IB Biology HL: Viruses, Origin of Cells, and Cell Structure

Comprehensive vocabulary review flashcards covering viral biology, hypotheses of abiogenesis and prebiotic evolution, LUCA, endosymbiosis, and cell differentiation.

Protein Capsid

The external protein coat of a virus that encloses and protects its genetic material.

Viral Glycoproteins

Proteins anchored on the outer surface of a virus that facilitate attachment to host cell receptors.

Viral Envelope

A lipid bilayer membrane surrounding the capsid that increases structural flexibility and provides extra protection.

Bacteriophage

A specialized virus that infects bacterial cells and possesses distinct head, sheath, and tail structures, often exhibiting a lysogenic cycle.

Reverse Transcriptase

A specialized enzyme found in retroviruses such as HIV that synthesizes DNA from an RNA template.

Antigenic Drift

Gradual mutation and sequence changes occurring in RNA viruses like influenza that generate new strains capable of evading host immune defenses and vaccines.

Lytic Cycle

A viral reproduction pathway wherein the virus attaches, injects genetic material, directs viral transcription and translation, replicates genomes, self-assembles into mature virions, and lyses the host cell.

Lysogenic Cycle

A viral life cycle in which the viral genome integrates into the host cell chromosome, remaining dormant without causing immediate cell lysis.

Escape Hypothesis

The theory of viral origins stating that mobile genetic elements escaped from cellular organisms and gained the capability to replicate independently and infect other cells.

Horizontal Gene Transfer

The exchange of genetic material between unrelated organisms, allowing viruses to acquire host genes over time that alter their evolutionary trajectory and behavior.

Prebiotic Earth Conditions

Environmental conditions of early Earth marked by the absence of oxygen, frequent electrical discharge, high UV radiation, elevated temperatures, and reducing gases like methane, hydrogen, ammonia, $CO_2$, and water vapor.

Spontaneous Generation

The disproven hypothesis that complex living organisms can arise naturally from non-living matter, refuted experimentally in the $19\text{th}\text{ century}$ by Louis Pasteur.

Miller-Urey Experiment

An experimental demonstration confirming that simulated conditions of prebiotic Earth can drive abiotic synthesis of organic compounds, including amino acids and fatty acids.

Protocell

A hypothetical early developmental stage of life composed of self-assembling fatty acid bilayers forming vesicles that encapsulate genetic material.

RNA World Hypothesis

The concept that RNA served as the initial genetic material due to its ability to store information, self-replicate, mutate, and act catalytically as enzymes.

Last Universal Common Ancestor (LUCA)

The most recent single common ancestor from which all extant life on Earth descended, estimated to have lived approximately $4\text{ billion}$ years ago.

Molecular Clock (Molecular Rocks)

A technique comparing differences in genetic mutations across lineages to estimate divergence times, dating the emergence of LUCA to approximately $4\text{ billion}$ years ago.

Hydrothermal Vents

Deep-sea fissures rich in methane, $CO_2$, and hydrogen that provide optimal thermodynamic energy and conditions for vesicle assembly and synthesis of proteins and nucleotides.

Endosymbiotic Theory

The evolutionary model stating that eukaryotic cells originated when an ancestral prokaryote engulfed an aerobic heterotroph (giving rise to mitochondria) and later a photosynthetic bacterium (giving rise to chloroplasts).

Stem Cells

Unspecialized cells with self-renewal capabilities that can undergo continuous cell division and differentiate into multiple cell lineages.

Cell Differentiation

The process whereby undifferentiated cells express specific subsets of genes to develop specialized functional adaptations, tissues, and organs in multicellular organisms.

Epigenetic Regulation

Chemical modifications such as DNA methylation and histone alterations that regulate gene transcription patterns without changing the primary nucleotide sequence.