04 Molecular Basis of Inheritance - part 03 - DNA Replication

04 Molecular Basis of Inheritance - part 03 - DNA Replication


DNA Replication :
  • The DNA molecule regulates and controls all the activities of the cell. Because of its unique structure, it is able to control the synthesis of other molecules of the cell. 
  • At the same time when the cell reproduces, the DNA also should duplicate itself to distribute equally to the daughter cells. 
  • As a carrier of genetic information, DNA has to perform two important functions : 
a. Heterocatalytic function : 
  • When DNA directs the synthesis of chemical molecules other than itself, then such functions of DNA are called heterocatalytic functions. 
  • Eg. Synthesis of RNA (transcription), synthesis of protein (Translation), etc. 
b. Autocatalytic function : 

  • When DNA directs the synthesis of DNA itself, then such function of DNA is called autocatalytic function. Eg. Replication. 
The process by which DNA duplicates itself is called replication. 

  • Through replication, it forms two copies that are identical to it. 
  • In eukaryotic organisms, replication of DNA takes place only once in the cell cycle. It occurs in the S- phase of interphase in the cell cycle. 
  • DNA replicates through Semiconservative mode of replication. 
  • The model for Semiconservative replication was proposed by Watson and Crick, on the basis of antiparallel and complementary nature of DNA strands. 
  • The process of semicoservative replication is as below: 










1. Activation of Nucleotides: 

  • The four types of nucleotides of DNA i.e. dAMP, dGMP, dCMP and dTMP are present in the nucleoplasm. 
  • They are activated by ATPin presence of an enzyme phosphorylase. 
  • This results in the formation of deoxyribonucleotide triphosphates i.e. dATP, dGTP, dCTP and dTTP. The process is known as Phosphorylation. 
2. Point of Origin or Initiation point: 
  • It begins at specific point `O' -origin and terminates at point `T'. Origin is flanked by `T' sites. 
  • The unit of DNA in which replication occurs, is called replicon. 
  • In prokaryotes, there is only one replicon however in eukaryotes, there are several replicons in tandem.
  • At the point `O', enzyme endonuclease nicksone of the strands of DNA, temporarily. 
  • The nick occurs in the sugar-phosphate back bone or the phosphodiester bond. 
3. Unwinding of DNA molecule: 
  • Now enzyme DNA helicase operates by breaking weak hydrogen bonds in the vicinity of `O. 
  • The strands of DNA separate and unwind. 
  • This unwinding is bidirectional and continues as `Y' shaped replication fork. Each separated strand acts as template. 
  • The two separated strands are prevented from recoiling (rejoining) by SSBP (Single strand binding proteins). 
  • SSB proteins remain attached to both the separated strands so as to facilate synthesis of new polynucleotide strands.
4. Replicating fork: 

  • The point formed due to unwinding and separation of two strands appear like a Y-shaped fork, called replicating/ replication fork. 
  • The unwinding of strands imposes strain which is relieved by super-helix relaxing enzyme. 
5. Synthesis of new strands: 
  • Each separated strand acts as mould or template for the synthesis of new complementary strand. 
  • It begins with the help of a small RNA molecule, called RNA primer. 
  • RNA primer get associated with the 3' end of template strand and attracts complementary nucleotides from surrounding nucleoplasm. 
  • These nucleotides molecules bind to the complementary nucleotides on the template strand by forming hydrogen bonds (i.e. A=T or T=A; G = C or C = G). 
  • The newly bound nucleotides get interconnected by phosphodiester bonds, forming a polynucleotide strand. 
  • The synthesis of new complementary strand is catalyzed by enzyme DNA polymerase. 
  • The new complementary strand is always formed in 5'-3' direction. 
6. Leading and Lagging strand: 
  • The template strand with free 3' end is called leading template and with free 5' end is called lagging template. 
  • The process of replication always starts at C-3 end of template strand and proceeds towards C-5 end. 
  • As both the strands of the parental DNA are antiparallel, new strands are always formed in   5'  - 3' direction. 
  • One of the newly synthesized strand develops continously towards replicating fork is called leading strand. 
  • Another new strand develop discontinuouslyaway from the replicating fork is called lagging strand. 
Maturation of Okazaki fragments : 
  • DNA synthesis on lagging template takes place in the form of small fragments, called Okazaki fragments (named after scientist Okazaki). 
  • Okazaki fragments are joined by enzyme DNA ligase. 
  • RNA primers are removed by DNA polymeraseand replaced by DNA sequence with the help of DNA polymerase-I in prokaryotes and DNA polymerase-Ī± in eukaryotes. 
  • Finally, DNA gyrase (topoisomerase) enzyme forms double helix to form daughter DNA molecules. 
7. Formation of daughter DNA molecules:
  • At the end of the replication, two daughter DNA molecules are formed. 
  • In each daughter DNA, one strand is parental and the other one is totally newly synthesized. 
  • Thus, 50% is contributed by mother DNA. Hence, it is described as semiconservative replication.

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