DNA replication

DNA replication

The process of synthesis of DNA molecule from preexisting dna is called dna replication.

DNA replication may occur by following process:

1     1.semi-conservative replication

         2.conservative replication

         3.dispersive replication

Semi-conservative mode of DNA replication

Enzymes involved in semi conservative DNA replication:

Helicase- helicase enzyme breaks the hydrogen bonds formed between bases of nucleotides

Topoisomerase- topoisomerase enzyme breaks and reseals one strand of DNA

DNA polymerase- it connects the nucleotides to form DNA polymer nucleotides. It forms RNA primer in DNA strand which is like a knot formed o thread. Due to its formation, newly formed DNA strands cannot separate.

Ligase- ligase binds the fragments of DNA strand together

Repair enzyme- this enzyme cuts off wrong base of nucleotide and rejoin correct base in newly formed DNA

Process:

DNA replication in  eukaryotes may begin at several points. in replication ,the helicase enzyme breaks the hydrogen bond between the bases of nucleotides. due to breaking of hydrogen bonds of nucleotides, the two strands separate. The process of separation of DNA strands also supported by enzyme topoisomerase. This enzyme breaks and reseals one strand of dna molecule.

Now the bases of DNA strands become exposed and joins with the free nucleotides present in nucleoplasm only in 5’-3’ direction. This process or formation of polynucleotide chain takes place in the presence of enzyme DNA polymerase. The four types of nucleotides are present in nucleus in the form of datp, dgtp; dttp and dctp.THESE molecules are triphosphates and break down into monophosphates damp, dtmp, dcmp and dgmp with the release of energy. These monophosphates nucleotides make hydrogen bonds with the complementary nucleotides of DNA strand.

According to base pairing rule the nucleotides are arranged. As DNA strand formation occurs only in 5’-3’ direction,in on strand of DNA a continuous strand is formed called leading strand ehereas in other strand of DNA,fragments of strands are formed,these fragments are called okazaki fragments named after Japanese scientist okazaki.the strand with okazaki fragments is called lagging strand.

Here,both DNA strands take part in replication process,so they are called as templates.the segments of strands are joined by the help of enzyme ligase.at the beginning of strand formation, RNA primer is formed at the DNA strand and it is due to the enzyme primase.sometime a wrong base maybe linked with the DNA nucleotide. When it occurs,the  repair enzyme replaces the wrong base by correct base.

In the above process,newly formed DNA have one strand of old DNA and one of new formed polynucleotide chain. Hence the process is called as semi conservative process of DNA replication.

It means half of parent DNA is conserved in new formed DNA.