Biology Notes FA FSc Chapter No 5 A Cellular Life

1st Year biology notes chapter no 5 a celular life for fbise, kpk, punjab and sindh boards.

Short Questions

Q.2 i) Give a brief status of viruses in classification.

Answer:
Viruses are at the borderline of living and nonliving. Due to their crystalline nature, they are considered non-living. They are acellular i.e. they do not have cellular organization yet show some characters of living organisms (e.g. they possess DNA). Viruses contain either RNA or DNA, normally encased in a protein coat. They reproduce only in living cells, where they cause a number of diseases. They are not considered as organisms and thus are not included in the five-kingdom classification system.
Classification of viruses:
Viruses can be broadly classified based on morphology and the type of host they infect. On the basis of morphology there are three classes of viruses which are:
a. Spherical Virus. e. g. Poliovirus

b. Tadpole shaped virus. e.g. Bacteriophage.
c. Rod-shaped virus. e.g. tobacco mosaic virus.
Viruses can be classified on the basis of a host:
a. Animals viruses:
They are parasites of animals and human beings and causes diseases in them. Common diseases in man are polio, smallpox, measles, mumps, and influenza, etc. They contain DNA but most animal viruses contain RNA as genetic material, but not both of them.
b. Plant viruses:
Plant viruses are a parasite on plants and cause diseases in them.
c. Bacteriophage (phage):
This virus is a parasite only on bacteria

Q.2 ii) Describe the differences between a retrovirus and a typical bacteriophage.

Answer:

Retrovirus	Bacteriophage
A retrovirus is a type of RNA virus that inserts a copy of its genome into the DNA of a host cell that it invades, thus changing the genome of that cell	A bacteriophage is any group of viruses that infect bacteria is called bacteriophages
Retrovirus infects in which RNA genome of the retrovirus is reverse transcribed into DNA which permanently attaches to the host genome.	Bacteriophages exhibit either lytic or lysogenic life cycles
Retroviruses enter by endocytosis or fusion	Bacteriophages are penetrated and viral DNA is injected into a host
Retrovirus contains their own reverse transcriptase enzyme.	Bacteriophages do not contain reverse transcriptase
Retroviral biosynthesis occurs in the nucleus in DNA viruses and in the cytoplasm in RNA viruses.	Bacteriophage synthesis occurs in the cytoplasm
Retroviruses are enveloped and bud out and non-enveloped viruses rupture the plasma membrane	Bacteriophages are released from the lysis of the host cell.

What are the consequences of a viral DNA becoming incorporated into a human egg or sperm cell?

Viruses have evolved intricate mechanisms to gain entry into the host cell.

Long Questions

Q.3 i) Explain how a virus manages to survive inside a host cell protected from the immune system?

Answer:
Virus mechanism of evasion of the immune response:
Viruses have evolved many mechanisms by which they can tackle the human immune system. Some of the prominent ways are as under:

Any foreign agent entering the body faces phagocytosis which is carried out by macrophages and neutrophils. In certain viruses capsules, protein, and fibrin coats do not bind the adhesion molecules used by macrophages and neutrophils so they are safe from being phagocytosed.
Some viruses cover, like bacteria, their cell walls with host proteins. So in this way body immune system is unable to recognize them as a foreign body.
Many viruses produce mutants (antigenic variations) on a regular basis. And vaccine developed to control the spread of one mutant virus becomes ineffective against the new mutant so controlling them is a continuous challenge. For example the influenza virus and HIV.
Viruses are highly adaptable and have developed ways to avoid detection by the human immune system and to avoid T cells. Some viruses stop MHC molecules from getting to the cell surface to display viral peptides. If this happens, the T cell doesn’t know there’s a virus inside the infected cell.
The degree to which viral antigens are exposed to the host immune defences is governed by the obligate intracellular replication of viruses. This exposure varies according to the virus-host cell interactions. However, human immune cell specializes in killing cells that are infected and termed as a natural killer cell or NK cell. When the NK cell finds a cell displaying fewer than normal MHC molecules it releases toxic substances, in a similar way to cytotoxic T cells, which kill the virally-infected cell.

Q.3 ii) Outline the usage of bacteriophage in genetic engineering.

Answer:
The bacteriophage is a virus which infects to bacteria. A bacteriophage can also be used for gene transformation in bacteria. The bacteriophage is also used for gene transformation in bacteria. Bacteriophages have several potential applications in the modern biotechnology industry and genetic engineering.
Bacteriophages in recent times are probably the most significant source of variation for bacterial genomes. Bacteriophages have been proposed as delivery vehicles for protein and DNA vaccines; as gene therapy delivery vehicles; as alternatives to antibiotics; for the detection of pathogenic bacteria; and as tools for screening libraries of proteins, peptides or antibodies. This diversity and the ease of their manipulation and production means that they have potential uses in research, therapeutics and manufacturing in both the biotechnology and medical fields.

Method of use of bacteriophage in genetic engineering:
Isolated genes cannot replicate themselves, a gene to be cloned must be inserted into the DNA of suitable cloning vector like bacteriophages. The virus DNA molecule is used to transfer a DNA fragment from a test tube into a living cell. Cloning vectors are capable of multiplying inside of living cells. Phages can be used as cloning vectors to introduce recombinant DNA into bacterial cells. Once inside a cell, the recombinant DNA may begin replication and new phages, each containing the gene of interest, are formed. The bacterial cellular machinery synthesizes the vector system proteins and DNA, but the bacterium is destroyed when the phages are released.

Q.3 iii) Explain the opportunistic diseases that may attack an AIDS victim.

Answer:
Opportunistic Invasions:
Opportunistic infections are so named because they take advantage of a weakened immune system. Because AIDS is a serious condition that weakens the body immune system, leaving it unable to fight off illness. Illnesses that are deemed to be opportunistic infections and lead to an AIDS diagnosis includes, but is not limited to,

Pulmonary tuberculosis
Candidiasis of the esophagus, trachea, bronchi or lungs
Toxoplasmosis of the brain
Severe bacterial infections
Recurrent pneumonia

Besides vision loss, nerve damage and brain impairment can also occur. Signs of brain deterioration include troubles thinking, loss of coordination and balance and behavioral changes.

Q.3 iv) Describe the causative agent, symptoms, treatment, and prevention of leaf curl virus disease of cotton.

Answer:
Leaf curl disease of cotton:
Cotton leaf curl disease is caused by a complex of begomovirus species, all of which incite similar symptoms in cotton and are transmitted by the whitefly Bemisia tabaci.
Symptoms:
The first symptoms of infection in cotton appear within 2-3 weeks of inoculation and are initially characterized by deep downward cupping of the youngest leaves. This is followed by either upward or downward curling of the leaf margins and swelling, darkening and formation of enations on the veins, which frequently (depending on cultivar) develop into cup-shaped, leaf-like structures.
Prevention and control:

Use resistant or tolerant cultivars
Protect seedlings from whiteflies
Use only good seeds and healthy transplants
Immediately remove infected-looking plants and bury them
Do not plant cotton near tomato and/or other crops susceptible to whiteflies
Practice crop rotation by planting crops that are not susceptible to whitefly
Control Weeds

Q.3 v) Write a brief note on prions and viroids.

Answer:
i. Prions:
“Prions are small proteinaceous infectious disease-causing agents.”
Prions are infectious protein particles thought to be responsible for a group of transmissible neurodegenerative diseases. Most evidence indicates that the infectious prion proteins are modified forms of normal proteins coded for by a host gene in the brain. It is thought that the normal prion protein, expressed on stem cells in the bone marrow and on cells that will become neurons, plays a role in the maturation of neurons.
For example:
In Scrapie, the central nervous system of sheep and goats is affected. The transmission of scrapie is mainly due to the unhygienic way of feeding on infected food.
Example of prion-related diseases in humans:
Humans are susceptible to several prion diseases:
CJD: Creutzfeld-Jacob Disease
GSS: Gerstmann-Straussler-Scheinker syndrome
FF1: Fatal Familial Insomnia
Alpers Syndrome:
These original classifications were based on a clinical evaluation of a patients family history symptoms and are still widely used, however, the more recent and accurate molecular diagnosis of the disease is gradually taking the place of this classification. The diseases are characterized by loss of motor control, dementia, paralysis wasting and eventually death.
Humans might be infected by prions in 2 ways: Acquired infection (diet and following medical procedures such as surgery, growth hormone injections, corneal transplants) i.e. infectious agent implicated. Apparent hereditary mendelian transmission where it is an autosomal and dominant trait.
2. Viroids:
“Viroid is an infectious agent that consists solely of a single strand of RNA and causes disease in certain plants. Viroids lack the protein coat (known as a capsid) of viruses and are the smallest known infectious agents.”
Structure of viroids:
Viroids are even more simple than viruses. They are small, circular, single-stranded molecules of infectious RNA lacking even a protein coat.
Plant diseases caused by viroids:
They are the cause of a few plant diseases such as:
1. Potato spindle-tuber disease
2. Cucumber pale fruit
3. Citrus exocortis disease
4. Cadang-cadang (coconuts)
The first viroid discovered was the potato spindle tuber viroid (PSTV)I which causes a disease in potatoes. It has been reported that the infectious agent for the disease was not a conventional virus but free RNA. PSTV is a circular RNA molecule whose nucleotide sequence and secondary structure has been established.
Disease caused in humans:
The only human disease known to be caused by a viroid is hepatitis D.