Characteristics of Life

Introduction to the Study of Life

Biology is the study of living organisms and the processes that define life. Understanding what constitutes a living thing is foundational to all biological sciences. This section introduces the five key characteristics that all organisms share and explores how certain infectious agents, such as prions, challenge our definitions of life.

Five Characteristics of Living Things

• Cellular Organization: All living things are composed of one or more cells. Cells are the basic unit of life, enclosed by a cell membrane that regulates the movement of substances in and out.

• Reproduction: Organisms reproduce to create new individuals. Reproduction can be asexual (offspring arise from a single organism) or sexual (offspring arise from the combination of genetic material from two parents).

• Genetic Information: Living things possess hereditary information encoded in genes (segments of DNA). Genes determine the traits of an organism and are passed from one generation to the next.

• Evolution: Populations of organisms evolve over time through changes in the frequency of heritable traits. Evolution requires genetic diversity, often arising from mutations.

• Energy Acquisition and Use: Organisms acquire energy from their environment and convert it into a usable form (such as ATP) through processes like photosynthesis (autotrophy) or by consuming other organisms (heterotrophy).

Comparison Table: Living Organisms vs. Prions

Prion Diseases

Chronic Wasting Disease (CWD)

Chronic Wasting Disease is a fatal neurological disease affecting deer and related species. It is characterized by progressive brain degeneration, leading to abnormal behavior, weight loss, and death. The disease is distributed across North America, as shown in epidemiological maps.

• Agent: Caused by prions (misfolded proteins).

• Symptoms: Neurological decline, wasting, and death.

• Transmission: Spread through contact with infected animals or contaminated environments.

Creutzfeldt-Jakob Disease (CJD)

Creutzfeldt-Jakob Disease is a rare, fatal neurological disorder in humans. It leads to rapid brain shrinkage and deterioration, with characteristic spongiform changes visible in brain tissue.

• Agent: Caused by prions.

• Symptoms: Rapid cognitive decline, motor dysfunction, and death.

• Pathology: Spongiform changes in brain tissue (visible under microscope).

Prions: Infectious Proteins

Prions are infectious agents composed solely of misfolded proteins. Unlike viruses, bacteria, or other pathogens, prions lack genetic material and cellular structure. They cause disease by inducing normal proteins in the host to misfold, leading to accumulation of abnormal proteins and cell death.

• Structure: Prions are altered forms of normal proteins, often with increased β-pleated sheet content.

• Mechanism: Prions propagate by converting normal proteins into the misfolded prion form.

• Diseases: Responsible for transmissible spongiform encephalopathies, such as CWD and CJD.

Why Prions Are Not Organisms:

• Prions do not consist of cells.

• They lack hereditary genetic information (no DNA or RNA).

• They do not acquire or use energy.

• They do not reproduce in the biological sense, but rather induce misfolding of host proteins.

Key Terms and Concepts

• Cell: The basic unit of life, enclosed by a membrane.

• Gene: A segment of DNA that encodes information for a trait.

• Mutation: A change in the DNA sequence, which can create new alleles.

• Allele: An alternative form of a gene.

• Autotrophy: Energy acquisition by producing own food (e.g., photosynthesis).

• Heterotrophy: Energy acquisition by consuming other organisms.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell.

Example: Evolution of Drug Resistance

Evolution can be observed in the development of drug resistance in bacteria. When a population is exposed to antibiotics, most bacteria die, but those with mutations conferring resistance survive and proliferate. This leads to a new population dominated by resistant bacteria.

• Mutation:

• Selection:

• Proliferation:

Energy Conversion Equation

Energy acquired by organisms is converted to ATP through cellular respiration:

Summary

• Living organisms share five key characteristics: cellular organization, reproduction, genetic information, evolution, and energy use.

• Prions, though infectious, are not considered living because they lack cells and genetic material.

• Chronic Wasting Disease and Creutzfeldt-Jakob Disease are caused by prions, which induce misfolding of host proteins, leading to fatal neurological decline.

Additional info: Prions challenge the traditional definition of life and are important in understanding the boundaries between living and non-living infectious agents.