Question

a) During DNA replication, why is it that the entire molecule does not open in one go?

b) Explain replication fork.

c) What are the two functions that the monomers (dNTPs) play?

Answer 1

a) Entire DNA will not open up at a time:

DNA is a long thread, approximately 2.2 m long in humans. So the entire DNA molecule will not open up at a because of the following reasons:

• If the entire DNA molecule will unwind at a time, it becomes entangled and difficult for replication enzymes to carry out replication.
• Unwinding of DNA by helicase enzymes is an energy requiring process. So to open an entire DNA molecule at a time a lot of energy is needed, which is not feasible for the cell.
• Unwinding of DNA strands creates supercoiling at the downstream end. So the entire DNA molecule does not open at a time.

b) Replication fork:

Replication fork is the structure formed during DNA replication by helicase enzymes.

• In this structure, a small stretch of DNA is unwound by helicase by breaking hydrogen bonds between the DNA fragments while the rest of the DNA remains unwound.
• It resembles a two pronged fork, therefore called a replication fork.

c) Two roles of dNTPs:

dNTPs play the following major two roles in DNA replication:

1. Substrates for replication

• They act as the substrate for DNA dependent DNA polymerase.
• This means deoxyribonucleotide monophosphates are the monomers which are added in the newly synthesized strand one by one.
• dNTPs act as precursor molecules for the generation of dNMPs.
• dNTP → dNMP + 2Pi + Energy
  

2. They provide energy

• dNTPs provide energy for the polymerization reaction as DNA replication is an energy requiring process.
• DNA polymerase does not use any other ATP molecules for this process.
• It uses the energy from breakdown of phosphate bonds in dNTPs to add new deoxyribonucleotides in the newly synthesized strand.
• dNTP → dNMP + 2Pi + Energy