Breeding and Biotechnology Questions

About Breeding and Biotechnology

Medium ~100 min study

Feeding a growing population is a practical problem, and this chapter is about the scientific answers to it. Crop improvement means developing varieties that yield more, taste better, resist disease and mature in less time. The Green Revolution showed what such improvement could do on a national scale, and much of the first half of the chapter explains the breeding methods that made it possible.

The second half moves from breeding to engineering. Breeding works by crossing organisms and selecting among the results, which limits it to characters already present in the species. Genetic engineering removes that limit by transferring a gene directly from one organism to another. Restriction enzymes cut the DNA, a vector carries the desired gene, and the combination is recombinant DNA. That single technique underlies transgenic crops, medicines produced in bacteria and much else.

Three shorter topics complete the chapter. Stem cells are unspecialised cells that can be directed into treatments. DNA fingerprinting compares repetitive sequences that differ between individuals, which makes it valuable in forensic work. Genetically modified organisms bring the chapter back to agriculture, with examples such as golden rice and Bt crops.

What you'll learn

• Define crop improvement and state what breeders aim to achieve.
• Compare the methods of plant breeding and say which suits an inexperienced farmer.
• Distinguish inbreeding from outbreeding and explain heterosis.
• Describe the steps of genetic engineering and name the enzyme that cuts DNA.
• Explain what recombinant DNA is and how a vector is used.
• State what stem cells are and how they are used in therapy.
• Explain the principle of DNA fingerprinting and name its forensic use.
• Give examples of genetically modified crops and say what gene each carries.

Before you start

• The structure of DNA and the idea that genes carry information.
• Basic understanding of heredity and of variation between individuals.
• Familiarity with the parts of a flower and with how plants are propagated.

Breeding and Biotechnology explained

What this chapter covers

The Green Revolution and crop improvement

Crop improvement is the development of improved crop varieties possessing higher yield, better quality, resistance to diseases and shorter duration. The Green Revolution applied this on a large scale, and IR 8, the variety known as miracle rice, became its best known product. Breeding programmes targeted high yield, disease resistance, pest resistance and improved nutritional quality in turn.

Methods of plant breeding

Several methods are described. New varieties may simply be introduced from elsewhere. Selection picks out desirable plants, and it takes more than one form: mass selection works on a mixed population and needs no special skill, while pureline and clonal selection are more demanding. Polyploidy breeding multiplies whole chromosome sets, mutation breeding induces heritable changes, and hybridisation crosses different varieties to combine their qualities.

Animal breeding

When breeding takes place between animals of the same breed it is called inbreeding, and a cross between different breeds is called outbreeding. The superiority of the hybrid obtained by cross breeding is called heterosis or hybrid vigour, and obtaining it is the main purpose of outbreeding.

Genetic engineering and gene cloning

Genetic engineering is the manipulation and transfer of genes from one organism to another. Restriction endonucleases act as molecular scissors, cutting DNA at particular sites. The desired gene is joined to a vector DNA, and the product of that joining is recombinant DNA. Gene cloning then multiplies the inserted gene inside a host organism.

Biotechnology in medicine

Biotechnology uses living organisms, and also enzymes obtained from them, to make products useful to human beings. In medicine this includes producing hormones and other therapeutic substances in engineered organisms rather than extracting them from natural sources.

Stem cells

Stem cells are undifferentiated or unspecialised masses of cells. Because they have not yet committed to a particular type, they can be directed towards replacing damaged tissue, and their use in treatment is known as stem cell therapy.

DNA fingerprinting

The human genome has three billion base pairs, so comparing two people base by base is impractical. DNA fingerprinting instead examines variable number tandem repeats, repetitive stretches also called satellite DNA whose number differs from one individual to another. This makes the method quick and reliable, and it is widely used in forensic applications such as identifying a culprit.

Genetically modified organisms

An organism carrying a modified endogenous gene or an introduced foreign gene is called transgenic, and equally genetically modified. Golden rice carries a beta carotene gene so the grain supplies the precursor of vitamin A. Bt crops carry a gene from the bacterium Bacillus thuringiensis which produces a protein toxic to insects, giving the plant its own pest resistance.

Common mistakes to avoid

• Treating hybridisation as suitable for an inexperienced farmer. Mass selection is the simple method; hybridisation needs skill.
• Calling vector DNA alone recombinant DNA. Recombinant DNA is the vector joined to the desired gene.
• Confusing inbreeding with outbreeding. Same breed is inbreeding; different breeds is outbreeding.
• Choosing polymorphic for the basis of DNA fingerprinting. The chapter builds it on repetitive sequences, the tandem repeats called satellite DNA.
• Assuming biotechnology uses only living organisms. It uses enzymes obtained from organisms as well.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

What is crop improvement?

Crop improvement is the development of improved crop varieties possessing higher yield, better quality, resistance to diseases and shorter duration. It is achieved through plant breeding methods such as introduction, selection, polyploidy breeding, mutation breeding and hybridisation, and in modern practice also through genetic engineering.

Which method of crop improvement suits an inexperienced farmer?

Mass selection suits an inexperienced farmer because it requires no specialised training. The farmer simply retains seed from the best plants in a mixed population for the next sowing. Clonal selection, pureline selection and hybridisation all require technical knowledge and controlled conditions, which is why they are unsuitable.

What is the difference between inbreeding and outbreeding?

Inbreeding takes place when animals of the same breed are mated with each other. Outbreeding is the cross between animals of different breeds. Outbreeding is generally preferred when the aim is heterosis, because the hybrid produced by crossing different breeds often shows superiority over both parents.

What is heterosis?

Heterosis, also called hybrid vigour, is the superiority of the hybrid obtained by cross breeding over its parents. The hybrid may grow faster, yield more or resist disease better than either parent, and obtaining this advantage is the main reason cross breeding is carried out.

What is recombinant DNA?

Recombinant DNA, written rDNA, is formed by joining a desired gene to a vector DNA. Restriction endonucleases cut both the source DNA and the vector so that they can be joined together. Neither the vector alone nor the desired gene alone is recombinant DNA; the term names the combination of the two.

What are restriction endonucleases used for?

Restriction endonucleases are enzymes that cut DNA at specific recognition sequences, which is why they are described as molecular scissors. They make genetic engineering possible, because a gene can only be moved from one organism to another once the DNA carrying it has been cut cleanly at chosen points.

What is the principle behind DNA fingerprinting?

DNA fingerprinting is based on identifying repetitive sequences of DNA. These are variable number tandem repeats, also known as satellite DNA, and the number of such repeats differs from one individual to another. Since the human genome has three billion base pairs, comparing these repeats is far quicker than comparing whole genomes.

What are stem cells?

Stem cells are undifferentiated or unspecialised masses of cells, meaning they have not yet developed into any particular cell type. Because of this they can be guided to form the tissue that is needed, and their use in treating disease or damage is called stem cell therapy.