Q23 of 42 Page 1

Describe the roles of (a) high temperature, (b) primers, and (c) bacterium Thermus aquaticus in carrying the process of polymerase chain reaction.

OR


How does β-galactosidase coding sequence act as a selectable marker? Why is it a preferred selectable marker to antibiotic resistance genes? Explain.


Polymerase chain reaction:


(a) High temperature- the heating breaks the hydrogen bonds to make ssDNA. The DNA molecule with more G-C requires higher temperatures.


(b) Primers – Primers (forward and reverse) are synthetic oligonucleotides. They are complementary to the sequences present on the desired DNA segment.


(c) Bacterium Thermusaquaticus –repeated amplification is achieved by the use of a thermostable DNA polymerase which is isolated from bacterium, Thermos aquaticus.


OR


• Alternative selectable markers that differentiate between the recombinants from non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate.


• In this, a recombinant DNA is inserted within the coding sequence of an enzyme, â-galactosidase. This results into inactivation of theenzyme, which is referred to as insertional inactivation.


• Blue coloured colonies observed in the presence of a chromogenic substrate if the plasmid in the bacteria does not have an insert.


• The presence of insert results into the insertional inactivation of the β-galactosidase and the colonies do not produce any colour, these are identified as recombinant colonies. Thus it is preferred a selectable marker to antibiotic resistance genes.


More from this chapter

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21

Give reasons why:

(a) DNA cannot pass into a host cell through the cell membrane.


(b) Proteases are added during isolation of DNA for genetic engineering.


(c) Single cloning site is preferred in a vector.


 22

State the medicinal value and the bioactive molecules produced by Penicilliumnotatum, Monascusperpureus and Trichodermapolysporum.

 24

Answer the following questions based on Meselson and Stahl’s experiment on E. coli:

(a) Write the name of the chemical substance used as the only source of nitrogen in the experiment.


(b) Why did they allow the synthesis of the light and the heavy DNA molecules in the organism?


(c) How did they distinguish the heavy DNA molecules from the light DNA molecules? Explain.


(d) Write the conclusion the scientists arrived at, at the end of the experiment.


 25

Describe the process of megasporogenesis upto fully developed embryo sac formation in an angiosperm.

OR


Given below is the diagram of a human ovum surrounded by a few sperms. Study the diagram and answer the following questions:



(a) Which one of the sperms would reach the ovum earlier?


(b) Identify ‘D’ and ‘E’. Mention the role of ‘E’.


(c) Mention what helps the entry of sperm into the ovum and write the changes occurring in the ovum during the process.


(d) Name the specific region in the female reproductive system where the event represented in the diagram takes place.