GENETIC CODE PROPERTIES

Through the experiments it has been proved that the mRNA codons of the genetic code have the following properties :

The code is triplet : Triplet code consists of 4x4x4 = 66 codons may code for 20 essential amino acids. The triple code of mRNA has been accepted.

The code is degenerate : There are 64 codons in the genetic code for 20 amino acids of which 4 codons are the signals. Therefore, 61 codons are to code for amino acids.It means that more than one codon may be coding for individual amino acid.

The code is non-overlapping : The genetic code is non-overlapping which means that the same letter does not take part in the formation of more than one codon.

The code is non-ambiguous : A particular codon will always code for the same amino acid. It may also be that the same amino acid may be coded by two different codons. However, when one codon codes for two amino acids, it is called ambiguous.

The code is comma less : The genetic code is without comma i.e. no punctuations are required between the two codons. There are no demarcating signals between two codons. This result is continuous coding of amino acid without interruption.No codons are left uncoded.which will be like UUUCUCGUAUCC.

The code has polarity : The code has polarity which is read between the fixed start and stop codons. The start codon is also known as initiation codon, and stop codon as termination codon.

The code is Universal : Though the genetic code has been worked out by using in vitro systems of microorganisms, yet there is no doubt of being its universal for all groups of micro organisms.

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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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