Q.2. Write short note on ‘Plasmids’. (2012)
Ans. Plasmids :-
Plasmids are genetic elements present in the bacterial cell which can be transferred from one cell to another cell by contact. The plasmids synthesize certain bacteriocines and provide resistance to antibiotics.
In Ecoli these are three classes of plasmide: -
1. F. ( i. e. fertility ) factors : They promote sexual conjugation. All E. coli cells
do not have these. Those which have them are called F+ and those that lack it are called F - F-, F+ forms, from sex pili.
2. R (i.e. resistance) factors : R factors offer bacteria resistance to certain drugs.
3. Col ( i.e. , collcinergic ) factors : Give to the bacteria capacity to secrete colitis which have an antibiotic function.
Q.3. Write an essay on reproduction in bacteria.
Related Questions -
Q. Write short note ontramsformation and tramsduction. (2012)
Q. Describe asexual reproduction in bacteria. ( 2001,06, 12)
Q. Write a note on endospore formation (2010)
Q. Describe ‘transformation’ in bacteria. (2010)
Q. Describe the different mehods of sexual reproduction in bacteria. (2013)
Ans. In bacteria reproduction is of three types:
1.Vegetative Reproduction: -
(a) By Budding:- In this method reproduction takes place by formation of buds.
(b) By Fission:- It is main and general method of reproduction. In the fission method the division of cell occurs across the long axis. During favourable condition cell increase its size and protoplast moves towards the centre and divides in two parts resulting formation of two new cells.
Fig. - Bateria: Reproduction by binary fission
2.Asexual Reproduction
(a) By conidia. Certain filammentous bacteria like streptomyces produce spore like structures in chains. These structures are called conidia. Each conidia later on develops into a new bacterial cell.
(b) By Zoospores: - Under unfavourable environment conditions bacteria form motile spores and these spores develop into bacterial cells e.g. -Rhizobium.
(c) By Endospores: - Under unfavourable conditions the protoplasm of cell condenses and forms a spherical mass. It develops a resistent wall and this structure is called endospore. In favourable conditions it develop into new bacterial cell. e.g. Bacillus, Clostridium.
3. Sexual Reproduction: -
In general bacteria are said not to reproduce sexually. But if at all genetic recombination is considered within sexual reproduction it is found 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 abaout 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 casulated 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 genetic transformation which occurs only in certain strains of Pneumococcous pneumoniae, Neisseria, Pseudomonas etc.
(b) Transduction: -
It is called when transfer of genetic material 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 transducible 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 and transduction.
The recipient bacteria are called F-, factor and donor bacteria are called either F+, or H fr (high frequency of recombination). The process is comleted into follwing four steps -
(i) Specific pairing (pairing between F- and F- or H fr)
(ii) Transfer of genetic material from donor to recipient.
(iii) Stage of genetic recombination and
(iv) Expression of new phenotype in the recombinat cell.
Q.4. Which technique is used for bacterial staining?
Related Question -
Q. Why are some bacteria gram positive and some gram negative (2007)
Q. Explain difference between gram positive and gram negative bacteria. (2011)
Q. Write short note on staining of bacteria. (2013)
Ans. Gram staining technique is used for bacterial staining. This technique was introduced by Christian Gram in 1884. In this method bacterial smears are heat fixed on a slide. They are then stained with crystal - violet followed by iodine. They stained deep purple. When treated with alcohol or acetone, some bacteria decolourise. Bacteria which retain the colour termed as Gram positive whereas decolourise Gram negative.
The reasons why bacteria respond differently to the Gram stain are not completely understood. It has be postulated that since Gram negative bacteria have relatively a high lipid content in their cell walls, the alcohol dissolves the lipids, the allows the leakage of crystal violet iodine complex. The Gram positive bacteria with less lipids in their cell walls are less susceptible to the action of alcohol. Due to this reason some bacteria shows Gram positive and some Gram negative staining characters.
Differences between Gram + ve and Gram - ve bacteria
