Q24 of 42 Page 1

Explain the process of DNA replication with the help of a replicating fork.

OR


(a) Dihybrid cross between two garden pea plant one homozygous tall with round seeds and the other dwarf with wrinkled seeds was carried.


(i) Write the genotype and phenotype of the F1 progeny obtained from this cross.


(ii) Give the different types of gametes of the F1 progeny.


(iii) Write the phenotypes and its ratios of the F2 generation obtained in this cross along with the explanation provided by Mendel.


(b) How were the observations of F2 progeny of dihybrid crosses in Drosophila by Morgan different from that of Mendel carried in pea plants ? Explain giving reasons.


DNA replication is the process of producing two identical replicas of DNA from one original DNA molecule. The process occurs in the following steps:

a. Replication of DNA originates at a specific region called “origin of replication”.


b. The whole DNA does not unwind but a small opening, called replication fork is present in the DNA helix.


c. The polymerization is catalysed by DNA dependent DNA polymerase. The polymerization occurs in 5’ to 3’ direction on the template strand only.


d. The replication is continuous on one strand (leading strand) having polarity 3’ to 5’ whereas it is discontinuous on the other strand (lagging strand).


e. The discontinuous synthesized fragments are later joined by the enzyme DNA ligase.



Figure 1. DNA replication at replication fork.


OR


a.


Figure 2. Parents and gametes in dihybrid cross.


i. The genotype of the F1 progeny obtained: TtRr
The phenotype of the F1 progeny obtained: All progeny is tall with round seeds.


ii. The different gametes of F1 progeny are: TR, Tr, tR and tr.


iii. The phenotypes and phenotypic ratio obtained in F2 progeny are:


9 : 3 : 3 : 1


Tall round Tall Wrinkled Dwarf Round Dwarf wrinkled


Mendel gave the law of independent assortment on the basis of this cross. According to this law, when two pairs of traits are combined in a hybrid, segregation of one pair of character is independent of the other pair of characters.


b. Mendel did not observe the phenomenon of linkage in pea plants which was observed by Morgan in Drosophila. The ratio of F2 progenyobtained in Morgan’s experiment were different from that obtained by Mendel.


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Explain the application of biotechnology in producing Bt cotton.

OR


Unless the vector and source DNA are cut, fragments separated and joined, the desired recombinant vector molecule cannot be created.


(a) How are the desirable DNA sequences cut ?


(b) Explain the technique used to separate the cut fragments.


(c) How are the resultant fragments joined to the vector DNA molecule ?


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Mention the site of fertilization of a human ovum. List the events that follow in sequence until the implantation of the blastocyst.

OR


(a) Draw a diagram of a fertilized embryo sac of a dicot flower. Label all its cellular components.


(b) Explain the development of a mature embryo from this embryo sac.