physically (example shown): a short RNA ribozyme folds into a shape that recognizes a target RNA (green molecule) and catalytically splits it into two pieces by hydrolyzing the phosphodiester bond.
Mechanism highlights: ribozyme binds a substrate RNA, undergoes a reaction that cleaves the bond, typically by adding water across the bond (hydrolysis).
Basic components: RNA bases (A, U, G, C), a ribose sugar, a phosphate backbone; the bond involved is the phosphodiester linkage.
Lab exploration: artificial ribozymes can be assembled from U, G, C, and A to attempt ligation (condensation) instead of hydrolysis. The ligase-like activity would join nucleotides to a growing RNA strand, analogous to RNA polymerase in modern cells.
Current limitation: the artificial ribozyme can add only one nucleotide at a time and then stops; real RNA polymerase runs continuously.
Conceptual scenario for origin of replication: in a volcanic or pond setting, ribozymes could catalyze RNA ligation to form longer RNA strands, and those strands could then guide the synthesis of more RNA or RNA-like catalysts, creating a feedback loop toward increasingly complex molecules.
Template-based replication basics (complementary templating):
DNA has two strands that separate; each strand serves as a template to make a complementary copy using base pairing (A with T, G with C in DNA; A with U in RNA).
The concept is summarized as complementary templating and base pairing: ATGC in DNA pairs as TACG on the complementary strand; in RNA, AUGC pairs with UACG depending on the template.
DNA replication enzymes and transcription/translation:
DNA polymerase uses a template strand to synthesize a new complementary strand via base pairing.
RNA transcription uses DNA as a template to produce RNA, which is then translated into proteins by the ribosome.
The universal base-pairing rules: DNA (ATGC), RNA (AUGC).
Artificial ribozyme as a first step toward origin-of-life experiments: a ribozyme that adds a nucleotide to a growing RNA strand demonstrates a plausible primitive catalytic capability that could support a primitive RNA-based replication system.
Minimal Genomes, Knockouts, and Synthetic Biology (Craig Venter)
Craig Venter’s genome project and minimal cell concept:
After sequencing the human genome, Venter explored the minimal molecular makeup needed for a cell to live by progressively removing genes from a chosen organism and testing viability.
Organism chosen for knockout studies: Mycoplasma genitalium, a gram-positive parasitic bacterium with about 525 protein-coding genes.
Knockout approach: remove genes one by one to see if the organism survives; the goal is to identify a minimal essential gene set.
Outcome: reduced gene count to roughly 265–350 essential genes while the organism remained viable, indicating a core set needed for life.
Shift to synthetic biology and a “synthetic chromosome” approach:
Venter sequenced the entire genome of Mycoplasma mycoides and used synthetic gene machines to assemble a synthetic chromosome from pieces online (gene synthesis services).
Yeast cells were used to assemble the synthetic chromosome segments via recombination.
The synthetic chromosome was then introduced into a recipient cell (Mycoplasma capricolum) whose genome had been destroyed by UV irradiation, effectively replacing its genome with the synthetic one.
Result: the cell replicated and produced its own proteins using the synthetic chromosome; a blue reporter gene was added to visibly verify chromosome uptake.
Important caveat: this experiment did not create life from scratch; it inserted artificial DNA into a recipient cell and demonstrated the feasibility of large-scale genome engineering. It showcased the potential of synthetic biology and chromosome-scale engineering rather than “creating life” from nothing.
Everyday context: recombinant DNA techniques are widely used (e.g., insulin production in bacteria) to illustrate practical applications of synthetic DNA in biology.
Conceptual takeaway: these studies demonstrate the possibility of designing and assembling genomes and chromosomes, highlighting the boundary between natural life and engineered genetic systems.
Protobionts, Lipid Membranes, and the Origins of Compartmentalization
Protobionts and liposomes:
Early Earth could readily assemble lipid molecules into spherical compartments called liposomes or protobionts, which approximate primitive cell-like membranes.
Dyson (1999) demonstrated laboratory-made artificial liposomes containing active enzymes such as phosphorylase and amylase.
Inside a liposome, glucose-6-phosphate could diffuse in, and in the presence of phosphorylase (which functions in carbohydrate metabolism), starch could be produced; amylase could convert starch to maltose, creating a concentration gradient driving diffusion of maltose outward.
Significance of protocell models:
These experiments show how a lipid boundary could compartmentalize chemistry, concentrating reactants and enabling metabolism in a controlled microenvironment.
Protocells or protobionts illustrate a step toward cellularity by providing a separate internal environment for reactions while enabling exchange with the exterior.
Limitations: the liposome approach demonstrates compartmentalization and some metabolic reactions, but it does not yet show a complete path from lipids and small molecules to a fully functioning, self-replicating cell with genetic information and translation.
Metabolism and Energy Transformation in Cells
Three fundamental energy-transforming mechanisms in cells:
1) Photosynthesis: captures light energy to ionize a chlorophyll molecule, initiating electron flow and energy capture; the energy in photons drives electron transfer, enabling downstream energy storage.
2) Glycolysis: oxidation of glucose to extract electrons to be transferred to other carriers; this process yields ATP and NADH at multiple steps and initiates energy extraction within the cell.
3) Electron transport and ATP synthesis: electrons flow through a chain of membrane-bound proteins (located in the mitochondria outer membrane and other membranes), ultimately transferring energy to ADP to form ATP; this energy powers cellular work.Autotrophs vs. heterotrophs:
Autotrophs capture energy and build organic molecules from inorganic carbon sources (e.g., CO₂) via processes like photosynthesis.
Heterotrophs consume already-formed organic molecules (e.g., glucose) and oxidize them to CO₂ and H₂O, releasing energy that the organism uses for metabolism.
Metabolic pathways and regulation:
Metabolic pathways are sequences of enzyme-catalyzed steps transforming substrates to products; pathway rate is regulated by substrate availability and feedback mechanisms.
Anabolic (biosynthetic) pathways build complex molecules from simpler ones (e.g., photosynthesis synthesizes glucose from CO₂ and H₂O).
Catabolic (degradative) pathways break down molecules to release energy (e.g., glycolysis and the citric acid cycle).
Example metabolic pathway (glycolysis) and energy accounting (as discussed in lecture):
Overall energy content of glucose: per mole.
End products of glycolysis include: two pyruvates, two ATP, and two NADH (which carry energy forward to the electron transport chain).
In the lecture, it was noted that from glucose, the initial energy release is far greater than the immediate yields of glycolysis, highlighting that glycolysis is only part of the total energy extraction from glucose.
The two pyruvates are each about , two NADH account for about , and two ATP are produced directly in glycolysis.
Overall efficiency of cellular metabolism is given as roughly –, far less than a modern car engine, but sufficient to power living systems.
Metabolic regulation via feedback inhibition and feedback activation:
Negative feedback: the end product inhibits an earlier enzyme in the pathway, reducing flux when the product accumulates (classic example: ATP inhibiting the initial enzyme in glycolysis; or isoleucine inhibiting threonine deaminase).
Positive feedback: a product enhances the activity of an enzyme, increasing the flux through the pathway; less common but can occur in certain regulatory networks.
Core idea emphasized: the shape of molecules governs biological activity, and changes in shape (via allosteric regulation or feedback) control metabolic flow.
Analogy used: a Ford assembly line where slowing the end of the line (decreased output) feeds back to slow the start of the line, illustrating pathway regulation through substrate/product concentrations.
Key takeaway: cells regulate thousands of interdependent metabolic pathways, coordinating energy capture and biosynthesis to sustain life.
DNA, RNA, Translation, and the Three Domains of Life
DNA as the genetic material and the flow of information:
DNA is duplicated via templated polymerization (two strands separate, each makes a complementary copy).
Transcription converts DNA to RNA; translation reads RNA codons to assemble proteins.
The genetic code maps codons (triplets of nucleotides) to amino acids; for example, UUU and UUC code for phenylalanine, among others. AUG is a start codon, and the code is largely universal across organisms.
Prokaryotes vs eukaryotes in gene expression:
Prokaryotes: no nuclear membrane; transcription and translation can occur simultaneously, often with little RNA processing.
Eukaryotes: nucleus encloses DNA; transcription followed by RNA processing (splicing, editing) before the mRNA is exported to the cytoplasm for translation.
Universal genetic code and shared biochemistry:
All organisms use the same twenty amino acids and the same basic nucleotides (A, T, G, C in DNA; A, U, G, C in RNA).
The shared set of metabolic pathways (e.g., glycolysis) and central dogma processes reflect deep common ancestry.
Three domains of life (Woese framework):
Bacteria, Archaea, and Eukaryota
The analysis of ribosomal RNA sequences, particularly small subunit rRNA, revealed this three-domain structure.
Carl Woese’s work showed that life groups into distinct domains with deep evolutionary splits, leading to the concept that all life shares a common ancestor (LUCA: Last Universal Common Ancestor).
Ribosome as a tool for taxonomy and evolutionary relationships:
The ribosome is a ribonucleoprotein complex with two subunits; it contains ribosomal RNA (rRNA) sequences that are highly informative for phylogeny.
In the lecture, eight species’ ribosomal RNA sequences from the small subunit were compared to infer relationships and support the three-domain view.
The Ribosome, Domain Classification, and LUCA
Carl Woese’s contribution (1991):
Sequenced small subunit ribosomal RNA from many species and used sequence divergences to infer evolutionary relationships.
Found three fundamental groups: Bacteria, Archaea, and Eukaryotes, supporting a three-domain model.
Implications for life’s origins:
The ribosome’s RNA component is central to the universal mechanism of protein synthesis and provides a molecular clock for deep evolutionary relationships.
The evidence from rRNA supports a common ancestry among all life and helps locate the divergence points that gave rise to bacteria, archaea, and eukaryotes.
Toward a Universal View of Life and the Workshop Note
The instructor emphasizes that life is a self-sustaining chemical system capable of metabolism, growth, replication, and evolution, driven by catalytic chemistry and energy transformations.
Summary of the quest to understand the origin of life:
Start with simple molecules and polymers forming under plausible prebiotic conditions.
Explore RNA’s catalytic potential (ribozymes) and the possibility of RNA-based replication before DNA/protein-based life.
Investigate how lipid membranes could encapsulate chemistry to form protocells with internal chemistry and exchange with the environment.
Use modern biology (genomics, synthetic biology) to test minimal genomes and the feasibility of constructing life-like systems, while recognizing the distinction between engineering life and creating life from non-living matter.
Final reminder from the instructor: there will be a workshop this afternoon revisiting Miller–Urey-type experiments and the molecules involved; print the workshop materials to prepare.
Quick Reference: Key Equations and Concepts (LaTeX)
Polymerization (dehydration synthesis):
Hydrolysis (reverse of dehydration):
Photosynthesis (illustrative overall equation):
Energy content (glucose):
Glycolysis outputs (as discussed):
Two pyruvate:
Two NADH:
Two ATP:
General nucleotide/base concepts:
DNA: ATGC; RNA: AUGC; complementary base pairing drives templated polymerization and transcription/translation.
Metabolic regulation concepts:
Negative feedback: end product inhibits the first enzyme (e.g., ATP feedback on initial glycolysis enzyme; isoleucine feedback on threonine deaminase).
Positive feedback: product enhances enzyme activity, increasing production rate.
Protocell concept:
Lipid bilayer compartments (liposomes) can house metabolic enzymes and substrates, enabling selective permeability and concentration of reactions inside a boundary.
Connections and Real-World Relevance
The polymerization and dehydration/hydrolysis concepts underpin digestion, nutrient processing, and macromolecule assembly in modern biology.
RNA’s catalytic roles foreshadow RNA-based life and the evolution of the genetic code, highlighting the link between chemistry and biology.
The minimal genome and synthetic chromosome work illustrate how biology can be engineered, with profound implications for medicine, industry, and our understanding of life’s boundaries.
The three-domain tree of life informs taxonomy, comparative biology, and the search for life beyond Earth by providing a framework for assessing relatedness among organisms through molecular signatures.
Lipid-based protocells remind us that life’s emergence likely required a boundary to separate internal chemistry from the environment, enabling complex reaction networks to persist and evolve.