MALARIA

Protozoa are unicellular eukaryotes and in most cases considerably larger than bacteria, but still of microscopic dimensions. The phylum protozoa comprises mainly four groups namely sarcodina, Mastigephora, ciliophora and sporozoa. The causative organism of Malaria (plasmodium) is included under the group sporozoa.

The name malaria expresses the association of this disease in bad air itself. Female mosquitoes of the genus Anapheles transmit this protozoan parasite.

Malaria is characterized by intermittent fever with anemia due to destruction of red blood cells. Four species of plasmodium cause Malaria. Plasmodium vivax causes most widely distributed malarial fever. Fever occurs after every two days. Plasmodium falciparum causes most severe type of disease common in tropics. Fever occurs for every two days or less. Plasmodium malariae is much less common and is mainly found in subtropical and tempereate zones. Fever reoccurs at three days interval. Plasmodium ovale predominant in west Africa and rare in other areas.

Infection: The pathogen completes its life cycle in man and female anopheles mosquito.

Life cycle in man : The pathogen enters human blood stream in the form of small spindle shaped sporozoites when the female Anapheles mosquito in which the parasite has completed its sexual cycle. From the blood the sporozoites enter liver where they develop into large multicellular shizonts. About 5 to 10 days after the infection shizonts breakdown into many merozoites and these merozoites enter blood stream which infect red blood corpuscles. Within the RBC they form trophozoites which are released after the lysis of red blood corpuscles and reinfect new red blood cells. Thus erythrocytic cycle reoccurs at usually 48 hours cycle. Some of the merozoites which invade RBCs become either male or female gametocytes.

Life cycle in mosquito : When these gametocytes sucked by mosquitoes, those gametocytes fuse to form zygote in the intestine of mosquito an it develop into oocyst. Mature oocysts rupture and release sporozoites which make their way to infect salivary glands and then ready to initiate a fresh human infection.

Lab diagnosis : T he diagnostic test for Malaria include straining blood smears with Giemsas strain. Plasmodium can be recognized in stained blood film.

Treatment : Chloroquine is the best drug for the treatment.

Prophylaxis : This include elimination of mosquito breeding places and personal protection against mosquito bites

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MECHANISM OF DNA REPLICATION

A. Kornberg (1992) has nicely discussed the DNA replication. In E.coli DNA replication has been investigated most extensively. It is thought that in eukaryotes probably similar mechanism operates. However, it has been found that in E.coli replication always starts at a very unique site called the origin. In E.coli the replicating apparatus contains an enzyme complex at the point where DNA thread is attached to plasmid membrane. Through this replicating point DNA thread moves and replication is accomplished. In eukaryotes enzyme moves along the DNA thread. It has earlier been described that E.coli possesses three types of DNA polymerases, each reads DNA template in 3’-->5’direction and catalyses the synthesis of DNA in 5’-->3’direction. The polymerases read deoxyribonucleotide triphoshates (dATP , dGRP, dTTP) as substrate and a DNA template. To the 3’ end of growing point, the nucleotides are added after interaction of 3’-OH end of dexyribose with alpha (first) phosphate group of substrate releasing pyrophosphate.

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THE NUCELIC ACIDS : DNA AND RNA

The nucleic acids found in viruses and all living organisms (microorganisms, plants and animals) carry the genetic informations. A nucleic of three main constituents: (i) a cyclic five carbon sugar, (ii) a purine or pyrimidine base, and (iii) a phosphate. The sugar is ribose or deoxyribose. Based on the types of sugars, the nucleic acids of two types, ribose nucleic acid (RNA) and deoxyribose sugar (DNA). Most of the organisms contain DNA but few phages, and plants and animal viruses contain RNA as genetic material.

Generally, nucleic acids are associated with protein to from nucleoprotein. In 1868, for the first time F. Miescher isolated nucleic acid from white blood cells that was acidic in nature to which he called nuclein. Purine and pyrimidines were isolated by Fischer in 1880. In1881, Zacharis identified nuclein with chromatin. Altaman in 1899 replaced the term nucleic acid and was awarded Nobel prize for demonstrating the presence of two purines (adenine and guanine) and two pyrimidines (thymine and cytosine). During 1910s, P.A. Levene discovered the phosphate and pentose sugar called dexoyribose molecules, in 1943, three American microbiologist, Ostwald Avery, Colin MacLeod and Maclyn McCarty, for the first time presented the evidence that DNA and F.H.C Crick (a British physicist) presented the double helix model of DNA and they were awarded Nobel Prize.

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ORGNIZATION OF DNA IN EUKARYOTIC CELL

In addition to organization of DNA in prokaryotes , in eukaryotes the DNA helix is highly organized into the well defined DNA-protein complex termed as nucleosomes. Among the proteins the most prominent are the histones. The histones are small and basic proteins rich in amino acids such as lysine and/or arginine. The histones play an important role in determining of eukaryotic chromosomes by determining the conformation known as chromatin. The nucleosomes are the repeating units of DNA organization which are often termed as beads. The DNA isolated from chromatin looks like string or beads. The 146 base pairs of DNA lie in the helical path and the histone-DNA assembly is known as the nucleosome core particle. The stretch of DNA between the nucleosome is known as ‘linker’ which varies in length from 14 to over 100 base pairs. The H1 is associated with the linker region and helps the folding of DNA into complex structure called chromatin fibers which visible as chromosomes during cell division.

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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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