Genetic Engineering

DNA ligase joins broken or separate DNA fragments by forming phosphodiester bonds between them. This is why it is used to construct recombinant DNA molecules. Option B is incorrect because restriction endonucleases cut DNA at specific recognition sites. Exam tip: Remember ligase as the biological ‘glue’ that seals DNA fragments.

The direct answer is A: sticky ends easily join complementary DNA ends. A restriction enzyme can cut two DNA molecules in a staggered way, leaving short single-stranded overhangs. These overhangs are called sticky ends because they can temporarily pair through complementary bases: A pairs with T, and G pairs with C. Step by step: the same or compatible restriction enzyme makes matching cuts; the exposed bases pair by hydrogen bonds; DNA ligase seals the sugar-phosphate backbone; recombinant DNA is formed. Option A is correct because complementary ends fit together and make joining easier and more specific. Option B is wrong because sticky ends do not destroy DNA; restriction enzymes make the cuts, while sticky ends help joining. Option C is wrong because they do not heat the cell. Option D is wrong because they do not inactivate proteins. Memory cue: sticky ends “stick” by complementary base pairing before ligase seals them.

A host cell can copy foreign DNA and sometimes express it as protein. Bacteria are common hosts.

Cells with antibiotic resistance genes survive in antibiotic medium. This helps identify transformed cells.

Gene cloning means making many copies of a particular gene. It is useful for study and production.

PCR uses repeated temperature changes, so a heat-stable enzyme is needed. Taq polymerase is a famous example.

Gel electrophoresis separates DNA fragments according to size. Smaller fragments usually move farther.

Agarose gel has pores through which DNA fragments move at different speeds. This causes size-based separation.

DNA fingerprinting identifies a person's unique DNA pattern. It is used in identification and forensic work.

Recombinant technology can produce human-like insulin on a large scale. It is a famous application of biotechnology.

The plasmid is cut at the correct site so the foreign gene can be inserted. Using the same restriction enzyme helps matching ends.

The governing concept is gene selection in genetic engineering. A desired gene is a gene identified and selected because its information can produce a useful or required characteristic. For example, researchers may select a human insulin gene to enable a microorganism to produce insulin, or a gene associated with resistance to a particular pest. The term does not mean that the gene is always beneficial in every context; it means that it is wanted for a stated experimental or practical objective. Thus option A is correct. Option B is too absolute, option C contradicts the fact that genes are DNA sequences, and option D has no biological basis.

Biosafety aims to protect organisms and the environment from possible risks. It is important in genetic engineering.

In microinjection, DNA is directly injected into a cell with a very fine needle. It can be considered a direct method.

In the gene gun method, DNA-coated tiny particles are delivered into plant cells. It is useful for plants.

A transgenic organism may show a new trait because of a foreign gene. It is also called a genetically modified organism.

Genetic modification can add traits such as pest resistance to crops. In exams, remember both benefits and safety concerns.

The direct answer is A: Bt crops are related to pest resistance. Bt refers to Bacillus thuringiensis, a bacterium that produces particular insecticidal proteins. Scientists introduce a suitable Bt gene into a crop so that the plant can produce a protein harmful to certain target insect larvae when they feed on it. The exact effect depends on the Bt gene and insect, so it is not resistance to every pest. Option A is correct because pest or insect resistance is the major agricultural feature of Bt crops. Option B is wrong because blood clotting is a human or animal physiological process, not the purpose of Bt crops. Option C is wrong because human hair colour has no connection with Bt technology. Option D is wrong because boiling milk is a food-processing activity, not a genetic-engineering trait. Memory cue: Bt means bacterial insect protection in a crop.

A promoter is a DNA region that can control the start of gene expression. Link it with start control in exams.

A restriction enzyme recognizes and cuts DNA at a restriction site. It is important in making recombinant DNA.

The direct answer is A: screening is necessary because not all cells take up foreign DNA. During transformation, DNA is mixed with or introduced into many cells, but uptake is not guaranteed. Also, a cell may take up a vector without receiving the desired insert, or the insert may be arranged incorrectly. Therefore, selection first helps identify cells with a suitable selectable marker, and screening checks whether the desired recombinant DNA is actually present. Option A is correct. Option B is wrong because cells are not always identical in their DNA uptake or recombinant status. Option C is biologically impossible in this context: DNA does not become water. Option D is not the general reason for screening; an enzyme may produce a colour signal in some tests, but colour change is only a possible detection method, not the fundamental reason. Memory cue: transformation is not guaranteed, so screen to find the true recombinant cells.

The direct answer is A, genetically identical copies. In biotechnology, a clone is a cell, organism, or DNA molecule produced from one original unit and having the same or essentially the same genetic material. In gene cloning, a selected gene is inserted into a suitable vector, placed in a host, and copied as the host multiplies. Therefore the important idea is genetic sameness, not merely physical similarity. Option A is correct because it gives the standard meaning of clone. Option B is wrong because clones are not defined as always different organisms; difference is the opposite of the usual meaning. Option C is wrong because cells generally contain DNA, and absence of DNA does not create a clone. Option D is wrong because cloning concerns genetic copies and can involve living cells or organisms, not only dead tissue. Remember: one original genetic pattern producing matching copies is cloning.

The governing concept is aseptic technique and prevention of contamination. Genetic engineering experiments often use small quantities of DNA, living host cells, enzymes, and selective culture media. If bacteria, fungi, nucleases, or other unwanted microorganisms enter through an unsterile tool, they may destroy DNA, compete with the intended host, alter the culture, or produce misleading results. Sterilising tools and maintaining clean working conditions therefore protect the sample and preserve experimental reliability. Option A is correct. Sterility does not make DNA shiny, lengthen primers, or increase plasmid mass; those changes require entirely different chemical or molecular processes.

When a foreign gene makes protein in a host, the product is a recombinant protein. Insulin is a famous example.

After gel electrophoresis, DNA fragments are stained with dyes such as ethidium bromide or SYBR Green. These dyes make the DNA bands visible under ultraviolet or blue light. Boiling, burying, or covering the DNA with paper does not reveal the bands. Exam tip: Each visible band represents DNA fragments of a particular size range.