General Characteristics of Viruses

 General Characteristics of Viruses

Viral structure: Typical viral components are shown in Fig. 2. These components are a nucleic acid core and a surrounding protein coat called a capsid. In addition some viruses have a surrounding lipid bilayer membrane called an envelope.

Fig. 2. The components of helical virus

A.  Nucleic acid

• Viral genomes are either DNA or RNA (not both)

• Nucleic acid may be single- or double-stranded

Fig. 3. Types of virus genomes 

B.  Capsid

• Protein coat

• Protection of Nucleic Acid

• Provides Specificity for Attachment

• Capsomeres are subunits of the capsid

Fig. 4. Capsid structure

C.  Envelope

• Outer covering of some viruses

• Envelope is derived from the host cell plasma membrane when the virus buds out

• Some enveloped viruses have spikes, which are viral glycoproteins that project from the envelope

• Naked (non-enveloped) viruses are protected by their capsid alone

Fig. 5. Enveloped helical virus

 

 

2.  Size of viruses:

• Determined by electron microscopy

• Ranges from 20 to 14000 nm in length

Fig. 6. Size of different viruses

 

3.  Shape of viruses:

 

Four basic morphologies

• Icosahedral - efficient means to conserve and enclose space; form capsomers (planar faces formed by association of proteins) 

• Helical - capsid is shaped like a hollow protein tube 

• Enveloped - outer covering derived from the host cell's nuclear or plasma membrane and often possessing spikes or peplomer projections involved in attachment and entry into a host cell sometimes via their enzymatic activity 

• Complex symmetry - viruses that fit neither of the above categories or which may employ portions in combination, e.g., bacteriophage

Fig.7. Types of viral symmetry

 

4.  Host Range: The specific types of cells a virus can infect in its host species represent the host range of the virus.

 

• Animal virus

• Plant virus

• Bacterial virus (bacteriophage)

Host range is determined by attachment sites (receptors)   

 

Important points to remember: 

• VIRION – a complete single viral particle

• Obligatory intracellular parasites

• Contain DNA or RNA

• Do not undergo binary fission

• Sensitive to interferon

• Contain a protein coat

• Some are enclosed by an envelope

• Some viruses have spikes

• Most viruses infect only specific types of cells in one host

• Host range is determined by specific host attachment sites and cellular factors (receptors)

• Viruses replicate through replication of their nucleic acid and synthesis of the viral protein.

• Viruses do not multiply in chemically defined media

• All ss-RNA viruses with negative polarity have the enzyme transcriptase (RNA dependent RNA polymerase) inside virions.

• Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase.  

Ecological Pyramids

Ecological Pyramids: In a food chain, producers and consumers at different trophic level are connected in terms of number, biomass and energy. These properties reduces from producers to consumers and representing these parameters for food chain gives a pyramid with a broad base and a tapering apex (Figure 39.6). Ecological pyramids can be of three types:

(a)     Pyramid of Numbers

(b)     pyramid of biomass

(c)     pyramid of energy

Example of inverted ecological pyramid is provided by parasitic food chains (Figure 39.7). A single mango tree supports large number of birds, which in turn supports a large number of parasites like lice and bugs. Hyperparasites, such as bacteria and fungus are the greatest in the number and occupy the top of the inverted pyramids.

 

Flow of energy in food chain: Sun is the ultimate source of energy on earth and plants utilizes it to produce food for rest of the member of the ecosystem. Only the 1% of the total energy fall on green part of leaves is changed into the potential energy of the organic substances, the rest of the energy dissipates as heat. To explain the flow of energy, lindermann proposed the law of ten per cent law. This law proposed that during transfer of food energy from one trophic level to the other, only 10% is stored at higher trophic and the rest 90% is lost in respiration, decomposition and waste in the form of heat (Figure 39.8). For example, 5000 jules fall on leaves, it will convert only 50 jules into the chemical form (food). It will be eaten by rabbit, he will get only 5 jules (10% of 50 jules) on next trophic level. Rabbit will be consumed by carnivorous, and they can be able store only 0.5 jules (10% of 5 jules).

Ecological Equilibrium: Ecosystem always remains in the state of equilibrium. The equilibrium is dynamic is nature and biotic components appear and disappear time to time due to their death or predator. In addition, decomposer converts the complex organic matter of dead plant and animals into the simple inorganic substances. These simple inorganic substances pass through the soil, plants and animals in a cyclic manner, and this keeps the life going on in an ecosystem. Thus, both biotic and abiotic components are in a dynamic state.

 

ECOLOGY TERMINOLOGY

•  Ecology is a branch of biology concerned with the study of the interactions of living organisms with each other and with their environment.
• An ecosystem is a community of organisms that interact with their environment.
• Biosphere is a region of the earth where life can exist.(atmosphere, hygrosphere, lithosphere)
• A habitat is a place where an organism lives.
• An abiotic factor is anything that is non-living and has an effect on living organisms in an ecosystem. The two main types are:

1. Climatic factors are weather conditions that have an effect on living organisms in an ecosystem.
2. Edaphic factors are anything relating to the soil or geology of land that have an effect on living organisms in an ecosystem.

• A biotic factor is anything that is living and has an effect on living organisms in an ecosystem. (e.g presence of predator, presence of pathogenic organisms).
• Pathogenic: capable of producing disease.
• A grazing food chain is a relationship of the sequence of predator-prey relationships in an ecosystem.
• A food web consists of two or more interconnected food chains.
• An ecological pyramid of numbers shows the numbers of organisms at each trophic level in a food chain. (May be upright, partially upright or inverted in shape.
• Niche refers to the functional role an organism plays in its habitat.
• A population is a group of organisms living in a habitat that belong to the same species.
• A community is a group of organisms living in a habitat that belong to many different species.
• Competition is the struggle between organisms for a resource that is in limited supply.

1. Contest competition is the direct fight between two organisms for a resource that is in short supply. (e.g. two stags fighting for a mate)
2. Scramble competition is the struggle amongst a number of organisms for a resource that is in short supply. Each organism gets a small share of the resource. (e.g. a pack of vultures competing for a portion of the kill made by a large predator)

• A resource is a stock or supply (such as food) that can be drawn on.
• Predation is the catching, killing and eating of another organism.
• Symbiosis is the biological relationship in which two species live in close proximity to each other and interact regularly in such a way as to benefit one or both of the organisms.
• Mutualism is when both of the organisms benefit from the presence of each other, e.g. N2-fixing bacteria that live in root nodules of legume plants (such as peas) assimilate NO3- from N2.
• Parasitism is where one organism, called the parasite, lives in or on another organism, called the host, and the host is harmed. (e.g. aphids are parasites of plants, athletes foot and mosquitoes)