Q24 of 42 Page 1

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.


a) 15NH4Cl (15N is the heavy isotope of nitrogen) as the only nitrogen source in the experiment.


b) To prove the semiconservative nature of DNA.


c) The heavy DNA molecule distinguished from normal DNA by centrifugation in a cesium chloride (CsCl) density gradient. After that, they transferred the E.coli into a medium with normal 14NH4Cl and let them grow (E.coli divides in 20 minutes). They have taken samples at definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices.


d) They concluded that the DNA replicates semiconsevatively.


More from this chapter

All 42 →
22

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

 23

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.


 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.


 26

Study the schematic representation of the genes involved in the lac operon given below and answer the questions that follow:


(a) Identify and name the regulatory gene in this operon. Explain its role in ‘switching off’ the operon.


(b) Why is the lac operon’s regulation referred to as negative


regulation?


(c) Name the inducer molecule and the products of the genes ‘z’ and ‘y’ of the operon. Write the functions of these gene products.


OR


(a) How does the Hardy-Wienberg equation explain genetic equilibrium?


(b) Describe how this equilibrium is disturbed that may lead to founder effect.