Characteristics of Life

Introduction

The study of biology begins with understanding what defines living organisms. This section explores the essential characteristics that distinguish living things from non-living entities, using prion diseases as a case study to highlight the boundaries of life.

Five Characteristics of Living Things

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

• Reproduction: Living things reproduce, either sexually (involving two parents and genetic recombination) or asexually (involving a single parent and identical offspring).

• Genetic Information: Organisms possess hereditary information encoded in genes (segments of DNA), which determine traits and are passed to offspring.

• Evolution: Populations of organisms evolve over time through changes in the frequency of heritable traits, often driven by mutations and natural selection.

• Energy Acquisition and Use: Organisms acquire energy (e.g., through photosynthesis or consumption) and convert it to a usable form, such as ATP, to power cellular processes.

Comparison of Prokaryote and Eukaryote Cells

• Similarities: Both types have a cell membrane, genetic material, and perform basic life functions.

• Differences: Prokaryotes lack a nucleus and membrane-bound organelles, while eukaryotes possess both.

Prion Diseases: Chronic Wasting Disease & Creutzfeldt-Jakob Disease

Introduction

Prion diseases are fatal neurological disorders affecting both animals and humans. They provide a unique perspective on the definition of life, as prions are infectious agents that challenge traditional biological criteria.

Chronic Wasting Disease (CWD)

• Description: A fatal neurological disease affecting deer and related species.

• Symptoms: Progressive brain degeneration, leading to wasting and death.

• Distribution: CWD is found in various regions across North America.

Creutzfeldt-Jakob Disease (CJD)

• Description: A fatal neurological disease in humans.

• Symptoms: Rapid brain shrinkage and deterioration, with characteristic spongiform pathology visible in brain tissue.

Prions: The Infectious Agent

• Definition: Prions are misfolded proteins that act as infectious agents, causing diseases such as CWD and CJD.

• Mechanism: Prions reproduce by inducing normal proteins in the host to misfold, disrupting their function and leading to cell death.

• Structure: Infectious prion proteins have abnormal β-pleated sheet structures, compared to the normal α-helix configuration.

Why Prions Are Not Considered Organisms

• Lack of Cellular Structure: Prions are not made of cells.

• No Genetic Material: Prions do not contain DNA or RNA.

• Limited Life Characteristics: While prions can reproduce (by converting normal proteins), their method is fundamentally different from cellular organisms.

Table: Comparison of Prions and Living Organisms

Example: Prion-Induced Disease Progression

• In CWD and CJD, prions accumulate in neural tissue, causing progressive degeneration and fatal outcomes.

• Prion diseases are transmissible and can spread within populations, but prions themselves do not meet the criteria for life.

Key Terms

• Prion: Infectious protein causing neurodegenerative disease.

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

• Gene: DNA segment encoding hereditary information.

• Evolution: Change in heritable traits in a population over time.

Summary

Prion diseases illustrate the boundaries of life by demonstrating how infectious agents lacking cellular structure and genetic material can cause disease, yet are not considered living organisms. Understanding the characteristics of life is fundamental to biology and helps distinguish living entities from non-living agents like prions.