Fun Times of DNA Replication

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In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from a single original DNA molecule. This method occurs in all living organisms and is the basis for biological inheritance. The cell possesses the distinctive property of division, which makes replication of DNA essential. DNA is composed of a double helix of two complementary strands. During replication, these strands are separated. Each strand of the original DNA molecule then functions as a template for the creation of its counterpart, a procedure referred to as semiconservative replication. Cellular proofreading and error-checking mechanics ensure near perfect fidelity for DNA replication.

In a cell, DNA replication starts at specific locations, or origins of replication, in the genome. Unwinding of DNA at the source and synthesis of new strands leads to replication forks growing bi-directionally from the origin. A number of proteins are associated with the replication fork to help in the initiation and continuation of DNA synthesis. Most prominently, DNA polymerase synthesizes the new strands with the addition of nucleotides that match each (template) strand. DNA replication may also be performed in vitro (artificially, outside a mobile ). DNA polymerases isolated from cells and artificial DNA primers can be used to initiate DNA synthesis at known sequences in a template DNA molecule. The polymerase chain reaction (PCR), a frequent laboratory technique, cyclically applies such artificial synthesis to amplify a specific target DNA fragment from a pool of DNA. DNA usually exists as a double-stranded structure, with both strands coiled together to form the characteristic double-helix. Each single strand of DNA is a series of four kinds of nucleotides. Nucleotides in DNA have a deoxyribose sugar, a phosphate, and a nucleobase.

The four kinds of nucleotide correspond to the four nucleobases adenine, cytosine, guanine, and thymine, commonly abbreviated as A,C, G and T. Adenine and guanine are purine bases, while cytosine and thymine are pyrimidines. These nucleotides form phosphodiester bonds, producing the phosphate-deoxyribose backbone of the DNA double helix with the nuclei bases pointing inward (i.e., toward the opposing strand). Nucleotides (bases) are matched between strands through hydrogen bonds to form base pairs. Adenine pairs with thymine (two hydrogen bonds), and guanine pairs with cytosine (more powerful: three hydrogen bonds).

DNA strands have a directionality, and the different ends of a single strand are known as the”3′ (three-prime) end” and the”5′ (five-prime) end”. By convention, if the base sequence of a single strand of DNA is given, the left end of the sequence is the 5′ end, while the ideal end of the sequence is the 3′ end. These conditions refer to the carbon atom in deoxyribose to which the next phosphate from the series attaches. Directionality has consequences in DNA synthesis, because DNA polymerase can synthesize DNA in just 1 direction by adding nucleotides to the 3′ end of a DNA strand. The pairing of complementary bases in DNA (via hydrogen bonding) means that the data contained within each strand is redundant.

Phosphodiester (intra-strand) bonds are stronger than hydrogen (inter-strand) bonds. This allows the strands to be separated from one another. The nucleotides on a single strand can therefore be used to reconstruct nucleotides on a recently synthesized partner strand. Everything required to know about DNA structure and its replication.

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