Summarize the following text to main points and sub-points and make detailed explain for them:

VIRUSES 
Viruses are Obligate Intracellular Parasites. 
Viruses are a cellular (no cytoplasm) microorganisms. Typically 
they are made of relatively small genetic material that is wrapped 
in a protein coat. Accordingly, viruses contain neither enzymes 
nor metabolites necessary for metabolic activities (some viruses 
might have one or more enzymes). 
Outside a living host cells, viruses do not show any sign of 
biological or metabolic activity (that is why many scientists look 
at them as dead being just crystals of genetic material). 
Nonetheless, as soon as viruses enter a suitable living host cell 
they exhibit several signs of life including transcription, 
Replication and protein synthesis etc.; but In most case they 
utilize the host enzymatic content as well as metabolites pool in 
order to carry out these vital processes. Viral activities inside the 
host cell frequently results in considerable damage to their host. 
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Classification of Viruses: 
Different types of viruses prefer specific types of hosts. 
Pathogenic viruses can be categorized as 
1) Animal (humans included) viruses, 
2) Plant viruses, or
 3) Bacterial viruses (bacteriophages). 
Viruses are classified mainly according to their genetic 
material into either: 
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DNA viruses: these contain either double-strand DNA 
(dsDNA) or single strand DNA (ss-DNA). 
RNA viruses: Similarly they are either ds-RNA or ss-
RNA viruses, 
Sometimes viruses are categorized into groups 
according to peculiar biological activities: For 
example: 
Some viruses are mainly transmitted by arthropods 
(insect), therefore they were clustered under the 
category ARBO viruses (Arthropod-Born Viruses) 
Hepatitis viruses were grouped together because they 
cause similar clinical Liver disease, hepatitis. Note that 
in the same group we find viruses that belong to DNA 
and RNA type members. Similarly, oncogenic viruses 
are malignancy-related viruses and they belong to 
both DNA and RNA viruses. 
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Morphology of Viruses: 
Most viruses exist as highly symmetric and organized 
structures. Structurally, a virus is made up of a nucleic acid 
genome (DNA or RNA) surrounded by a protein coat called 
the capsid. According to the shape of the viral capsid viruses 
acquire their final morphology. 
The basic morphologies of viruses are: 
Icosahedral (twenty-sided) shape 
Helical shape 
Complex 
Some viruses have an additional envelope (from host origin) 
that surrounds the capsid. 
According to the presence or absence of the envelop viruses 
are described as either enveloped or Naked (no envelop) 
viruses. Few Bacterial viruses are enveloped. 
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Bacteriophages: 
These are viruses that infect and can induce 
lysis of bacterial cells. 
When a suspension of viruses is spread on 
the surface of an agar plate that was 
inoculated with the proper bacterial host 
cells, then incubated together overnight, 
zones of lysis (clear zones of no bacterial 
growth) known as plaques appear in the 
bacterial culture. These plaques are 
considered viral colonies. 
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Intracellular Viral Multiplication: 
The type of bacteriophage that has infected that 
bacterium determines the ultimate fate of a virus-
infected bacterial cell
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Some viruses are so described as virulent. These 
eventually lead to lysis of the infected cells. 
This type of virus replication is known as lytic 
cycle of virus. 
Other bacteriophages, on the other hand, are 
temperate i.e. moderate or self-restrained. As a 
temperate phage enters its host cell it integrates 
itself in the bacterial chromosome, hence do not 
damage the host cell at that point. This Pathway 
of viral replication is known as lysogenic cycle 
and the integrated virus is named 
PROPHAGE. 
The phenomenon is called LYSOGENY, and the 
bacterium as lysogenic. 
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However, at another point of time the bacterial cell loses 
immunity (that forced the virus to lysogenize at the 
beginning) and the viral genome excises (cuts) itself out to 
initiate a typical lytic cycle of the virus. 
Lysogenic bacteria may acquire some important new 
properties as a result of the incorporated virus genome in 
their chromosomes. 
One of the most important of these properties is the 
acquisition of the Ability to produce a potent exotoxin e.g. 
Erythrogenic toxin produced by lysogenized Streptococcus 
pyogenes strains which cause scarlet fever; 
Botulinum food poisoning exotoxin (the most potent 
biological toxin) produced by lysogenic Clostridium 
botulinum. 
Acquisition of a new property by lysogeny is described as 
lysogenic conversion.
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Stages of Viral Replication in Lytic Cycle : 
1- Adsorption (attachment) to host cell surface: 
Each virus infects a specific host cell because each virus 
carries a specific type of molecules that can attach only and 
specifically to certain host cell surface receptors. 
Cell receptors of some viruses could be located in the cell 
wall, others could be in the flagella, sex pili or cytoplasmic 
membranes (in case of wall-less mycoplasma). 
Attachment molecules (ligands) in the virus particles are 
located on their surfaces or at specialized structures e.g. at 
the tips of the tail fibrils of T-even phages (T2, T4, and T6). 
The latter phages ligands have affinity to certain lipoproteins 
(receptors) attached to t11e cell walls of Gram negative 
bacteria. They both interact by weak noncovalent binding. 
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2- Penetration of the host cell: 
The penetration of the host cell and the entry of the viral 
genome follow viral attachment into the cytoplasm of the host 
cell. 
In T -4 phage, penetration is brought about as a result of the 
contact of the base plate with the surface of the host cell 
followed by contraction of the tube leading to puncture of the 
cell wall. 
This mechanical effect results in the injection of the contents 
of the virus head i.e. the genome, through the tail tube into 
the interior of the cell. 
This method is sometimes described as drilling a hole in the 
cell wall. 
Chi (X) phage injects its genome inside the Shaft of the 
flagellum, which then migrate to the cytoplasm. 
3- Intracellular viral multiplication and assembly: 
General scheme for the viral growth cycle is as follows: 
Early viral mRNA synthesis → Early viral protein synthesis → 
Viral genome replication → Late viral mRNA synthesis → Late 
viral protein synthesis
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Microbial Genetics
Chromosomes of procaryotes and eucaryotes
• Each bacterial cell contains only one 
chromosome, consisting of a single molecule of 
double-stranded deoxyribonucleic acid (DNA) in 
the form of a closed circle.
• The procaryotic chromosome is naked, lacking 
the nuclear membrane found in eucaryotic cells.
• Because its length is about 1200 times that of the 
entire cell, the bacterial chromosome is twisted, 
coiled and packaged into a highly compact form.
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• In addition to its chromosome, a bacterial 
cell may contain one or more interesting 
DNA structures called “plasmids”. 
Plasmids are double-stranded DNA 
molecules that are much smaller than the 
chromosome and can replicate independently 
of the chromosome. 
Most plasmids are circular DNA molecules. 
Plasmids have been used extensively in 
genetic engineering techniques.
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• The chromosomes of eucaryotic 
microorganisms (fungi, algae, and 
protozoa) differ from bacterial 
chromosomes in several aspects:
1- Each chromosome is linear rather than 
circular.
2- The DNA molecules are at least 10 
times longer than those in bacteria.
3- There is actually more than one 
chromosome per cell (most eucaryotic 
cells are diploid.
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Nucleic acids
• DNA and another substance called ribonucleic 
acid (RNA), are called nucleic acids.
• DNA is the substance that contains the hereditary 
information of a cell, whereas RNA is usually 
involve deciphering the information in DNA and 
carry out its instructions.
Deoxyribonucleic acid
A single molecule of DNA contains a vast library 
of hereditary information, but it has a simple 
chemical structure with the following features:
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I. Each DNA molecule is composed of building 
units called nucleotides”
2- Each nucleotide is constructed of three parts:
a. One molecule of a class of nitrogen-
containing compounds called "nitrogenous 
bases"
b. One molecule of the pentose sugar 
deoxyribose.
c. One phosphate group.
3- A cell links these parts together to form a 
nucleotide using energy from food sources.
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4- Four kinds of nitrogenous bases occur in DNA. 
Two are Adenine (A) and Guanine (G), which are 
called “purine bases”. 
The other two are Cytosine (C) and Thymine (T), 
which are called "pyrimidine bases”.
5- A cell links together thousands of nucleotides to 
form a single strand of DNA. Two things are 
interesting about this strand:
a. Each phosphate is attached to two deoxyribose.
b. The deoxyribose and the phosphates alternate to 
form a “backbone” from which project the purines 
and pyrimidines.
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6- Two DNA strands are cross-linked by 
means 

Summary :