All Free Biology MCQs with Answers

• A. Clostridium septicum
• B. Xanthomonas citri
• C. Bacillus coagulens
• D. Agrobacterium tumefaciens

Explanation: Agrobacterium tumefaciens has been extensively used in genetic engineering experiments. It is the causative agent of crown gall, an important disease of many commercial crops. This disease has come to be recognized in recent years as being caused by a DNA plasmid (Ti plasmid) carried by bacterium and transferred to the plant cells. Following the discovery of the relationship between crown gall and the Ti plasmid, this plasmid has come to be widely used in plant genetic engineering as a vector in order to inject a novel gene in a host plant to form a transgenic plant.

• A. Restriction endonuclease
• B. DNA ligase
• C. Transcriptase
• D. Primase

Explanation: DNA restriction endonuclease is an important class of restriction exonucleases, class II, which cut double­stranded DNA molecules only at sites characterized by a specific nucleotide sequence. Restriction enzymes are isolated from bacterial cells and are tools for molecular biologists. Several hundred restriction enzymes are now known, each with a specific sequence requirement dictating where it will cut DNA. Some, such as Hind III, makes staggered cuts leaving 'sticky ends', three nucleotides long protruding on one strand from each severed terminus; others make clean cuts in both strands at the same place and thus generate 'blunt ends'. Digesting DNA with a restriction enzyme, therefore, creates a characteristic set of fragments, which can be isolated by electrophoresis and subsequently analysed.

• A. Bacteriophage transforms in wild.
• B. It is not mutated.
• C. Both strains have similar cistrons.
• D. Both strains have different cistrons.

Explanation: A mutant strain of T4 - bacteriophage, RII, fails to lyse the E.coli but when two strains R-­II (X) and R-­IIY are mixed then they lyse the E.coli because both strains have different cistrons.

• A. Minimal medium and Streptomycin
• B. Complete medium and Streptomycin
• C. Only minimal medium
• D. Only complete medium.

Explanation: If Streptomycin resistant mutant is to be obtained, the material should be allowed to grow on medium lacking streptomycin so that both mutant and wild types may grow. These colonies are imprinted on Petri plates to form the master pattern and other plates having streptomycin can then be pressed on velveteen to get an impression. The plate now containing only mutants for Streptomycin resistance will grow.

• A. 50 kilo base
• B. 500 kilo base
• C. 5000 kilo base
• D. 5 kilo base.

Explanation: A plasmid is a DNA molecule separate from the chromosomal DNA and capable of autonomous replication. In many cases, it is typically circular and double­-stranded. It usually occurs in bacteria and is sometimes found in eukaryotic organisms. The size of plasmids varies from 1 to over 400 kilobase pairs (kbp). There may be one copy, for large plasmids, to hundreds of copies of the same plasmid in a single cell. The term plasmid was first introduced by the American molecular biologist Joshua Lederberg in 1952.

• A. It is circular DNA which have capacity to join to eukaryotic DNA.
• B. It can move between prokaryotic and eukaryotic cells.
• C. Both ends show replication.
• D. It has antibiotic resistance gene.

Explanation: Plasmid and bacteriophage are used as vectors in genetic engineering. Plasmid is an autonomously replicating circular extra chromosomal DNA found in bacteria. They can be transferred from cell to cell in a bacterial colony. This characteristic is being used in biotechnology for transferring desirable gene into the target gene of the host. Bacteriophage is a bacterial virus that can infect it, quickly multiply within and destroy (lyse) their host (virus) cells. During infection, bacteriophages inject their DNA into these cells. The injected DNA selectively replicate and are expressed in the host that results in a multiplication of phages that ultimately burst out of the cell (by lysis). This ability to transfer DNA from the phage genome to a specific host during the infection process gave scientists the idea that specially designed phage vectors could be used for gene cloning.

• A. Mitochondrial gene
• B. Plasmid gene
• C. Bacterial gene
• D. None of these.

Explanation: The DNA plasmid replicates in a semi­ conservative manner. The initiation of replication is controlled by the plasmid gene and elongation and termination are controlled by bacterial genes.

• A. Ligase
• B. Restriction endonulcease
• C. Primase
• D. Exonucleases.

Explanation: Recombinant DNA is the product obtained after isolating a specific DNA segment and then inserting it into another DNA molecule at a desired position. Restriction endonucleases are the enzymes that digest DNA at specific sites to isolate a specific DNA segment. Thus they are required for producing recombinant DNA.

• A. Nitrobacter and Azotobacter
• B. Rhizobium and Diplococcus
• C. Nitrosomonas and Kliebsiella
• D. Escherichia and Agrobacterium

Explanation: E.coli contains many important standard cloning vectors widely used in gene cloning experiments like pBR322 contains origin of replication (ori). Other cloning vectors like PACYC177, pBR324, PRK 64.6 contain ampicillin resistance gene and they are also found in E.coli. Among higher plants, Ti plasmid of Agrobacterium tumefaciens and Ri plasmid of A. rhizogenes is the best known vector. T­-DNA from Ti or Ri plasmid of Agrobacterium is considered to be a very potential vector for cloning experiments with higher plants.

• A. Used for in vitro DNA synthesis.
• B. Used in genetic engineering.
• C. Synthesized by bacteria.
• D. Present in mammalian cells for degradation of DNA.

Explanation: Restriction endonucleases were found by Arber in 1962 in bacteria. They act as "molecular scissors" or chemical scalpels. They recognize the specific base sequence at palindrome sites in DNA duplex and cut its strands. For example, restriction endonuclease EcoRI found in the colon bacteria E.coli reocgnizes the base sequence GAATTC in DNA duplex and cuts its strands between G and A.