New research has given a clearer picture of how DNA replication begins at the molecular level, showing how inactive MCM2-7 double hexamers are remodelled into active CMG helicases. The work shows that helicase activation is coupled to the initial melting and unwinding of DNA. The institution and journal behind the study are not named.
Helicase is an enzyme that unwinds and separates the two strands of DNA, allowing the genetic information in them to be copied. DNA normally exists as a double helix, with complementary bases on its two strands held together by hydrogen bonds. These strands have to be separated during replication.
Replicative helicases are loaded onto DNA at or near replication origins. They then move along the DNA using energy from nucleotide hydrolysis, which drives the separation of the strands. As a helicase moves, it destabilises the base-pairing interactions between complementary bases and exposes the individual strands.
This unwinding produces a Y-shaped replication fork, where the separated parental strands become available to the replication machinery.
Unwinding generates torsional stress and supercoiling ahead of the replication fork. Topoisomerases relieve this stress and prevent excessive twisting, tangling or breakage of the DNA.
Each separated parental strand then acts as a template for the synthesis of a new complementary DNA strand by the replication machinery.
DNA replication is tightly regulated so that it begins at the correct stage of the cell cycle. The newly detailed step, in which inactive MCM2-7 double hexamers become active CMG helicases, is part of that control. Because activation is linked to the first melting and unwinding of DNA, the switch from a loaded but inactive helicase to a working one coincides with the opening of the DNA itself.
With reference to the activation of helicases at the start of DNA replication, consider the following statements:
Which of the statements given above are correct?
Consider the following statements:
Statement-I: Topoisomerases are required ahead of the replication fork during DNA replication.
Statement-II: Unwinding of DNA by helicase generates torsional stress and supercoiling ahead of the replication fork.
Which one of the following is correct in respect of the above statements?
Which one of the following best describes the replication fork as per the article?
