All Free Biology MCQs with Answers

• A. Make cuts at specific positions within the DNA molecule.
• B. Recognize a specific nucleotide sequence for binding of DNA ligase.
• C. Restrict the action of the enzyme DNA polymerase.
• D. Remove nucleotides from the ends of the DNA molecule.

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 recognizes the base sequence GAATTC in DNA duplex and cuts its strands between G and A.

• A. Nucleoids
• B. Lomasomes
• C. Mesosome
• D. Plasmids

Explanation: Plasmid is a small circular double-stranded DNA molecule present in the cytoplasm of the bacterial cell. It can replicate independently of the bacterial chromosome. Due to this characteristic of plasmid, it is used as the vector (vectors are for the transferring of a piece of DNA to the target gene) in gene cloning.

• A. Meloidogyne incognita
• B. Agrobacterium tumefaciens
• C. Penicillium expansum
• D. Trichoderma harzianum

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. Construction of recombinant DNA by joining with cloning vectors.
• B. Isolation of DNA molecules.
• C. Cutting of DNA into fragments.
• D. Separation of DNA fragments according to their size.

Explanation: Electrophoresis is a technique used for the separation of substances of different ionic properties. Since the DNA fragments are negatively charged molecules, they can be separated by allowing them to move towards the anode. DNA fragments move towards the anode according to the size of their molecules through the pores of agarose gel. Thus, the smaller fragments move farther away as compared to larger fragments.

• A. DNA polymerase
• B. Exonucleases
• C. DNA ligase
• D. Endonucleases

Explanation: The construction of the first recombinant DNA emerged from the possibility of linking a gene encoding antibiotic resistance with a native plasmid. The cutting of DNA at specific locations became possible with the discovery of the so­-called 'molecular scissors' - restriction enzymes. The cut piece of DNA was then linked with the plasmid DNA. This plasmid DNA acts as a vector to transfer the piece of DNA attached to it. The linking of the antibiotic resistance gene with the plasmid vector became possible with the enzyme DNA ligase, which acts on cut DNA molecules and joins their ends. This makes a new combination of circular autonomously replicating DNA created in vitro and is known as recombinant DNA.

• A. Synthesizes DNA
• B. Cuts the DNA molecule randomly.
• C. Cuts the DNA molecule at specific sites
• D. Restricts the synthesis of DNA inside the nucleus.

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.

• A. Crown gall bacterium and Caenorhabditis elegans
• B. Escherichia coli and Agrobacterium tumefaciens
• C. Vibrio cholerae and a tailed bacteriophage
• D. Diplococcus sp. and Pseudomonas sp.

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. Are present in mammalian cells for degradation of DNA when the cell dies
• B. Are used in genetic engineering for ligating two DNA molecules
• C. Are used for in vitro DNA synthesis
• D. Are synthesized by bacteria as part of their defense mechanism.

Explanation: Restriction endonucleases are enzymes that digest double stranded DNA following recognition of specific nucleotide sequences. This is achieved by cleaving the two phosphodiester bonds, one within each strand of the DNA duplex. They are found in bacteria and their function in bacteria is to cut up any invading virus as a part of its defense mechanism, thus restricting the multiplication of viruses in the bacterial cell. Different species of bacteria produce different restriction endonucleases.

• A. Enhancer
• B. Transgene
• C. Promoter
• D. Reporter

Explanation: The plants, in which a functional foreign gene has been incorporated by any biotechnological methods that generally is not present in the plant, are called transgenic plants. When plant cells are transformed by any of the transformation methods it is necessary to isolate the transformed cells/tissue. There are certain selectable marker genes present in vectors that facilitate the selection process. In transformed cells, the selectable marker genes are introduced through a vector. There is a number of marker genes that are commonly described as reporter genes/screenable genes. Some of the reporter genes which are most commonly used in plant transformation are cat, gus, lux, nptII., etc.

• A. They are small and therefore, easy to study.
• B. They are uniparental in origin and do not take part in recombination
• C. Their structure is known in greater detail
• D. They can be studied from the samples of fossil remains.

Explanation: Sequence of both mtDNA and Y chromosomes are considered for the study of human evolution because they are uniparental in origin. mtDNA is inherited along with the maternal cytoplasm and the Y chromosome is inherited from the father. So they do not take part in recombination. In addition, mtDNA has a higher mutation rate than nuclear DNA so that it is more useful for short term evolutionary studies.