Variety of Life - Theory & Question Answers Chapter # 05

 Biology XI Notes - Variety of Life - Theory & Question Answers

Chapter # 05
Theory & Question Answers
Section III - Biodiversity

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VARIETY OF LIFE

Classification / Taxonomy: Classification or Taxonomy is defined as techniques of describing, naming, and classifying living organisms on the basis of the similarities and dissimilarities of characters.

Character: A character can be defined as any attribute as a descriptive phrase, referring to form, structure, or behavior of a specific organism for a particular purpose; thus, a character is anything or any feature whose expression can be measured or assessed.

Bases Of Classification: Following are the bases of classification.

• Homology:

  • The living organisms of a particular group have many fundamental similarities in their structure. It is believed that they originated from the same structure in a common ancestor and were thus once controlled by the same gene. Structures that are similar because of the common origin but may differ functionally are said to be homologous, e.g., the flipper of a whale, limbs of a cat and man, and the wings of a bat are homologous. All these organisms are placed in class Mammalia. The study or phenomenon of homologous origin is said to be homology.

• Biochemistry:

  • The bacteria-like organisms, which look alike, can also be classified by comparing the chemical substances which they contain. By using techniques such as chromatography, electrophoresis, comparison of sequence of amino acids in proteins, or order of bases in DNA, help to classify organisms and to determine their evolutionary relationship.

• Cytology:

  • Microscopic study of cells can be useful in the classification of living organisms at the kingdom, generic, and species levels. Prokaryotic and eukaryotic organisms have been identified on cytological bases. Electron microscopic studies proved that bacteria and cyanobacteria have incomplete nuclei, so they are placed in kingdom Monera. The number of chromosomes can enable entomologists to classify locusts and grasshoppers. Seeds and pollen grains surface studies can be used in classifying flowering plants.

• Genetics:

• The main and final tool helping in classifying organisms is genetics. All morphological, cytological, and biochemical characters are based upon genetic constitution.

Concept Of Species And Hierarchy Of Biological Classification:

• Species:

  • It is the basic unit of classification. A species is a group of organisms that have numerous physical features in common and are normally capable of interbreeding and producing viable fertile offspring.

• Genera: (Singular Genus)

  • Closely related species are grouped together into genera.

• Family:

  • Closely related genera are grouped into a family.

• Order:

  • Many families with certain similar characters are grouped in order.

• Class:

  • It is a group of similar orders.

• Phyla / Division:

  • Many closely related classes constitute a division (in plants) and phyla (in animals).

• Kingdom:

  • It is the highest level of classification. All phyla or divisions are included in the kingdom.

Taxonomic Hierarchy: The ascending series of successively larger, more inclusive, groups of organisms make up the Taxonomic hierarchy. Each grouping of organisms within the hierarchy is called a Taxon, and each taxon has a rank and a name.

Nomenclature: The system used to give scientific names to living organisms is called nomenclature.

Binomial Nomenclature: It is the modern system of naming species, introduced by a Swedish biologist Linnaeus. According to him, every species shall have a particular name consisting of two words—a substantive or generic and another adjective or specific.

The first substantive or generic part is the name of the genus to which the species belongs, while the second adjective or specific part is a designation for that particular species based on certain definite and specific characters which differentiate the species from other species.

Rules Of Binomial Nomenclature:

• The first word (generic part) of the name is capitalized.
• Both words of the name are printed in italics.
• If the name is handwritten or typed, it is underlined.

Two To Five System Of Classification:

• Two Kingdoms System:

• Until quite recently, living organisms were divided into two kingdoms.

• The animal kingdom:
• This kingdom contained mainly motile organisms which fed heterotrophically. Unicellular heterotrophs (Protozoa) were put in this kingdom.

• The Plant Kingdom:
  • This kingdom contained mainly static organisms which fed autotrophically by photosynthesis. Unicellular autotrophs (Protophyta) were put in the plant kingdom with the algae. Fungi and bacteria were attached to the plant kingdom because they possessed a rigid cell wall.

Problems With Two Kingdom System: There are three main problems with having only two kingdoms.

• The first problem concerns the unicellular flagellates, such as the protozoa in the animal kingdom. However, some euglenoids, including euglena itself, contain chlorophyll and feed autotrophically by photosynthesis.
• The second problem concerns fungi, which were put in the plant kingdom. Fungi are really very different from green plants. Not only do they lack chlorophyll and feed heterotrophically, but their cellular structure differs from that of plants in several ways.
• The third problem concerns bacteria. The electron microscope has shown bacteria to have a simple prokaryotic cell structure and markedly different from all eukaryotes.

The Five Kingdom System: To cope with these problems, a number of alternative classificatory schemes have been suggested, all involving more than two kingdoms.

Robert. H. Whittaker’s Scheme (1969): The most supportive scheme of classification was proposed by Robert. H. Whittaker in 1969. He based his classification on two main criteria:

• The Level Of Organization:

  • Prokaryotes
  • Unicellular eukaryotes
  • Multicellular eukaryotes

• The Modes Of Nutrition:

  • Ingestive heterotrophs
  • Absorptive heterotrophs
  • Photosynthetic autotrophs

On this basis, Whittaker proposed the following five kingdoms.

• Kingdom Prokaryotae (Monera):

  • Level of organization - prokaryotes
  • Mode of Nutrition - Feed by a variety of different methods

• Kingdom Protista:

  • Level of organization - Unicellular eukaryotes
  • Mode of Nutrition - Feed by a variety of different methods
  • The protozoa and unicellular algae are brought together in this kingdom.

• Kingdom Fungi:

• Level of organization - Multicellular eukaryotes
• Mode of Nutrition - Absorptive heterotrophs.
• The fungi are put in this kingdom.

• Kingdom Plantae:

  • Level of organization - Multicellular eukaryotes
  • Mode of Nutrition - Photosynthetic autotrophs.

• Kingdom Animalia:

• Level of organization - Multicellular eukaryotes
• Mode of Nutrition - Ingestive heterotrophs.
• These are motile organisms.

Objections (Difficulties): The five-kingdom system solves many difficulties, but it also creates some major snags related to the protist kingdom. Separating the unicellular algae from the simple multicellular algae is not entirely satisfactory as they have certain features in common. Modification of Whittaker’s scheme was put forward by L. Margulis and K. Schwartz. The two American biologists L. Margulis and K. Schwartz suggested that all algae, unicellular and multicellular, should be included in the Protist kingdom, and this kingdom should be called the Protoctista. According to them, there are five kingdoms of living organisms. According to the modification of Whittaker’s scheme put forward by L. Margulis and K. Schwartz, there are five kingdoms of living organisms as listed below.

• Kingdom Prokaryotae: (Monera)

  • It includes almost all the prokaryotes, e.g., bacteria and cyanobacteria, etc.

• Kingdom Protoctista: (Protista)

  • It includes all the unicellular eukaryotic organisms, which are no longer classified as animals, plants, or fungi, e.g., Euglena, Paramecium, Chlamydomonas, Plasmodium, etc. Multicellular algae and primitive fungi have also been included.

• Kingdom Fungi:

  • It includes non-chlorophyllous, multicellular (except yeast) organisms having a chitinous cell wall and a coenocytic body called mycelium, e.g., Agaricus (mushroom), yeast, etc. They are absorptive heterotrophs.

• Kingdom Plantae:

  • It includes all the eukaryotic multicellular chlorophyllous photosynthetic autotrophs having cell walls made up primarily of cellulose, zygote retained to become an embryo, and exhibiting heterotrophic alternation of generation, e.g., Moss, Fern, Pine, Apple, etc.

• Kingdom Animalia:

  • It includes all eukaryotic, non-chlorophyllous, multicellular, ingestive heterotrophs with no cell wall, e.g., Hydra, earthworm, human, etc.

Viruses: The term virus is derived from a Latin word Venome, which means poison. They were first reported by a Russian biologist Iwanowsky in 1892. In 1935, Wendy Stanley succeeded in isolating tobacco Mosaic Virus (TMV) from infected tobacco leaves and prepared their pure crystal.

Characteristics Of Viruses:

• Viruses are non-cellular obligate parasites.
• They have a protein coat and a nucleic acid core.
• They range in size from 20 nm to 250 nm.
• Viruses may be virulent, i.e., destroying the host cell, or they may also be temperate, i.e., becoming integrated into their host genomes and remaining stable for long periods of time.
• They exhibit the process of replication.

Structure: They appear like little spheres or golf balls, rod-shaped, like tadpoles, and may be polyhedral. A virus consists of the following parts:

Viral Genome:

• It is a nucleic acid core. It may consist of a single or several molecules of DNA or RNA. The smallest viruses have only four genes, while the largest have several hundreds.

Capsid:

• An outer protein coat that encloses the viral genomes is called a capsid. The capsid is made up of protein subunits called capsomeres. The capsid is a thick protective sheath.

Viral Envelopes:

• Some viruses have accessory structures called viral envelopes. They are membranes cloaking their capsids. The presence and absence of envelopes classify viruses as enveloped and unenveloped viruses. Viral envelopes help viruses infect their hosts.

Tail Fibres:

• Tail fibers are additional protein fibers arising from the base plate. They help in attachment with the host. Bacteriophages contain several tail fibers.

Classification Of Viruses: The diversity of viruses is great, mostly related to the modes of origin. Viruses are classified into the following eight groups.

Unenveloped Plus-Strand RNA Viruses:(Polio Viruses, Rhino Viruses)They are called plus strand because they act directly as mRNA after infecting the host cell, attaching to the host’s ribosomes and being translated. As indicated by their name, these viruses lack envelopes and consist only of a nucleic acid core surrounded by a protein capsid. They infect plants and bacteria, causing polio and cold in human beings.

Unenveloped Plus-Strand RNA Viruses: (Polio Viruses, Rhino Viruses)
They are called plus strand because they act directly as mRNA after infecting the host cell, attaching to the host’s ribosomes and being translated. As indicated by their name, these viruses lack envelopes and consist only of a nucleic acid core surrounded by a protein capsid. They infect plants and bacteria, causing polio and cold in human beings.

• Enveloped Plus-Strand RNA Viruses: (Hepatitis A and C Viruses)
  The enveloped plus strand RNA viruses, all of which parasitize animals, are distinguished from the members of the preceding group by their lipid-rich envelopes. They infect arthropods and vertebrates, causing leukemia and yellow fever in human beings.

• Minus-Strand RNA Viruses: (Rhabdo Viruses And Pox Viruses)
  Minus-strand RNA viruses are distinguished from plus-strand RNA viruses because they carry the RNA strand complementary to the mRNA that carries the genetic information of the appropriate mRNA, which then functions in the cell. They infect plants and animals, causing flu, mumps, and rabies in human beings.

• Retrovirus:
  A virus that is replicated in a host cell via the enzyme reverse transcriptase to produce DNA from its RNA genome. They are enveloped viruses. Retroviruses are either single-stranded RNA (e.g., HIV) or double-stranded DNA (e.g., Hepatitis B) viruses.

• Double-Strand RNA Viruses: (Reo Viruses)
  These are double-stranded, icosahedral RNA viruses that infect plants and animals, causing Colorado tick fever in human beings.

• Small-Genome DNA Viruses: (Parvo Viruses)
  Many DNA viruses have small genomes; some of these viruses have single-stranded DNA, while others have double-stranded DNA. Among them are the parvoviruses, which infect animals. They are icosahedral and about 20 nanometers in diameter. They infect animals causing viral hepatitis and warts in human beings.

• Medium-Genome And Large-Genome DNA Viruses: (Herpes Viruses)

         The herpes viruses, one of the major groups of large-genome, double-stranded DNA viruses.                 They cause herpes, shingles, cancer, and poxes in human beings.

• Bacteriophage:
  A long DNA molecule is coiled within the head. They infect bacteria only.

Life Cycle Of Bacteriophage: Bacteriophage can reproduce by two alternative mechanisms:

• Lytic cycle
• Lysogenic cycle

Lytic Cycle: A viral reproductive cycle that culminates in the death of the host cell is known as the lytic cycle. This type of life cycle is referred as lytic because it always leads to the lysis (break / open) of the host bacterial cell. Virus produces only by lytic cycle is a virulent virus. Life cycle of lytic phage can be explained by considering T₄ phage and it infects the bacterium Escherichia coli.

Steps Of Lytic Cycle: Lytic cycle completed into following steps.

• Attachment:

  • The T₄ phage uses its tail fibers to stick to specific receptor sites on the outer surface of an E. coli.

• Injection:

  • From tail fibers, an enzyme lysozyme is released which dissolves the bacterium surface. Sheath contracts and a pore is made into the bacterial cell wall; through this pore, viral DNA is injected into the host bacterial cell. The empty capsid of the phage is left as a ‘ghost’ outside the host.

• Replication:

  • Viral DNA controls all the metabolic activities of the bacterial cell and directs the production of DNA and phage proteins.

• Assemblage:

  • The phage parts come together. Three separate sets of proteins assemble to form phage heads, tails, and tail fibers forming daughter phages.

• Lysis:

• These phages then direct production of lysozyme, an enzyme that digests the bacterial cell wall. With a damaged wall, osmosis causes the cell to swell and finally to burst, releasing 100 to 200 phage particles.

The Lysogenic Cycle: The lysogenic cycle replicates the viral genome without destroying the host. Viruses that are capable of using both modes of reproduction within a bacterium are called temperate viruses (Lambda). The lambda or temperate phage resembles T₄, but its tail has only one short tail fiber.

Steps Of Lysogenic Cycle:

• Adsorption:

  • Infection of an E. coli cell by X begins when the phage binds to the surface of the cell.

• Injection:

  • The Lambda injects its DNA into the host. The DNA molecule forms a circle.

• Prophage Formation:

  • The circular DNA molecule is incorporated by genetic recombination into a specific site on the host cell’s.

Biology XI Notes - Variety of Life - Theory & Question Answers

The Lysogenic Cycle: The lysogenic cycle replicates the viral genome without destroying the host. Viruses that are capable of using both modes of reproduction within a bacterium are called temperate viruses (Lambda). The lambda or temperate phage resembles T₄, but its tail has only one short tail fiber.

Steps Of Lysogenic Cycle:

• Adsorption:

  • Infection of an E. coli cell by X begins when the phage binds to the surface of the cell.

• Injection:

  • The Lambda injects its DNA into the host. The DNA molecule forms a circle.

• Prophage Formation:

  • The circular DNA molecule is incorporated by genetic recombination into a specific site on the host cell’s chromosome (chromatin body). It is known as Prophage.

• Replication:

  • The phage genome is mostly silent within the bacterium; every time the E. coli cell prepares to divide, it replicates the phage DNA along with its own and passes on the copies to the daughter cells. It is usually an environmental trigger, such as radiation or the presence of certain chemicals, which switches the virus from the lysogenic to the lytic mode.

Viroids And Prions:

• Viroids:

  • Viroids are the class of pathogens as small and simple as viruses. These are tiny molecules of naked circular RNA that infect plants, can replicate, and these RNA molecules can disrupt the metabolism of a plant cell and stunt the growth of the whole plant.

• Prions:

• Prions are infectious proteins that cause a number of degenerative brain diseases, including scrapie in sheep and the mad-cow disease. According to one hypothesis, a prion is a misfolded form of protein that converts the normal protein to the prion version.

Viral Diseases: (Transmission / Spread And Control)

Animal Diseases:

• Poliomyelitis:

  • Caused by: Polio viruses (unenveloped plus-strand RNA viruses)

  • Pathology: Virus affects the central nervous system. The destruction of motor neurons in the spinal cord results in paralysis.

  • Symptoms: Nausea, fever, headache are initial symptoms.

  • Treatment and control; Oral vaccination: Colds, Immunization methods are very difficult if not impossible.

  • Caused by: Rhinoviruses (unenveloped plus-strand RNA virus)

  • Pathology: Virus infects the upper respiratory tract.

  • Symptoms: Sneezing, cough, headache, nasal discharge, and sore throat.

• Dengue, Yellow Fever And Encephalitis:

  • Caused by: Arboviruses (enveloped plus-strand RNA viruses) are arthropod-borne viruses. Encephalitis is a sudden onset of severe headache, chills, fever, nausea, and vomiting. Some types of encephalitis may result in death, coma, and paralysis.

  Yellow fever is an acute, mosquito-borne illness; severe cases are characterized by jaundice and hemorrhage. Dengue is a mosquito-borne infection characterized by fever, muscle, and joint pain.

• Rabies:

  • Caused by: Rhabdovirus (minus-strand RNA virus) spread by small mammals such as dogs, raccoons, foxes.
  • Pathogenesis: It is an acute infection of the central nervous system.
  • Symptoms: Nervous dysfunction, increased salivation, and perspiration hydrophobia.
  • Treatment And Control: Control by vaccinating pets and humans.

• Measles And Mumps:

  • Caused by: Paramyxoviruses (minus-strand RNA virus)
  • Symptoms of Measles: Fever, redness of eyes, sneezing, rashes on the body.
  • Mumps: Characterized by the enlargement of one or more parotid glands.
  • Treatment And Control: The development of MMR vaccines has reduced the incidence of these diseases in humans.

• Flu:

  • Caused by: Flu viruses (minus-strand RNA viruses)
  • Symptoms: Chills, headache, dry cough, high fever, and muscular pain.

• AIDS:

• Acquired immune deficiency syndrome.
• Caused by: HIV (retroviruses)

Plant Diseases: Plant viruses can stunt any plant growth and diminish crop yields. One of the best-known plant diseases is called tobacco mosaic virus.

Transmission: There are two major routes by which a plant viral disease can spread.

• Horizontal Transmission:
  • A plant is infected from an external source of virus through injured parts or through insects.
• Vertical Transmission:
  • A plant inherits a viral infection from a parent.

Control: Agriculturists have not yet devised a cure for most viral diseases of plants. They have focused largely on reducing the incidence and transmission of diseases and producing virus-resistant varieties of crop plants.

Human Immune Deficiency Virus (HIV) - A Retrovirus:

AIDS - Acquired Immune Deficiency Syndrome:

Pathology: It is a disorder that impairs the body’s lymphocytic cell T4 immune system in humans. The body’s immune system breaks down, leaving the patient exposed to a variety of diseases.

The Infectious Agent: The infectious agent that causes AIDS is the glycoprotein of the viral envelope.

Transmission: The HIV can only survive in body fluids and is transmitted by blood or semen. 90% transmission occurs by sexual contact. People can contract the disease through:

• Intimate Sexual Contact:

  • It passes from the infected partner to an unaffected partner through sexual contact.

• Infected Blood Entering The Blood Stream:

  • By intravenous drug users using unsterilized needles and syringes.
  • By infected blood transfusion to uninfected individuals.
  • Close contact between infected and non-infected people through cuts and open wounds.
  • Through infected mother to the baby by placenta or by feeding milk.

Sign And Symptoms:

• Short flu.
• Skin cancer - Kaposi’s sarcoma.
• Weight loss.
• Fever.
• Diarrhea.
• Dementia.
• Septicaemia (blood poisoning)
• Severity of immune deficiency varies
• Irreversible dementia. Mental agility and behavioral changes.

Control - Treatment And Prevention:

• Best known drug - Azidothymidine or Zidovudine (formerly known as AZT)
• Ribavarin - antiviral drug
• Sumarin - antiparasitic drug

Prevention:

• Reduction of transmission by the use of clean needles and sterilized syringes.
• Education about the disease plays an important role in the prevention of disease.

Hepatitis: Hepatitis is an inflammation of the liver.

Causative Agent Of Hepatitis: Viral infection, toxic agents, or drugs.

Symptoms: It is characterized by jaundice, abdominal pain, liver enlargement, fatigue, fever.

Types Of Hepatitis:

• Hepatitis A:

  • It is caused by an enveloped RNA virus. Transmitted by contact with faces from infected individuals.

• Hepatitis B (Serum Hepatitis):

  • It is caused by Retrovirus. Enveloped double-stranded DNA virus. The hepatitis B virus poses a serious public health problem. It is transmitted through sexual contacts, skin contacts, blood transfusion.

• Hepatitis C:

  • Hepatitis C is caused by enveloped plus-strand RNA viruses. Viruses pass through blood transfusion, from mother to child during pregnancy, and by sexual contact.