Explain semiconservative mode of DNA replication.

Explanation

The process of DNA replication in eukaryotic cells takes place inside the nucleus and occurs in the following steps :
1. Activation of nucleotides :
(1) In the nucleus , there are all the four types of nucleotides in the form of their monophosphates such as DAMP , DTMP , DCMP and dGMP. They are activated into their triphosphates respectively , viz,... DATP ,DTTP, DCTP and dGTP.
(2) This transformation is done with the help of ATP and in the presence of enzyme phosphorlase. This process is called activation of nucleotides.
2. Origin or initiation process :
(1) DNA repliacation starts at the specific point called origin or initiation point .
(2) In eukaryotic cells, there are several such points. Enzyme endonuclease acts at such points and causes a nick or incision . This initiates breaks in the hydrogen bonds between the two strands resultign into unzipping of the DNA strands. In this way the DNA molecule is split.
3. Unwinding of DNA strands :
(1) The enzyme endonculease frees the DNA strands , later on the enzyme helicase or DNA unwinding protein (rep protein) separates the DNA strands further. Due to this separation, DNA looks like an inverted Y. This structure is called replication fork.
(2) The spearated strands are prevented from coiling back with the help of SSBP. (Single Strand DNA Binding protein) or helix destabilizing protein.
4. Synthesis of new Strands :
(1) Each separated strand acts like a template for the synthesis of a new strand which is complementary to the old one. This process is helped by a small RNA molecule called RNA primer.
(2) The enzyme RNA primase controls the synthesis of RNA primer. RNA primer attaches itself to 3 and of the template strand and attracts the complementary nucleotides from the surrounding nucleoplasm.
(3) The entire process is due to the influence of enzyme DNA poltmerase. The new nucleotides are arranged on a leading strand in ` 5' to 3'` direction.
5. Formation of leading and lagging strands :
(1) The enzyme DNA polmerase can catalyze the polymerization reaction only in the one direction , i.e, ` 5' to 3'` . therefore, the template which is opening from ` 3' to 5'` direction acts as a leading template. Its opposite strand is leading or continuous strand as here the synthesis occurs in ` 5' to3'` direction and is in a continuous way. On a leading strand the synthesis takes place at a faster rate and in a continuous manner.
(2) The other strand which opens from ` 5' to 3'` is the lagging template. It synthesizes its comple-mentary strand which has ` 3' to 4' ` orientation. Therefore, here synthesis does not take place in a continuous manner and is also at slower pace. This strand is therefore called lagging strand.
(3) The lagging strand is synthesized in the form of short DNA fragments which are in `5' to 3'` direction. These DNA fragments are called Okazaki fragments which are later joined together by the enzyme DNA ligase.
(4) During synthesis each Okazaki fragment requires RNA primer. Which is later removed by RNAase enzymes.
6. Formation of daughter DNA molecules :
(1) Each old strand produces a new complementary stand. Both old and new strand then undergo coiling and two indentical DNA molecules are produced from one DNA molecule at the end of DNA replication process.
(2) This type of replication is called semi- conservative replication as 50% part of moter DNA molecule is retained and only 50% part is newly synthesized .
` (##NVT_21_BIO_XII_C12_E02_002_S01.png" width="80%">