32. (a) Study the structure shown in the figure (two parental DNA strands with the strands separated at the lower end: left strand with \(5^{\prime}\) at the top and \(3^{\prime}\) at the lower end, right strand with \(3^{\prime}\) at the top and \(5^{\prime}\) at the lower end).
(i) Identify the structure shown above and name the process taking place.
(ii) Write the source of energy for the process and list the enzymes involved.
(iii) Mention the difference in the process based on polarity. What terms are used for such synthesis ?
(iv) Redraw the structure after completion of the process and label the newly synthesised parts.
Show Hint
The Y-shaped structure is a replication fork. Polymerase works only \(5^{\prime}\) to \(3^{\prime}\), so one strand is made continuously and the other in fragments.
Step 1: Part (i): Identify the structure:
The strands are joined as a duplex at the top and are pulled apart into two arms at the bottom, so the shape is a Y.
This Y shape is a replication fork. The process taking place is DNA replication.
Step 2: Part (ii): Energy and enzymes:
Energy comes from the deoxyribonucleoside triphosphates (dATP, dGTP, dCTP, dTTP). Each loses two phosphates when it is added to the new strand, and this releases the energy.
Enzymes: helicase (opens the double helix), topoisomerase (relieves the strain), primase (makes the RNA primer), DNA polymerase (adds nucleotides) and DNA ligase (joins fragments).
Step 3: Part (iii): Difference based on polarity:
DNA polymerase can add nucleotides only in the \(5^{\prime}\) to \(3^{\prime}\) direction.
The fork moves upward in the figure, towards the intact duplex. The left strand, read towards the fork, runs \(3^{\prime}\) to \(5^{\prime}\), so its new strand grows continuously towards the fork. This is the leading strand.
The right strand, read towards the fork, runs \(5^{\prime}\) to \(3^{\prime}\). Its new strand can only be made in short pieces called Okazaki fragments, which DNA ligase joins. This is the lagging strand.
The terms are continuous synthesis (leading strand) and discontinuous synthesis (lagging strand).
Step 4: Part (iv): Redrawn structure after replication:
A labelled sketch is expected. Two daughter DNA molecules are drawn, each with one old strand and one new strand.
Left old strand (\(5^{\prime}\) top, \(3^{\prime}\) bottom) is paired with a new leading strand: its end next to the old \(5^{\prime}\) is \(3^{\prime}\), and its other end is \(5^{\prime}\). It is labelled "newly synthesised leading strand, continuous".
Right old strand (\(3^{\prime}\) top, \(5^{\prime}\) bottom) is paired with a new lagging strand made of Okazaki fragments joined by ligase: \(5^{\prime}\) at the top, \(3^{\prime}\) at the bottom. It is labelled "newly synthesised lagging strand, discontinuous".
RNA primers are shown at the start of each fragment.
Final Answer:
Structure: replication fork. Process: DNA replication. Energy: dNTPs. Leading strand is continuous and lagging strand is discontinuous, because polymerase works only \(5^{\prime}\) to \(3^{\prime}\).
