Cellular Life: Properties & Organization

Topic Learning Objectives

• Understand the 5 properties of cellular life
• Classify organisms as living or non-living based on cellular life properties
• Describe the basic features of viruses
• Explain why a virus is not a cellular organism based on morphological and molecular components
• Compare viruses to prokaryotic and eukaryotic cells, identifying common and differing features
• Identify which living properties a virus performs in conjunction with a host

Characteristics of Life: Organization

• Life can be studied from molecules to the entire living planet
• The levels of the organization of life are biosphere, ecosystems, communities, populations, organisms, organs and organ systems, tissues, cells, organelles, and molecules
• The fundamental unit of life is the "cell"
• Cells is the smallest unit that can perform all activities required for life
• Cell theory claims all living organisms are made of one or more cells
• All cells come from preexisting cells.

Characteristics of Life: Information

• Life processes involve the expression and transmission of genetic information
• Within cells, chromosomes contain DNA made of hundreds/thousands of genes
• Cells can replicate DNA and divide
• All living organisms can reproduce asexually or sexually

Characteristics of Life: Metabolism

• Life necessitates the transfer and transformation of energy and matter
• All of life's activities require energy
• The primary source of energy for life is the Sun
• In photosynthesis, sunlight transforms into chemical energy
• Inorganic chemicals are other sources of energy

Characteristics of Life: Homeostasis

• Organisms tend toward homeostasis: an equilibrium of parameters defining their internal environment
• Homeostasis is achieved through the interactions of molecules and (in multicellular organisms) the cells, tissues, and organs
• Feedback regulation contributes to homeostasis
• The output of a process regulates that very process
• Feedback regulation can be positive or negative

Characteristics of Life: Evolution

• All life adapts to the environment via evolutionary processes
• All cellular life is hypothesized to have descended from one common ancestor (LUCA) approximately 3.8-4 billion years ago

Features of LUCA

• Possesses cellular characteristics
• Cell membrane
• Cytoplasm
• RNA, DNA, Ribosomes
• Electron transport chain
• ATP Synthase

Viruses and Life

• Life definitions relying on listed features exclude viruses
• Viruses are not made of cells
• Viruses cannot self-replicate
• Viruses depend on their host for resources and energy for all processes
• It is not currently understood how, or if, viruses relate to cellular life
• Viruses are described as “a kind of borrowed life”, a gray zone between life-forms and chemicals
• NASA's definition of life is broader, defining “self-sustaining chemical systems capable of Darwinian evolution” allowing for virus inclusion.

Understanding Viruses

• A virus is an infectious particle consisting of genes packaged in a protein coat
• Some are surrounded by a membranous envelope
• The smallest virus is 20 nm in diameter, the largest is 1,500 nm

Discovery of Viruses

• The first, well-studied virus was tobacco mosaic virus
• This stunts plant leaves and causes yellowed mottling
• Initially, it was hypothesized to be a very small bacterium that could not grow on nutrient media
• Beijerinck hypothesized a particle smaller and simpler than a bacterium
• Scientist Wendell Stanley crystallized the infectious TMV particle
• This allowed it to be visualized by an electron microscope

Viral Features

• Viral genomes may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA or single-stranded RNA
• Typically, genomes are organized as a single linear or circular molecule of nucleic acid

Viral Envelopes

• A viral envelope - a membranous envelope - surrounds the capsid of some viruses
• A viral envelope is derived from the membranes of the host cell
• It contains host phospholipids, membrane proteins and viral glycoproteins

Viral Capsids

• A capsid is the protein shell enclosing the viral genome
• Their shape varies (e.g. rod or polyhedral)
• Capsids are built from smaller subunits called capsomeres

Virus Replication

• Infection begins with a virus binding to a host cell
• Genome entry hinges on virus and host cell types
• After entry, virus-encoded proteins commandeer the host
• These proteins reprogram the cell to copy the genome and make viral proteins
• The host provides biochemicals and cellular machinery necessary for viral proliferation

Viruses as Parasites

• Viruses are obligate intracellular parasites that can only replicate within a host
• Virus host range is limited because of virus host recognition systems
• Virus host recognition system typically requires a “lock-and-key" fit between viral surface proteins and host receptor molecules

Virus Self-Assembly

• After production, viral nucleic acids and capsomeres spontaneously self-assemble into new viruses
• The replication cycle ends with hundreds/thousands of viruses exiting from the infected cell(often damaging or destroying it)
• Viral progeny can potentially infect new cells

Virus Facts

• A virus is an infectious particle of protein coat and a nucleic acid core
• Viruses can not reproduce by themselves
• Contains DNA or RNA
• Are host specific
• Have high mutation rates

Bacteriophages

• Bacteriophages are viruses that infect bacteria
• An elongated capsid head encloses their DNA
• A protein tail piece attaches the phage to the host and injects virus DNA

Bacteriophage Reproduction

• The bacteriophage reproduction cycle involves the capsid combining with a receptor, followed by viral DNA penetration into the host, which triggers biosynthesis and maturation of viral components
• It ends with release of new viruses from the host cell
• In a lysogenic cycle, viral DNA is passed on when bacteria reproduce

Animal Virus Reproduction

• Animal virus reproduction retrovirus goes like the following:
• Attachment
• Entry
• Uncoating
• Reverse transcriptase
• Replication
• Host DNA
• Biosynthesis
• Maturation
• Release

Prions

• Prions are renegade proteins that may be variants of "normal" proteins
• Mechanism of infection largely unknown
• Mad Cow disease
• Creutzfeldt-Jakob disease
• Chronic wasting disease
• Irreversible Neurodegeneration