Foundations of General Biology: Cell Theory, Evolution, and Biological Classification

Key Themes in Biology

Defining Life and the Study of Biology

Biology is the scientific study of life and living organisms. To determine if something is alive, biologists consider several fundamental characteristics shared by all living things.

• Cellular Organization: All living organisms are made up of membrane-bound cells.

• Replication: All organisms are capable of reproduction.

• Evolution: Populations of organisms are continually evolving.

• Information: Organisms process hereditary information encoded in genes as well as information from the environment.

• Energy: All organisms acquire and use energy.

Example: A bacterium is considered alive because it is made of cells, can reproduce, evolves over time, processes genetic information, and uses energy.

Living or Not?

• Viruses: Not considered alive because they lack cellular structure and must use host organisms to reproduce and obtain energy.

• Antlers: Not alive; they are non-living structures produced by living organisms.

• Sponges: Animals; they are alive.

Theories in Biology

Scientific Theories

A scientific theory is a well-substantiated explanation for a broad set of phenomena, supported by a wide body of evidence.

• Cell Theory: What are organisms made of?

• Evolution by Natural Selection: Where do organisms come from?

• Chromosome Theory of Inheritance: How is hereditary information transmitted from one generation to the next?

Cell Theory

Historical Development

• Robert Hooke: First to use a microscope to describe cells (1665).

• Antony van Leeuwenhoek: First to describe single-celled organisms; founder of microbiology.

• Schleiden and Schwann (1830s): Proposed that all life is composed of cells.

Principles of Cell Theory

• All organisms are made of cells.

• All cells come from preexisting cells.

• Cells are highly organized compartments bounded by a plasma membrane and containing concentrated chemicals in an aqueous solution.

Louis Pasteur's Experiment

• Hypothesis: Cells arise from cells; cells do not arise by spontaneous generation.

• Experiment: Used swan-necked flasks to show that sterilized broth remained free of cells unless exposed to preexisting cells.

Chromosome Theory of Inheritance

Genetic Information and Heredity

• Hereditary or genetic information is encoded in genes, which are located on chromosomes.

• Chromosomes contain molecules of DNA.

• DNA is the hereditary material; genes are segments of DNA that code for cell products.

Structure of DNA

• James Watson and Francis Crick proposed that DNA is a double-stranded helix.

• Each strand is made up of four building blocks: A (adenine), T (thymine), C (cytosine), G (guanine).

• The sequence of these nucleotides encodes genetic information.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information in cells: from DNA to RNA to protein.

• Transcription: Creating a messenger RNA (mRNA) molecule from a DNA template.

• Translation: Converting the mRNA sequence into a protein.

Mutations in DNA can cause changes in genotype (genetic makeup) and phenotype (physical characteristics).

• Genotype: The genetic constitution of an organism.

• Phenotype: The observable traits or characteristics of an organism.

Evolution by Natural Selection

Principles of Evolution

• All species are related by common ancestry.

• Characteristics of species can be modified from generation to generation by natural selection ("descent with modification").

Natural Selection and Populations

• Natural selection explains how evolution occurs.

• Two conditions for natural selection:

  • Individuals must vary in characteristics that are heritable.

  • Certain versions of these traits help individuals survive and reproduce more successfully.

• Population: An interbreeding group of individuals of the same species in the same area.

Fitness and Adaptation

• Fitness: The ability of an individual to produce offspring.

• Adaptation: A trait that increases fitness in a particular environment.

The Tree of Life and Biological Classification

The Tree of Life

• All species, past and present, trace their ancestry to a single common ancestor.

• The cell theory and theory of evolution by natural selection support this concept.

Phylogenetic Tree of Life

• Taxonomy: The effort to name and classify organisms.

• Taxon (plural: taxa): A named group at any level.

• Phylogenetic Tree: A diagram showing evolutionary relationships among species.

• Branches that share a recent common ancestor represent taxa that are closely related.

Domains of Life

• Three major domains: Bacteria, Archaea, and Eukarya.

• Eukaryotes: Organisms with membrane-bound nuclei (e.g., plants, animals, fungi, protists).

• Prokaryotes: Organisms without membrane-bound nuclei (Bacteria and Archaea).

Linnaean System of Classification

• Developed by Carolus Linnaeus; still in use today.

• A species is made up of individuals that regularly breed together or have distinct characteristics.

• Binomial nomenclature: Each species is given a two-part Latin name (genus + specific epithet).

Summary Table: Key Biological Concepts

Concept	Definition	Example
Cell Theory	All organisms are made of cells; all cells come from preexisting cells	Bacteria, plants, animals
Chromosome Theory of Inheritance	Genetic information is encoded in genes located on chromosomes	DNA in human cells
Evolution by Natural Selection	Species change over time due to differential survival and reproduction	Antibiotic resistance in bacteria
Taxonomy	Classification and naming of organisms	Homo sapiens (humans)

Key Equations and Terms

• Central Dogma:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

• Fitness: $\text{Fitness} = \text{Number of offspring produced by an individual}$

Additional info: Eukaryotes are more closely related to Archaea than to Bacteria. Fungi and animals are more closely related to each other than to plants.