Biological Classification

During replication of a bacterial chromosome DNA synthesis starts from a replication origin site and moves in a bidirectional way. This means that two replication forks move away from the origin site in opposite directions, leading to the synthesis of two new DNA strands simultaneously. RNA primers are involved in the initiation of DNA synthesis, but they are eventually replaced by DNA nucleotides during the process. Telomerase is not involved in bacterial DNA replication as bacteria have circular chromosomes and do not have the telomere sequences found in eukaryotic chromosomes.

Cycas belongs to Order Cycadales of gymnosperms. Its leaves show circinate vernation (i.e., the leaves are coiled in young stage). The coralloid roots in Cycas arise from the lateral branches of the normal roots and contain blue-green algae like Nostoc and Anabaena.

A well developed flower like that of angiosperms is absent in Cycas. It has compact cones containing microsporophylls and megasporophylls. The megaspores are loosely arranged on the megasporophyll. The male cone is a compact structure. Vessels in xylem are absent and it contains only tracheids for conduction of water.

Retroviruses have RNA as the genetic material and hence they exhibit reverse transcription whereby DNA is synthesized on RNA template. They have reverse transcriptase as the enzyme.

Virus is a nucleoprotein entity which becomes active only inside a living cells utilizing the latter machinery for multiplication. Capsid is the protein covering the genetic material.

Prokaryotes (bacteria) have only circular chromosomes.

The Kingdom Monera includes all prokaryotes. They are basically unicellular but can be mycelial, colonial and filamentous. They contain peptidogycan in cell wall. Naked circular DNA coiled to form nucleoid without association with histones, ribosomes 70S, thylakoids present in photoautotrophs but other membrane bound organelles are absent.

Nutrition is of various types - parasitic, chemoautotrophic, photoautotrophic and saprobic. Some monerans have the ability to fix nitrogen. Due to presence of these characters in archaea and nitrogen-fixing organisms they are placed under monera.

All others fungi, plantae, protists and animalia are eukaryotic.

Phenetic classification is based upon observable characteristics of an organism. Phylogenetic system of classification is a system indicating the evolutionary or phylogenetic relationship of organisms

Viruses can live only inside the host cell, using their machinery for its own metabolism.

Tobacco mosaic virus is 300 nm long and 20 nm in diameter.

Normally bacteria cannot survive in antibiotic containing medium but if it does so it must have acquired resistance against that antibiotic. These are well adapted to grow in streptomycin containing medium and thus are more evolved. So due to natural selection only the more evolved and better adapted species is able to survive.

In addition to the nucleoid, bacterial cytoplasm normally contains many small, separate pieces of DNA, called plasmids. These circular DNA units are 1/100 the size of the main nuclear DNA (nucleoid) and are also not enclosed in a membrane structure.

When found in cytoplasm, entirely independent of the bacterial chromosome, they replicate autonomously. Sometimes it becomes integrated into the main DNA and replicates with it. During conjugation, the plasmids, sometimes called episomes, help in the transfer of the genetic material between different bacteria. It may carry some genes of resistance to a variety of antibiotics.

The five kingdom classification is a mode of classification based on the following criteria.
• Complexity of cell structure
• Complexity of body structure
• Modes of nutrition
• Ecological life styles
• Phylogenetic relationship

Semilog of per minute growing bacterium when plotted against time, would yield ascending straight line.

In transduction, genetic material of one bacterial cell goes to other bacterial cell by agency of bacteriophages or phages (viruses, infecting bacteria).

Transduction was first of all reported in Salmonella typhimurium by Zinder and Lederberg (1952). Transduction is used for gene mapping and analysis in bacteria and also for strain construction

The growth curve for bacteria is sigmoid. First phase is the phase of slow growthLag phase. Second phase is the period of accelerated growth - Log phase. Third phase is the phase when growth rate becomes stable - Stationary phase. Finally growth rate declines.

Aspergillus flavus produces carcinogenic fungus toxin (Aflatoxin) during storage condition of crop plant.

Ustilago causes loose smut of wheat, as a result the grain and flower get converted into powdered mass.

The fungal hyphae secrete enzymes which convert insoluble complex food material in the substratum to the soluble ones. The hyphae wall of intracellular hyphae come in contact with the host protoplasm and obtain food by direct diffusion.

Cauliflower mosaic virus contain dsDNA. It is circular and shows semidiscontinuous type of replication.

Archaebacteria are the most ancient and halogenous group of bacteria and are called living fossils.

Using Gram stain, developed by Danish physician, Christian Gram in 1884, two kinds of bacteria were noted - those species of bacteria that are decolorized by alcohol are called gram negative and those that retain the stain are called gram positive.

This property of bacteria is related with the structure and compositional differences between the walls of gram positive and gram negative forms. In the cell wall of Gram +ve bacteria, both horizontal and vertical peptide linkages are present, due to which mesh is dense and hence the stain does not come out. Further outer layer of cell wall of Gram +ve bacteria is made of teichoic acid.

In the cell wall of Gram –ve bacteria, either horizontal or vertical peptide linkage are present, due to which mesh is loose and hence stain comes out. Further outermost layer of cell wall of Gram –ve bacteria is made of lipopolysaccharides.

Cyanobacteria are a group of photosynthetic bacteria that are capable of performing oxygenic photosynthesis. During this process, they use sunlight to convert carbon dioxide and water into organic compounds, while producing oxygen as a byproduct. This oxygen production is a key characteristic of oxygenic photosynthesis, and is what allows cyanobacteria to release oxygen into the atmosphere.

In addition to oxygenic photosynthesis, many species of cyanobacteria also have the ability to fix atmospheric nitrogen into a form that can be used by other organisms. This process is facilitated by an enzyme called nitrogenase, which converts nitrogen gas into ammonia, a compound that can be taken up by plants and other organisms. Cyanobacteria are the primary nitrogen fixers in many ecosystems, and they play a critical role in maintaining the balance of nutrients in many environments.

So, to summarize, cyanobacteria are oxygenic because they produce oxygen during photosynthesis, and they are capable of fixing nitrogen because they contain the enzyme nitrogenase. Option A, "Oxygenic with nitrogenase," is the correct answer.

Black rust of wheat is caused by Puccinia graminis. The symptoms are seen in stem or leaf sheath as brownish spot.