DNA REPLICATION


DNA replication is the process by which a DNA molecule makes an exact copy of itself. It takes place in cells during the interphase stages of mitosis and meiosis. The process uses the enzymes, DNA polymerase, DNA ligase and DNA helicase; also used are ATP and a DNA template.

The mechanism for the replication of a DNA molecule is semi-conservative. This means that, parental strands separate, and strand serves as the template for making a new complementary strand. Thus, in each replica, one strand is derived intact from the parent molecule, and the other strand is newly synthesized.

The process of replication of the DNA  involves breakages of the hydrogen bonds between complete two strands, under the in helicase, in short sections, from one  of the molecule. After a breakage occurs, complementary strands separate from each other: This process is called unwinding or unzipping. Each split in the DNA molecule is called a replication fork. At these forks, each separated strand serves as a template for the synthesis of a new complementary strand.

There is linking of free deoxyribonucleotide units that are present in the nucleus to complementary bases on each of the DNA template strands. The newly assembled deoxyribonecleotides are linked together by phosphodiester bonds. DNA polymerase, which analysis both processes, works only in a 5’ to 3’ direction.

While the process of synthesizing a new strand is continuous along one strand. it is discontinuous along the other strand. The strand that is continuously synthesized is referred to as the leading sirand; and the strand synthesized discontinuously is referred to as the lagging strand. The leading strand has a 5' phosphoric acid group at the end where unzipping begins, the lagging strand has 3'-0H group at this end. Each of the newly synthesized strands is of opposite polarity to a template strand.

The synthesis of the continuous strand begins from where the DNA molecule iirst forks and proceeds backwards to the end where unzipping begins. Subsequent segment of new strands are synthesized beginning from where the DNA molecule forks again, in other words, synthesis of discontinuous segments is in the opposite direction with respect to the replication fork. These segments are called Okazaki fragments; they are joined together by DNA ligase to form a complete new strand.

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