By Momina Shehzadi | BS Zoology Graduate, Pakistan In this article, we will learn Fungi - Morphology, Reproduction, Physiology and Importance - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students. Fungi are found everywhere around us. They are present in soil, in water, in air, on dead plants, on dead animals, on our food, on our skin and even inside our body. When bread becomes old, green or black spots appear on it. These spots are fungi. When we see mushroom growing after rain, that is also a fungus. When yeast is used to make bread soft, that is also a fungus. Fungi are very different from bacteria and plants. They are eukaryotic organisms. It means their cells have a true nucleus and other membrane bound organelles. They do not have chlorophyll, so they cannot make their own food like plants. They get their food from other living or dead things. That is why they are called heterotrophic organisms. The study of fungi is called mycolog...
By Momina Shehzadi | BS Zoology Graduate, Pakistan
In this article, we will learn Virus - General Characteristics, Structure, Classification and Bacteriophage - in simple words with diagrams. This topic is very important for BS Zoology and Microbiology students.
INTRODUCTION
Viruses are smallest infectious agents which can only be seen under electron microscope. The word virus is derived from Latin word meaning poison. The study of viruses is called Virology.
Viruses are acellular, meaning they do not have cellular organization like bacteria. They are obligate intracellular parasites, meaning they can only reproduce inside living host cell. Outside host cell they are like non-living chemicals and inside host cell they show living characters like reproduction and mutation.
Because of this dual nature, viruses are considered as connecting link between living and non-living. They are not included in any of five kingdoms of Whittaker classification.
GENERAL CHARACTERISTICS OF VIRUSES
1. Size and Shape: Viruses are very small, ranging from 20 nanometer to 300 nanometer in size. Smallest virus is Parvovirus which is 20 nm and largest virus is Poxvirus which is 300 nm. They are smaller than bacteria. They show different shapes like rod shaped in Tobacco Mosaic Virus, spherical in Poliovirus, bullet shaped in Rabies virus and complex in Bacteriophages.
2. Acellular Organization: Viruses do not have cellular organization. They do not have cytoplasm, nucleus, ribosomes or mitochondria. They are made of only nucleic acid and protein.
3. Genetic Material: Viruses contain either DNA or RNA as genetic material, never both. DNA can be single stranded or double stranded. Similarly RNA can be single stranded or double stranded. This is unique feature because all other living organisms have both DNA and RNA.
4. No Metabolic Machinery: Viruses do not have enzymes for energy production or protein synthesis. They depend completely on host cell machinery for reproduction. They have no ribosomes of their own.
5. Obligate Parasites: Viruses are obligate intracellular parasites. They can only reproduce inside living host cell. Outside host they remain inactive and can form crystals. Wendell Stanley first crystallized Tobacco Mosaic Virus.
6. Host Specificity: Each virus infects specific host and specific cells of host. For example, Bacteriophages infect bacteria only, Hepatitis B virus infects liver cells only, Poliovirus infects nerve cells. This specificity is due to presence of specific receptors on host cells.
7. Cannot Grow on Artificial Media: Viruses cannot be grown on artificial culture media like bacteria. They can only be grown inside living cells, embryonated chicken eggs, tissue cultures or laboratory animals.
8. Filterable: Viruses are filterable because they can pass through bacteria proof filters. This property helped in discovery of viruses.
STRUCTURE OF VIRUS - VIRION
A complete virus particle is called virion. Each virion consists of following parts.
1. Nucleic Acid Core or Genome
Genome is central core of virus which carries genetic information. It can be DNA or RNA. Genome can be linear or circular, single stranded or double stranded. DNA viruses include Herpes virus, Smallpox virus, Hepatitis B virus. RNA viruses include Influenza virus, Poliovirus, HIV, TMV, SARS CoV 2.
Some viruses like Retroviruses have enzyme reverse transcriptase which converts RNA into DNA.
2. Capsid
Capsid is protein coat surrounding nucleic acid. It is made of many small subunits called capsomeres. Each capsomere is made of one or more polypeptide chains. Capsid protects nucleic acid from damage by enzymes and environmental factors.
Capsid also helps virus to attach to host cell. Capsid proteins determine antigenic properties of virus and stimulate immune response in host.
On basis of arrangement of capsomeres, capsid shows three types of symmetry.
a) Helical Symmetry: Capsomeres arranged in helical manner around nucleic acid. Example Tobacco Mosaic Virus which is rod shaped with 2130 capsomeres.
b) Icosahedral Symmetry: Capsid is 20 sided polyhedron with 12 vertices. Most animal viruses have icosahedral symmetry. Example Poliovirus, Herpes virus, Adenovirus.
c) Complex Symmetry: Some viruses have complex structure with head and tail. Example Bacteriophages have icosahedral head and helical tail.
3. Envelope
Some viruses have an outer covering called envelope outside capsid. Envelope is made of lipid bilayer derived from host cell membrane during budding process. Envelope contains viral proteins and glycoproteins which form spikes or peplomers.
Enveloped viruses include HIV, Influenza virus, Herpes virus, Corona virus. Non enveloped viruses include Poliovirus, Adenovirus, TMV.
Envelope helps virus in attachment to host cell and entry. Enveloped viruses are more sensitive to detergents and drying because lipid envelope is easily destroyed.
4. Enzymes
Some viruses contain enzymes inside virion. For example Lysozyme in bacteriophages which helps to break bacterial cell wall. Reverse transcriptase in retroviruses which makes DNA from RNA. RNA polymerase in some RNA viruses for RNA replication.
CLASSIFICATION OF VIRUSES
Viruses are classified on different basis.
A. Classification on Basis of Host
1. Bacteriophages or Bacterial Viruses: Viruses that infect bacteria. Example T4 phage, Lambda phage, M13 phage. Bacteriophages are widely used in molecular biology.
2. Plant Viruses: Viruses that infect plants and cause mosaic, leaf curl and stunted growth. Example Tobacco Mosaic Virus, Potato Virus, Tomato Mosaic Virus.
3. Animal Viruses: Viruses that infect animals and humans and cause many diseases. Example Influenza virus, Poliovirus, HIV, Rabies virus, Hepatitis virus.
4. Insect Viruses: Viruses that infect insects. Example Baculoviruses which infect caterpillars and are used as biopesticides.
B. Classification on Basis of Nucleic Acid
1. DNA Viruses: Contain DNA as genetic material. DNA can be single stranded or double stranded. Examples are Herpesviridae, Poxviridae, Hepadnaviridae, Parvoviridae. Most DNA viruses replicate in nucleus of host cell.
2. RNA Viruses: Contain RNA as genetic material. RNA can be single stranded or double stranded, positive sense or negative sense. Examples are Orthomyxoviridae which includes Influenza virus, Coronaviridae, Retroviridae which includes HIV, Picornaviridae which includes Poliovirus.
RNA viruses mutate faster than DNA viruses because RNA polymerase has no proofreading.
C. Classification on Basis of Morphology
1. Helical Viruses: Example Tobacco Mosaic Virus.
2. Icosahedral Viruses: Example Poliovirus.
3. Enveloped Viruses: Example HIV, Influenza.
4. Complex Viruses: Example Bacteriophages, Poxviruses.
D. Baltimore Classification - Modern Classification
David Baltimore classified viruses into 7 groups on basis of mRNA synthesis pathway.
Group I: Double stranded DNA viruses. Example Herpes virus.
Group II: Single stranded DNA viruses. Example Parvovirus.
Group III: Double stranded RNA viruses. Example Rotavirus.
Group IV: Positive sense single stranded RNA viruses. Example Poliovirus.
Group V: Negative sense single stranded RNA viruses. Example Influenza virus, Rabies virus.
Group VI: Single stranded RNA with DNA intermediate Retroviruses. Example HIV.
Group VII: Double stranded DNA with RNA intermediate. Example Hepatitis B virus.
REPRODUCTION OF VIRUSES
Viruses reproduce only inside host cell by taking over host machinery. General steps are adsorption, penetration, uncoating, replication, assembly and release.
IMPORTANCE OF VIRUSES
Harmful Effects: Viruses cause many serious diseases in humans like common cold, influenza, AIDS, COVID 19, polio, measles, hepatitis, rabies. In plants they cause loss of crop yield. In bacteria they can kill useful bacteria.
Useful Effects: Viruses are used in genetic engineering as vectors to transfer useful genes. Bacteriophages are used in phage therapy to kill antibiotic resistant bacteria. Viruses are used to prepare vaccines. They are used in biological control of pests.
CONCLUSION
Viruses are unique infectious agents at boundary of living and non-living. Their study is essential to understand infectious diseases, vaccines and biotechnology. For BS Zoology students, it is very important to understand structure and classification because many exam questions come from this chapter.
FAQs for Students - Virus Structure and Classification
Q1: Why viruses are not considered living cells?
Ans: Because they are acellular, do not have cytoplasm, ribosomes, mitochondria and cannot reproduce outside host cell. Outside host they are like crystals and show no metabolism.
Q2: What is difference between DNA virus and RNA virus?
Ans: DNA virus contains DNA as genetic material, replicates in nucleus and mutates slowly. RNA virus contains RNA, replicates in cytoplasm and mutates fast. Example of DNA virus is Herpes virus and RNA virus is Influenza virus.
Q3: What is virion and viroid?
Ans: Virion is complete infectious virus particle with nucleic acid and capsid. Viroid is only small circular RNA without capsid which infects plants.
Q4: What is envelope in virus and its function?
Ans: Envelope is outer lipid bilayer present in some animal viruses derived from host membrane. It contains spikes for attachment to host cells and helps in entry. Enveloped viruses are sensitive to soap and disinfectants.
Q5: What are bacteriophages and their types?
Ans: Viruses that infect bacteria are called bacteriophages. On basis of life cycle they are lytic phages which kill host immediately and temperate phages which integrate DNA into host genome as prophage.
Q6: What is Baltimore classification?
Ans: Baltimore classification is modern classification of viruses into 7 groups based on type of nucleic acid and mechanism of mRNA synthesis. It was proposed by David Baltimore who got Nobel Prize.
Also Read: What is Microbiology? - History and Scope of Microbiology
Written by Momina Shehzadi, BS Zoology Graduate from Pakistan.
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