Exploring Biology: DNA Replication and Structure

In molecular biology, DNA replication is the biological course of action of producing two identical replicas of DNA from one original DNA molecule. This course of action occurs in all living organisms and is the basis for biological inheritance. The cell possesses the distinctive character of division, which makes replication of DNA basic. DNA is made up of a double helix of two complementary strands. During replication, these strands are separated. Each strand of the original DNA molecule then serves as a template for the production of its style, a course of action referred to as semiconservative replication. Cellular proofreading and error-checking mechanisms ensure near perfect fidelity for DNA replication.

In a cell, DNA replication begins at specific locations, or origins of replication, in the genome. Unwinding of DNA at the origin and combination of new strands results in replication forks growing bi-directionally from the origin. A number of proteins are associated with the replication fork to help in the lauching and continuation of DNA combination. Most prominently, DNA polymerase synthesizes the new strands by adding nucleotides that supplement each (template) strand. DNA replication occurs during the S-stage of interphase. DNA replication can also be performed in vitro (artificially, outside a cell). DNA polymerases secluded from cells and artificial DNA primers can be used to begin DNA combination at known sequences in a template DNA molecule. The polymerase chain reaction (PCR), a shared laboratory technique, cyclically applies such artificial combination to amplify a specific target DNA break up 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 chain of four types of nucleotides. Nucleotides in DNA contain a deoxyribose sugar, a phosphate, and a nucleobase.

The four types 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, creating the phosphate-deoxyribose backbone of the DNA double helix with the nuclei bases pointing inward (i.e., toward the opposing strand). Nucleotides (bases) are equaled between strands by hydrogen bonds to form base pairs. Adenine pairs with thymine (two hydrogen bonds), and guanine pairs with cytosine (stronger: three hydrogen bonds).

DNA strands have a directionality, and the different ends of a single strand are called the 3 (three-chief) end and the 5 (five-chief) end. By convention, if the base ordern of a single strand of DNA is given, the left end of the ordern is the 5 end, while the right end of the ordern is the 3 end. The strands of the double helix are anti-similar with one being 5 to 3, and the opposite strand 3 to 5. These terms refer to the carbon atom in deoxyribose to which the next phosphate in the chain attaches. Directionality has consequences in DNA combination, because DNA polymerase can synthesize DNA in only one direction by adding nucleotides to the 3 end of a DNA strand. The pairing of complementary bases in DNA (by hydrogen bonding) method that the information 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 consequently be used to reconstruct nucleotides on a newly synthesized partner strand. Everything needed to know about DNA structure and its replication.