DIFFERENT FORMS OF DNA

The most common form of DNA which has right handed helix and proposed by Watson and Crick is called B-form of DNA or B-DNA. In addition, the DNA may be able to exist in other forms of double helical structure. These are A and C forms of double helix which vary from B-from in spacing between nucleotides and number of nucleotides per turn, rotation per base pair, vertical rise per base pair and helical diameter.

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Cell Membrane

The cell membrane is the bounding layer of cytoplasm of the cell. This is also called as plasma membrane or cytoplasmic membrane. The cell membrane is a vital structure and critical barrier that separates the inside of the cell from the outer environment. Unlike the cell wall, it shrinks
under high osmotic pressure. The most widely accepted current model for cell membrane structure is the fluid mosaic model proposed by S Janathan Singer and Garth Nicholson.

Cell membrane is a composite, unit membrane which is tri laminar in nature and 7-8nm in thickness. Cell membrane is largely lipo protein in nature. It is composed of 20 - 30 percent phospholipids such as phosphatidyethanolamine, phosphatidyl-serine and phosphatidylcholine and 60 to 70 percent proteins. The phospholipids are amphipathic and form a lipid bylayer with hydrophobic groups ( fatty acids ) directing inward and hydrophilic groups
( glycerols ) directing outward. However in archaeobacteria the cell membrane is of mono lipid layer. The lipid matrix is fluid in nature and allow the componants to move around laterally. Of the total membrane proteins, about 70 - 80 percent proteins are embedded in the lipid matrix and are called as integral or intrinsic proteins. These integral proteins cannot be removed easily from the cell membrane about 20 - 30 percent of the total membrane proteins are loosely connected to the membrane and can be removed easily. These are called as peripheral or extrinsic proteins.

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ULTRA STRUCTURE OF BACTERIAL CELL

Microorganisms exist as either prokaryotes or eukaryotes. The prokaryotic cells are morphologically much simpler than eukaryotic cells. The prokaryotic cells lack internal membrane bound organelles that are present in the cytoplasm of eukaryotic cells such as mitochondria and chloroplast etc. Though simple, prokaryotic cell contains a variety of structures, however, all the structures are not found in every prokaryotic organism, A prokaryotic cell is bounded by a chemically complex cell wall. A cell membrane lies below the cell wall and surrounds the inner cytoplasmic matrix. This cytoplasmic matrix include unbounded nucleoid, ribosomes and some inclusion bodies. External to the cell wall there occurs extra components possessing different functions. The most typical example for prokaryotic cell organization is the bacterial cell. Structurally a bacterial cell consists of Structures external to the cell wall, cell wall and structures internal to the cell wall.

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VIRUS

Virus literally means ‘poison’. They are also called as filterable molecules during olden days as they are able to pass through filter pores which do not permit bacteria to pass. They are the submicroscopic entities. Andre Luoff, a virologist and Nobel Laureate, defined the viruses as “Viruses are Viruses”.
The general characteristics of Viruses are :
1. Viruses are a cellular organisms that are infectious in nature.
2. They are 10 to 100 times smaller than most bacteria and range from 20 to 200nm size.
3. They exist in two states namely extra cellular and intracellular.
4. They are obligate intracellular parasites
5. Viruses are incapable of independent growth in artificial media. They lack metabolic machinery of their own to generate energy or to synthesize proteins. Viruses depend on host cell machinery to carry out vital metabolic functions. However, they consist some genetic information for reproduction.
6. They reproduce only in living host cells such as plants, animals and microorganisms.
7. They structurally complete nature and infectious virus is called viron.
8. They possess either DNA or RNA as genetic material but never both.
9. The nucleic acid can be either single stranded or double stranded.
10. Genetic material is enclosed in a highly specialized protein coat called capsid.
11. The capsid may be naked or enveloped.
12. Some viruses such as bacteriophases possess a banal structure of head and tail attached.
13. They are neither prokaryotic or eukaryotic. They do not consist cytoplasm and cytoplasmic organelles.
14. Several viruses cause different diseases in plants, animals and human beings.

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SEXUAL REPORDUCTION IN BACTERIA

In bacteria sexual reproduction is of three types :

Transformation : The process of uptake of a complete or fragment of a naked DNA molecule by a competent cell from the medium and incorporation of this molecule into recipient chromosome in a heritable form is called as transformation. This type of gene transfer is discovered and demonstrated by Fred Griffith. The process of transformation is random and any portion of a genome may be transferred between bacteria. The process of transformation is an important and natural route of genetic exchange in bacteria. The transformation, as natural phenomenon, is found only in certain gram +ve and gram –ve bacteria such as Streptococcus, Bacillus, Thermoactinomyces Haemophilus, Azotobacter, Pseudomonas etc. In laboratories, the transformation is carried out artificially by treating the cells with calcium chloride. This treatment makes the cell membrane more permeable to DNA. However, the frequency of the transformation increases with high concentration of DNA.

Conjugation : The process of transfer of genetic material from one cell to another cell through direct cell-cell contact is called as conjugation. This process of conjugation was discovered in E.coli by J.Lederberg and E.L. Tatum in 1976. Later, scientists namely Hayes, Jacols and wollman proved that gene transfer in this process occurs only in one direction and it is non- reciprocal. This one-way gene transfer takes place from male or donor cell to female or recipient cell. Normally much longer DNA fragment is transferred in this conjugation process when compared to that of transformation process. Besides the main chromosome, some of the bacterial cells contain one or more. Small DNA molecules called as plasmids. This extra chromosomal genetic material can also be transferred from one cell to another cell through conjugation process.

Transduction : Transduction is defined as a process of genetic transfer from donor bacterial cell to recipient bacterial cell mediated through a bacteriophage. This process is first discovered by Norton Zinder and Joshua Lederberg in Salmonella, by a variety of bacteriophages. Transduction is one of the most common mechanism for gene exchange and recombination in bacteria. There are two type of transduction processes namely generalized transduction and specialized transduction.

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