Q.15. Describe various modes of reproduction in bacteria. (2009)
Realted Questions -
Q. Write short note on binary fission. (2013)
Ans. Bacteria reproduce rapidly even fastest of all living organisms. They reproduce by asexual methods but also sexually. Sexual reproduction is being very primitive and uncommon.
Asexual Reproduction: -
(i) By Binary Fission: -
It is a very common method occurring during favourable conditions. It is almost a simple multiplication of cells by division i.e. cell simply divides into two equal parts. It is almost a simple multiplication of cells by division i.e cell simply divides into two equalparts. It is completed in three stages- DNA multiplication, DNA partitioning and cross wall formation. The division is brought about by a constriction across the long axis of the cell in middle which deepens inwards.
In a few cases during early stages of cell division generally cross wall formation remains incomplete and the two daughter cells re still jointed. The cell division continues so as to form a long chain of cells. The connecting link between these cells is called plasmodesmata.
Each cell division requires 20 to 30 minutes which would mean two cells in twenty minutes. Since in the cell division the entire parent bacteria is transformed into two daughter bnacteria the bacteria are called immortals.
(ii) Endospore Formation: -
A few bacteria are capable to form spores which are formed single inside the cell. Such spores are called endospores and these are highly resistant and even cannot be killed by boiling for several minutes. Endospores are formed in Bacillus and Clostridium.
Formation of endospore-During endospore formation accumulation of polybetahydroxybutyric acid and redistribution and concentration of nucleic acid at one end of cell occur. From the mesosome cell membrane protrudes inside and forms two layered septum enclosing a part of cytoplasm and some ribosomes, mesosomes and enzymes. This newly formes structure is a complete cell in itself and termed as endospore.
Structure of the Endospore: -
The endospores are spherical, ellipsoidal or cylindrical in shape and may be present in terminal or in central region. The thin and delicate outer layer is exosporium which is followed by a spore coat. Spore coat may be single or many layers thick and composed of material similar to cell wall. The inner region is distingulshable into the cortex and the region enclosed in core wall which predominantly contains nucleic acid and ribosomes. The cortex region consists of dipicolinic acid.
Germination of Endospore: -
It occurs during favourable conditions when it imbines water, accelartes enzymatic action and intakes salts and nutrients. The endospore becomes turgid and bursts. The inner content emerges out and works as vegetative cell.
Sexual Reproduction: -
In general bacteria are said not to reproduce sexually. But if at all genetic recombination is considered within sexual reproduction it is fouind in bacteria. In recombination process the genetic material of donor cell is transferred to competent recipient cell and becomes integrated part of it. The process can be brought about by one of the following three phenomena-
(a) Transformation: -
The phenomena was discovered by Griffith which explains transfer of free DNA molecules into recipient cells. In this experiment he found that when a mouse is injected virulent non capsulated bacteria, it remains alive but when non capsulated bacteria together with heat killed capsulated bacteria are injected many of the mouse died and found some of the heat killed cells release DNA fragments which are transformed into some of the non capsulated bacteria.
This process is known as genetaic transformation which occurs only in certain strains of Pneumococcous pneumoniae, Neisseria, Pseudomonas etc.
(b) Transduction: -
It is called when transfer of genetic naterial between two bacteria occurs through the agency of certain temperate viruses. It was discovered by Zinder and Laderberg and has been reported in Escherichia coli, Salmonella, shigella, Staphylococcous and Bacillus. Neither all the bacteria are transucible nor all the temperate viruses are capable of transduction. During transduction only restricted group of bacterial genes(specialized transduction) or any portion of bacterial genes (generalized transduction) are transferred.
(iii) Bacterial Conjugation: -
The transfer of genetic material takes place by cell to cell contact of donor and recipient cells. It involves the transfer of relatively larger portion of genetic material unlike smaller portions as in transformation an transduction. Lederberg and Tautum discovered the process while working on starin K-12 of Escherichia coli. Later it was also reportd in Salmonella, Pseudomonas and Vibrio. The recipient bacteria are called F- (Ffactor) and donor bacteria are called eeither F+ or Hfr(High frequency of recombination). The process is completed into following four steps.
(1) Specific pairing (pairing between F-and F- or Hfr),
(2) Transfer of genetic material from donor to recipient
(3) Stage of genetic recombination and
(4) Expression of new phenotype in the recombinat cell.
Q.16. Write short note on Amphitrichous bacteria. (2012)
Ans. Some bacteria have the ability to move independently from place to place-said to be motile and others that can not move about independently are non-motile.
The true motility of bacteria is achieved by means of flagella (little whips). These are delicate, hair like process which extend out from one or more parts of the cell bodies of the motile bacteria. The flagella arise from the cytoplasm and are usually much longer than the bacterium.
A flagella is 4 - 5m long and can be divide into three parts:
(a) A basal granule.
(b) A hook that pentrates the celiwall and connects the main filament and basal granule.
Depending on the pattern of flagella attachment to bacterial cell and also the number of flagella, bacteria may be divided into different groups:
(a) Monotrichous - bacterium having a single flagella at one pole (end).
(b) Lophotrichous - bacterium with a group of flagella at one pole.
(c) Amphitrichous - bacterium with single or many flagellum at each pole.
(d) Peritrichous - flagella occur on he entire surface of bacterium.
(e) Atrichous - bacterium without flagella.
Q.17. Write short note on the following -
(a) Rhizobium (2013)
(b) Acetobacter (2013)
Ans. (a) Rhizobium: -
Some bacteria live symbiotically in other plants and animals e.g. Rhizobium leguminosrum, which is found in roots of the leguminous plants. These bacteria in the roots of leguminous plants, convert atmospheric nitrogen compound, which is utilized by the leguminous plants and in turn higher plants provide food and shelter to the bacteria.
These bacteria are capable of converting atmospheric free nitrogen into nitrates, the compounds which are useful for the growth of plants. The nitrogen fixing bacteria are the natural fertilizers of the soil. Because of their ability of fixing the atmospheric nitrogen, the leguminous plants are commonly used in crop rotation.
(b) Acetobacter: -
Acetobacter is a genus of acetic acid bacteria characterized by the ability to convert ethanol to acetic acid in the presence of oxygen. There are several species within this genus, and there are other bacteria capable of forming acetic acid under various conditions; but all of the Acetobacter are known by this characteristic ability.
Acetobacter are of particular importance commercially, because they are used in the production of vinegar (intentionally converting the ethanol in the wine to acetic acid), and they can destroy wine which they infect by producing excessive amounts of acetic acid or ethyl acetate, both of which can render the wine unpalatable. Acetobacter are also used to intentionally acidify beer during long maturation periods in the production of traditional Flemish Sour Ales.
The growth of Acetobacter in wine can be suppressed through effective sanitation, by complete exclusion of air from wine in storage, and by the use of moderate amounts of sulfur dioxide in the wine as a preservative. Acetobacter can be easily distinguished in the laboratory by the growth of colonies on a medium containing about 7% ethanol, and enough calcium carbonate to render it partially opaque. When Acetobacter colonies form enough acetic acid from the ethanol, the calcium carbonate around the colonies dissolves, forming a very distinct clear zone.
Q.18. Draw ultra microscopic structure of rod shaped bacteria with labelled diagram only. (2016) Ans.
