The various steps involved in this process are summarized as follows:

1. The mechanism of replication starts at a specific point of the DNA molecule. This is called the origin.

2. At the origin, the DNA strand breaks because of an incision (nick). This is made by an enzyme called incision enzyme (endonuclease).

3. The hydrogen bonds joining the two strands are broken by the enzyme.

4. The two strands start unwinding. This takes place with the help of a DNA unwinding protein. The two polynucleotide strands are thus separated.

5. The point where the two strands separate appears like a fork or a y-shape. This is described as a replicating fork.

6. A new strand is constructed on each old strand. This takes place with the help of a small RNA primer molecule which is complimentary to the DNA at that point. Each old DNA strand acts as a template (site) for the construction of new strand. The RNA primer attaches itself to the old strand and attracts the enzymes which add new nucleotides. The deoxyribose nucleotides are present in the surrounding nucleoplasm. Appropriate nucleotides are selected from the nucleoplasm, and are attached by H-bonds to their respective complementary bases on the old strand. A new DNA strand is thus constructed opposite to each old strand.

7. The formation of new complementary strand always begins at the 3' end of the template strand (original strand) and progresses towards the 5' end (i.e in 3' ® 5' direction). Since the new strand is antiparallel to the template strand, it is obvious that the new strand itself is always developed in the, 5'®3' direction. For this reason when the two original strands separate (then with respect to the origin of separation), one acts as 3'®5' template while the other acts as 3'®5' template. Of the two, the replication of 3'®5' template begins first. Hence the new strand formed on it is called the leading strand. The other template (5'®3') must begin replication at the fork and progress back toward the previously transcribed fragment. The new strand formed on it is called the lagging strand.

8. Replication of the lagging strand takes place in small fragments called Okazaki fragments. These are then connected together by the enzyme polynucleotide ligase.

9. Replication may take place in only one direction on the DNA helix (unidirectional) or in two directions (bidirectional).

10. At the end of the process, two double stranded DNA molecules are formed from the original DNA molecule.

11. In each newly formed DNA, one strand is old while the other is new. Hence, it is described as semi-conservative replication.

The semi-conservative nature of DNA replication was confirmed by Meselson and Stahl (1958) with the help of an experiment. They marked the DNA in Esherichia coli with heavy isotope of nitrogen (15N) and then traced it in the following generations of the E-coli progeny. It was demonstrated that every daughter DNA is an exact copy of the parent DNA and that each daughter DNA has one strand of the parent (old strand) and the other strand in newly formed.

Table of Contents

8.0 Introduction
8.1 Packaging of Hereditary Material
8.2 The Structure of DNA
8.3 Replication Of DNA In Eukaryotes
8.4 Replicatin of Pokaryotic Chromosome
8.5 Plasmids
8.6 RNA: Structure and Types
8.7 The Genetic Code
8.8 The Central Theme of Protein Synthesis

Chapter 9

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