Chromosome of Cyanobacteria

The DNA in cyanobacteria is organized into a complex helical and folded structure and is distributed uniformly throughout the cytoplasm. It is likely that DNA is associated with histone like proteins and RNA. The size of the genome varies widely in cyanobacteria with molecular weight ranging between 1.6x109 and 8x109 daltons. Most of the unicellular cyanobacteria possess genomes of about 1.6x109 – 2.7x109 daltons. Filamentous cyano bacteria how ever have larger genomes.
Viral Genome
The core of the virion is made up of nucleic acid, either DNA or RNA never both. Four types of nucleic acid are formed in viruses with reference to number of strands. They formed in viruses with reference to number of strands.
They are:
1- Single stranded DNA ( ssDNA) Eg.Ø X 174
2- Double stranded DNA (dcDNA) Eg. Herpes virus
3- Single Stranded RNA (ssRNA) eg. TMV
4- Double stranded RNA (dsRNA) eg. Reovirus
The ssDNA may be linear (Parvovirus) or circular (Ø X 174). The ssDNA becomes double-stranded during replication when it is called replicative form (RF).
Double stranded DNA (dsDNA) is found in many animal viruses and bacteriophages. It maybe linear (in bacteriophages), cross-linked (Vaccinia virus) or closed circular duplex (Papova virus)
Single- stranded RNA (ssRNA) found in a variety of animal viruses and plant viruses maybe plus (infectious) RNA as in RNA bacteriophage, togaviruses etc or minus (non- infectious) as in rhabdoviruses and paramyxoviruses. Plus ssRNA directly acts as mRNA and is translated to proteins on host ribosomes where as minus ssRNA first transcribe in mRNA through an RNA/DNA intermediate and then gets translated to proteins.
Double- stranded RNA is found in animal viruses like reovirus.
In general, plant viruses have RNA (ss/ds) as genetic material except for canlimovirus and geminivirus which contain DNA. Animal viruses have DNA (ds) and RNA (ss/ds). Bacteriophages have DNA (ss/ds) or RNA (ss/ds). Most phages are DNA viruses.

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Posted in: Microbiology

CHROMOSOME

The term chromosome was introduced by waldeyer in 1888. The identification of chromosomes as vehicles of hereditary was put forth by Sutton, boven and others in 1903 - chromosome theory of hereditary. The chromosomes are seen as microscopic thread-like structures in the nucleus of eukaryotes and in the nucleotide region of prokaryotes.

Prokaryotic chromosome

Prokaryotic cell contains neither a distinct membrane around nucleus nor a mitotic apparatus. The Prokaryotic chromosome, almost always a single circle of double stranded DNA is located in an irregularly shaped region called the nucleoid (also called the nuclear body, chromatin body, nuclear region). The nucleoid can be observed under the electron microscope and also under the light microscope after staining with the Feulgen stain, which specifically reacts with DNA. A cell can have more than one nucleoid when cell division occurs after DNA replication. Some bacteria like Agrobacterium tumefaciens are found to have more than 4 chromosomes.

Sinorhizobium meliloti is found to have 3 chromosomes. But usually, most prokaryotes are haploid with only a single chromosome.

In actively growing bacteria, the nucleoid has projections that extend into the cytoplasmic matrix. These may contain DNA that is being actively transcribed to produce mRNA.

Electron microscopic studies have shown the nucleoid in contact with either the mesosome or the plasma membrane which maybe involved in separation of DNA into daughter cells during division. Chemical analysis of nucleoids reveals that they are composed of about 60% DNA, some RNA and a small amount of protein.

In E.coli, a rod shaped bacterium of about 2-6 m length and 0.5- 1.5 m diameter, the closed DNA circle measures approximately 1400 m representing 4x10⁶ bp. Therefore it needs to be efficiently packaged to fit within the nucleoid. The DNA is looped and coiled extensively (about 45 supercoiled loops) probably with the aid of nucleoid proteins which differ from the histone like protein (Hu) have been found to be associated with isolated DNA. Perhaps these proteins act as repressors and prevent some sections of the chromosome from being transcribed.

Recently electron microscope observations have shown the presence of nucleosome-like structure in the E.coli chromosome. DNA is found to constitute 80% by weight of the chromosome and in addition proteins (10% largely RNA polymerase enzyme) and RNA (30% newly transcribed mRNA, tRNA, rRNA) core is present which acts as a scaffold and holds the loops and determines their position. The chromosome appears to be a highly regular. Then negative charge of the DNA neutralized by polyamines such as spermine and spermidine and by Mg²⁺ as well as basic proteins. A dense region containing membranous material is seen in the central part and is likely to represent fragments of plasma membrane,(mesosomes), to which the chromosomes is attached in the intact cell. The DNA has in addition to the four usual bases small amounts of methylated bases such as 6-methylaminopurine and 5-methylcytosine.

In archaeobecteria like Thermoplasma acidophilus, DNA is found condensed by wrapping around a proteinaceous core, quite unusual for a prokaryote. It contains a single, small, basic histone-like protein (termed HTa) that forms nucleosome like structure containing a core of four molecules of HTa around which a 40 base pair length DNA is wrapped.

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Posted in: Microbiology

RNA

RNA Structure and function:

The tertiary structure of RNA is similar to DNA, but there are several important differences:

• RNA usually forms intramolecular base pairs
• the information carried by RNA is not redundant because of these intramolecular base pairs.
• the major and minor grooves are less pronounced
• the structural, informational adaptor and information transfer roles of RNA are all involved in decoding the information carried by DNA

The 4 types of RNA

• tRNA (transfer RNA)
• mRNA
• rRNA
• snRNA

tRNA

tRNA is the information adapter molecule. It is the direct interface between amino-acid sequence of a protein and the information in DNA. Therefore it decodes the information in DNA. There are > 20 different tRNA molecules. All have between 75-95 nt.

All tRNA's from all organisms have a similar structure, indeed a human tRNA can function in yeast cells.

There are 4 arms and 3 loops. The acceptor, D, T pseudouridine C and anticodon arms, and D, T pseudouridine C and anticodon loops. Sometimes tRNA molecules have an extra or variable loop (this is shown in yellow in the adjacent figure).

tRNA is synthesized in two parts. The body of the tRNA is transcribed from a tRNA gene. The acceptor stem is the same for all tRNA molecules and is added after the body is synthesized. It is replaced often during lifetime of a tRNA molecule.

The adjacent image is a 3-D model of a yeast tRNA molecule which can code for ser. The model and the schematic above share the same color coding. You can rotate the molecule in the y axis to get better views of the structure.

Observe how the molecule is folded with the D and T pseudo-U C loops in contact, and with the acceptor stem and the anticodon loop at opposite ends.

The acceptor stem is the site at which a specific amino acid is attached by an amino-acyl-tRNA synthase. The anticodon reads the information in a mRNA sequence by base pairing.

Notice how the overall gross structure of the helix resembles that of DNA. Observe that the phosphoryl groups (shown in orange) are not on the outside of the helix like they are in DNA but are located in the groove. bases are paired similarly to DNA. In this image the acceptor stem is on the left and the anticodon loop is at the bottom. The D loop is in front of the T pseeudoU C loop at the top right.

mRNA

Messenger or mRNA is a copy of the information carried by a gene on the DNA. The role of mRNA is to move the information contained in DNA to the translation machinery.

mRNA is heterogeneous in size and sequence. It always has a 5 ' cap composed of a 5' to 5' triphosphate linkage between two modified nucleotides: a 7-methylguanosine and a 2 ' O-methyl purine. This cap serves to identify this RNA molecule as an mRNA to the translational machinery. In addition, most mRNA molecules contain a poly-Adenosine tail at the 3' end. Both the 5' cap and the 3' tail are added after the RNA is transcribed and contribute to the stability of the mRNA in the cell.

mRNA is not made directly in a eukaryotic cell. It is transcribed as heterogeneous nuclear RNA (hnRNA) in the nucleus. hnRNA contains introns and exons. The introns are removed by RNA splicing leaving the exons, which contain the information, joined together. In some cases, individual nucleotides can be added in the middle of the mRNA sequence by a process called RNA editing. In the figure the exons are represented as the region of variable sequence.

hnRNA and mRNA are never found free in the cell. Like DNA, they are bound by cations and proteins. These complexes are termed ribonucleoproteins or RNPs. The variability in sequence and structure means that no structure has been determined for a mRNA.

rRNA and ribosome synthesis

Ribosomal RNA (rRNA) is a component of the ribosomes, the protein synthetic factories in the cell. Eukaryotic ribosomes contain four different rRNA molecules: 18 s, 5.8 s, 28 s, and 5 s rRNA. Three of the rRNA molecules are synthesized in the nucleolus, and one is synthesized elsewhere. rRNA molecules are extremely abundant. They make up at least 80% of the RNA molecules found in a typical eukaryotic cell.

Synthesis of the three nucleolar rRNA molecules is unusual because they are made on one primary transcript that is chopped up into three mature rRNA molecules. These rRNA molecules and the 5 s rRNA combine with the ribosomal proteins in the nucleolus to form pre 40 s and pre 60 s ribosomal subunits. These pre-subunits are exported to the nucleus where they mature and assume their role in protein synthesis.

The rRNA molecules have several roles in protein synthesis. First, the 28 s rRNA has a catalytic role, it forms part of the peptidyl transferrase activity of the 60 s subunit. Second, 18s rRNA has a recognition role, involved in correct positioning of the mRNA and the peptidyl tRNA. Finally, the rRNA molecules have a structural role. They fold into three-dimensional shapes that form the scaffold on which the ribosomal proteins assemble. The model on the left shows a the three dimensional structure that the 5 s rRNA from the African frog, Xenopus laevis is thought to adopt.

snRNA

Small nuclear RNA (snRNA) is the name used to refer to a number of small RNA molecules found in the nucleus. These RNA molecules are important in a number of processes including RNA splicing (removal of the introns from hnRNA) and maintenance of the telomeres, or chromosome ends. They are always found associated with specific proteins and the complexes are referred to as small nuclear ribonucleoproteins (SNRNP) or sometimes as snurps.

Antibodies against snurps are found in a number of autoimmune diseases.

http://www.biochem.uwo.ca/meds/medna/RNA.html

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Paramecium

The paramecium is larger than the amoeba. It can be found in ponds with scum on them. It has more of a shape than an amoeba, looking like the bottom of a shoe. It is covered with tiny hairs that help it move. These hairs are called cilia. The paramecium is able to move in all directions with its cilia.

The paramecium eats tiny algae, plants, etc. The cilia propel the food into a tiny mouth opening of the paramecium. The food is then shoved down a little tube called a gullet that leads to the protoplasm or stuffing of the cell. The food is held in little cells called vacuoles. It has two other vacuoles at either end of its body to get rid of excess water and wastes. As with the amoeba, oxygen and carbon dioxide pass through the cell membrane of the paramecium.

The paramecium has two nuclei, a big and small one. The big one operates as the director of the cell's activities, rather like a little brain. The smaller one is used for reproduction. The paramecium splits in half (fission) just as the ameba does. First the smaller nucleus splits in half and each half goes to either end of the paramecium. Then the bigger nucleus splits and the whole paramecium splits. Occasionally two paramecium exchange material and form a new paramecium. This is called conjugation.

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Posted in: Microbiology

Tobacco mosaic virus (TMV)

TMV was the first virus to be discovered by the Dimitri Ivanowski in 1892 and crystallized by the W.M Stanley. It causes mosaic disease in tobacco plant. It belongs to tobamo virus group.

It is rod shaped, containing no envelope measuring about 300nm in length and 15-17 nm in diameter.

It has a protein capsid constituting of 95% of virus and a core of nucleic acid. The capsid is in the form of a tube with a cavity measuring about 2nm in diameter. It is composed of 2130 identical capsomers which are closely packed and arranged in the form of a regular spiral or helix.

There are about 49 capsomers for every 3 turns that is about 16 per turn of the helix. There are 130 turns in a complete virus capsid.

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Microsystis

Kingdom: Monera

Division: Cynophysia

Class: Cynophyceae

Order: Chrococcales

Family: Chrococcaceae

Genus: Microsystis

Microsystis is a free floating or planktonic blue green algae commonly found in fresh water bodies such as ponds pools lakes, etc. which maybe contaminated by sewage water. It generally forms dense water grooms. It is colonial form and the colonies are irregular. Each colony consists of a large number of densely packed small cells that are evenly distributed through out a common thin watery, gelatinous matrix. On mucilaginous matrix which is colourless and homogenous. The colony has many air space also called pseudo vacuoles or gas vacuoles which gives bouncy and allows the algae to float on the surface of water. The gas vacuole also helps in exchange of gases.

The cells are usually spherical. Each cell is typically cyanophychian with a central colourless incipient nucleus (centroplasm) and a peripheral highly pigmented chromoplasm. The chromoplasm contains phycocyanin (blue pigments), chlorophyll (green pigments) in large amounts and phycoerythrin (red pigments) in small amounts. The centroplasm consists of naked DNA without a nuclear membrane.

REPRODUCTION

It is by cell division of cells along the three planes. Sometimes a few cells may separate from the colony and develop into a new colony

ECONOMIC IMPORTANCE

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• Microsystis is known to cause to form water blooms and there for deprives the aquatic animals of the oxygen by preventing exchange of gas between the water and the atmosphere.

• It produces neurotoxin which causes nerve disorder in animals consuming water contaminated with microsystis. swimming in water with microsystis causes skin irritation and diseases
• Since microsystis grows abundantly in polluted water, it is used as a biological indicator for water pollution.

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Economic importance of genus Aspergillus.

Aspergillus has both harmful ans useful activities from the view of the human.

Harmful activities

Many species of Aspergillus such as A. glqaus A.flavus A. repns are responsible
of spoilage of exposed food stuff like jams, jellies, bread, tobacco and
many other product like leather & textiles. Many of the species are pathogenic to animals as well as human beigns. A. flavus,

A. fumigates and A.niger causes diseases of respiratory tracks commonly refered to as
aspergilloses. Aspergilloses is reported in birds, cattle, sheep, horses and human begins.

Symptoms of Aspergillosis resembles those of tuberculosis. Diesease of the human ear called otomycosis is caused by A.niger, A.flavus and A.fumigatus.

A.flavus produces a toxic substance called afflotoxin which has some carcinogenic effects and may cause cancer of liver in human begin and animals.

Conidia of Aspergillus are abundant in air. They usually spoil the laboratory culture. Many plant diseases-crown rot of ground nut and ball rot of cotton are caused by
species of Aspergillus.

Aspergillus nidulaus


A. niger

Useful effects

Several species are employed in cheese manufacturing.

A.oryzae is used in the preparation of wine from rice and soya bean sauces. Some specises of aspergillus are the source of certain antibiotics like Flavicein,Aspergillin, Geodin, Funagalin, Patulin, Ustin etc.

A. niger is used in bio-assay of metals as it can detect copper even in traces.

A.gowssipii is used in the production of vitamin B. Some species are used in the production of fats. Several species are used in the industrial production of organic acids like citric acid and gluconic acid.

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Posted in: Microbiology