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    Deoxyribonucleic Acid (DNA) is a double stranded helical structure with each strand containing permutations of nucleotides that contain information of protein synthesis. Nucleotides are made of a five-carbon sugar, nitrogenous base and phosphate group. There are four types of nitrogenous bases that are complementarily paired to each other: adenine pairs with thymine by two hydrogen bonds and cytosine and guanine pair with three hydrogen bonds.  

    One parent strand of DNA undergoes replication to produce two strands of DNA identical to the parent. DNA replication is called ‘semiconservative’ as it uses the unwound parent strands as templates to produce complementary strands that join together to form a helix. The two strands that form DNA run in anti-parallel directions, and it unzips to produce replication forks on each side which serve as the template for DNA replication. DNA is read in the 3’ to 5’ direction whilst the complementary strand is produced in the 5’ to 3’ direction. There are three stages of DNA replication: initiation, elongation and termination.

    In initiation, DNA helicase identifies an initiator protein where it begins to break hydrogen bonds between the complementary bases, effectively ‘unzipping’ the double-stranded parent DNA and producing the replication fork with a leading and lagging strand. In elongation, the enzyme primase adds RNA primers on the template strands for nucleotide attachment. Then, DNA polymerase matches free nucleotides to the template strands and adds phosphodiester bonds to produce a complementary strand. Part of the DNA polymerase complex acts to proofread the new DNA strand for any mutations in base sequence. In termination, DNA ligase binds to a termination sequence to stop DNA replication. The two strands then wind to form two identical DNA helices. 

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