top of page

DNA REPLICATION

(in prokaryotes)


important points
  • Autocatalytic- DNA itself catalyzes the formation of new DNA.

  • Semi-conservative- One strand is newly synthesized over an existing parental template strand.

  • Semi-discontinuous.

  • Template dependent process.

  • Based on complimentary base pairing. (Purine-pyrimidine)


SITE OF DNA REPLICATION
  • Occurs in the cytoplasm of Prokaryotes as they lack a membrane bound nucleus.


REQUIREMENTS FOR DNA REPLICATION


1) Origin of Replication site
  • Starting point of DNA replication.

  • Prokaryotes have only one Ori site in their genome.

  • Eukaryotes have multiple Ori sites in their genome.


2) SUBSTRATES
  • Ribonucleoside triphosphates (NTPs)- For RNA primer formation.

  • Deoxyribonucleoside triphosphates (dNTPs)- For DNA polymerization.

  • Template DNA- Used for complimentary base pairing.


3) ENZYMES
  • Helicase- ATP dependent unwinding of DNA helix.

  • DNA gyrase- Creates nicks to release supercoiling caused by unwinding.

  • Primase- RNA primer formation. (DNA dependent RNA polymerase)

  • DNA ligase- Joining of DNA fragments.

  • DNA polymerase I

    • 5'-> 3' polymerase activity- for polymerization.

    • 5'-> 3' exonuclease activity- for primer removal.

    • 3'-> 5' exonuclease activity- for proof reading.

  • DNA polymerase II

    • 5'-> 3' polymerase activity- for polymerization.

    • 5'-> 3' exonuclease activity- absent

    • 3'-> 5' exonuclease activity- for proof reading.

  • DNA polymerase III

    • 5'-> 3' polymerase activity- for polymerization. (main polymerase)

    • 5'-> 3' exonuclease activity- absent

    • 3'-> 5' exonuclease activity- for proof reading.


4) PROTEINS
  • Single Strand Binding Proteins (SSBP)


5) IONS
  • Magnesium ions.



STEPS OF DNA REPLICATION

1) Unwinding of DNA helix
  • Starts at Ori site.

  • Helicase binds to the Ori site and unwinds the double stranded DNA into single strands.

  • Helicase uses ATP to break H-bonds between the bases, so only short stretches of DNA undergoes unwinding due to high energy requirements.

  • Single stands are stabilized by binding of SSBP to prevent it from rejoining.

  • Unwinding of DNA results in formation of Replication fork.


2) RELEASING OF SUPERCOILING
  • Unwinding of DNA at Ori site creates supercoiling in the upstream region.

  • Supercoiling is released by DNA gyrase by creating nicks (breaking of phosphodiester bonds) in the DNA and rejoining it.


3) ADDITION OF RNA PRIMERS
  • Enzyme Primase adds a very short fragment of RNA over the template strands to initiate DNA replication.

  • RNA primers provides the free -OH group required for DNA polymerase enzyme to start adding deoxyribonucleotides based on complimentary base pairing.


4) FORMATION OF NEW STRANDS
  • Occurs in only 5'-> 3' direction.

  • Directional activity of DNA polymerase makes the DNA replication process very complex as the existing DNA strands have anti-parallel polarity.

  • One of the strand is synthesized (over the 3'-> 5' template strand) continuously and requires only one primer. This strand is called the Leading Strand and its synthesis is fast.

  • The synthesis of other strand (over the 5'-> 3' template) is however discontinuous and requires multiple primers. This strand is called Lagging Strand and is initially synthesized in the form of relatively short fragments of DNA called Okazaki fragments.

  • The synthesis of Lagging strand is slow and it lags behind the Leading strand.


5) REMOVAL OF RNA PRIMERS
  • The DNA polymerase I removes the RNA primers by 5'-> 3' exonuclease activity and also adds the deoxyribonucleotides to replace the RNA primers.

  • The Okazaki fragments are now joined by DNA ligase to complete the DNA replication process.


 


47 views0 comments

Recent Posts

See All

Comments


AD SPACE

bottom of page