DNA Replication Process in Prokaryotes and Eukaryotes MCQ - Practice Questions with Answers

• The process of DNA replication occurs in three stages:
  • Initiation
  • Elongation
  • Termination

Initiation:

• DNA replication initiates at the particular regions on the DNA strand called the origin of replication.
• The process begins with the activity of DNA helicase that unwinds the two helical strands. Such strands are now called template strands and form a structure called the replication fork.
• Specialized proteins called single-stranded binding proteins now bind to the template strands and prevent the reformation of H-bonds between the separated template strands.
• RNA primase now synthesizes RNA primers that serve as ‘kick-starters’ for the activity of DNA polymerase because It requires a free 3'-OH group to which it can add nucleotides by forming a phosphodiester bond between the 3'-OH end and the 5' phosphate of the next nucleotide.

Elongation:

• DNA polymerase is able to add nucleotides only in the 5' to 3' direction (a new DNA strand can be only extended in this direction). 
• However, the DNA double helix is antiparallel; that is, one strand is in the 5' to 3' direction and the other is oriented in the 3' to 5' direction.
• So, the parental template strand with 3’ to 5’ polarity provides for uninterrupted and continuous synthesis of daughter strand in 5’ to 3’ direction. 
• Such a daughter strand in 5’ to 3’ direction is called the leading strand and is synthesized in the direction of the replication fork. 
• Only a single RNA primer is required for the polymerization of the leading strand.
• The other parental template strand with 5’ to 3’ direction serves for interrupted polymerization of daughter strand in 3’ to 5’ direction.
• Multiple RNA primers are synthesized over 5’ to 3’ template strand and each of these is then elongated in the form of Okazaki fragments.
• Such a daughter strand which is synthesized in 3’ to 5’ direction is called lagging strand. It is synthesized away from the direction of the replication fork.
• A protein called the sliding clamp holds the DNA polymerase in place as it continues to add nucleotides. 
• The sliding clamp is a ring-shaped protein that binds to the DNA and holds the polymerase in place. 

Termination:

• After the process of polymerization is completed in the given replication fork, the primers are removed by the exonuclease activity of DNA pol I, and the gaps are filled in by deoxyribonucleotides.
• The nicks that remain between the newly synthesized DNA (that replaced the RNA primer) and the previously synthesized DNA are sealed by the enzyme DNA ligase that catalyzes the formation of a phosphodiester linkage between the 3'-OH end of one nucleotide and the 5' phosphate end of the other fragment.
   

• Eukaryotic genomes are larger and complex as compared to the prokaryotic genome.
• The number of DNA polymerases in eukaryotes is much more than prokaryotes: 14 are known, of which five are known to have major roles during replication and have been well studied. 
• They are known as pol α, pol β, pol γ, pol δ, and pol ε.
• The essential steps of replication are the same as in prokaryotes. 
• The helicase using the energy from ATP hydrolysis opens up the DNA helix. 
• Replication forks are formed at each replication origin as the DNA unwinds. 
• The opening of the double helix causes over-winding, or supercoiling, in the DNA ahead of the replication fork. 
• These are resolved with the action of topoisomerases. 
• Primers are formed by the enzyme primase and using the primer, DNA pol can start synthesis. 
• While the leading strand is continuously synthesized by the enzyme pol δ, the lagging strand is synthesized by pol ε.
•  A sliding clamp protein is known as PCNA (Proliferating Cell Nuclear Antigen) holds the DNA pol in place so that it does not slide off the DNA. 
• RNase H removes the RNA primer, which is then replaced with DNA nucleotides. 
• The Okazaki fragments in the lagging strand are joined together after the replacement of the RNA primers with DNA. 
• The gaps that remain are sealed by DNA ligase, which forms the phosphodiester bond.

Telomeres in Eukaryotic Chromosomes:

• The ends of the linear chromosomes are known as telomeres, which have repetitive sequences that code for no particular gene. 
• In humans, a six base pair sequence, TTAGGG, is repeated 100 to 1000 times. 
• The telomerase enzyme contains a catalytic part and a built-in RNA template. 
• It attaches to the end of the chromosome, and complementary bases to the RNA template are added on the 3' end of the DNA strand. 
• Once the 3' end of the lagging strand template is sufficiently elongated, DNA polymerase can add the nucleotides complementary to the ends of the chromosomes. 
• Thus, the ends of the chromosomes are replicated.