DNA Synthesis

Deoxyribonucleic acid (DNA) synthesis is a process by which strands of the building blocks of DNA (nucleic acids) are created. In a cell, DNA synthesis takes place in a process known as replication. Using genetic engineering and enzyme chemistry, scientists have also developed human-made methods for synthesizing DNA.

The DNA molecule was discovered by Francis Crick, James Watson, and Maurice Wilkins. In 1953, Watson and Crick used x-ray crystallography data from Rosalind Franklin to show that the structure of DNA is a double helix, which resembles a ladder whose long strands are twisted so that the structure somewhat resembles a spiral staircase. For this work, Watson, Crick, and Wilkins received the Nobel Prize for physiology or medicine in 1962.

DNA is a long chain polymer made up of chemical building blocks called nucleotides. It is the genetic material in most living organisms that carries information related to protein synthesis. Typically, DNA exists as two chains of nucleotides that are chemically linked following base pairing rules. Each nucleotide is made up of a type of sugar molecule, a phosphate group, and one of four nitrogen-containing bases. The bases are adenine (A), guanine (G), thymine (T), and cytosine (C). The bases can form bonds with one another, but only in a very specific way. Adenine links with thymine and guanine links with cytosine. The linked bases are oriented towards the inside of the DNA molecule, akin to the rungs of a ladder. The sugar portion of the molecule makes up the long strands, analogous to the sides of the ladder. The phosphate group holds the whole structure together by connecting the sugars. The order in which the nucleotides are linked is known as the sequence that is determined by DNA sequencing.

In a eukaryotic cell, DNA is synthesized prior to cell division by a process called replication. At the start of replication the two strands of DNA are separated by various enzymes. Each strand then serves as a template for producing a new strand. Replication is catalyzed by an enzyme known as DNA polymerase. This molecule brings complementary nucleotides to each of the DNA strands. The nucleotides connect to form new DNA strands, which are exact copies of the original strand known as daughter strands. Since each daughter strand contains half of the parent DNA molecule, this process is known as semi-conservative replication (this means that half of the new DNA molecule is derived from the older strand, with the other half being newly made). The process of replication is important because it provides a method for cells to transfer an exact duplicate of their genetic material from one generation of cell to the next.

After the nature of DNA was determined, scientists began to examine the cellular genes. When a certain gene was isolated, it became desirable to synthesize copies of that molecule. One of the first ways in which a large amount of a specific DNA was synthesized was though genetic engineering. Genetic engineering begins by combining a gene of interest with a bacterial plasmid. A plasmid is a small stretch of DNA that is found in many bacteria. The resulting hybrid DNA is called recombinant DNA. This new recombinant DNA plasmid is then injected into a bacterial cell. The cell is then cloned by allowing it to grow and multiply in a culture. As the cells multiply so do the copies of the inserted gene. When the bacteria has multiplied enough, the multiple copies of the inserted gene can then be isolated. This method of DNA synthesis can produce billions of copies of a gene in a couple of weeks.

In 1985, researchers developed a new process for synthesizing DNA called polymerase chain reaction (PCR). This method is much faster than previous known methods producing billions of copies of a DNA strand in just a few hours. It begins by putting a small section of double stranded DNA in a solution containing DNA polymerase, nucleotides, and primers. The solution is heated to separate the DNA strands. When it is cooled, the polymerase creates a copy of each strand. The process is repeated every five minutes in an automated machine until the desired amount of DNA is produced.