Question

A section of DNA appears as shown above.
When replication occurs, if this molecule is being unzipped in from left to right, which strand will be the leading strand, and why?
3'...ATGGCATCG... 5
5'...TACCGTAGC...3’
A. The top strand, because DNA polymerase reads the template strand in a 5’-3’ direction forming the new strand in a 3’-5’ direction.
B. The bottom strand, because DNA polymerase reads the template strand in a 3’-5’ B direction making the new strand in segments running 3-5.
C. The top strand, because DNA polymerase replicates the template strand in a continuous 5 - 3 direction.
D. The top strand, because DNA polymerase replicates the template strand in a continuous 5 - 3 direction.

Answer 1

Hint:
DNA replication is the process of making another copy of DNA from itself, and this process mainly takes place during cell division such as mitosis, where duplication of DNA is necessary to have some sets of DNA in offsprings like Parents.

Complete answer:
-The first step in the DNA replication process is to unzip the double-stranded DNA molecule. Where the unzipping can be done by enzyme helicase.
-Helicase breaks down the hydrogen bonds present between the complementary base pairs of strands so that two strands of the DNA separates, and it forms the Y shaped replication fork, where both separated strands act as template strands.
-The strand that is present toward the replication fork which is from 3’ to 5’, and this strand is called as a leading strand, and another one is the strand that directed away from the replication fork which is from 5’ to 3’, and this strand is called as lagging strand.
-In the leading strand, a small piece of RNA, which is called a primer, comes along and binds to the end of the leading strand, where the primer acts as the starting point of DNA synthesis.
-And an enzyme called DNA polymerase binds to the leading strand, and it walks along the strand, and it’s adding new complementary bases to the strand in a 5’ to 3’ direction.
-In the leading strand, adding complementary base pairs occurs continuously, but is the lagging strand it is discontinuous because, primase act at different points and the DNA formed as Okazaki fragments, where they should be sealed further by the enzyme ligase.
-So, after the formation of the complementary base pairs, proofreading formed strands occurs, if everything is ok, then sealing takes place, and two strands are formed.
So the correct option is A.

Note: The replication of DNA is said to be semiconservative because, in the process of replication, the half strand is obtained from the parent molecule because replication occurs and complementary base pairs are added for both strands, where the strand the adding from 5’to 3’ occurs, then that strand is leading strand.