Q25 of 42 Page 1

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.


• The process of formation of megaspores from the megaspore mother cell is called megasporogenesis.


• Ovules differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus.


• It is large in size cell containing dense cytoplasm and a prominent nucleus.


• The MMC undergoes meiotic division and four megaspores produce from meiosis.


• In most of the flowering plants, one of the megaspores is functional while the other three degenerate and the functional megaspore develops into the female gametophyte (embryo sac).


• This method of embryo sac formation from a single megaspore is termed monosporic development.


OR


a) A sperm reaches the ovum earlier.


b) D-Cells of the corona radiate and E- Zonapellucida. When sperm comes in contact with the zonapellucida layer, it induces changes in the membrane that ensures the only one sperm can fertilize an ovum.


c) The secretion of the acrosome helps the sperm enter into the cytoplasm of the ovum through the zonapellucida and the plasma membrane. This induces the meiotic division of the secondary oocyte completion. The second meiotic division is unequal and results in the formation of a second polar body and a haploid ovum. The haploid nucleus of the sperms and the ovum fuse together to form a diploid zygote.


d) Ampullary region of the fallopian tube where the fertilization takes place.


More from this chapter

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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.


 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.


 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.


 27

a) Study the flow chart given below and complete the equation that follows by identifying 1, 2, 3 and 4.


(b) Mention the different ways by which the population density of different species can be measured.


OR


(a) ‘The pyramid of energy is always upright.’ Explain.


(b) Explain with the help of labelled diagrams, the difference between an upright pyramid of biomass and an inverted pyramid of biomass.