What enzyme is responsible for breaking of bonds between nucleotides?

Helicases are enzymes that bind and may even remodel nucleic acid or nucleic acid protein complexes. There are DNA and RNA helicases. DNA helicases are essential during DNA replication because they separate double-stranded DNA into single strands allowing each strand to be copied. During DNA replication, DNA helicases unwind DNA at positions called origins where synthesis will be initiated. DNA helicase continues to unwind the DNA forming a structure called the replication fork, which is named for the forked appearance of the two strands of DNA as they are unzipped apart. The process of breaking the hydrogen bonds between the nucleotide base pairs in double-stranded DNA requires energy. To break the bonds, helicases use the energy stored in a molecule called ATP, which serves as the energy currency of cells. DNA helicases also function in other cellular processes where double-stranded DNA must be separated, including DNA repair and transcription. RNA helicases are involved in shaping the form of RNA molecules, during all processes involving RNA, such as transcription, splicing, and translation.

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Key enzyme involved in DNA replication, it is responsible for ‘unzipping’ the double helix structure by breaking the hydrogen bonds between bases on opposite strands of the DNA molecule.

This page was last updated on 2014-11-24

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Posted April 29, 2022

Hydrogen bonds in DNA replication are broken down by the helicase class of enzymes. This causes the double-stranded DNA molecule to unzip into two single strands, allowing each strand to be replicated. 

A DNA molecule is made up of two long strands of nucleotides held together by hydrogen bonds between complementary nitrogenous bases, resulting in a structure that resembles a coiled ladder. In order to facilitate DNA replication, the two strands of the molecule must be peeled apart. This can only be done by breaking the hydrogen bonds that hold together the nitrogenous bases. When DNA helicase unwinds, it acts as a wedge between the two DNA strands. As it keeps unwinding further, helicase, powered by ATP, keeps breaking apart the hydrogen bonds separating the two DNA strands in a manner similar to a zip opening. This causes the two strands of DNA to separate at the beginning of DNA replication. 

DNA Helicase-Polymerase Coupling in Bacteriophage DNA Replication

BrdU [5-Bromo-2'-deoxyuridine] *CAS 59-14-3*

Replication:DNA copied into DNA

Created by George Rice, Montana State University

In general, DNA is replicated by uncoiling of the helix, strand separation by breaking of the hydrogen bonds between the complementary strands, and synthesis of two new strands by complementary base pairing. Replication begins at a specific site in the DNA called the origin of replication. DNA replication is bidirectional from the origin of replication. To begin DNA replication, unwinding enzymes called DNA helicases cause the two parent DNA strands to unwind and separate from one another at the origin of replication to form two "Y-shaped" replication forks. These replication forks are the actual site of DNA copying. Helix destabilizing proteins bind to the single-stranded regions so the two strands do not rejoin. Enzymes called topoisimerases produce breaks in the DNA and then rejoin them in order to relieve the stress in the helical molecule during replication. As the strands continue to unwind and separate in both directions around the entire DNA molecule, the hydrogen bonding of free DNA nucleotides with those on each parent strand produce new complementary strands. As the new nucleotides line up opposite each parent strand by hydrogen bonding, enzymes called DNA polymerases join the nucleotides by way of phosphodiester bonds. Actually, the nucleotides lining up by complementary base pairing are deoxynucleoside triphosphates, composed of a nitrogenous base, deoxyribose, and three phosphates. As the phosphodiester bond forms between the 5' phosphate group of the new nucleotide and the 3' OH of the last nucleotide in the DNA strand, two of the phosphates are removed providing energy for bonding. Finally each parent strand serves as a template to synthesize a complementary copy of itself, resulting in the formation of two identical DNA molecules.

The following resources were originally accessed through the BioSciEd Net (BEN) digital resources collection, which is the National Science Digital Library (NSDL) Pathway for biological sciences education. For more teaching resources, please visit BEN to use their searchable database. BEN is free to use, but requires registration.

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