Characteristics of Living Things

• There are 9 main characteristics of living things:

  • Organization of structures

  • Evolution of populations

  • Having DNA

  • Ability of reproduction

  • Growth/Development

  • Response to Environment

  • Metabolism

  • Maintaining homeostasis

  • Contains one or more cells

• A considerable debate is if viruses are living organisms are not → the consensus has become chiefly that they are not

  • This is because they don’t always possess DNA genomes and cannot reproduce on their own

  • The only characteristics of life they possess are the evolution of populations and organization of structures

• Viruses are very relevant to COVID-19

  • COVID-19 stands for Coronavirus Disease 2019

  • SARS-CoV-2: Severe Acute Respiratory Syndrome Coronavirus 2, the virus that causes COVID-19

History of Cell Biology

1. Zacharias Jansen invented the first microscope in the late 1500s.

2. Robert Hooke published Micrographia and coined the term “cell” to describe the compartments he saw in cork slices in 1665.

3. Antonie van Leeuwenhoek improved lens quality and observed living cells in the late 1600s.

4. The compound microscope was invented and lens quality improved in the 1830s.

5. Robert Brown found the nucleus in 1831.

6. Theodor Schwann and Matthias Schleiden put forth their cell theory in 1838

   1. Their cell theory had three tenets:

      1. The cell is a unit of structure, function, and organization in all living things

      2. The cell is both a distinct entity and a building in making organisms

      3. Cells form by free cell formation, similar to how crystals form

7. Rudolf Virchow revised the third tenet of the cell theory to say that all cells come from pre-existing cells in 1855

8. The Miller-Urey experiment performed by Stanley Miller and Harold Urey suggested organic compounds can be formed in abiotic conditions in 1953.

Early Life and the Miller-Urey Experiment

Early Earth and the Evolution of the First Cells

• The four steps that led to the evolution of the first cells in order were:

  • Abiotic synthesis of simple organic compounds

  • Abiotic synthesis of organic macromolecular polymers

  • Evolution of an information storage molecule capable of replication

  • Formation of a membrane surrounding the information storage molecule

• The atmosphere of early earth was much different than it is today

  • The atmosphere of early earth was made of mostly reduced gases like Hydrogen gas (H2), Methane (CH4), Ammonia (NH3), and Water Vapor (gaseous H2O)

  • The current composition of the atmosphere is 78% Nitrogen gas (N2), 21% Oxygen gas (O2), 0.9% Argon (Ar), and 0.1% other gases

    • These other gases include Carbon Dioxide (CO2), Water Vapor (H2O), Neon (Ne), Helium (He), and Ozone (O3)

The Miller-Urey Experiment

• This experiment tested the hypothesis that lightning energy powered the formation of simple organic compounds from reduced atmospheric gases.

• They tried to replicate early earth conditions by exposing a mixture of gases that was like the types and proportions of gases found in early earth to electric discharge.

• After conducting the experiment, they found the simple amino acids alanine and glycine after a week of exposing the gases to electric discharge.

  • These results suggested organic compounds can be made in abiotic conditions

The Modern Cell Theory

• The modern cell theory still has 3 tenets:

  • The cell is a building block of all living things.

  • Cells can exist as a singular entity or function as a unit in making organisms.

  • All cells come from pre-existing cells.

Classification of Life

• All life can be classified into one of three domains:

  • Eukarya- all the eukaryotes

  • Archaea- prokaryotic organisms including extremophiles

  • Bacteria- prokaryotic organisms

• These three domains of life were made by biologists Carl Woese and George Fox

• Organisms were reclassified into these three domains based on similarities in rRNA gene sequences

  • rRNA gene sequences were used because:

    • all living things have rRNA because all living things have ribosomes

    • these gene sequences do not evolve rapidly

    • in vitro replication of these sequences through PCR is easy

• After analyzing rRNA gene sequences, Woese and Fox found out about three main things

  • There are 2 separate groups of prokaryotes: bacteria and archaea

  • Eukaryotes and archaea are more closely related to each other than either is to bacteria

  • Protists should not be in one kingdom

    • Protists: any eukaryotes that aren’t animals, fungi, or plants

    • Some protists aren’t even closely related to each other