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Acellular Life notes

MDCAT Biology

Acellular life includes viruses, viroids, satellites and prions. Viruses are non-cellular infectious particles that reproduce only inside living cells, while HIV causes AIDS by attacking helper T lymphocytes and weakening the immune system.

Meaning and General Features of Viruses

Viruses are acellular infectious particles. They are not made of cells and do not have cytoplasm, cell membrane, ribosomes or independent metabolic machinery. A virus can reproduce only inside a living host cell, so it is called an obligate intracellular parasite.

Outside a host cell, a virus exists as an inactive particle called a virion. Inside a suitable cell, it uses the host cell's enzymes, ribosomes and energy to make new viral particles. Viruses are therefore placed at the boundary between living and non-living matter.

  • A virus contains genetic material surrounded by a protective protein coat.
  • Viral genetic material may be DNA or RNA, but a single virus generally contains only one type of nucleic acid.
  • Viruses do not grow successfully in pure culture test tubes because they require living cells for reproduction.
  • Viruses are not destroyed by antibiotics such as penicillin. Antibiotics act against bacteria and do not normally kill viruses.
  • The smallest viruses are about 20 nm in size.
  • Viruses can be crystallised outside a host cell, showing some non-living characteristics.
  • Inside a host cell, viruses show living characteristics by reproducing and causing infection.

Discovery, Structure and Classification of Viruses

In 1892, D. J. Ivanovsky showed that the infectious agent causing tobacco mosaic disease could pass through porcelain filters that retained bacteria. Charles Chamberland developed the porcelain bacterial filter used in such investigations and showed that bacteria could not pass through it. Later, the infectious agent was recognised as a virus.

The complete virus particle is called a virion. Its viral genome and protein coat together form the nucleocapsid. Some viruses also possess an outer envelope containing lipids and viral proteins.

  • The viral genome is the genetic material of the virus and may be DNA or RNA.
  • The capsid is the protective protein coat around the viral genome.
  • The nucleocapsid is made up of the genome and capsid.
  • Capsid proteins are arranged into smaller units called capsomeres.
  • The matrix protein lies between the nucleocapsid and the envelope in an enveloped virus.
  • An envelope is an outer lipid membrane containing viral glycoproteins or spikes.
  • Herpes virus is an example of an enveloped virus.
  • Influenza virus has a single-stranded RNA genome.

Viral Envelopes and Reproduction

Some viruses acquire an envelope from the host cell membrane as newly formed viruses leave the cell. This process is called budding. The envelope may also be obtained from other host cell membranes, such as the nuclear membrane or membranes associated with the endoplasmic reticulum and Golgi apparatus, depending on the virus.

Viral reproduction involves attachment to a suitable host cell, entry of the viral genome or whole virus, production of viral components, assembly of new virions and release. The exact steps differ according to the type of virus.

  • Enveloped viruses possess a lipid envelope; non-enveloped viruses do not.
  • Viral spikes help a virus attach to specific receptors on a host cell.
  • Viruses can bud from the plasma membrane, nuclear membrane or other internal membranes of the host cell.
  • Budding may allow an enveloped virus to leave the cell without immediate complete destruction of the cell.
  • Viruses show host specificity because their attachment proteins recognise particular receptors.
  • The host range of a virus depends partly on the availability of suitable receptors on host cells.

Bacteriophages and Their Structure

Viruses that infect bacteria are called bacteriophages, or phages. A typical bacteriophage has a head and a tail. The head contains the viral nucleic acid, while the tail helps the virus attach to the bacterial cell and inject its genetic material.

The typical bacteriophage has a tadpole-like shape. After attachment to the bacterial surface, the phage injects its nucleic acid into the bacterium. The protein coat usually remains outside the bacterial cell.

  • A bacteriophage is a virus that infects a bacterial cell.
  • A typical bacteriophage is shaped like a tadpole.
  • The head of a bacteriophage contains its genetic material.
  • The capsid protects the phage genome.
  • The tail, tail fibres and base plate help in attachment to the bacterial surface.
  • The phage injects its nucleic acid into the bacterium while most of its protein coat remains outside.
  • Bacteriophages may follow a lytic cycle or a lysogenic cycle.

Lytic and Lysogenic Cycles

In the lytic cycle, a bacteriophage attaches to a bacterial cell and injects its genetic material. The phage genome directs the host cell to make viral nucleic acid and proteins. These components are assembled into new phages. The bacterial cell finally breaks open, or lyses, and releases the new phages.

In the lysogenic cycle, the phage DNA joins the bacterial chromosome. The integrated phage DNA is called a prophage. It is copied whenever the bacterial chromosome replicates and can later become active and enter the lytic cycle.

  • The lytic cycle ends with the death and lysis of the bacterial cell.
  • The cycle in which the phage kills the bacterium is called the lytic cycle.
  • The lysogenic cycle begins when phage DNA becomes integrated into the bacterial chromosome.
  • Phage DNA integrated into a bacterial chromosome is called a prophage.
  • In the lysogenic cycle, the prophage is copied along with the bacterial DNA.
  • A change in conditions may activate a prophage and cause it to enter the lytic cycle.
  • Lytic cycle and lysogenic cycle differ because the lytic cycle immediately produces new phages and destroys the host, whereas the lysogenic cycle initially keeps the host cell alive.

Other Acellular Infectious Agents

Viroids are much simpler than viruses. They are small, circular, single-stranded RNA molecules that lack a protective protein coat. They mainly infect plants. The term viroid was introduced after their discovery by Theodor O. Diener. D. J. Ivanovsky discovered a filterable infectious agent, but he did not discover viroids.

Prions are infectious proteins. They contain no DNA or RNA. Abnormally folded prion proteins can cause serious diseases of the nervous system. Satellite viruses are defective viruses that require the help of another, unrelated virus in the same host cell.

  • Viroids consist of naked, circular, single-stranded RNA.
  • Viroids lack a protective protein coat.
  • Viroids mainly cause diseases in plants.
  • Theodor O. Diener is associated with the discovery of viroids.
  • Prions are infectious proteins that lack nucleic acid.
  • Prions are known to cause mad cow disease, also called bovine spongiform encephalopathy.
  • A satellite virus is a defective virus that needs an unrelated helper virus for essential functions.
  • Viroids, prions and satellite viruses are different from complete viruses in structure and dependence on host or helper agents.

Viral Diseases and Prevention

Viruses cause diseases in humans, animals and plants. Their effects range from mild illness to chronic disease. Transmission may occur through air droplets, contaminated food or water, blood, sexual contact, insect vectors or direct contact, depending on the virus.

Vaccination exposes the immune system to a harmless form or component of a pathogen. It stimulates the formation of immune memory and helps prevent disease. Edward Jenner developed the first successful vaccination against smallpox. Smallpox was a viral disease, so the first infectious disease against which an effective preventive method was developed was a viral disease.

  • Edward Jenner is associated with the discovery of vaccination.
  • Smallpox was the viral disease involved in Jenner's first successful vaccination method.
  • Rubella is commonly called German measles.
  • Dengue fever is transmitted to humans by the bite of Aedes mosquitoes.
  • Hepatitis A usually causes an acute liver infection and does not cause chronic liver disease.
  • Hepatitis B and Hepatitis C may cause chronic liver disease.
  • Vaccines help the body produce specific immune memory before natural infection occurs.
  • Viral disease prevention may include vaccination, safe water, hygiene, insect control and avoiding contact with infected body fluids.

HIV Infection and AIDS

HIV means human immunodeficiency virus. It is a retrovirus that contains RNA and uses the enzyme reverse transcriptase to make a DNA copy inside the host cell. The DNA copy can become integrated into the host cell's genetic material. HIV mainly infects helper T lymphocytes, although it can also infect macrophages and some other immune cells.

AIDS means acquired immunodeficiency syndrome. It develops when HIV infection causes severe weakening of the immune system. The reduced number and impaired function of helper T cells make the person vulnerable to opportunistic infections and certain cancers.

  • The major cell infected by HIV is the helper T lymphocyte.
  • HIV is a retrovirus with an RNA genome.
  • Reverse transcriptase makes DNA from the viral RNA template.
  • HIV can integrate its DNA into the genetic material of the host cell.
  • Helper T lymphocytes coordinate immune responses by activating other immune cells.
  • AIDS is the advanced stage of HIV infection and is characterised by serious immune deficiency.
  • HIV is transmitted through infected blood, semen, vaginal fluids and breast milk.
  • HIV is not spread by ordinary casual contact such as shaking hands, sharing food or sitting near an infected person.

HIV Transmission, Effects and Prevention

HIV enters the body through infected body fluids and targets cells that carry suitable receptors. As helper T lymphocytes are gradually destroyed or impaired, the immune response becomes weaker. The infected person may remain without obvious symptoms for a period, but the virus continues to multiply and damage the immune system.

There is no generally available preventive vaccine against HIV. One reason is that HIV changes its surface proteins and genetic material rapidly. This high mutation rate makes it difficult to design a vaccine that provides lasting protection against all viral variants. Prevention therefore depends on reducing exposure and using safe medical practices.

  • HIV may be transmitted by sharing contaminated needles or syringes.
  • Transfusion of infected blood can transmit HIV if blood is not properly screened.
  • Unprotected sexual contact can transmit HIV.
  • An infected mother may transmit HIV to her child during pregnancy, childbirth or breastfeeding.
  • HIV infection can be reduced by screened blood, sterile medical equipment and safer sexual practices.
  • HIV cannot be prevented by antibiotics because HIV is a virus, not a bacterium.
  • The absence of a widely effective HIV vaccine is partly related to the virus's high mutation rate.
  • Antiretroviral medicines can reduce viral multiplication and help infected people live longer, healthier lives.

Key terms

Virus
A non-cellular infectious particle that reproduces only inside a living host cell.
Virion
A complete virus particle outside a host cell.
Capsid
The protective protein coat surrounding a viral genome.
Capsomere
A protein subunit that forms part of a viral capsid.
Nucleocapsid
The combination of a viral genome and its capsid.
Viral envelope
An outer lipid membrane containing viral proteins and often acquired from the host cell.
Bacteriophage
A virus that infects and reproduces inside a bacterial cell.
Lytic cycle
A phage life cycle in which new phages are produced and the bacterial cell is lysed.
Lysogenic cycle
A phage life cycle in which phage DNA joins the bacterial chromosome and forms a prophage.
Prophage
Phage DNA integrated into the chromosome of a bacterial cell.
Viroid
A small naked, circular, single-stranded RNA agent that mainly infects plants.
Prion
An infectious, abnormally folded protein that can cause nervous system diseases.
Satellite virus
A defective virus that requires an unrelated helper virus to complete essential functions.
HIV
Human immunodeficiency virus, a retrovirus that attacks immune cells, especially helper T lymphocytes.
AIDS
Acquired immunodeficiency syndrome caused by advanced HIV infection and severe weakening of immunity.
Helper T lymphocyte
An immune cell that coordinates immune responses by activating other immune cells.
Reverse transcriptase
An enzyme used by retroviruses to make DNA from an RNA template.
Vaccine
A preparation that stimulates specific immune memory and helps prevent a particular infection.

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Virus Basics

  • Viruses are acellular, non-cellular infectious particles.
  • A complete, mature, infectious virus particle is called a virion.
  • Viruses cannot replicate independently because they lack metabolic machinery.
  • An isolated virus is non-living because it cannot metabolize.
  • All viruses contain nucleic acid and a protein coat; lipids are not universal.

Structure and Types

  • The protein coat surrounding viral nucleic acid is the capsid.
  • Capsids consist of protein subunits called capsomeres.
  • Herpes virus contains 162 capsomeres.
  • Poxviruses are the largest animal viruses.
  • Viroids are naked, infectious RNA molecules lacking a protective protein coat.

Bacteriophages

  • Bacteriophages are viruses that infect bacteria.
  • Bacteriophages replicate inside bacterial cells.
  • The lytic cycle produces new phages and destroys the bacterial cell.
  • The lysogenic cycle integrates phage DNA into bacterial DNA.
  • Integrated phage DNA is called a prophage.
  • Bacteria protect themselves by fragmenting viral DNA with restriction endonucleases.

Human Viral Diseases

  • Rabies is mainly transmitted through the bite of an infected animal, especially a dog.
  • Mumps and measles are caused by paramyxoviruses.
  • HCV is an enveloped RNA virus causing hepatitis C.
  • Hepatitis A does not cause chronic liver disease.
  • Smallpox was eradicated globally through effective vaccination.
  • WHO declared smallpox completely eradicated in 1980.

HIV and AIDS

  • HIV is a retrovirus classified in the lentivirus subgroup.
  • The HIV matrix protein is p17.
  • HIV mainly destroys CD4 helper lymphocytes.
  • The interval before detectable antibodies appear is the window period.
  • HIV transmission occurs through infected blood, semen, vaginal fluids, and breast milk.
  • Shaking hands or sharing utensils does not transmit HIV.

HIV Replication and Prevention

  • HIV reverse transcriptase forms DNA from its RNA template.
  • Provirus integration occurs in the host-cell nucleus.
  • Integrated HIV DNA directs formation of viral components.
  • New virions assemble and bud from the host-cell membrane.
  • Avoiding contaminated blood, unsterilised instruments, and unsafe sexual contact reduces transmission.

Must remember

  • Virion means a complete, mature, infectious virus particle.
  • Lipids are absent from some viruses, but nucleic acid and protein are universal.
  • Herpes virus has 162 capsomeres.
  • HIV matrix protein is p17.
  • HIV belongs to the lentivirus subgroup of retroviruses.
  • HIV destroys CD4 helper lymphocytes.
  • Provirus integration occurs in the nucleus.
  • Hepatitis A does not cause chronic liver disease.
  • Smallpox was declared eradicated in 1980.
  • Viroids lack a protective protein coat.

Acellular Life mnemonics

Order of the steps in the lytic cycle

A Proud Biologist Assembles Rockets.

  • A: Attachment
  • P: Penetration
  • B: Biosynthesis
  • A: Assembly
  • R: Release

Use this order for the complete lytic cycle. Do not confuse it with the shorter DNA-injection sequence of a bacteriophage.

How a bacteriophage injects DNA

Landing Tigers Pierce DNA.

  • L: Landing of the phage on the bacterium
  • T: Tail contraction
  • P: Penetration
  • D: DNA injection

This sequence describes the attack and injection stage, not the complete lytic cycle. T2 phage is a dsDNA phage.

HIV transmission, provirus and AIDS infection

HIV passes by Pregnancy, Delivery and Breastfeeding; its DNA Integrates.

  • P: Transmission during pregnancy
  • D: Transmission during delivery
  • B: Transmission through breastfeeding
  • I: Provirus means viral DNA integrated into the host chromosome

Remember P-D-B for mother-to-child transmission. Aspergillosis can occur as an opportunistic fungal infection in AIDS patients.

Basic facts about viruses and related pathogens

Prions are Simplest; Viruses Need Hosts, Borrow Ribosomes and Break cell theory.

  • P: Prions are the simplest disease-causing pathogens
  • V: Virology is the study of viruses
  • N: Viruses are nutritionally obligate parasites and need a host
  • B: Viruses use the host cell's ribosomes
  • B: Viruses are an exception to cell theory

Viruses cannot carry out independent metabolism or protein synthesis. They depend on the host cell for these functions.

Examples, shapes and capsid facts

T2 Doubles, TMV Twists, Polio Polygons; Flu has RNA and an Envelope; Tetanus is not Viral.

  • T2 Doubles: T2 phage contains double-stranded DNA
  • TMV Twists: Tobacco mosaic virus has a helical shape
  • Polio Polygons: Poliovirus has a polyhedral shape
  • Flu has RNA and an Envelope: influenza is caused by an enveloped RNA virus
  • Tetanus is not Viral: tetanus is a bacterial disease, not a viral disease
  • Adenovirus has 252 capsomeres
  • Icosahedral capsids have 20 triangular faces, with capsomeres arranged as pentamers and hexamers
  • A viral envelope is derived from the host cell membrane
  • Hepatitis D is caused by a virus

Use the examples to match a virus with its nucleic acid, envelope or shape. The 252 value refers specifically to the adenovirus capsid.

Biology shortcuts

Recognising acellular organisms

If an entity has no cellular organisation and depends on a host for replication, identify it as a virus. Viruses are non-cellular and are not placed among cellular organisms.

  • Check whether it has cytoplasm, ribosomes and independent metabolism.
  • Viruses contain nucleic acid inside a protein coat but lack cellular structure.
  • Example: An infectious particle without cytoplasm or ribosomes is a virus.
  • Answer: Non-cellular virus.

This shortcut does not apply to bacteria, which are cellular prokaryotes.

Separating prokaryotic and eukaryotic cells

Use ribosomes as the common feature. Both cell types have ribosomes, but only eukaryotes have a membrane-bound nucleus and membrane-bound organelles.

  • Look for the feature present in both groups.
  • Ribosomes occur in prokaryotes and eukaryotes for protein synthesis.
  • Example: Which structure is common to both? Ribosomes.
  • Answer: Ribosomes.

Do not use mitochondria, chloroplasts or a membrane-bound nucleus as common features.

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