Microscope Types: Optical, Compound and Electron

Types of Microscopes

• Microscopes are categorized by application area, performance, cost, or interaction type with the object (light or electrons).

Optical Microscopes

• Optical microscopes use visible light or UV light (in fluorescence microscopy) to sharply magnify samples.
• The light rays refract with optical lenses.

Compound Microscopes

• Compound Microscopes are built with two lens systems for greater magnification
• They include an objective and an ocular (eye piece).

Stereo Microscope

• Stereo microscopes, also known as dissecting microscopes, are optical microscopes that magnify up to 100x.
• They provide a 3-dimensional view of the specimen and are highly useful for observing opaque objects.

Electron Microscopes

• Electron microscopes are the most advanced microscopes in modern science.
• They function using a beam of electrons to strike and magnify objects.
• Electron microscopes are specifically designed for studying cells, small particles of matter, and large objects.

Virus Overview

• Viruses exhibit a variety of shapes and sizes.
• The size of viruses is measured in nanometers, where one nanometer is one-billionth of a meter.
• Viruses are neither eukaryotic nor prokaryotic.
• The typical size of viruses ranges from 20 to 750nm, which is about 45,000 times smaller than the width of a human hair.
• A light microscope is usually limited to a resolution of about 200nm, so most viruses need to be viewed with an electron microscope.

Virus Structure

• A virus's basic structure includes a genetic information molecule and a protective protein layer.
• The core is composed of nucleic acids, which are the genetic information in the form of RNA or DNA.
• The capsid is the protein layer surrounding and protecting the nucleic acids.
• The term virion refers to a single virus in its complete form, when it has achieved full infectivity outside the cell.
• Virus structures can be icosahedral, enveloped, complex, or helical.
• Viruses are obligate cellular parasites, and thus, they can only replicate inside living cells.
• Viral replication is achieved through the replication of their nucleic acid and synthesis of the viral protein.
• Viruses do not multiply in chemically defined media, nor do they undergo binary fission.

Terminology

• A virion is a complete virus particle.
• A capsid is the protein coat that surrounds nucleic acid.
• Nucleocapsid refers to the nucleic acid plus the capsid.
• Capsomeres are structural protein units making up the capsid.
• Defective viruses cannot replicate on their own and require helper viruses.
• A nanometer is a milli-micron.
• Viruses consist of nucleic acid, either DNA or RNA, surrounded by a protein coat called the capsid.
• Capsids protect nucleic acid from inactivation by outer physical conditions.
• Capsomeres are small structural units which make up the capsid
• Some viruses may contain a lipoprotein envelope, which is composed of both virally coded protein and host lipid.
• The viral envelope is covered with glycoprotein spikes

Virus Structure Types

Icosahedral

• These viruses appear spherical.
• An icosahedron is comprised of equilateral triangles fused together in a spherical shape.
• The genetic material is fully enclosed inside the capsid.
• Viruses with icosahedral structures are released into the environment when the cell dies, breaks down, and lyses, thus releasing the virions.
• Examples: Poliovirus, rhinovirus, and adenovirus.

Enveloped

• An enveloped virus structure is a conventional icosahedral structure that is surrounded by a lipid bilayer membrane.
• The envelope is formed when the virus exits the cell.
• Examples include the influenza virus, Hepatitis C, and HIV.

Helical

• This virus structure has a capsid with a central cavity or a hollow tube, made by proteins arranged in a circular fashion, creating a disc-like shape.
• The disc shapes attach to each other, forming a tube where the nucleic acid is located in the middle.
• All filamentous viruses are helical in shape and are typically 15-19nm wide and ranging in length from 300 to 500nm depending on the genome size.
• Example: Tobacco mosaic virus

Complex

• These virus structures have a combination of icosahedral and helical shapes and may have a complex outer wall or head-tail morphology.
• Head-tail morphology is unique to viruses that only infect bacteria, known as bacteriophages.
• The head of the virus has an icosahedral shape with a helical shaped tail.
• Bacteriophages use their tail to attach to the bacterium, create a hole in the cell wall, and then insert its DNA into the cell using the tail as a channel.
• Poxvirus is one of the largest viruses in size and has a complex structure and a unique outer wall and capsid. One of the most famous types of poxviruses is the variola virus, which causes smallpox.

Virus Replication Steps

• Adsorption (attachment)
• Penetration
• Uncoating
• Replication of the viral genome
• Transcription of the viral genome into m-RNA
• Translation of m-RNA into viral proteins
• Protein synthesis
• Viral assembly

Virus Replication Stages in Detail

• Adsorption requires the virus to recognize and bind to specific cellular receptors with glycoproteins on the infected cell's surface.
• Penetration occurs through two main methods.
• Enveloped viruses that can form syncytia (multi-nucleated giant cells) enter the cell by fusing their viral envelope with the cell's plasma membrane.
• Remaining enveloped viruses enter via endocytosis.
• In endocytosis, the cell membrane invaginates to form vesicles in the cytoplasm
• Infected viruses are engulfed inside the vesicles.
• The nucleocapsid is later released into the host cytoplasm.
• Unenveloped viruses enter cells either by endocytosis, in which the endosome lyses (seen in adenoviruses), or by forming a pore in the cell membrane.
• The viral RNA is then released inside the cell (seen in picornaviruses).
• Uncoating: The viral genome is released from its protective capsid to enable nucleic acid replication.
• Transcription: This is the synthesis of m-RNA.
• Translation: The viral genome is translated using cell ribosomes into structural and non-structural proteins.
• Replication: The viral nucleic acid is replicated.
• Assembly: New virus genomes and proteins are assembled to form new virus particles.
• Release: Enveloped viruses are released through budding from the infected cells, while unenveloped viruses are released through rupture of the infected cells.

Reproduction of Viruses

Lytic Cycle:

• After attachment to a host cell, a virus injects its nucleic acid.
• The host cell's normal operations are overtaken, and numerous copies of the viral protein coat and nucleic acid are produced.
• Once produced, protein coats and nucleic acids are assembled into new viruses.
• The host cell fills with newly assembled viruses and then bursts.
• The host cell dies, and the released viruses start searching for the next host cell.
• This type of viral reproduction is a lytic cycle.

Lysogenic Cycle:

• Viruses, such as herpes and HIV, may enter the host cell but remain hidden for years.
• The viral nucleic acid integrates into the host cell's chromosome.
• The functions of the cell are unaffected during the lysogenic cycle.
• At some point, the virus becomes active.
• It separates itself from the host cell's genetic material, takes over the cell's functions to produce new viruses, and destroys the host cell as the new viruses are released.
• This type of viral reproduction is a lysogenic cycle.