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Molecular basis of Inheritance — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular basis of Inheritance4 Marks
Question
How are DNA molecule replicate? Describe it.###Describe the process of DNA replication.
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Answer

DNA Replication : DNA is the only molecule on earth which is capable of self-replication. It occurs during S-phase of the cell cycle when chromatin remain in extended form. During replication, the weak hydrogen bonds between complementary nitrogenous bases break thus separate and uncoil two polynucleotide strands of DNA molecule. Each strand act as a template for synthesis of new complementary strands. After the completion of replication, each daughter DNA molecule has one parental and one newly synthesized strand thus restoring 50% characteristics of parent DNA. This scheme of replication of DNA is known as 'Semi-conservative DNA replication'.
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The Mechanism of DNA Replication : DNA replication is a complex process involving many enzymes. The first step is unwinding of DNA molecule at a specific point called initiation point. The enzyme topoisomerase helps in making a nick in the DNA molecule. The two strands of DNA start separating by breaking of hydrogen bonds between complementary nitrogenous bases. Each strand serve as a template for synthesis of new strand. The point of origin is marked as replication fork as it resamble a fork on either side of point of replication specific initiator proteins recognise point of initiation on DNA. RNA polymerase initiates synthesis of RNA primer on DNA template.
With the help of enzyme DNA polymerase a new DNA strand is synthesized in 5'-3' direction on 3'-5' template DNA. This is called leading strand and it is synthesized as one piece. On the other parent 5'-3' strand short DNA segments are formed in 5'-3' direction starting from RNA primer. These DNA segments are called Okazaki fragments and have 1000-2000 nucleotides. These fragments are later joined together forming a continuous lagging strand. The enzyme DNA ligase help in joining the fragments. (fig.)
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