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LIFE CYCLE OF VIRUSES

INTRODUCTION TO VIRUSES

Viruses are simple, non-cellular entities consisting of one or more molocules of either DNA or RNA enclosed in a coat of protein.Some viruses are enveloped by capsid some are naked. They can reproduce only within living cells and are obligate intra-cellular parasites. Viruses are smaller than prokaryotic cells ranging in size from 0.02 to 0.03 micrometre. A fully assembled infectious virus is called a Virion.The main function of virion is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed by the host cell.Each viral species has very limited host range.

TYPES OF VIRUSES BASED ON HOST

Prokaryotic viruses : Viruses that infect prokaryotes like bacteria.

Eukaryotic viruses : Viruses that infect Eukaryotes like animals and plants.

LIFE CYCLE OF PROKARYOTIC VIRUSES(BACTERIOPHAGE)-  The general life cycle of prokaryotic viruses are examplified by bacteriophage.For a virus to replicate it must induce a living host cell to synthesys all essential components needed to make more virions.These components must then be assembled into new virions and escape from the cell in order to infect other cells. There are to different viral life cycle Lytic and Lysogenic.

LYTIC LIFE CYCLE

Lytic or vegitative life cycle culminates in lysis or rupture of host cell and release numerous viral progeny. These are also called as virulent bacteriophages because they cause death of bacteria.

Lytic life cycle consists of following steps:

• Adsorption or docking with the host receptor protein.

• Entry or peneteration of the viral nucleic acid into the host cytoplasm.

• Biosynthesis of the viral components.

• Assembly (Maturation) of the viral components into complete viral units. After assembly viral proteins cause the lysis of host cell.

• Release of the completed virus from the host cell.

Example of Lytic phage :- T-even phages.

LYSOGENIC LIFE CYCLE

In the Lysogenic cycle the viral DNA is inserted into the host's DNA and it reproduces as the host's DNA replicates. A lysogenis virus can remain in this state for numerous replications of host cell DNA untill it excises itself from the host DNA and undergo a lytic life life cycle.When the genetic material of these phages is inserted into the DNa of host cells it is said to be in the prophage state. A cell that contains a prophage is known as lysogen.These are also called as temperate phages.

Example: lambda phage

SHOWING PROKARYOTIC VIRUSE LIFE CYCLE (LYTIC AND LYSOGENIC)

LIFE CYCLE OF EUKARYOTIC VIRUSES

ENVELOPED VIRUSES

• Adsorption begins with the docking proteins of the virus locking into specific receptor sites on the surface of the target host cells.

• In the case of some enveloped eukaryotic viruses the viral membrane fuses with the cell membrane and in effect opens up so as to allow the virus to enter the cell cytoplasm. Once in the cytoplasm the protein capsid is removed and the viral genome is freed into the cytoplasm. Depending on the nature of the virus the viral genome may stay in the cytoplasm or it may enter the nucleus before beginning the next stage of the life cycle.

• In another case the cell engulfs the entire enveloped eukaryotic virus, taking it into the cytoplasm,called as endocytosis. Once inside the cytoplasm the enveloped eukaryotic virus is said to be in a vesicle. Subsequently, the two lipid membranes of the vesicle fuse, releasing the capsid covered virus. The capsid is then removed and the viral genome is freed for the next stage of viral replication.

• The actual processes from biosynthesis to maturation and release varies depending on the virus type. However, at the core, the viral genomic material uses cellular metabolism for its own selfish purposes. Finally the mature virus or virions. are released into the environment to begin the cycle again.

SHOWING EUKARYOTIC VIRUS (RETROVIRUS)LIFE CYCLE

NAKED VIRUSE

The naked virus replication is less elaborate, but follows the same course. After binding to the host receptors the naked viral capsid penetrates into the host cytoplasm, the genome is released and the steps of viral replication ensue. Since the final release of enveloped eukaryotic viruses involves taking a portion of the cell membrane the process is called budding. For example, the HIV virus buds out of the cell. Whether release involves lysis or budding, a cell eventually dies due to the viral infection and destruction of its metabolism

Examples of eukaryotic viruses are :

Animal viruses - adenovirus, retrovirus

Plant viruses - tobacco mosaic virus

Insect viruses - baculovirus

SUMMARY

Viruses replicate inside the host using host machinary. Different viruses replicate in specific host by differenet mechanisms.General life cycle of viruses include genome entry to host and then replicate in host , assembly of new and then release.

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Competive enzyme inhibtion

Animal growth

QUESTION -Discuss growth process in animals

ANSWER-

INTRODUCTION

Growth occurs by two main processes: increase in the number of cells and increase in the size of cells.In animals, growth takes place all over the animal's body until it reaches full size. Cells in certain areas, such as the skin, still retain the ability to grow and divide.

Mitosis is the mechansim involved in all these processes, it is the type of cell division1 where 2 new cells are produced from a single cell. The two new cells have the same number of chromosomes as the parent cell and are identical to it and to each other.

Multicellular organisms have a huge variety of different types of cell; humans have around 200 different cell types, all of which are derived from a single fertilised egg cell. New cells produced by mitosis differentiate and become specialised. Each cell type has specific genes "switched on" or "switched off" so that it can carry out its particular function.

PROCESS OF GROWTH

Growth occurs during two periods: prenatal and postnatal. During prenatal growth, cells begin dividing after fertilization. Along with dividing, they begin differentiating. Differentiating is when the cells take on a certain shape and function. Three layers of cells form: the ectoderm (outer layer), the mesoderm (middle layer), and the endoderm (inner layer). The ectoderm forms the brain and nervous system. The mesoderm forms the cardio-vascular system, voluntary muscles, bone, and connective tissue. The endoderm develops into the digestive system.

During postnatal growth, the animal grows in size. This occurs in to stages. The first stage is from birth to ½ the animal’s mature size. During this period, the animal grows fast and efficiently. During the second growth period, growth is slowed. This usually occurs around puberty. The tissues slowly grow until they reach maturity. The skeleton system of the animal grows rapidly after birth. When the bones have begun to mature, the muscles of the animal begin to grow. Finally, after the muscle growth, the animal begins to deposit fat.

Late maturing animals take a longer time to reach their maximum size. This delays the on set of puberty. Their bones grow for a longer period of time. Because of this, it takes longer for the animal to reach their mature size. It also takes more time for the animal to deposit fat. Early maturing animals do not take as long to reach maturity. They enter puberty earlier, and put on muscle and fat at a younger age.

Plant growth question - Courtsey Gervaise

QUESTION - Discuss growth processes in plants with special consideration of primary and secondary growth.

ANSWER-

INTRODUCTION TO PLANT GROWTH

Plants typically continue to grow through out their lives. Growth can be defined as permanent and irreversible change in the size and number of a cell , organ, or complete organism. Growth is the process by which a plant increases in the number and size of leaves and stems. There are different phases of cell growth – cell division , cell enlargement , cell maturation. As the cells keep multiplying, the plant grows upward and outward. New leaves will appear on it, even as flowers also appear on some plants. Plants that receive proper nutrients grow to their full height and maturity. However, the height and maturity of the plant will also depend on the variety to which it belongs.

The growth of plants depends on various factors. Some of the most important factors are age of the plant, climatic conditions, growth habits of plants, and the season. The two important activities photosynthesis and respiration play a major role in the process of growth.

Growth Requirement Of Plants

• Sunlight is one of the most important nutrients required by plants. The stems of the plant may become thin, if there is a lack of sunlight.

• All plants require sufficient water. However, the amount of water required for a given plant may differ from one variety to the other. Dryness of soil is an indicator that more water is required.

• Fertile soil is another major requirement for plant growth. The soil should be rich in minerals, such as nitrogen, phosphorous and potassium , to support plant growth.

• Adequate air and right temperature are equally essential for plant growth. The plant should get sufficient air for growth and a temperature that suits their deve

TWO TYPES OF PLANT GROWTH

1. PRIMARY GROWTH -

 The primary growth refers to increase in length of plant, number of leaves, and production of branches.

It refers to the growth taking place in apical surface where higher amount of cell division takes place in apical meristem. The high pace of cell division results in cell elongation and takes place in undifferentiated regions referred as meristems.

The primary growth in roots is concentrated at root tip and is covered by the presence of root cap. The root cap produces polysaccharide and help in rapid rate of growth. The growth at the apical meristem in roots takes place at rapid phase and results in replacement of cells at root cap.

2.SECONDARY GROWTH- Secondary growth refers to increase in girth and growth associated with girth.

Secondary growth in plants takes place in vascular cambium and at a later stage it produces secondary xylem cells in the inside of the meristem and formation of secondary phloem cells on the outside of the meristem.

It is found that the secondary growth disturbs the epidermis in terms of rupture it results in formation of cork cambium which is in general dark and gives rise to thickened cork cells. The thickened cork cells are known for their role to protect.

In some cases the secondary growth can be anomalous where in regular pattern of vascular cambium producing xylem to the inside and phloem to the outside is not followed.

DIAGRAM 

 

Cell biology question on cell divison

QUESTION- What is the functional difference between mitosis and meiosis?

ANSWER

MITOSIS  - Baisic function of mitosis is cellular multiplication for fundamental process or of growth     development of multicellular organisms,tissue renewing, asexual reproduction.

MEOSIS - Basic function of meiosis is gamete formation in gametic meiosis ,and spore formation in sporic meiosis i.e meosis is for sexual reproduction .

Cell biology question - Courtesy Gervaise

QUESTION- Eukaryotic cells are complex in organization and structure, especially when you consider their small size. Consider the organelle contents of the typical eukaryotic cell. Select two organelles and compare them relative to structure and function. Describe what would be the specific probable effects on the cell if each of these organelles were lost.

ANSWER - 

 TWO SELECTED ORGANELLES FROM EUKARYOTES ARE Mitochondria and chloroplast

COMPARISON IN TABLE BELOW:

DIAGRAMATIC COMPARISON
 

EFFECT ON CELL IF MITOCHONDRIA IS LOST

If there is no mitochondria in the cell of an organism then there will be no energy in the cell due to the mitochondria being the powerhouse of the cell. Mitochondria is the place for respiration in the cell during which ATP is released as energy to utilized by cell for all its basic functions. If no mitochondria ,no energy, no life. Cell would die.

EFFECT ON CELL IF CHLOROPLAST IS LOST

If a plant cell lost its chloroplast, it would not be able to photosynthesize and create sugars for growth. Without photosynthesis ,there would be no food that can be breakdown to get energy out of it. S no photosynthesis , no, food, no energy . Cell would die.