Everything you need to know about microbial genetics. Some of the frequently asked questions are as follows:-
Q.1. What are the main types of plasmids? Name the organisms in which they are found.
Ans: The following are the main types of plasmids:
(i) Conjugative plasmids, e.g., F plasmid in Escherichia coli, pfd m, K in Pseudomonas, P in Vibrio cholera, and SCP in Streptomyces.
(ii) R plasmids, which show resistance to a wide variety of antibiotics, resistance to mercury, cadmium, nickel, cobalt, zinc and arsenic, e.g., in Enteric bacteria, Staphylococcus and Pseudomonas.
(iii) Plasmids for bacteriocin and antibiotic production: The bacteriocins are ribosomally synthesized peptides but the Escherichia coli produces colicins coded col plasmids, and Bacillus subtilis produces subtilisin. They are also produced by Clostridum, enteric bacteria and Streptomyces.
(iv) Plasmids to control physiological functions include:
(a) Utilization of lactose, sucrose, urea and fixation of nitrogen, e.g., enteric bacteria and Rhizobium.
(b) Degradation of octane, camphor, naphthalene and salicylate, e.g., Pseudomonas.
(c) Production of pigments, e.g., Erwinia and Staphylococcus.
(d) Nodulation and symbiotic nitrogen fixation Rhizobium.
(v) Virulence plasmids
(a) Enterotoxin, K antigen, endotoxin in Escherichia coli.
(b) Tumorigenic plasmid Agrobacterium tumefaciens.
(c) Adherence to teeth (dextran), e.g., in Streptococcus mutans.
(d) Coagulase, hemolysin, fibrinolysin and enterotoxin, e.g., in Staphylococcus aureus.
Q.2. What is generalized transduction?
Ans: It is the genetic transfer of host genes (host DNA derived from any portion of host genome) becoming part of DNA of the mature virus particles in place of the virus genome. It was first discovered and reported in Salmonella typhimurium with phage P 22. It has also been studied in Escherichia coli with phage PI.
Q.3. What is specialized (restricted) transduction?
Ans: The specialized (restricted) transduction occurs only in some temperate viruses where DNA from a specific region of the host chromosome is integrated into the virus genome usually replacing some of the genes. It was first discovered in Escherichia coli bringing transduction of the galactose genes by the temperate phage lambda. The region in which lambda integrates is immediately adjacent to the cluster of host genes which control the enzymes involved in galactose utilization, and the DNA of lambda is inserted into the host DNA at that site.
Q.4. Why is altered phage particle lambda dgal or λ dgal so called?
Ans: Because it is defective for galactose. It is a defect of the phage genes lost and does not make mature phage. But a helper phage can provide those functions missing in the defective particle.
Q.5. What is abortive transduction?
Ans: The fate of transduced DNA in the recipient cell (now called a transductant) depends on various factors. If DNA is a complete replicon (e.g., a plasmid) it may be stably inherited by the transductant. But if the DNA is a fragment of a chromosome or plasmid it may meet one of the three possible fates.
1. It may be completely degraded by the recipient cell’s restriction endonuclease system.
2. It may undergo recombination with a homologous region of the parents chromosome (or plasmid) such that at least some of the genes it carries can be stably inherited also called complete transduction.
3. It may persist in the cell in a stable but not replicating form which is referred to as abortive transduction. The transduced DNA in an abortive transductant may exist as a circular DNA protein complex.
Q.6. What are the plasmids with the ability to integrate into the host chromosome, called?
Q.7. What is the curing of plasmid?
Ans: The process of eliminating plasmids from the host cell by various treatments is called curing. The treatments are use of acridine dyes and electroporation. The curing takes place by inhibition of replication of plasmid without parallel inhibition of chromosome replication.
Q.8. How do R plasmids work?
Ans: R plasmids carry antibiotic resistance genes. These genes encode proteins that either inactivate the antibiotic or affect its uptake into the cell.
Q.9. Give an example of R plasmid.
Ans: R 100 is an 89.3 kilobase pair plasmid that carries resistance genes for sulphonamides, streptomycin, spectinomycin, fusidic acid, chloramphericol and tetracycline. R 100 is also known to carry several genes conferring resistance to mercury.
R 100 can be transferred between enteric bacteria of the genera Escherichia, Klebsiella, Proteus, Salmonella and Shigella but not to non-enteric bacterium Pseudomonas Many drug resistant elements in R plasmids such as those in R 100 are transposable elements and can be used in transposon mutagenesis. R plasmids with genes for resistance to Kanamycin penicillin and neomycin too are known. Many R plasmids and F plasmids have related tra regions and recombination can occur between F and R plasmids.
Q.10. What are bacteriocins? How are they named?
Ans: Bacteriocins are ribosomally synthesized peptides. Many of them require postranslational modification for ‘ activity. The bacteriocins are named in accordance with the species of organisms that produce them. Thus in Escherichia coli they are colicins coded by Col plasmids and Bacillus subtilis produces subtilisin.
Q.11. How are colicins harmful?
Ans: The colicins kill cells by disrupting some critical cell function. Some colicins form channels in the cell membrane which allow potassium ions and protons to leak out resulting in loss of the cell’s energy forming ability. An interesting fact is that colicin E2 is a DNA endonuclease which can cleave cellular DNA and colicin E3 is a nuclease that can cleave at a specific site in 16S rRNA and inactivates ribosomes.
Q.12. Name a colicin that cuts into a specific site is 16S rRNA.
Ans: Colicin E3.
Q.13. What are the two main functions of pili?
Ans: They are:
(1) To make specific contact with the receptor on the recipient bacterial cell, and
(2) To retract after that to bring the two cells together such that a conjugation bridge is formed.
Q.14. Is F plasmid an episome?
Ans: The F plasmid which can integrate into the host chromosome, is an episome.
Q.15. What are F plasmids?
Ans: The cells having an un-integrated F plasmid are called F1” and the cells which can work as recipients of Hfr are called F”. The cells designated as F” lack F plasmid. The F plasmids result in three distinct changes in the properties of a cell.
(1) Ability to produce F pilus,
(2) Mobilization of DNA for transfer to another cell and
(3) Brings about alteration of surface receptors and as a result the cell cannot behave as a recipient in conjugation.
Q.16. What is genetic map?
Ans: The arrangement of gene loci on a chromosome is known as genetic map.
Q.17. What is transposition? Give its importance.
Ans: Transposition is the process by which a gene moves from one place to another in the genome. It is important for genetic evolution and in genetic analysis.
Q.18. What are transposable elements?
Ans: They are the genetic elements on which the movement or transposition of genes depends. These have also been called “jumping genes”.
Q.19. What are different classes of transposable elements?
Ans: The transposable elements in bacteria are of three types.
(1) Insertion sequences (IS)
(2) Transposons Tn), and
(3) Some special viruses (e.g. Mu).
In eukaryotes transposable elements are in yeast (e.g., sigma and Ty), in fruit fly (e.g., copia, P), in maize (e.g., Ac) and in humans (e.g., retrovirus sarcoma, human immunodeficiency virus or HIV).
Q.20. Give the mechanisms of transposition.
Ans: One of the essential features of transposition is the inverted repeats found at the ends of transposable elements. The second essential feature is an enzyme called transposase which recognizes the aforesaid repeats. The enzyme transposase on recognizing DNA, cuts it and then ligates the DNA.
While a transposable element becomes inserted into another DNA (referred to as target DNA) a short sequence in the target DNA at the site of interaction is duplicated. The target DNA sequence was not present in the transposon but the transposable element causes duplication of this DNA by the insertion process.
The duplication of target sequence apparently arises, as single stranded breaks are generated by the transponase.
There are two categories of mechanisms of transposition:
(1) Conservative, in which the transposable element is excised from one location in the chromosome and gets inserted at a second location, e.g., in Tn 5.
(2) Replicative, in which the transposon is duplicated and a new copy is inserted at another location, e.g., in the transposon bacteriophage Mu.
Q.21. Name three often studied composite transposons.
Ans: These are Tn 5, Tn 9 and Tn 10 which carry genes for resistance to kanamycin, chloramphenicol and tetracycline respectively.
Q.22. What are the simplest transposons called?
Ans: The simplest transposons are called insertion sequences (IS).
Q.23. What is general recombination (or homologous recombination)?
Ans: In general recombination genetic exchange between homologous DNA sequences from two different sources takes place. As homologous DNA sequences have the same sequence, the base pairing takes place over an extended length of the two DNA molecules.
Q.24. What characteristics do insertion sequences (IS) and transposons have in common?
Ans: They show some common characteristics which are:
(1) Both have inverted repeats at their ends.
(2) Both encode for ‘transposes’ which is an enzyme required in transposition.
Q.25. What are the use of transposable elements?
Ans: Transposable elements can be used as biological mutagens.
Q.26. How are locations of various genes mapped on a bacterial chromosome?
Ans: The genes in a bacterial genetic map are located using the mechanisms of transformation, transduction and conjugation, e.g., a circular reference map of Escherichia coli strain K-12 has been prepared by giving map distances in minutes of transfer with 100 minutes for the whole chromosome. In it “zero time” is set arbitrarily for threonine Operon which is the first genetic transfer. This can be detected using the original Hfr strain.
Using a combination of conjugation and transduction over 1400 genes has been located on the circular chromosome of E. coli. It was found that some set of related genes, e.g., trp genes (genes involved in biosynthesis of amino acid tryptophan are tightly clustered while arg genes (involved in arginine biosynthesis) are scattered.
Q.27. What is regulon?
Ans: A set of operons controlled by the same regulatory proteins is a regulo.
Q.28. What is operon?
Ans: A cluster of genes (often in prokaryotic cells) whose expression is controlled by a single operator is operon.
Q.29. Give the range and size of prokaryotic chromosomes.
Ans: Mycoplasma genitalium a parasitic bacterium has 580 kilobase pairs in its chromosome and is the smallest chromosome while Myxococcus Xanthus has an unusually large chromosome of 9500 kilo base pairs and that Escherichia coli is formed of 4700 kilobase pairs.
Q.30. What is the genetic proof that mitochondria and chloroplasts of cells of eukaryotes have evolved in the past from endosymbiosis bacteria?
Ans: The genomes of these organelles are usually circular like that of prokaryotes.
Q.31. What is signal hypothesis?
Ans: Signal hypothesis means that the translocation of a protein from its site of synthesis into or through a membrane requires that the protein be synthesized in a precursor form (preprotein) having a specific N-terminal sequence of amino acid residues (= signal peptide, signal sequence and leader peptide) that is essential for initiation of translocation but is excised during translocation.
Q.32. What is a signal recognition particle (SRP)?
Ans: It is a particle in eukaryotes that is involved in translocation of proteins across the endoplasmic reticulum. An SRP contains 6 polypeptides together with 7 S RNA that is essential for the function.
Q.33. What is apoptosis?
Ans: It is genetically programmed cell death.