Question

What is the basis for the difference in the synthesis of the leading and lagging strand of DNA molecules?
A. Origins of replication occur only at the 5’ end of the molecule
B. DNA ligase works only in the 3’ 5’ direction
C. DNA polymerase can add new nucleotides only to the 3’ end of the growing strand
D. Helicases and single-strand binding proteins work at the 5’ end

Answer 1

Hint: DNA replication produces two types of new strands of DNA. One is continuous (leading strand) and other is discontinuous strand (lagging strand). DNA ligase joins lagging strands from 3’ to 5’ end while DNA polymerase helps in the synthesis of leading strands which run from 5’ to 3’ end.

Complete answer:
DNA replication is the process by which new strands are synthesized from the template strand. DNA has a double helical structure. One strand binds to the other strand with the help of hydrogen bonds. DNA consists of many sites at which replication starts that are known as the origin of replication. One strand of the DNA runs from 3’ to 5’ end while the other end runs from 5’ to 3’ end. The 5’end has phosphate group on 5 carbon of the deoxyribose; the 3’ end contains hydroxyl group on 3 carbon of the deoxyribose.
During replication, the template strand forms two new strands: one strand is completely synthesized that is leading strand while the other strand has discontinuous fragments. These fragments are called okazaki fragments. The leading strand synthesized in the 5’ to 3’ direction. The lagging strand synthesized in the 3’ to 5’ direction. Nucleotides of the leading strand are joined by the DNA polymerase. It only attaches at the 3’end of the growing strand. The fragments of the lagging strand are joined by DNA ligase.
From the above information we have found that ‘DNA polymerase can join new nucleotides only to the 3’ end of the growing strand’.

Hence, the correct answer is option (C).

Note: Replication starts by unzipping of DNA helix. It is done by the helicase enzyme. A forked like structure appears when the process of DNA replication starts. DNA polymerase plays a very important role in this process as well as in transcription also.