DNA replication.

1) The process by which DNA molecule produces daughter DNA molecules which are exact copies of each other and that of parental DNA or original DNA strand is called as DNA replication.

2) Semi – Conservative Method: It is a type of replication in which one strand of daughter DNA molecule is parental or conserved and other strand is newly synthesized. (Half old & half new).

The process can be explained as follows:-

A) Initiation point or Origin:

The process of replication starts at specific point of DNA called as Origin of replication.

Eukaryotic DNA shows more than one points of origin whereas prokaryotic DNA shows single point of origin.

B) Direction:

The process of replication is bidirectional i.e. proceeds simultaneously on both strands.

C) Unwinding:

1) Near point of origin the two DNA strands are cut with help of enzyme Restriction Endonucleases.

2) Enzyme DNA helicase unwinds the two DNA strands and make them straight. This enzyme is also called as DNA un-winding protein.

3) After this, H2 bonds between the two strands break in the presence of enzyme topoisomerase. These two strands separate from each other and N – Base are exposed.

D) Replication Fork:

The point at which the two strands are separated producing inverted ‘Y’ shaped structure is called as replication fork.

E) Synthesis of New Strand:

1) Each separated strand with its exposed N – Base acts as ‘templet’ or ‘Mould’ for synthesis of new strand.

2) It requires RNA primer i.e. small RNA sequence which remains attached to templet strand and helps to attract complimentary nucleotides.

3) Finally, nucleotides link together with help of phosphodiester linkages producing a long chain.

4) Enzyme RNA Primase is required for synthesis of RNA primer. Its activity is continued by DNA polymerase enzyme.

5) Newly synthesized strand remain connected with parental strands and shows complimentary base sequence with parental strand.

6) At the completion of process newly synthesized strands and conserved parental strands which are straight start coiling in the presence of enzyme DNA gyrase.

F) Leading strand and Lagging strand:-

1) The synthesis of new strand always begins near 3’ end of parental strand in direction 5’ to3 because polymerase can add new nucleotides only in direction 5’ to 3’

2) Out of two separated strands from parental DNA, one is termed as leading templet while other is termed as lagging templet.

3) Leading templet replicates continuously producing leading strand which grows continuously towards the fork.

4) Lagging templet replicates discontinuously producing lagging strand which grows away from the fork. 

5) Okazaki Fragments: Formation of lagging strand is discontinuous in the form of small fragments called as Okazaki fragments.

These fragments further join to form a continuous strand with the help of enzyme DNA ligase (Connects Okazaki fragments).

3. Significance of Replication

A) It is prerequisite for cell division.

B) It ensures equal distribution of DNA into daughter cell.

C) As a result of replication transfer of genetic information from one generation to other takes place.

Role of various enzymes involved in DNA replication.

The various enzymes involved in DNA replication are as follows:-

1) Restriction endonucleases: Cut DNA strands set the point of origin to initiate DNA replication.

2) DNA Helicase: It is also called as DNA unwinding protein and it makes DNA molecule straight.

3) Topoisomerase: It separates two DNA strands by breaking H– bonds between them and produces replication fork.

4) RNA Primase: It brings about synthesis of RNA primer, a short sequence nucleotides.

5) DNA polymerase: It adds new nucleotides in the direction 5’ to 3’ and helps in synthesis of polynucleotide chain of DNA.

6) DNA Ligase: It joins Okazaki fragments produced in the form lagging strand.

7) DNA Gyrase: It brings about coiling of daughter DNA molecule at end of replication.

Messelson and Stahl’s experiment

1) Messelson & stahl performed experiment to prove semi conservative mode of replication of DNA.

2) For experiment, bacterial DNA (E-Coli) was used.

3) This DNA was cultured over the nutrient media containing isotope of Nitrogen i.e. N¹⁵

4) Bacterial DNA was allowed to replicate over the same medium for several generations.

5) It was found that both strands contain N¹⁵ as constituent of N – Bases.

6) Then this bacterial DNA was cultured over the nutrient media containing N14 and allowed to replicate.

7) It was observed that daughter molecules with N14 & N15 strands were produced.

8) Thus, experiment proves that during each replication out of 2 stands produced, only one is newly synthesized and other is parental or conserved.

DNA replication is semi conservative

1)  In each daughter DNA molecule one strand is newly synthesized and other is parental strand.

2)  Originality of DNA molecule is thus maintained and conserved.

3)  Newly synthesized strand is complimentary to templet, parental strand and identical to other DNA strand. (Parental strand).

Hence, DNA replication is semiconservative.