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.
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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