Question

During DNA replication, the addition of nucleotides on the lagging strand occurs.
A. Towards the replication fork.
B. At a faster rate than leading occurs
C. Continuously
D. Discontinuously

Answer 1

Hint:- DNA replication is the process of forming two identical copies of DNA strands from one molecule of DNA. DNA replication occurs during the S-phase of cell division. In cell division, all the components of the cell, including DNA are duplicated and are given equally to the two daughter cells. DNA is a long polymer of deoxyribonucleotides.

Complete answer:
DNA replication is a very fast and efficient process which takes place in the nucleus. A set of enzymes are involved in the replication process. Some of these enzymes are DNA helicases, topoisomerases, RNA primase and DNA dependent DNA polymerase. As the DNA molecules are two long strands of DNA and therefore, cannot be separated in their entire length due to high energy requirements. Small opening within the DNA strand where the replication occurs is called a replication fork.
The replication does not initiate randomly at any place in the DNA. There are definite regions in the DNA where replication originates. These regions are called the origin of replication or ORI. Eukaryotic DNA has multiple origins of replication.
DNA replication begins with a cut made by an endonuclease enzyme at the origin of replication. DNA helicase enzyme breaks the hydrogen bond between the DNA strands thereby unwinding the DNA helix and separating the two strands. For the initiation of DNA analysis, a short segment of RNA called the RNA primer is required which is synthesized by RNA primase enzyme in the 5’-3' direction.
The new strand produced complementarity to the template strand with 3’-5' polarity is synthesized in a continuous manner and is called the leading strand . New strands produced complementarity to the template strand having 5’-3' polarity will be discontinuous. They are synthesized as fragments, called Okazaki fragments. The Okazaki fragments are later joined by the DNA ligase enzyme. This strand is called the lagging strand. This happens because the DNA dependent DNA polymerase enzyme catalyses polymerization only in one direction, that is 5’-3'. Therefore, the nucleotides are added against the replication fork in the lagging strand and is slower than the leading strand, as the lagging strand requires the action of DNA ligase to complete replication.
Thus, the right option is D.

Note:- DNA replication is an energetically expensive process. Deoxyribonucleoside triphosphates and high energy phosphates act as substrates. The unwinding of DNA imposes tension at the distal end of DNA molecule which is relieved by DNA gyrase or topoisomerase enzyme. To prevent the recoiling of DNA strands, single strand binding protein (SSB) binds to the template strand.