Biology Notes - A2.3 Viruses

Virus Structure

Non-cellular infectious particles, not considered alive.

Size of Viruses

Diameter between 20 and 300 nm.

Microscopy

Viruses can only be seen with an electron microscope.

Acellular Nature

Viruses have no cellular structures and no metabolism.

Common Features

Small size, few molecules, fixed size, and a nucleic acid core.

Nucleic Acid Types

Viruses contain either DNA or RNA, which can be single-stranded or double-stranded.

Structure of Nucleic Acids

Can have a linear or circular structure.

Capsid

A protein coat that encapsulates the viral nucleic acid.

Attachment Proteins

Proteins that allow viruses to bind to and enter host cells.

Lipid Envelope

An additional layer some viruses have, derived from the host cell's membrane.

Parasitic Nature

Viruses can only reproduce by infecting living cells.

Energy from Host Cells

Viruses do not respire; they take energy from the host to replicate.

Virus Shapes

Can be threadlike, polyhedral, or spherical.

Host Specificity

Each virus can attach and infect specific types of host cells.

Example - Bacteriophage Lambda

Infects E. coli and has a double-stranded DNA genome.

Bacteriophage Tail Mechanics

The tail contracts to insert the viral DNA into the host.

Example - Coronaviruses

Cause respiratory diseases; have single-stranded RNA and a spherical shape.

Glycoproteins in Coronaviruses

Project from the surface and form a 'corona'.

Examples of Coronaviruses

SARS-CoV-2, MERS, and SARS.

Example - HIV

Transmitted through body fluids; contains two RNA strands.

Reverse Transcriptase

Enzyme allowing the production of DNA from viral RNA, unique to retroviruses.

Viral Replication

Occurs via the lytic and lysogenic pathways.

Lytic Cycle Steps

Attachment, injection, biosynthesis, assembly, and lysis.

Lysogenic Pathway

Viral nucleic acid integrates with host DNA and remains dormant.

Triggers for Lytic Pathway

Environmental changes such as UV exposure can activate dormant viruses.

Theories of Virus Origin

Escape theory, regressive theory, virus-first theory.

Antigenic Drift

Accumulation of small genetic changes over time.

Antigenic Shift

Major genetic changes resulting from co-infection of host cells.

Vaccine Challenges

Vaccines for rapidly evolving viruses need frequent updates.

Isolation Strategy

To control fast-evolving viruses, infected individuals may need isolation